build: add entt library

include/entt/.clang-format

@@ -0,0 +1,2 @@
+DisableFormat: true
+SortIncludes: false

include/entt/config/config.h

@@ -0,0 +1,134 @@
+#ifndef ENTT_CONFIG_CONFIG_H
+#define ENTT_CONFIG_CONFIG_H
+
+#if __has_include(<entt/ext/config.h>)
+#    include <entt/ext/config.h>
+#endif
+
+#include <version>
+#include "version.h"
+
+// NOLINTBEGIN(cppcoreguidelines-macro-usage)
+
+#ifdef ENTT_USE_STL
+#    define ENTT_FORCE_STL
+#endif
+
+#if defined(__cpp_exceptions) && !defined(ENTT_NO_EXCEPTION)
+#    define ENTT_THROW throw
+#    define ENTT_TRY try
+#    define ENTT_CATCH catch(...)
+#else
+#    define ENTT_THROW
+#    define ENTT_TRY if(true)
+#    define ENTT_CATCH if(false)
+#endif
+
+#if defined(__cpp_consteval)
+#    define ENTT_CONSTEVAL consteval
+#else
+#    define ENTT_CONSTEVAL constexpr
+#endif
+
+#ifdef ENTT_USE_ATOMIC
+#    include "../stl/atomic.hpp"
+#    define ENTT_MAYBE_ATOMIC(Type) stl::atomic<Type>
+#else
+#    define ENTT_MAYBE_ATOMIC(Type) Type
+#endif
+
+#ifndef ENTT_ID_TYPE
+#    include "../stl/cstdint.hpp"
+#    define ENTT_ID_TYPE stl::uint32_t
+#else
+#    include "../stl/cstdint.hpp" // provides coverage for types in the std namespace
+#endif
+
+#ifndef ENTT_SPARSE_PAGE
+#    define ENTT_SPARSE_PAGE 4096
+#endif
+
+#ifndef ENTT_PACKED_PAGE
+#    define ENTT_PACKED_PAGE 1024
+#endif
+
+#ifdef ENTT_DISABLE_ASSERT
+#    undef ENTT_ASSERT
+#    define ENTT_ASSERT(condition, msg) (void(0))
+#elif !defined ENTT_ASSERT
+#    include <cassert>
+#    define ENTT_ASSERT(condition, msg) assert(((condition) && (msg)))
+#endif
+
+#ifdef ENTT_DISABLE_ASSERT
+#    undef ENTT_ASSERT_CONSTEXPR
+#    define ENTT_ASSERT_CONSTEXPR(condition, msg) (void(0))
+#elif !defined ENTT_ASSERT_CONSTEXPR
+#    define ENTT_ASSERT_CONSTEXPR(condition, msg) ENTT_ASSERT(condition, msg)
+#endif
+
+#define ENTT_FAIL(msg) ENTT_ASSERT(false, msg);
+
+#ifdef ENTT_NO_ETO
+#    define ENTT_ETO_TYPE(Type) void
+#else
+#    define ENTT_ETO_TYPE(Type) Type
+#endif
+
+#ifdef ENTT_NO_MIXIN
+#    define ENTT_STORAGE(Mixin, ...) __VA_ARGS__
+#else
+#    define ENTT_STORAGE(Mixin, ...) Mixin<__VA_ARGS__>
+#endif
+
+#ifdef ENTT_STANDARD_CPP
+#    define ENTT_NONSTD false
+#else
+#    define ENTT_NONSTD true
+#    if defined __clang__ || defined __GNUC__
+#        define ENTT_PRETTY_FUNCTION __PRETTY_FUNCTION__
+#        define ENTT_PRETTY_FUNCTION_PREFIX '='
+#        define ENTT_PRETTY_FUNCTION_SUFFIX ']'
+#    elif defined _MSC_VER
+#        define ENTT_PRETTY_FUNCTION __FUNCSIG__
+#        define ENTT_PRETTY_FUNCTION_PREFIX '<'
+#        define ENTT_PRETTY_FUNCTION_SUFFIX '>'
+#    endif
+#endif
+
+#ifndef ENTT_EXPORT
+#    if defined _WIN32 || defined __CYGWIN__ || defined _MSC_VER
+#        define ENTT_EXPORT __declspec(dllexport)
+#        define ENTT_IMPORT __declspec(dllimport)
+#        define ENTT_HIDDEN
+#    elif defined __GNUC__ && __GNUC__ >= 4
+#        define ENTT_EXPORT __attribute__((visibility("default")))
+#        define ENTT_IMPORT __attribute__((visibility("default")))
+#        define ENTT_HIDDEN __attribute__((visibility("hidden")))
+#    else /* Unsupported compiler */
+#        define ENTT_EXPORT
+#        define ENTT_IMPORT
+#        define ENTT_HIDDEN
+#    endif
+#endif
+
+#ifndef ENTT_API
+#    if defined ENTT_API_EXPORT
+#        define ENTT_API ENTT_EXPORT
+#    elif defined ENTT_API_IMPORT
+#        define ENTT_API ENTT_IMPORT
+#    else /* No API */
+#        define ENTT_API
+#    endif
+#endif
+
+#if defined _MSC_VER
+#    pragma detect_mismatch("entt.version", ENTT_VERSION)
+#    pragma detect_mismatch("entt.noexcept", ENTT_XSTR(ENTT_TRY))
+#    pragma detect_mismatch("entt.id", ENTT_XSTR(ENTT_ID_TYPE))
+#    pragma detect_mismatch("entt.nonstd", ENTT_XSTR(ENTT_NONSTD))
+#endif
+
+// NOLINTEND(cppcoreguidelines-macro-usage)
+
+#endif

include/entt/config/macro.h

@@ -0,0 +1,11 @@
+#ifndef ENTT_CONFIG_MACRO_H
+#define ENTT_CONFIG_MACRO_H
+
+// NOLINTBEGIN(cppcoreguidelines-macro-usage)
+
+#define ENTT_STR(arg) #arg
+#define ENTT_XSTR(arg) ENTT_STR(arg)
+
+// NOLINTEND(cppcoreguidelines-macro-usage)
+
+#endif

include/entt/config/version.h

@@ -0,0 +1,18 @@
+#ifndef ENTT_CONFIG_VERSION_H
+#define ENTT_CONFIG_VERSION_H
+
+#include "macro.h"
+
+// NOLINTBEGIN(cppcoreguidelines-macro-*,modernize-macro-*)
+
+#define ENTT_VERSION_MAJOR 4
+#define ENTT_VERSION_MINOR 0
+#define ENTT_VERSION_PATCH 0
+
+#define ENTT_VERSION \
+    ENTT_XSTR(ENTT_VERSION_MAJOR) \
+    "." ENTT_XSTR(ENTT_VERSION_MINOR) "." ENTT_XSTR(ENTT_VERSION_PATCH)
+
+// NOLINTEND(cppcoreguidelines-macro-*,modernize-macro-*)
+
+#endif

include/entt/container/dense_set.hpp

@@ -0,0 +1,890 @@
+#ifndef ENTT_CONTAINER_DENSE_SET_HPP
+#define ENTT_CONTAINER_DENSE_SET_HPP
+
+#include <compare>
+#include "../config/config.h"
+#include "../core/bit.hpp"
+#include "../core/compressed_pair.hpp"
+#include "../core/type_traits.hpp"
+#include "../stl/bit.hpp"
+#include "../stl/cmath.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/limits.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/tuple.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+static constexpr stl::size_t dense_set_placeholder_position = (stl::numeric_limits<stl::size_t>::max)();
+
+template<typename It>
+class dense_set_iterator final {
+    template<typename>
+    friend class dense_set_iterator;
+
+    static_assert(stl::is_pointer_v<It>, "Not a pointer type");
+
+public:
+    using value_type = stl::remove_const_t<stl::remove_pointer_t<It>>::second_type;
+    using pointer = const value_type *;
+    using reference = const value_type &;
+    using difference_type = stl::ptrdiff_t;
+    using iterator_category = stl::random_access_iterator_tag;
+
+    constexpr dense_set_iterator() noexcept
+        : it{} {}
+
+    constexpr dense_set_iterator(const It iter) noexcept
+        : it{iter} {}
+
+    template<typename Other>
+    requires (!stl::same_as<It, Other> && stl::constructible_from<It, Other>)
+    constexpr dense_set_iterator(const dense_set_iterator<Other> &other) noexcept
+        : it{other.it} {}
+
+    constexpr dense_set_iterator &operator++() noexcept {
+        return ++it, *this;
+    }
+
+    constexpr dense_set_iterator operator++(int) noexcept {
+        const dense_set_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    constexpr dense_set_iterator &operator--() noexcept {
+        return --it, *this;
+    }
+
+    constexpr dense_set_iterator operator--(int) noexcept {
+        const dense_set_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    constexpr dense_set_iterator &operator+=(const difference_type value) noexcept {
+        it += value;
+        return *this;
+    }
+
+    constexpr dense_set_iterator operator+(const difference_type value) const noexcept {
+        dense_set_iterator copy = *this;
+        return (copy += value);
+    }
+
+    constexpr dense_set_iterator &operator-=(const difference_type value) noexcept {
+        return (*this += -value);
+    }
+
+    constexpr dense_set_iterator operator-(const difference_type value) const noexcept {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
+        return it[value].second;
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return stl::addressof(operator[](0));
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return operator[](0);
+    }
+
+    template<typename Other>
+    [[nodiscard]] constexpr stl::ptrdiff_t operator-(const dense_set_iterator<Other> &other) const noexcept {
+        return it - other.it;
+    }
+
+    template<typename Other>
+    [[nodiscard]] constexpr bool operator==(const dense_set_iterator<Other> &other) const noexcept {
+        return it == other.it;
+    }
+
+    template<typename Other>
+    [[nodiscard]] constexpr auto operator<=>(const dense_set_iterator<Other> &other) const noexcept {
+        return it <=> other.it;
+    }
+
+private:
+    It it;
+};
+
+template<typename It>
+class dense_set_local_iterator final {
+    template<typename>
+    friend class dense_set_local_iterator;
+
+    static_assert(stl::is_pointer_v<It>, "Not a pointer type");
+
+public:
+    using value_type = stl::remove_const_t<stl::remove_pointer_t<It>>::second_type;
+    using pointer = const value_type *;
+    using reference = const value_type &;
+    using difference_type = stl::ptrdiff_t;
+    using iterator_category = stl::forward_iterator_tag;
+
+    constexpr dense_set_local_iterator() noexcept = default;
+
+    constexpr dense_set_local_iterator(It iter, const stl::size_t pos) noexcept
+        : it{iter},
+          offset{pos} {}
+
+    template<typename Other>
+    requires (!stl::same_as<It, Other> && stl::constructible_from<It, Other>)
+    constexpr dense_set_local_iterator(const dense_set_local_iterator<Other> &other) noexcept
+        : it{other.it},
+          offset{other.offset} {}
+
+    constexpr dense_set_local_iterator &operator++() noexcept {
+        return offset = it[static_cast<difference_type>(offset)].first, *this;
+    }
+
+    constexpr dense_set_local_iterator operator++(int) noexcept {
+        const dense_set_local_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return stl::addressof(it[static_cast<difference_type>(offset)].second);
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return *operator->();
+    }
+
+    template<typename Other>
+    [[nodiscard]] constexpr bool operator==(const dense_set_local_iterator<Other> &other) const noexcept {
+        return offset == other.offset;
+    }
+
+    [[nodiscard]] constexpr stl::size_t index() const noexcept {
+        return offset;
+    }
+
+private:
+    It it{};
+    stl::size_t offset{dense_set_placeholder_position};
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Associative container for unique objects of a given type.
+ *
+ * Internally, elements are organized into buckets. Which bucket an element is
+ * placed into depends entirely on its hash. Elements with the same hash code
+ * appear in the same bucket.
+ *
+ * @tparam Type Value type of the associative container.
+ * @tparam Hash Type of function to use to hash the values.
+ * @tparam KeyEqual Type of function to use to compare the values for equality.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Hash, typename KeyEqual, typename Allocator>
+class dense_set {
+    static constexpr float default_threshold = 0.875f;
+    static constexpr stl::size_t minimum_capacity = 8u;
+    static constexpr stl::size_t placeholder_position = internal::dense_set_placeholder_position;
+
+    using node_type = stl::pair<stl::size_t, Type>;
+    using alloc_traits = stl::allocator_traits<Allocator>;
+    static_assert(stl::is_same_v<typename alloc_traits::value_type, Type>, "Invalid value type");
+    using sparse_container_type = stl::vector<stl::size_t, typename alloc_traits::template rebind_alloc<stl::size_t>>;
+    using packed_container_type = stl::vector<node_type, typename alloc_traits::template rebind_alloc<node_type>>;
+
+    [[nodiscard]] stl::size_t value_to_bucket(const auto &value) const noexcept {
+        return fast_mod(static_cast<size_type>(sparse.second()(value)), bucket_count());
+    }
+
+    [[nodiscard]] auto constrained_find(const auto &value, const stl::size_t bucket) {
+        for(auto offset = sparse.first()[bucket]; offset != placeholder_position; offset = packed.first()[offset].first) {
+            if(packed.second()(packed.first()[offset].second, value)) {
+                return begin() + static_cast<iterator::difference_type>(offset);
+            }
+        }
+
+        return end();
+    }
+
+    [[nodiscard]] auto constrained_find(const auto &value, const stl::size_t bucket) const {
+        for(auto offset = sparse.first()[bucket]; offset != placeholder_position; offset = packed.first()[offset].first) {
+            if(packed.second()(packed.first()[offset].second, value)) {
+                return cbegin() + static_cast<const_iterator::difference_type>(offset);
+            }
+        }
+
+        return cend();
+    }
+
+    template<typename Other>
+    [[nodiscard]] auto insert_or_do_nothing(Other &&value) {
+        const auto index = value_to_bucket(value);
+
+        if(auto it = constrained_find(value, index); it != end()) {
+            return stl::make_pair(it, false);
+        }
+
+        packed.first().emplace_back(sparse.first()[index], stl::forward<Other>(value));
+        sparse.first()[index] = packed.first().size() - 1u;
+        rehash_if_required();
+
+        return stl::make_pair(--end(), true);
+    }
+
+    void move_and_pop(const stl::size_t pos) {
+        if(const auto last = size() - 1u; pos != last) {
+            size_type *curr = &sparse.first()[value_to_bucket(packed.first().back().second)];
+            packed.first()[pos] = stl::move(packed.first().back());
+            for(; *curr != last; curr = &packed.first()[*curr].first) {}
+            *curr = pos;
+        }
+
+        packed.first().pop_back();
+    }
+
+    void rehash_if_required() {
+        if(const auto bc = bucket_count(); size() > static_cast<size_type>(static_cast<float>(bc) * max_load_factor())) {
+            rehash(bc * 2u);
+        }
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Key type of the container. */
+    using key_type = Type;
+    /*! @brief Value type of the container. */
+    using value_type = Type;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = stl::ptrdiff_t;
+    /*! @brief Type of function to use to hash the elements. */
+    using hasher = Hash;
+    /*! @brief Type of function to use to compare the elements for equality. */
+    using key_equal = KeyEqual;
+    /*! @brief Random access iterator type. */
+    using iterator = internal::dense_set_iterator<typename packed_container_type::pointer>;
+    /*! @brief Constant random access iterator type. */
+    using const_iterator = internal::dense_set_iterator<typename packed_container_type::const_pointer>;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = stl::reverse_iterator<iterator>;
+    /*! @brief Constant reverse iterator type. */
+    using const_reverse_iterator = stl::reverse_iterator<const_iterator>;
+    /*! @brief Forward iterator type. */
+    using local_iterator = internal::dense_set_local_iterator<typename packed_container_type::pointer>;
+    /*! @brief Constant forward iterator type. */
+    using const_local_iterator = internal::dense_set_local_iterator<typename packed_container_type::const_pointer>;
+
+    /*! @brief Default constructor. */
+    dense_set()
+        : dense_set{minimum_capacity} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_set(const allocator_type &allocator)
+        : dense_set{minimum_capacity, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator and user
+     * supplied minimal number of buckets.
+     * @param cnt Minimal number of buckets.
+     * @param allocator The allocator to use.
+     */
+    dense_set(const size_type cnt, const allocator_type &allocator)
+        : dense_set{cnt, hasher{}, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function and user supplied minimal number of buckets.
+     * @param cnt Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param allocator The allocator to use.
+     */
+    dense_set(const size_type cnt, const hasher &hash, const allocator_type &allocator)
+        : dense_set{cnt, hash, key_equal{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator, hash
+     * function, compare function and user supplied minimal number of buckets.
+     * @param cnt Minimal number of buckets.
+     * @param hash Hash function to use.
+     * @param equal Compare function to use.
+     * @param allocator The allocator to use.
+     */
+    explicit dense_set(const size_type cnt, const hasher &hash = hasher{}, const key_equal &equal = key_equal{}, const allocator_type &allocator = allocator_type{})
+        : sparse{allocator, hash},
+          packed{allocator, equal} {
+        rehash(cnt);
+    }
+
+    /*! @brief Default copy constructor. */
+    dense_set(const dense_set &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    dense_set(const dense_set &other, const allocator_type &allocator)
+        : sparse{stl::piecewise_construct, stl::forward_as_tuple(other.sparse.first(), allocator), stl::forward_as_tuple(other.sparse.second())},
+          packed{stl::piecewise_construct, stl::forward_as_tuple(other.packed.first(), allocator), stl::forward_as_tuple(other.packed.second())},
+          threshold{other.threshold} {}
+
+    /*! @brief Default move constructor. */
+    dense_set(dense_set &&) noexcept = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    dense_set(dense_set &&other, const allocator_type &allocator)
+        : sparse{stl::piecewise_construct, stl::forward_as_tuple(stl::move(other.sparse.first()), allocator), stl::forward_as_tuple(stl::move(other.sparse.second()))},
+          packed{stl::piecewise_construct, stl::forward_as_tuple(stl::move(other.packed.first()), allocator), stl::forward_as_tuple(stl::move(other.packed.second()))},
+          threshold{other.threshold} {}
+
+    /*! @brief Default destructor. */
+    ~dense_set() = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This container.
+     */
+    dense_set &operator=(const dense_set &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This container.
+     */
+    dense_set &operator=(dense_set &&) noexcept = default;
+
+    /**
+     * @brief Exchanges the contents with those of a given container.
+     * @param other Container to exchange the content with.
+     */
+    void swap(dense_set &other) noexcept {
+        using stl::swap;
+        swap(sparse, other.sparse);
+        swap(packed, other.packed);
+        swap(threshold, other.threshold);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return sparse.first().get_allocator();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * If the array is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal array.
+     */
+    [[nodiscard]] const_iterator cbegin() const noexcept {
+        return packed.first().data();
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const noexcept {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() noexcept {
+        return packed.first().data();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last instance of the
+     * internal array.
+     */
+    [[nodiscard]] const_iterator cend() const noexcept {
+        return packed.first().data() + packed.first().size();
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const noexcept {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() noexcept {
+        return packed.first().data() + packed.first().size();
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * If the array is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first instance of the reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crbegin() const noexcept {
+        return stl::make_reverse_iterator(cend());
+    }
+
+    /*! @copydoc crbegin */
+    [[nodiscard]] const_reverse_iterator rbegin() const noexcept {
+        return crbegin();
+    }
+
+    /*! @copydoc rbegin */
+    [[nodiscard]] reverse_iterator rbegin() noexcept {
+        return stl::make_reverse_iterator(end());
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     * @return An iterator to the element following the last instance of the
+     * reversed internal array.
+     */
+    [[nodiscard]] const_reverse_iterator crend() const noexcept {
+        return stl::make_reverse_iterator(cbegin());
+    }
+
+    /*! @copydoc crend */
+    [[nodiscard]] const_reverse_iterator rend() const noexcept {
+        return crend();
+    }
+
+    /*! @copydoc rend */
+    [[nodiscard]] reverse_iterator rend() noexcept {
+        return stl::make_reverse_iterator(begin());
+    }
+
+    /**
+     * @brief Checks whether a container is empty.
+     * @return True if the container is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return packed.first().empty();
+    }
+
+    /**
+     * @brief Returns the number of elements in a container.
+     * @return Number of elements in a container.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return packed.first().size();
+    }
+
+    /**
+     * @brief Returns the maximum possible number of elements.
+     * @return Maximum possible number of elements.
+     */
+    [[nodiscard]] size_type max_size() const noexcept {
+        return packed.first().max_size();
+    }
+
+    /*! @brief Clears the container. */
+    void clear() noexcept {
+        sparse.first().clear();
+        packed.first().clear();
+        rehash(0u);
+    }
+
+    /**
+     * @brief Inserts an element into the container, if it does not exist.
+     * @param value An element to insert into the container.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    stl::pair<iterator, bool> insert(const value_type &value) {
+        return insert_or_do_nothing(value);
+    }
+
+    /*! @copydoc insert */
+    stl::pair<iterator, bool> insert(value_type &&value) {
+        return insert_or_do_nothing(stl::move(value));
+    }
+
+    /**
+     * @brief Inserts elements into the container, if they do not exist.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     */
+    void insert(stl::input_iterator auto first, stl::input_iterator auto last) {
+        for(; first != last; ++first) {
+            insert(*first);
+        }
+    }
+
+    /**
+     * @brief Constructs an element in-place, if it does not exist.
+     *
+     * The element is also constructed when the container already has the key,
+     * in which case the newly constructed object is destroyed immediately.
+     *
+     * @tparam Args Types of arguments to forward to the constructor of the
+     * element.
+     * @param args Arguments to forward to the constructor of the element.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    template<typename... Args>
+    stl::pair<iterator, bool> emplace(Args &&...args) {
+        if constexpr(((sizeof...(Args) == 1u) && ... && stl::is_same_v<stl::decay_t<Args>, value_type>)) {
+            return insert_or_do_nothing(stl::forward<Args>(args)...);
+        } else {
+            auto &node = packed.first().emplace_back(stl::piecewise_construct, stl::make_tuple(packed.first().size()), stl::forward_as_tuple(stl::forward<Args>(args)...));
+            const auto index = value_to_bucket(node.second);
+
+            if(auto it = constrained_find(node.second, index); it != end()) {
+                packed.first().pop_back();
+                return stl::make_pair(it, false);
+            }
+
+            stl::swap(node.first, sparse.first()[index]);
+            rehash_if_required();
+
+            return stl::make_pair(--end(), true);
+        }
+    }
+
+    /**
+     * @brief Removes an element from a given position.
+     * @param pos An iterator to the element to remove.
+     * @return An iterator following the removed element.
+     */
+    iterator erase(const_iterator pos) {
+        const auto diff = pos - cbegin();
+        erase(*pos);
+        return begin() + diff;
+    }
+
+    /**
+     * @brief Removes the given elements from a container.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     * @return An iterator following the last removed element.
+     */
+    iterator erase(const_iterator first, const_iterator last) {
+        const auto dist = first - cbegin();
+
+        for(auto from = last - cbegin(); from != dist; --from) {
+            erase(packed.first()[static_cast<size_type>(from) - 1u].second);
+        }
+
+        return (begin() + dist);
+    }
+
+    /**
+     * @brief Removes the element associated with a given value.
+     * @param value Value of an element to remove.
+     * @return Number of elements removed (either 0 or 1).
+     */
+    size_type erase(const value_type &value) {
+        for(size_type *curr = &sparse.first()[value_to_bucket(value)]; *curr != placeholder_position; curr = &packed.first()[*curr].first) {
+            if(packed.second()(packed.first()[*curr].second, value)) {
+                const auto index = *curr;
+                *curr = packed.first()[*curr].first;
+                move_and_pop(index);
+                return 1u;
+            }
+        }
+
+        return 0u;
+    }
+
+    /**
+     * @brief Returns the number of elements matching a value (either 1 or 0).
+     * @param key Key value of an element to search for.
+     * @return Number of elements matching the key (either 1 or 0).
+     */
+    [[nodiscard]] size_type count(const value_type &key) const {
+        return find(key) != end();
+    }
+
+    /**
+     * @brief Returns the number of elements matching a key (either 1 or 0).
+     * @param key Key value of an element to search for.
+     * @return Number of elements matching the key (either 1 or 0).
+     */
+    [[nodiscard]] size_type count(const auto &key) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
+        return find(key) != end();
+    }
+
+    /**
+     * @brief Finds an element with a given value.
+     * @param value Value of an element to search for.
+     * @return An iterator to an element with the given value. If no such
+     * element is found, a past-the-end iterator is returned.
+     */
+    [[nodiscard]] iterator find(const value_type &value) {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /*! @copydoc find */
+    [[nodiscard]] const_iterator find(const value_type &value) const {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /**
+     * @brief Finds an element that compares _equivalent_ to a given value.
+     * @param value Value of an element to search for.
+     * @return An iterator to an element with the given value. If no such
+     * element is found, a past-the-end iterator is returned.
+     */
+    [[nodiscard]] iterator find(const auto &value)
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /*! @copydoc find */
+    [[nodiscard]] const_iterator find(const auto &value) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
+        return constrained_find(value, value_to_bucket(value));
+    }
+
+    /**
+     * @brief Returns a range containing all elements with a given value.
+     * @param value Value of an element to search for.
+     * @return A pair of iterators pointing to the first element and past the
+     * last element of the range.
+     */
+    [[nodiscard]] stl::pair<iterator, iterator> equal_range(const value_type &value) {
+        const auto it = find(value);
+        return {it, it + !(it == end())};
+    }
+
+    /*! @copydoc equal_range */
+    [[nodiscard]] stl::pair<const_iterator, const_iterator> equal_range(const value_type &value) const {
+        const auto it = find(value);
+        return {it, it + !(it == cend())};
+    }
+
+    /**
+     * @brief Returns a range containing all elements that compare _equivalent_
+     * to a given value.
+     * @param value Value of an element to search for.
+     * @return A pair of iterators pointing to the first element and past the
+     * last element of the range.
+     */
+    [[nodiscard]] stl::pair<iterator, iterator> equal_range(const auto &value)
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
+        const auto it = find(value);
+        return {it, it + !(it == end())};
+    }
+
+    /*! @copydoc equal_range */
+    [[nodiscard]] stl::pair<const_iterator, const_iterator> equal_range(const auto &value) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
+        const auto it = find(value);
+        return {it, it + !(it == cend())};
+    }
+
+    /**
+     * @brief Checks if the container contains an element with a given value.
+     * @param value Value of an element to search for.
+     * @return True if there is such an element, false otherwise.
+     */
+    [[nodiscard]] bool contains(const value_type &value) const {
+        return (find(value) != cend());
+    }
+
+    /**
+     * @brief Checks if the container contains an element that compares
+     * _equivalent_ to a given value.
+     * @param value Value of an element to search for.
+     * @return True if there is such an element, false otherwise.
+     */
+    [[nodiscard]] bool contains(const auto &value) const
+    requires is_transparent_v<hasher> && is_transparent_v<key_equal> {
+        return (find(value) != cend());
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator cbegin(const size_type index) const {
+        return {packed.first().data(), sparse.first()[index]};
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator begin(const size_type index) const {
+        return cbegin(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the beginning of the given bucket.
+     */
+    [[nodiscard]] local_iterator begin(const size_type index) {
+        return {packed.first().data(), sparse.first()[index]};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator cend([[maybe_unused]] const size_type index) const {
+        return {};
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] const_local_iterator end(const size_type index) const {
+        return cend(index);
+    }
+
+    /**
+     * @brief Returns an iterator to the end of a given bucket.
+     * @param index An index of a bucket to access.
+     * @return An iterator to the end of the given bucket.
+     */
+    [[nodiscard]] local_iterator end([[maybe_unused]] const size_type index) {
+        return {};
+    }
+
+    /**
+     * @brief Returns the number of buckets.
+     * @return The number of buckets.
+     */
+    [[nodiscard]] size_type bucket_count() const {
+        return sparse.first().size();
+    }
+
+    /**
+     * @brief Returns the maximum number of buckets.
+     * @return The maximum number of buckets.
+     */
+    [[nodiscard]] size_type max_bucket_count() const {
+        return sparse.first().max_size();
+    }
+
+    /**
+     * @brief Returns the number of elements in a given bucket.
+     * @param index The index of the bucket to examine.
+     * @return The number of elements in the given bucket.
+     */
+    [[nodiscard]] size_type bucket_size(const size_type index) const {
+        return static_cast<size_type>(stl::distance(begin(index), end(index)));
+    }
+
+    /**
+     * @brief Returns the bucket for a given element.
+     * @param value The value of the element to examine.
+     * @return The bucket for the given element.
+     */
+    [[nodiscard]] size_type bucket(const value_type &value) const {
+        return value_to_bucket(value);
+    }
+
+    /**
+     * @brief Returns the average number of elements per bucket.
+     * @return The average number of elements per bucket.
+     */
+    [[nodiscard]] float load_factor() const {
+        return static_cast<float>(size()) / static_cast<float>(bucket_count());
+    }
+
+    /**
+     * @brief Returns the maximum average number of elements per bucket.
+     * @return The maximum average number of elements per bucket.
+     */
+    [[nodiscard]] float max_load_factor() const {
+        return threshold;
+    }
+
+    /**
+     * @brief Sets the desired maximum average number of elements per bucket.
+     * @param value A desired maximum average number of elements per bucket.
+     */
+    void max_load_factor(const float value) {
+        ENTT_ASSERT(value > 0.f, "Invalid load factor");
+        threshold = value;
+        rehash(0u);
+    }
+
+    /**
+     * @brief Reserves at least the specified number of buckets and regenerates
+     * the hash table.
+     * @param cnt New number of buckets.
+     */
+    void rehash(const size_type cnt) {
+        auto value = cnt > minimum_capacity ? cnt : minimum_capacity;
+        const auto cap = static_cast<size_type>(static_cast<float>(size()) / max_load_factor());
+        value = value > cap ? value : cap;
+
+        if(const auto sz = stl::bit_ceil(value); sz != bucket_count()) {
+            sparse.first().resize(sz);
+
+            for(auto &&elem: sparse.first()) {
+                elem = placeholder_position;
+            }
+
+            for(size_type pos{}, last = size(); pos < last; ++pos) {
+                const auto index = value_to_bucket(packed.first()[pos].second);
+                packed.first()[pos].first = stl::exchange(sparse.first()[index], pos);
+            }
+        }
+    }
+
+    /**
+     * @brief Reserves space for at least the specified number of elements and
+     * regenerates the hash table.
+     * @param cnt New number of elements.
+     */
+    void reserve(const size_type cnt) {
+        packed.first().reserve(cnt);
+        rehash(static_cast<size_type>(stl::ceil(static_cast<float>(cnt) / max_load_factor())));
+    }
+
+    /**
+     * @brief Returns the function used to hash the elements.
+     * @return The function used to hash the elements.
+     */
+    [[nodiscard]] hasher hash_function() const {
+        return sparse.second();
+    }
+
+    /**
+     * @brief Returns the function used to compare elements for equality.
+     * @return The function used to compare elements for equality.
+     */
+    [[nodiscard]] key_equal key_eq() const {
+        return packed.second();
+    }
+
+private:
+    compressed_pair<sparse_container_type, hasher> sparse;
+    compressed_pair<packed_container_type, key_equal> packed;
+    float threshold{default_threshold};
+};
+
+} // namespace entt
+
+#endif

include/entt/container/fwd.hpp

@@ -0,0 +1,38 @@
+#ifndef ENTT_CONTAINER_FWD_HPP
+#define ENTT_CONTAINER_FWD_HPP
+
+#include "../stl/functional.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+
+namespace entt {
+
+template<
+    typename Key,
+    typename Type,
+    typename = stl::hash<Key>,
+    typename = stl::equal_to<>,
+    typename = stl::allocator<stl::pair<const Key, Type>>>
+class dense_map;
+
+template<
+    typename Type,
+    typename = stl::hash<Type>,
+    typename = stl::equal_to<>,
+    typename = stl::allocator<Type>>
+class dense_set;
+
+template<typename...>
+class basic_table;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Type Element types.
+ */
+template<typename... Type>
+using table = basic_table<stl::vector<Type>...>;
+
+} // namespace entt
+
+#endif

include/entt/container/table.hpp

@@ -0,0 +1,434 @@
+#ifndef ENTT_CONTAINER_TABLE_HPP
+#define ENTT_CONTAINER_TABLE_HPP
+
+#include "../config/config.h"
+#include "../core/iterator.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/tuple.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename... It>
+class table_iterator {
+    template<typename...>
+    friend class table_iterator;
+
+public:
+    using value_type = decltype(stl::forward_as_tuple(*stl::declval<It>()...));
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using difference_type = stl::ptrdiff_t;
+    using iterator_category = stl::input_iterator_tag;
+    using iterator_concept = stl::random_access_iterator_tag;
+
+    constexpr table_iterator() noexcept
+        : it{} {}
+
+    constexpr table_iterator(It... from) noexcept
+        : it{from...} {}
+
+    template<typename... Other>
+    requires (stl::constructible_from<It, Other> && ...)
+    constexpr table_iterator(const table_iterator<Other...> &other) noexcept
+        : table_iterator{stl::get<Other>(other.it)...} {}
+
+    constexpr table_iterator &operator++() noexcept {
+        return (++stl::get<It>(it), ...), *this;
+    }
+
+    constexpr table_iterator operator++(int) noexcept {
+        const table_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    constexpr table_iterator &operator--() noexcept {
+        return (--stl::get<It>(it), ...), *this;
+    }
+
+    constexpr table_iterator operator--(int) noexcept {
+        const table_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    constexpr table_iterator &operator+=(const difference_type value) noexcept {
+        return ((stl::get<It>(it) += value), ...), *this;
+    }
+
+    constexpr table_iterator operator+(const difference_type value) const noexcept {
+        table_iterator copy = *this;
+        return (copy += value);
+    }
+
+    constexpr table_iterator &operator-=(const difference_type value) noexcept {
+        return (*this += -value);
+    }
+
+    constexpr table_iterator operator-(const difference_type value) const noexcept {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
+        return stl::forward_as_tuple(stl::get<It>(it)[value]...);
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return {operator[](0)};
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return operator[](0);
+    }
+
+    template<typename... Other>
+    [[nodiscard]] constexpr stl::ptrdiff_t operator-(const table_iterator<Other...> &other) const noexcept {
+        return stl::get<0>(it) - stl::get<0>(other.it);
+    }
+
+    template<typename... Other>
+    [[nodiscard]] constexpr bool operator==(const table_iterator<Other...> &other) const noexcept {
+        return stl::get<0>(it) == stl::get<0>(other.it);
+    }
+
+    template<typename... Other>
+    [[nodiscard]] constexpr auto operator<=>(const table_iterator<Other...> &other) const noexcept {
+        return stl::get<0>(it) <=> stl::get<0>(other.it);
+    }
+
+private:
+    stl::tuple<It...> it;
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Basic table implementation.
+ *
+ * Internal data structures arrange elements to maximize performance. There are
+ * no guarantees that objects are returned in the insertion order when iterate
+ * a table. Do not make assumption on the order in any case.
+ *
+ * @tparam Container Sequence container row types.
+ */
+template<typename... Container>
+class basic_table {
+    using container_type = stl::tuple<Container...>;
+
+public:
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = stl::ptrdiff_t;
+    /*! @brief Input iterator type. */
+    using iterator = internal::table_iterator<typename Container::iterator...>;
+    /*! @brief Constant input iterator type. */
+    using const_iterator = internal::table_iterator<typename Container::const_iterator...>;
+    /*! @brief Reverse iterator type. */
+    using reverse_iterator = internal::table_iterator<typename Container::reverse_iterator...>;
+    /*! @brief Constant reverse iterator type. */
+    using const_reverse_iterator = internal::table_iterator<typename Container::const_reverse_iterator...>;
+
+    /*! @brief Default constructor. */
+    basic_table()
+        : payload{} {
+    }
+
+    /**
+     * @brief Copy constructs the underlying containers.
+     * @param container The containers to copy from.
+     */
+    explicit basic_table(const Container &...container) noexcept
+        : payload{container...} {
+        ENTT_ASSERT((((stl::get<Container>(payload).size() * sizeof...(Container)) == (stl::get<Container>(payload).size() + ...)) && ...), "Unexpected container size");
+    }
+
+    /**
+     * @brief Move constructs the underlying containers.
+     * @param container The containers to move from.
+     */
+    explicit basic_table(Container &&...container) noexcept
+        : payload{stl::move(container)...} {
+        ENTT_ASSERT((((stl::get<Container>(payload).size() * sizeof...(Container)) == (stl::get<Container>(payload).size() + ...)) && ...), "Unexpected container size");
+    }
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_table(const basic_table &) = delete;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_table(basic_table &&other) noexcept
+        : payload{stl::move(other.payload)} {}
+
+    /**
+     * @brief Constructs the underlying containers using a given allocator.
+     * @param allocator A valid allocator.
+     */
+    explicit basic_table(const auto &allocator)
+        : payload{Container{allocator}...} {}
+
+    /**
+     * @brief Copy constructs the underlying containers using a given allocator.
+     * @tparam Allocator Type of allocator.
+     * @param container The containers to copy from.
+     * @param allocator A valid allocator.
+     */
+    template<class Allocator>
+    basic_table(const Container &...container, const Allocator &allocator) noexcept
+        : payload{Container{container, allocator}...} {
+        ENTT_ASSERT((((stl::get<Container>(payload).size() * sizeof...(Container)) == (stl::get<Container>(payload).size() + ...)) && ...), "Unexpected container size");
+    }
+
+    /**
+     * @brief Move constructs the underlying containers using a given allocator.
+     * @tparam Allocator Type of allocator.
+     * @param container The containers to move from.
+     * @param allocator A valid allocator.
+     */
+    template<class Allocator>
+    basic_table(Container &&...container, const Allocator &allocator) noexcept
+        : payload{Container{stl::move(container), allocator}...} {
+        ENTT_ASSERT((((stl::get<Container>(payload).size() * sizeof...(Container)) == (stl::get<Container>(payload).size() + ...)) && ...), "Unexpected container size");
+    }
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @tparam Allocator Type of allocator.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    template<class Allocator>
+    basic_table(basic_table &&other, const Allocator &allocator)
+        : payload{Container{stl::move(stl::get<Container>(other.payload)), allocator}...} {}
+
+    /*! @brief Default destructor. */
+    ~basic_table() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This container.
+     */
+    basic_table &operator=(const basic_table &) = delete;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This container.
+     */
+    basic_table &operator=(basic_table &&other) noexcept {
+        swap(other);
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given table.
+     * @param other Table to exchange the content with.
+     */
+    void swap(basic_table &other) noexcept {
+        using stl::swap;
+        swap(payload, other.payload);
+    }
+
+    /**
+     * @brief Increases the capacity of a table.
+     *
+     * If the new capacity is greater than the current capacity, new storage is
+     * allocated, otherwise the method does nothing.
+     *
+     * @param cap Desired capacity.
+     */
+    void reserve(const size_type cap) {
+        (stl::get<Container>(payload).reserve(cap), ...);
+    }
+
+    /**
+     * @brief Returns the number of rows that a table has currently allocated
+     * space for.
+     * @return Capacity of the table.
+     */
+    [[nodiscard]] size_type capacity() const noexcept {
+        return stl::get<0>(payload).capacity();
+    }
+
+    /*! @brief Requests the removal of unused capacity. */
+    void shrink_to_fit() {
+        (stl::get<Container>(payload).shrink_to_fit(), ...);
+    }
+
+    /**
+     * @brief Returns the number of rows in a table.
+     * @return Number of rows.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return stl::get<0>(payload).size();
+    }
+
+    /**
+     * @brief Checks whether a table is empty.
+     * @return True if the table is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return stl::get<0>(payload).empty();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * If the table is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first row of the table.
+     */
+    [[nodiscard]] const_iterator cbegin() const noexcept {
+        return {stl::get<Container>(payload).cbegin()...};
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const noexcept {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() noexcept {
+        return {stl::get<Container>(payload).begin()...};
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last row of the table.
+     */
+    [[nodiscard]] const_iterator cend() const noexcept {
+        return {stl::get<Container>(payload).cend()...};
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const noexcept {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() noexcept {
+        return {stl::get<Container>(payload).end()...};
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the beginning.
+     *
+     * If the table is empty, the returned iterator will be equal to `rend()`.
+     *
+     * @return An iterator to the first row of the reversed table.
+     */
+    [[nodiscard]] const_reverse_iterator crbegin() const noexcept {
+        return {stl::get<Container>(payload).crbegin()...};
+    }
+
+    /*! @copydoc crbegin */
+    [[nodiscard]] const_reverse_iterator rbegin() const noexcept {
+        return crbegin();
+    }
+
+    /*! @copydoc rbegin */
+    [[nodiscard]] reverse_iterator rbegin() noexcept {
+        return {stl::get<Container>(payload).rbegin()...};
+    }
+
+    /**
+     * @brief Returns a reverse iterator to the end.
+     * @return An iterator to the element following the last row of the reversed
+     * table.
+     */
+    [[nodiscard]] const_reverse_iterator crend() const noexcept {
+        return {stl::get<Container>(payload).crend()...};
+    }
+
+    /*! @copydoc crend */
+    [[nodiscard]] const_reverse_iterator rend() const noexcept {
+        return crend();
+    }
+
+    /*! @copydoc rend */
+    [[nodiscard]] reverse_iterator rend() noexcept {
+        return {stl::get<Container>(payload).rend()...};
+    }
+
+    /**
+     * @brief Appends a row to the end of a table.
+     * @tparam Args Types of arguments to use to construct the row data.
+     * @param args Parameters to use to construct the row data.
+     * @return A reference to the newly created row data.
+     */
+    template<typename... Args>
+    stl::tuple<typename Container::value_type &...> emplace(Args &&...args) {
+        if constexpr(sizeof...(Args) == 0u) {
+            return stl::forward_as_tuple(stl::get<Container>(payload).emplace_back()...);
+        } else {
+            return stl::forward_as_tuple(stl::get<Container>(payload).emplace_back(stl::forward<Args>(args))...);
+        }
+    }
+
+    /**
+     * @brief Removes a row from a table.
+     * @param pos An iterator to the row to remove.
+     * @return An iterator following the removed row.
+     */
+    iterator erase(const_iterator pos) {
+        const auto diff = pos - begin();
+        return {stl::get<Container>(payload).erase(stl::get<Container>(payload).begin() + diff)...};
+    }
+
+    /**
+     * @brief Removes a row from a table.
+     * @param pos Index of the row to remove.
+     */
+    void erase(const size_type pos) {
+        ENTT_ASSERT(pos < size(), "Index out of bounds");
+        erase(begin() + static_cast<difference_type>(pos));
+    }
+
+    /**
+     * @brief Returns the row data at specified location.
+     * @param pos The row for which to return the data.
+     * @return The row data at specified location.
+     */
+    [[nodiscard]] stl::tuple<const typename Container::value_type &...> operator[](const size_type pos) const {
+        ENTT_ASSERT(pos < size(), "Index out of bounds");
+        return stl::forward_as_tuple(stl::get<Container>(payload)[pos]...);
+    }
+
+    /*! @copydoc operator[] */
+    [[nodiscard]] stl::tuple<typename Container::value_type &...> operator[](const size_type pos) {
+        ENTT_ASSERT(pos < size(), "Index out of bounds");
+        return stl::forward_as_tuple(stl::get<Container>(payload)[pos]...);
+    }
+
+    /*! @brief Clears a table. */
+    void clear() {
+        (stl::get<Container>(payload).clear(), ...);
+    }
+
+private:
+    container_type payload;
+};
+
+} // namespace entt
+
+/*! @cond ENTT_INTERNAL */
+#include <utility>
+
+namespace std {
+
+template<typename... Container, typename Allocator>
+struct uses_allocator<entt::basic_table<Container...>, Allocator>
+    : entt::stl::bool_constant<(entt::stl::uses_allocator_v<Container, Allocator> && ...)> {};
+
+} // namespace std
+/*! @endcond */
+
+#endif

include/entt/core/algorithm.hpp

@@ -0,0 +1,143 @@
+#ifndef ENTT_CORE_ALGORITHM_HPP
+#define ENTT_CORE_ALGORITHM_HPP
+
+#include "../stl/algorithm.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+
+namespace entt {
+
+/**
+ * @brief Function object to wrap `stl::sort` in a class type.
+ *
+ * Unfortunately, `stl::sort` cannot be passed as template argument to a class
+ * template or a function template.<br/>
+ * This class fills the gap by wrapping some flavors of `stl::sort` in a
+ * function object.
+ */
+struct std_sort {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given binary comparison function.
+     *
+     * @tparam Compare Type of comparison function object.
+     * @tparam Args Types of arguments to forward to the sort function.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     * @param args Arguments to forward to the sort function, if any.
+     */
+    template<typename Compare = stl::less<>, typename... Args>
+    void operator()(stl::random_access_iterator auto first, stl::random_access_iterator auto last, Compare compare = Compare{}, Args &&...args) const {
+        stl::sort(stl::forward<Args>(args)..., stl::move(first), stl::move(last), stl::move(compare));
+    }
+};
+
+/*! @brief Function object for performing insertion sort. */
+struct insertion_sort {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given binary comparison function.
+     *
+     * @tparam Compare Type of comparison function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param compare A valid comparison function object.
+     */
+    template<typename Compare = stl::less<>>
+    void operator()(stl::random_access_iterator auto first, stl::random_access_iterator auto last, Compare compare = Compare{}) const {
+        if(first < last) {
+            for(auto it = first + 1; it < last; ++it) {
+                auto value = stl::move(*it);
+                auto pre = it;
+
+                // NOLINTBEGIN(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+                for(; pre > first && compare(value, *(pre - 1)); --pre) {
+                    *pre = stl::move(*(pre - 1));
+                }
+                // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+
+                *pre = stl::move(value);
+            }
+        }
+    }
+};
+
+/**
+ * @brief Function object for performing LSD radix sort.
+ * @tparam Bit Number of bits processed per pass.
+ * @tparam N Maximum number of bits to sort.
+ */
+template<stl::size_t Bit, stl::size_t N>
+requires ((N % Bit) == 0) // The maximum number of bits to sort must be a multiple of the number of bits processed per pass
+struct radix_sort {
+    /**
+     * @brief Sorts the elements in a range.
+     *
+     * Sorts the elements in a range using the given _getter_ to access the
+     * actual data to be sorted.
+     *
+     * This implementation is inspired by the online book
+     * [Physically Based Rendering](http://www.pbr-book.org/3ed-2018/Primitives_and_Intersection_Acceleration/Bounding_Volume_Hierarchies.html#RadixSort).
+     *
+     * @tparam It Type of random access iterator.
+     * @tparam Getter Type of _getter_ function object.
+     * @param first An iterator to the first element of the range to sort.
+     * @param last An iterator past the last element of the range to sort.
+     * @param getter A valid _getter_ function object.
+     */
+    template<stl::random_access_iterator It, typename Getter = stl::identity>
+    void operator()(It first, It last, Getter getter = Getter{}) const {
+        if(first < last) {
+            constexpr auto passes = N / Bit;
+
+            using value_type = stl::iterator_traits<It>::value_type;
+            using difference_type = stl::iterator_traits<It>::difference_type;
+            stl::vector<value_type> aux(static_cast<stl::size_t>(stl::distance(first, last)));
+
+            auto part = [getter = stl::move(getter)](auto from, auto to, auto out, auto start) {
+                constexpr auto mask = (1 << Bit) - 1;
+                constexpr auto buckets = 1 << Bit;
+
+                // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays, misc-const-correctness)
+                stl::size_t count[buckets]{};
+
+                for(auto it = from; it != to; ++it) {
+                    ++count[(getter(*it) >> start) & mask];
+                }
+
+                // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+                stl::size_t index[buckets]{};
+
+                for(stl::size_t pos{}, end = buckets - 1u; pos < end; ++pos) {
+                    index[pos + 1u] = index[pos] + count[pos];
+                }
+
+                for(auto it = from; it != to; ++it) {
+                    const auto pos = index[(getter(*it) >> start) & mask]++;
+                    out[static_cast<difference_type>(pos)] = stl::move(*it);
+                }
+            };
+
+            for(stl::size_t pass = 0; pass < (passes & ~1u); pass += 2) {
+                part(first, last, aux.begin(), pass * Bit);
+                part(aux.begin(), aux.end(), first, (pass + 1) * Bit);
+            }
+
+            if constexpr(passes & 1) {
+                part(first, last, aux.begin(), (passes - 1) * Bit);
+                stl::move(aux.begin(), aux.end(), first);
+            }
+        }
+    }
+};
+
+} // namespace entt
+
+#endif

include/entt/core/any.hpp

@@ -0,0 +1,623 @@
+#ifndef ENTT_CORE_ANY_HPP
+#define ENTT_CORE_ANY_HPP
+
+#include "../config/config.h"
+#include "../core/concepts.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/cstdint.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+#include "type_info.hpp"
+#include "type_traits.hpp"
+#include "utility.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+enum class any_request : stl::uint8_t {
+    info,
+    transfer,
+    assign,
+    compare,
+    copy,
+    move
+};
+
+template<stl::size_t Len, stl::size_t Align>
+struct basic_any_storage {
+    static constexpr bool has_buffer = true;
+    union {
+        const void *instance{};
+        // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+        alignas(Align) stl::byte buffer[Len];
+    };
+};
+
+template<stl::size_t Align>
+struct basic_any_storage<0u, Align> {
+    static constexpr bool has_buffer = false;
+    const void *instance{};
+};
+
+template<typename Type, stl::size_t Len, stl::size_t Align>
+// NOLINTNEXTLINE(bugprone-sizeof-expression)
+struct in_situ: stl::bool_constant<(Len != 0u) && alignof(Type) <= Align && sizeof(Type) <= Len && stl::is_nothrow_move_constructible_v<Type>> {};
+
+template<stl::size_t Len, stl::size_t Align>
+struct in_situ<void, Len, Align>: stl::false_type {};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief A SBO friendly, type-safe container for single values of any type.
+ * @tparam Len Size of the buffer reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+template<stl::size_t Len, stl::size_t Align>
+class basic_any: private internal::basic_any_storage<Len, Align> {
+    using request = internal::any_request;
+    using base_type = internal::basic_any_storage<Len, Align>;
+    using vtable_type = const void *(const request, const basic_any &, const void *);
+    using deleter_type = void(const basic_any &);
+
+    template<typename Type>
+    static constexpr bool in_situ_v = internal::in_situ<Type, Len, Align>::value;
+
+    template<cvref_unqualified Type>
+    static const void *basic_vtable(const request req, const basic_any &value, const void *other) {
+        switch(const auto *elem = static_cast<const Type *>(value.data()); req) {
+            using enum internal::any_request;
+        case info:
+            return &type_id<Type>();
+        case transfer:
+            if constexpr(stl::is_move_assignable_v<Type>) {
+                // NOLINTNEXTLINE(bugprone-casting-through-void)
+                *const_cast<Type *>(elem) = stl::move(*static_cast<Type *>(const_cast<void *>(other)));
+                return other;
+            }
+            [[fallthrough]];
+        case assign:
+            if constexpr(stl::is_copy_assignable_v<Type>) {
+                *const_cast<Type *>(elem) = *static_cast<const Type *>(other);
+                return other;
+            }
+            break;
+        case compare:
+            if constexpr(!stl::is_function_v<Type> && !stl::is_array_v<Type> && is_equality_comparable_v<Type>) {
+                return (*elem == *static_cast<const Type *>(other)) ? other : nullptr;
+            } else {
+                return (elem == other) ? other : nullptr;
+            }
+        case copy:
+            if constexpr(stl::is_copy_constructible_v<Type>) {
+                // NOLINTNEXTLINE(bugprone-casting-through-void)
+                static_cast<basic_any *>(const_cast<void *>(other))->initialize<Type>(*elem);
+            }
+            break;
+        case move:
+            ENTT_ASSERT(value.mode == any_policy::embedded, "Unexpected policy");
+            if constexpr(in_situ_v<Type>) {
+                // NOLINTNEXTLINE(bugprone-casting-through-void, bugprone-multi-level-implicit-pointer-conversion)
+                return ::new(&static_cast<basic_any *>(const_cast<void *>(other))->buffer) Type{stl::move(*const_cast<Type *>(elem))};
+            }
+        }
+
+        return nullptr;
+    }
+
+    template<cvref_unqualified Type>
+    static void basic_deleter(const basic_any &value) {
+        ENTT_ASSERT((value.mode == any_policy::dynamic) || ((value.mode == any_policy::embedded) && !stl::is_trivially_destructible_v<Type>), "Unexpected policy");
+
+        const auto *elem = static_cast<const Type *>(value.data());
+
+        if constexpr(in_situ_v<Type>) {
+            (value.mode == any_policy::embedded) ? elem->~Type() : (delete elem);
+        } else if constexpr(stl::is_array_v<Type>) {
+            delete[] elem;
+        } else {
+            delete elem;
+        }
+    }
+
+    template<typename Type, typename... Args>
+    void initialize([[maybe_unused]] Args &&...args) {
+        using plain_type = stl::remove_cvref_t<Type>;
+
+        vtable = basic_vtable<plain_type>;
+        underlying_type = type_hash<plain_type>::value();
+
+        if constexpr(stl::is_void_v<Type>) {
+            deleter = nullptr;
+            mode = any_policy::empty;
+            this->instance = nullptr;
+        } else if constexpr(stl::is_lvalue_reference_v<Type>) {
+            deleter = nullptr;
+            mode = stl::is_const_v<stl::remove_reference_t<Type>> ? any_policy::cref : any_policy::ref;
+            static_assert((stl::is_lvalue_reference_v<Args> && ...) && (sizeof...(Args) == 1u), "Invalid arguments");
+            // NOLINTNEXTLINE(bugprone-multi-level-implicit-pointer-conversion)
+            this->instance = (stl::addressof(args), ...);
+        } else if constexpr(in_situ_v<plain_type>) {
+            if constexpr(stl::is_trivially_destructible_v<plain_type>) {
+                deleter = nullptr;
+            } else {
+                deleter = &basic_deleter<plain_type>;
+            }
+
+            mode = any_policy::embedded;
+
+            if constexpr(stl::is_aggregate_v<plain_type> && (sizeof...(Args) != 0u || !stl::is_default_constructible_v<plain_type>)) {
+                ::new(&this->buffer) plain_type{stl::forward<Args>(args)...};
+            } else {
+                // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay)
+                ::new(&this->buffer) plain_type(stl::forward<Args>(args)...);
+            }
+        } else {
+            deleter = &basic_deleter<plain_type>;
+            mode = any_policy::dynamic;
+
+            if constexpr(stl::is_aggregate_v<plain_type> && (sizeof...(Args) != 0u || !stl::is_default_constructible_v<plain_type>)) {
+                this->instance = new plain_type{stl::forward<Args>(args)...};
+            } else if constexpr(stl::is_array_v<plain_type>) {
+                static_assert(sizeof...(Args) == 0u, "Invalid arguments");
+                this->instance = new plain_type[stl::extent_v<plain_type>]();
+            } else {
+                this->instance = new plain_type(stl::forward<Args>(args)...);
+            }
+        }
+    }
+
+    void invoke_deleter_if_exists() {
+        if(deleter != nullptr) {
+            deleter(*this);
+        }
+    }
+
+public:
+    /*! @brief Size of the internal buffer. */
+    static constexpr auto length = Len;
+    /*! @brief Alignment requirement. */
+    static constexpr auto alignment = Align;
+
+    /*! @brief Default constructor. */
+    constexpr basic_any() noexcept
+        : basic_any{stl::in_place_type<void>} {}
+
+    /**
+     * @brief Constructs a wrapper by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_any(stl::in_place_type_t<Type>, Args &&...args)
+        : base_type{} {
+        initialize<Type>(stl::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Constructs a wrapper taking ownership of the passed object.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value A pointer to an object to take ownership of.
+     */
+    template<typename Type>
+    requires (!stl::is_const_v<Type> && !stl::is_void_v<Type>)
+    explicit basic_any(stl::in_place_t, Type *value)
+        : base_type{} {
+        if(value == nullptr) {
+            initialize<void>();
+        } else {
+            initialize<Type &>(*value);
+            deleter = &basic_deleter<Type>;
+            mode = any_policy::dynamic;
+        }
+    }
+
+    /**
+     * @brief Constructs a wrapper from a given value.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     */
+    template<typename Type>
+    requires (!stl::same_as<stl::remove_cvref_t<Type>, basic_any>)
+    basic_any(Type &&value)
+        : basic_any{stl::in_place_type<stl::decay_t<Type>>, stl::forward<Type>(value)} {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    basic_any(const basic_any &other)
+        : basic_any{} {
+        other.vtable(request::copy, other, this);
+    }
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_any(basic_any &&other) noexcept
+        : base_type{},
+          vtable{other.vtable},
+          deleter{other.deleter},
+          underlying_type{other.underlying_type},
+          mode{other.mode} {
+        if(other.mode == any_policy::embedded) {
+            other.vtable(request::move, other, this);
+        } else if(other.mode != any_policy::empty) {
+            this->instance = stl::exchange(other.instance, nullptr);
+        }
+    }
+
+    /*! @brief Frees the internal buffer, whatever it means. */
+    ~basic_any() {
+        invoke_deleter_if_exists();
+    }
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This any object.
+     */
+    basic_any &operator=(const basic_any &other) {
+        if(this != &other) {
+            invoke_deleter_if_exists();
+
+            if(other) {
+                other.vtable(request::copy, other, this);
+            } else {
+                initialize<void>();
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This any object.
+     */
+    basic_any &operator=(basic_any &&other) noexcept {
+        if(this != &other) {
+            invoke_deleter_if_exists();
+
+            if(other.mode == any_policy::embedded) {
+                other.vtable(request::move, other, this);
+            } else if(other.mode != any_policy::empty) {
+                this->instance = stl::exchange(other.instance, nullptr);
+            }
+
+            vtable = other.vtable;
+            deleter = other.deleter;
+            underlying_type = other.underlying_type;
+            mode = other.mode;
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Value assignment operator.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @param value An instance of an object to use to initialize the wrapper.
+     * @return This any object.
+     */
+    template<typename Type>
+    requires (!stl::same_as<stl::remove_cvref_t<Type>, basic_any>)
+    basic_any &operator=(Type &&value) {
+        emplace<stl::decay_t<Type>>(stl::forward<Type>(value));
+        return *this;
+    }
+
+    /**
+     * @brief Returns false if a wrapper is empty, true otherwise.
+     * @return False if the wrapper is empty, true otherwise.
+     */
+    [[nodiscard]] bool has_value() const noexcept {
+        return (mode != any_policy::empty);
+    }
+
+    /**
+     * @brief Returns false if the wrapper does not contain the expected type,
+     * true otherwise.
+     * @param req Expected type.
+     * @return False if the wrapper does not contain the expected type, true
+     * otherwise.
+     */
+    [[nodiscard]] bool has_value(const type_info &req) const noexcept {
+        return (underlying_type == req.hash());
+    }
+
+    /**
+     * @brief Returns false if the wrapper does not contain the expected type,
+     * true otherwise.
+     * @tparam Type Expected type.
+     * @return False if the wrapper does not contain the expected type, true
+     * otherwise.
+     */
+    template<cvref_unqualified Type>
+    [[nodiscard]] bool has_value() const noexcept {
+        return (underlying_type == type_hash<Type>::value());
+    }
+
+    /**
+     * @brief Returns the object type info if any, `type_id<void>()` otherwise.
+     * @return The object type info if any, `type_id<void>()` otherwise.
+     */
+    [[nodiscard]] const type_info &info() const noexcept {
+        return *static_cast<const type_info *>(vtable(request::info, *this, nullptr));
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void *data() const noexcept {
+        if constexpr(base_type::has_buffer) {
+            return (mode == any_policy::embedded) ? &this->buffer : this->instance;
+        } else {
+            return this->instance;
+        }
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @param req Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void *data(const type_info &req) const noexcept {
+        return has_value(req) ? data() : nullptr;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @tparam Type Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    template<typename Type>
+    [[nodiscard]] const Type *data() const noexcept {
+        return has_value<stl::remove_const_t<Type>>() ? static_cast<const Type *>(data()) : nullptr;
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] void *data() noexcept {
+        return (mode == any_policy::cref) ? nullptr : const_cast<void *>(stl::as_const(*this).data());
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @param req Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] void *data(const type_info &req) noexcept {
+        return (mode == any_policy::cref) ? nullptr : const_cast<void *>(stl::as_const(*this).data(req));
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @tparam Type Expected type.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    template<typename Type>
+    [[nodiscard]] Type *data() noexcept {
+        if constexpr(stl::is_const_v<Type>) {
+            return stl::as_const(*this).template data<stl::remove_const_t<Type>>();
+        } else {
+            return (mode == any_policy::cref) ? nullptr : const_cast<Type *>(stl::as_const(*this).template data<stl::remove_const_t<Type>>());
+        }
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the wrapper.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&...args) {
+        invoke_deleter_if_exists();
+        initialize<Type>(stl::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Assigns a value to the contained object without replacing it.
+     * @param other The value to assign to the contained object.
+     * @return True in case of success, false otherwise.
+     */
+    bool assign(const basic_any &other) {
+        if(other && (mode != any_policy::cref) && (underlying_type == other.underlying_type)) {
+            return (vtable(request::assign, *this, other.data()) != nullptr);
+        }
+
+        return false;
+    }
+
+    /*! @copydoc assign */
+    // NOLINTNEXTLINE(cppcoreguidelines-rvalue-reference-param-not-moved)
+    bool assign(basic_any &&other) {
+        if(other && (mode != any_policy::cref) && (underlying_type == other.underlying_type)) {
+            return (other.mode == any_policy::cref) ? (vtable(request::assign, *this, stl::as_const(other).data()) != nullptr) : (vtable(request::transfer, *this, other.data()) != nullptr);
+        }
+
+        return false;
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        invoke_deleter_if_exists();
+        initialize<void>();
+    }
+
+    /**
+     * @brief Returns false if a wrapper is empty, true otherwise.
+     * @return False if the wrapper is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return has_value();
+    }
+
+    /**
+     * @brief Checks if two wrappers differ in their content.
+     * @param other Wrapper with which to compare.
+     * @return False if the two objects differ in their content, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const basic_any &other) const noexcept {
+        if(other && (underlying_type == other.underlying_type)) {
+            return (vtable(request::compare, *this, other.data()) != nullptr);
+        }
+
+        return (!*this && !other);
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A wrapper that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] basic_any as_ref() noexcept {
+        basic_any other = stl::as_const(*this).as_ref();
+
+        switch(mode) {
+            using enum any_policy;
+        case cref:
+        case empty:
+            other.mode = mode;
+            break;
+        default:
+            other.mode = any_policy::ref;
+            break;
+        }
+
+        return other;
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_any as_ref() const noexcept {
+        basic_any other{};
+        other.instance = data();
+        other.vtable = vtable;
+        other.underlying_type = underlying_type;
+        other.mode = any_policy::cref;
+        return other;
+    }
+
+    /**
+     * @brief Returns true if a wrapper owns its object, false otherwise.
+     * @return True if the wrapper owns its object, false otherwise.
+     */
+    [[nodiscard]] bool owner() const noexcept {
+        return (mode == any_policy::dynamic || mode == any_policy::embedded);
+    }
+
+    /**
+     * @brief Returns the current mode of an any object.
+     * @return The current mode of the any object.
+     */
+    [[nodiscard]] any_policy policy() const noexcept {
+        return mode;
+    }
+
+private:
+    vtable_type *vtable{};
+    deleter_type *deleter{};
+    id_type underlying_type{};
+    any_policy mode{};
+};
+
+/**
+ * @brief Performs type-safe access to the contained object.
+ * @tparam Type Type to which conversion is required.
+ * @tparam Len Size of the buffer reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ * @param data Target any object.
+ * @return The element converted to the requested type.
+ */
+template<typename Type, stl::size_t Len, stl::size_t Align>
+[[nodiscard]] stl::remove_const_t<Type> any_cast(const basic_any<Len, Align> &data) noexcept {
+    const auto *const instance = any_cast<stl::remove_reference_t<Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+/*! @copydoc any_cast */
+template<typename Type, stl::size_t Len, stl::size_t Align>
+[[nodiscard]] stl::remove_const_t<Type> any_cast(basic_any<Len, Align> &data) noexcept {
+    // forces const on non-reference types to make them work also with wrappers for const references
+    auto *const instance = any_cast<stl::remove_reference_t<const Type>>(&data);
+    ENTT_ASSERT(instance, "Invalid instance");
+    return static_cast<Type>(*instance);
+}
+
+/*! @copydoc any_cast */
+template<typename Type, stl::size_t Len, stl::size_t Align>
+// NOLINTNEXTLINE(cppcoreguidelines-rvalue-reference-param-not-moved)
+[[nodiscard]] stl::remove_const_t<Type> any_cast(basic_any<Len, Align> &&data) noexcept {
+    if constexpr(stl::is_copy_constructible_v<stl::remove_cvref_t<Type>>) {
+        if(auto *const instance = any_cast<stl::remove_reference_t<Type>>(&data); instance) {
+            return static_cast<Type>(stl::move(*instance));
+        }
+
+        return any_cast<Type>(data);
+    } else {
+        auto *const instance = any_cast<stl::remove_reference_t<Type>>(&data);
+        ENTT_ASSERT(instance, "Invalid instance");
+        return static_cast<Type>(stl::move(*instance));
+    }
+}
+
+/*! @copydoc any_cast */
+template<typename Type, stl::size_t Len, stl::size_t Align>
+[[nodiscard]] const Type *any_cast(const basic_any<Len, Align> *data) noexcept {
+    return data->template data<stl::remove_const_t<Type>>();
+}
+
+/*! @copydoc any_cast */
+template<typename Type, stl::size_t Len, stl::size_t Align>
+[[nodiscard]] Type *any_cast(basic_any<Len, Align> *data) noexcept {
+    if constexpr(stl::is_const_v<Type>) {
+        // last attempt to make wrappers for const references return their values
+        return any_cast<Type>(&stl::as_const(*data));
+    } else {
+        return data->template data<Type>();
+    }
+}
+
+/**
+ * @brief Constructs a wrapper from a given type, passing it all arguments.
+ * @tparam Type Type of object to use to initialize the wrapper.
+ * @tparam Len Size of the buffer reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Args Types of arguments to use to construct the new instance.
+ * @param args Parameters to use to construct the instance.
+ * @return A properly initialized wrapper for an object of the given type.
+ */
+template<typename Type, stl::size_t Len = basic_any<>::length, stl::size_t Align = basic_any<Len>::alignment, typename... Args>
+[[nodiscard]] basic_any<Len, Align> make_any(Args &&...args) {
+    return basic_any<Len, Align>{stl::in_place_type<Type>, stl::forward<Args>(args)...};
+}
+
+/**
+ * @brief Forwards its argument and avoids copies for lvalue references.
+ * @tparam Len Size of the buffer reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ * @tparam Type Type of argument to use to construct the new instance.
+ * @param value Parameter to use to construct the instance.
+ * @return A properly initialized and not necessarily owning wrapper.
+ */
+template<stl::size_t Len = basic_any<>::length, stl::size_t Align = basic_any<Len>::alignment, typename Type>
+[[nodiscard]] basic_any<Len, Align> forward_as_any(Type &&value) {
+    return basic_any<Len, Align>{stl::in_place_type<Type &&>, stl::forward<Type>(value)};
+}
+
+} // namespace entt
+
+#endif

include/entt/core/bit.hpp

@@ -0,0 +1,26 @@
+#ifndef ENTT_CORE_BIT_HPP
+#define ENTT_CORE_BIT_HPP
+
+#include "../config/config.h"
+#include "../stl/bit.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+
+namespace entt {
+
+/**
+ * @brief Fast module utility function (powers of two only).
+ * @tparam Type Unsigned integer type.
+ * @param value A value of unsigned integer type.
+ * @param mod _Modulus_, it must be a power of two.
+ * @return The common remainder.
+ */
+template<stl::unsigned_integral Type>
+[[nodiscard]] constexpr Type fast_mod(const Type value, const stl::size_t mod) noexcept {
+    ENTT_ASSERT_CONSTEXPR(stl::has_single_bit(mod), "Value must be a power of two");
+    return static_cast<Type>(value & (mod - 1u));
+}
+
+} // namespace entt
+
+#endif

include/entt/core/compressed_pair.hpp

@@ -0,0 +1,266 @@
+#ifndef ENTT_CORE_COMPRESSED_PAIR_HPP
+#define ENTT_CORE_COMPRESSED_PAIR_HPP
+
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/tuple.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename Type, stl::size_t>
+struct compressed_pair_element {
+    using reference = Type &;
+    using const_reference = const Type &;
+
+    // NOLINTNEXTLINE(modernize-use-equals-default)
+    constexpr compressed_pair_element() noexcept(stl::is_nothrow_default_constructible_v<Type>)
+    requires stl::default_initializable<Type> {}
+
+    template<typename Arg>
+    constexpr compressed_pair_element(Arg &&arg) noexcept(stl::is_nothrow_constructible_v<Type, Arg>)
+    requires (!stl::same_as<stl::remove_cvref_t<Arg>, compressed_pair_element>)
+        : value{stl::forward<Arg>(arg)} {}
+
+    template<typename... Args, stl::size_t... Index>
+    constexpr compressed_pair_element(stl::tuple<Args...> args, stl::index_sequence<Index...>) noexcept(stl::is_nothrow_constructible_v<Type, Args...>)
+        : value{stl::forward<Args>(stl::get<Index>(args))...} {}
+
+    [[nodiscard]] constexpr reference get() noexcept {
+        return value;
+    }
+
+    [[nodiscard]] constexpr const_reference get() const noexcept {
+        return value;
+    }
+
+private:
+    Type value{};
+};
+
+template<typename Type, stl::size_t Tag>
+requires is_ebco_eligible_v<Type>
+struct compressed_pair_element<Type, Tag>: Type {
+    using reference = Type &;
+    using const_reference = const Type &;
+    using base_type = Type;
+
+    constexpr compressed_pair_element() noexcept(stl::is_nothrow_default_constructible_v<base_type>)
+    requires stl::default_initializable<Type>
+        : base_type{} {}
+
+    template<typename Arg>
+    constexpr compressed_pair_element(Arg &&arg) noexcept(stl::is_nothrow_constructible_v<base_type, Arg>)
+    requires (!stl::same_as<stl::remove_cvref_t<Arg>, compressed_pair_element>)
+        : base_type{stl::forward<Arg>(arg)} {}
+
+    template<typename... Args, stl::size_t... Index>
+    constexpr compressed_pair_element(stl::tuple<Args...> args, stl::index_sequence<Index...>) noexcept(stl::is_nothrow_constructible_v<base_type, Args...>)
+        : base_type{stl::forward<Args>(stl::get<Index>(args))...} {}
+
+    [[nodiscard]] constexpr reference get() noexcept {
+        return *this;
+    }
+
+    [[nodiscard]] constexpr const_reference get() const noexcept {
+        return *this;
+    }
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief A compressed pair.
+ *
+ * A pair that exploits the _Empty Base Class Optimization_ (or _EBCO_) to
+ * reduce its final size to a minimum.
+ *
+ * @tparam First The type of the first element that the pair stores.
+ * @tparam Second The type of the second element that the pair stores.
+ */
+template<typename First, typename Second>
+class compressed_pair final
+    : internal::compressed_pair_element<First, 0u>,
+      internal::compressed_pair_element<Second, 1u> {
+    using first_base = internal::compressed_pair_element<First, 0u>;
+    using second_base = internal::compressed_pair_element<Second, 1u>;
+
+public:
+    /*! @brief The type of the first element that the pair stores. */
+    using first_type = First;
+    /*! @brief The type of the second element that the pair stores. */
+    using second_type = Second;
+
+    /**
+     * @brief Default constructor, conditionally enabled.
+     *
+     * This constructor is only available when the types that the pair stores
+     * are both at least default constructible.
+     */
+    constexpr compressed_pair() noexcept(stl::is_nothrow_default_constructible_v<first_base> && stl::is_nothrow_default_constructible_v<second_base>)
+    requires stl::default_initializable<first_type> && stl::default_initializable<second_type>
+        : first_base{},
+          second_base{} {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    constexpr compressed_pair(const compressed_pair &other) = default;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    constexpr compressed_pair(compressed_pair &&other) noexcept = default;
+
+    /**
+     * @brief Constructs a pair from its values.
+     * @tparam Arg Type of value to use to initialize the first element.
+     * @tparam Other Type of value to use to initialize the second element.
+     * @param arg Value to use to initialize the first element.
+     * @param other Value to use to initialize the second element.
+     */
+    template<typename Arg, typename Other>
+    constexpr compressed_pair(Arg &&arg, Other &&other) noexcept(stl::is_nothrow_constructible_v<first_base, Arg> && stl::is_nothrow_constructible_v<second_base, Other>)
+        : first_base{stl::forward<Arg>(arg)},
+          second_base{stl::forward<Other>(other)} {}
+
+    /**
+     * @brief Constructs a pair by forwarding the arguments to its parts.
+     * @tparam Args Types of arguments to use to initialize the first element.
+     * @tparam Other Types of arguments to use to initialize the second element.
+     * @param args Arguments to use to initialize the first element.
+     * @param other Arguments to use to initialize the second element.
+     */
+    template<typename... Args, typename... Other>
+    constexpr compressed_pair(stl::piecewise_construct_t, stl::tuple<Args...> args, stl::tuple<Other...> other) noexcept(stl::is_nothrow_constructible_v<first_base, Args...> && stl::is_nothrow_constructible_v<second_base, Other...>)
+        : first_base{stl::move(args), stl::index_sequence_for<Args...>{}},
+          second_base{stl::move(other), stl::index_sequence_for<Other...>{}} {}
+
+    /*! @brief Default destructor. */
+    ~compressed_pair() = default;
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This compressed pair object.
+     */
+    constexpr compressed_pair &operator=(const compressed_pair &other) = default;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This compressed pair object.
+     */
+    constexpr compressed_pair &operator=(compressed_pair &&other) noexcept = default;
+
+    /**
+     * @brief Returns the first element that a pair stores.
+     * @return The first element that a pair stores.
+     */
+    [[nodiscard]] constexpr first_type &first() noexcept {
+        return static_cast<first_base &>(*this).get();
+    }
+
+    /*! @copydoc first */
+    [[nodiscard]] constexpr const first_type &first() const noexcept {
+        return static_cast<const first_base &>(*this).get();
+    }
+
+    /**
+     * @brief Returns the second element that a pair stores.
+     * @return The second element that a pair stores.
+     */
+    [[nodiscard]] constexpr second_type &second() noexcept {
+        return static_cast<second_base &>(*this).get();
+    }
+
+    /*! @copydoc second */
+    [[nodiscard]] constexpr const second_type &second() const noexcept {
+        return static_cast<const second_base &>(*this).get();
+    }
+
+    /**
+     * @brief Swaps two compressed pair objects.
+     * @param other The compressed pair to swap with.
+     */
+    constexpr void swap(compressed_pair &other) noexcept {
+        using stl::swap;
+        swap(first(), other.first());
+        swap(second(), other.second());
+    }
+
+    /**
+     * @brief Extracts an element from the compressed pair.
+     * @tparam Index An integer value that is either 0 or 1.
+     * @return Returns a reference to the first element if `Index` is 0 and a
+     * reference to the second element if `Index` is 1.
+     */
+    template<stl::size_t Index>
+    requires (Index <= 1u)
+    [[nodiscard]] constexpr decltype(auto) get() noexcept {
+        if constexpr(Index == 0u) {
+            return first();
+        } else {
+            return second();
+        }
+    }
+
+    /*! @copydoc get */
+    template<stl::size_t Index>
+    requires (Index <= 1u)
+    [[nodiscard]] constexpr decltype(auto) get() const noexcept {
+        if constexpr(Index == 0u) {
+            return first();
+        } else {
+            return second();
+        }
+    }
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Type Type of value to use to initialize the first element.
+ * @tparam Other Type of value to use to initialize the second element.
+ */
+template<typename Type, typename Other>
+compressed_pair(Type &&, Other &&) -> compressed_pair<stl::decay_t<Type>, stl::decay_t<Other>>;
+
+/**
+ * @brief Swaps two compressed pair objects.
+ * @tparam First The type of the first element that the pairs store.
+ * @tparam Second The type of the second element that the pairs store.
+ * @param lhs A valid compressed pair object.
+ * @param rhs A valid compressed pair object.
+ */
+template<typename First, typename Second>
+constexpr void swap(compressed_pair<First, Second> &lhs, compressed_pair<First, Second> &rhs) noexcept {
+    lhs.swap(rhs);
+}
+
+} // namespace entt
+
+/*! @cond ENTT_INTERNAL */
+#include <utility>
+
+namespace std {
+
+template<typename First, typename Second>
+struct tuple_size<entt::compressed_pair<First, Second>>: integral_constant<entt::stl::size_t, 2u> {};
+
+template<entt::stl::size_t Index, typename First, typename Second>
+requires (Index <= 1u)
+struct tuple_element<Index, entt::compressed_pair<First, Second>>: conditional<Index == 0u, First, Second> {};
+
+} // namespace std
+/*! @endcond */
+
+#endif

include/entt/core/concepts.hpp

@@ -0,0 +1,17 @@
+#ifndef ENTT_CORE_CONCEPTS_HPP
+#define ENTT_CORE_CONCEPTS_HPP
+
+#include "../stl/type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @brief Specifies that a type is not a cv-qualified reference.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+concept cvref_unqualified = stl::is_same_v<stl::remove_cvref_t<Type>, Type>;
+
+} // namespace entt
+
+#endif

include/entt/core/enum.hpp

@@ -0,0 +1,102 @@
+#ifndef ENTT_CORE_ENUM_HPP
+#define ENTT_CORE_ENUM_HPP
+
+#include "../stl/concepts.hpp"
+#include "../stl/type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @brief Enable bitmask support for enum classes.
+ * @tparam Type The enum type for which to enable bitmask support.
+ */
+template<typename Type>
+struct enum_as_bitmask: stl::false_type {};
+
+/*! @copydoc enum_as_bitmask */
+template<typename Type>
+requires requires {
+    requires stl::is_enum_v<Type>;
+    { Type::_entt_enum_as_bitmask } -> stl::same_as<Type>;
+}
+struct enum_as_bitmask<Type>: stl::true_type {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The enum class type for which to enable bitmask support.
+ */
+template<typename Type>
+inline constexpr bool enum_as_bitmask_v = enum_as_bitmask<Type>::value;
+
+/**
+ * @brief Specifies that an enum class supports bitmask operations.
+ * @tparam Type Enum class type.
+ */
+template<typename Type>
+// check again that it is an enum to deal with incorrect specializations
+concept enum_bitmask = stl::is_enum_v<Type> && enum_as_bitmask_v<Type>;
+
+} // namespace entt
+
+/**
+ * @brief Operator available for enums for which bitmask support is enabled.
+ * @tparam Type Enum class type.
+ * @param lhs The first value to use.
+ * @param rhs The second value to use.
+ * @return The result of invoking the operator on the underlying types of the
+ * two values provided.
+ */
+template<entt::enum_bitmask Type>
+[[nodiscard]] constexpr Type operator|(const Type lhs, const Type rhs) noexcept {
+    return static_cast<Type>(static_cast<entt::stl::underlying_type_t<Type>>(lhs) | static_cast<entt::stl::underlying_type_t<Type>>(rhs));
+}
+
+/*! @copydoc operator| */
+template<entt::enum_bitmask Type>
+[[nodiscard]] constexpr Type operator&(const Type lhs, const Type rhs) noexcept {
+    return static_cast<Type>(static_cast<entt::stl::underlying_type_t<Type>>(lhs) & static_cast<entt::stl::underlying_type_t<Type>>(rhs));
+}
+
+/*! @copydoc operator| */
+template<entt::enum_bitmask Type>
+[[nodiscard]] constexpr Type operator^(const Type lhs, const Type rhs) noexcept {
+    return static_cast<Type>(static_cast<entt::stl::underlying_type_t<Type>>(lhs) ^ static_cast<entt::stl::underlying_type_t<Type>>(rhs));
+}
+
+/**
+ * @brief Operator available for enums for which bitmask support is enabled.
+ * @tparam Type Enum class type.
+ * @param value The value to use.
+ * @return The result of invoking the operator on the underlying types of the
+ * value provided.
+ */
+template<entt::enum_bitmask Type>
+[[nodiscard]] constexpr Type operator~(const Type value) noexcept {
+    return static_cast<Type>(~static_cast<entt::stl::underlying_type_t<Type>>(value));
+}
+
+/*! @copydoc operator~ */
+template<entt::enum_bitmask Type>
+[[nodiscard]] constexpr bool operator!(const Type value) noexcept {
+    return !static_cast<entt::stl::underlying_type_t<Type>>(value);
+}
+
+/*! @copydoc operator| */
+template<entt::enum_bitmask Type>
+constexpr Type &operator|=(Type &lhs, const Type rhs) noexcept {
+    return (lhs = (lhs | rhs));
+}
+
+/*! @copydoc operator| */
+template<entt::enum_bitmask Type>
+constexpr Type &operator&=(Type &lhs, const Type rhs) noexcept {
+    return (lhs = (lhs & rhs));
+}
+
+/*! @copydoc operator| */
+template<entt::enum_bitmask Type>
+constexpr Type &operator^=(Type &lhs, const Type rhs) noexcept {
+    return (lhs = (lhs ^ rhs));
+}
+
+#endif

include/entt/core/family.hpp

@@ -0,0 +1,35 @@
+#ifndef ENTT_CORE_FAMILY_HPP
+#define ENTT_CORE_FAMILY_HPP
+
+#include "../config/config.h"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Dynamic identifier generator.
+ *
+ * Utility class template that can be used to assign unique identifiers to types
+ * at runtime. Use different specializations to create separate sets of
+ * identifiers.
+ */
+template<typename...>
+class family {
+    static auto identifier() noexcept {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using value_type = id_type;
+
+    /*! @brief Statically generated unique identifier for the given type. */
+    template<typename... Type>
+    // at the time I'm writing, clang crashes during compilation if auto is used instead of value_type
+    inline static const value_type value = identifier();
+};
+
+} // namespace entt
+
+#endif

include/entt/core/fwd.hpp

@@ -0,0 +1,51 @@
+#ifndef ENTT_CORE_FWD_HPP
+#define ENTT_CORE_FWD_HPP
+
+#include "../config/config.h"
+#include "../stl/cstddef.hpp"
+#include "../stl/cstdint.hpp"
+
+namespace entt {
+
+/*! @brief Possible modes of an any object. */
+enum class any_policy : stl::uint8_t {
+    /*! @brief Default mode, no element available. */
+    empty,
+    /*! @brief Owning mode, dynamically allocated element. */
+    dynamic,
+    /*! @brief Owning mode, embedded element. */
+    embedded,
+    /*! @brief Aliasing mode, non-const reference. */
+    ref,
+    /*! @brief Const aliasing mode, const reference. */
+    cref
+};
+
+// NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+template<stl::size_t Len = sizeof(double[2]), stl::size_t = alignof(double[2])>
+class basic_any;
+
+/*! @brief Alias declaration for type identifiers. */
+using id_type = ENTT_ID_TYPE;
+
+/*! @brief Alias declaration for the most common use case. */
+using any = basic_any<>;
+
+template<typename, typename>
+class compressed_pair;
+
+template<typename>
+class basic_hashed_string;
+
+/*! @brief Aliases for common character types. */
+using hashed_string = basic_hashed_string<char>;
+
+/*! @brief Aliases for common character types. */
+using hashed_wstring = basic_hashed_string<wchar_t>;
+
+// NOLINTNEXTLINE(bugprone-forward-declaration-namespace)
+struct type_info;
+
+} // namespace entt
+
+#endif

include/entt/core/hashed_string.hpp

@@ -0,0 +1,260 @@
+#ifndef ENTT_CORE_HASHED_STRING_HPP
+#define ENTT_CORE_HASHED_STRING_HPP
+
+#include "../stl/cstddef.hpp"
+#include "../stl/cstdint.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename = id_type>
+struct fnv_1a_params;
+
+template<>
+struct fnv_1a_params<stl::uint32_t> {
+    static constexpr auto offset = 2166136261;
+    static constexpr auto prime = 16777619;
+};
+
+template<>
+struct fnv_1a_params<stl::uint64_t> {
+    static constexpr auto offset = 14695981039346656037ull;
+    static constexpr auto prime = 1099511628211ull;
+};
+
+template<typename Char>
+struct basic_hashed_string {
+    using value_type = Char;
+    using size_type = stl::size_t;
+    using hash_type = id_type;
+
+    const value_type *repr{};
+    hash_type hash{fnv_1a_params<>::offset};
+    size_type length{};
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Zero overhead unique identifier.
+ *
+ * A hashed string is a compile-time tool that allows users to use
+ * human-readable identifiers in the codebase while using their numeric
+ * counterparts at runtime.<br/>
+ * Because of that, a hashed string can also be used in constant expressions if
+ * required.
+ *
+ * @warning
+ * This class doesn't take ownership of user-supplied strings nor does it make a
+ * copy of them.
+ *
+ * @tparam Char Character type.
+ */
+template<typename Char>
+class basic_hashed_string: internal::basic_hashed_string<Char> {
+    using base_type = internal::basic_hashed_string<Char>;
+    using params = internal::fnv_1a_params<>;
+
+    struct const_wrapper {
+        // non-explicit constructor on purpose
+        constexpr const_wrapper(const base_type::value_type *str) noexcept
+            : repr{str} {}
+
+        const base_type::value_type *repr;
+    };
+
+public:
+    /*! @brief Character type. */
+    using value_type = base_type::value_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = base_type::size_type;
+    /*! @brief Unsigned integer type. */
+    using hash_type = base_type::hash_type;
+
+    /**
+     * @brief Returns directly the numeric representation of a string view.
+     * @param str Human-readable identifier.
+     * @param len Length of the string to hash.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const value_type *str, const size_type len) noexcept {
+        return basic_hashed_string{str, len};
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @tparam N Number of characters of the identifier.
+     * @param str Human-readable identifier.
+     * @return The numeric representation of the string.
+     */
+    template<stl::size_t N>
+    // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+    [[nodiscard]] static ENTT_CONSTEVAL hash_type value(const value_type (&str)[N]) noexcept {
+        return basic_hashed_string{str};
+    }
+
+    /**
+     * @brief Returns directly the numeric representation of a string.
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     * @return The numeric representation of the string.
+     */
+    [[nodiscard]] static constexpr hash_type value(const_wrapper wrapper) noexcept {
+        return basic_hashed_string{wrapper};
+    }
+
+    /*! @brief Constructs an empty hashed string. */
+    constexpr basic_hashed_string() noexcept
+        : basic_hashed_string{nullptr, 0u} {}
+
+    /**
+     * @brief Constructs a hashed string from a string view.
+     * @param str Human-readable identifier.
+     * @param len Length of the string to hash.
+     */
+    constexpr basic_hashed_string(const value_type *str, const size_type len) noexcept
+        // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay)
+        : base_type{str} {
+        // NOLINTBEGIN(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+        for(; base_type::length < len; ++base_type::length) {
+            base_type::hash = (base_type::hash ^ static_cast<id_type>(str[base_type::length])) * params::prime;
+        }
+        // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+    }
+
+    /**
+     * @brief Constructs a hashed string from an array of const characters.
+     * @tparam N Number of characters of the identifier.
+     * @param str Human-readable identifier.
+     */
+    template<stl::size_t N>
+    // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+    ENTT_CONSTEVAL basic_hashed_string(const value_type (&str)[N]) noexcept
+        // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay)
+        : base_type{str} {
+        for(; str[base_type::length]; ++base_type::length) {
+            base_type::hash = (base_type::hash ^ static_cast<id_type>(str[base_type::length])) * params::prime;
+        }
+    }
+
+    /**
+     * @brief Explicit constructor on purpose to avoid constructing a hashed
+     * string directly from a `const value_type *`.
+     *
+     * @warning
+     * The lifetime of the string is not extended nor is it copied.
+     *
+     * @param wrapper Helps achieving the purpose by relying on overloading.
+     */
+    explicit constexpr basic_hashed_string(const_wrapper wrapper) noexcept
+        : base_type{wrapper.repr} {
+        // NOLINTBEGIN(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+        for(; wrapper.repr[base_type::length]; ++base_type::length) {
+            base_type::hash = (base_type::hash ^ static_cast<id_type>(wrapper.repr[base_type::length])) * params::prime;
+        }
+        // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+    }
+
+    /**
+     * @brief Returns the size of a hashed string.
+     * @return The size of the hashed string.
+     */
+    [[nodiscard]] constexpr size_type size() const noexcept {
+        return base_type::length;
+    }
+
+    /**
+     * @brief Returns the human-readable representation of a hashed string.
+     * @return The string used to initialize the hashed string.
+     */
+    [[nodiscard]] constexpr const value_type *data() const noexcept {
+        return base_type::repr;
+    }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the hashed string.
+     */
+    [[nodiscard]] constexpr hash_type value() const noexcept {
+        return base_type::hash;
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] explicit constexpr operator const value_type *() const noexcept {
+        return data();
+    }
+
+    /**
+     * @brief Returns the numeric representation of a hashed string.
+     * @return The numeric representation of the hashed string.
+     */
+    [[nodiscard]] constexpr operator hash_type() const noexcept {
+        return value();
+    }
+
+    /**
+     * @brief Compares two hashed strings.
+     * @param other A valid hashed string.
+     * @return True if the two hashed strings are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const basic_hashed_string &other) const noexcept {
+        return value() == other.value();
+    }
+
+    /**
+     * @brief Lexicographically compares two hashed strings.
+     * @param other A valid hashed string.
+     * @return The relative order between the two hashed strings.
+     */
+    [[nodiscard]] constexpr auto operator<=>(const basic_hashed_string &other) const noexcept {
+        return value() <=> other.value();
+    }
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Char Character type.
+ * @param str Human-readable identifier.
+ * @param len Length of the string to hash.
+ */
+template<typename Char>
+basic_hashed_string(const Char *str, stl::size_t len) -> basic_hashed_string<Char>;
+
+/**
+ * @brief Deduction guide.
+ * @tparam Char Character type.
+ * @tparam N Number of characters of the identifier.
+ * @param str Human-readable identifier.
+ */
+template<typename Char, stl::size_t N>
+// NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+basic_hashed_string(const Char (&str)[N]) -> basic_hashed_string<Char>;
+
+inline namespace literals {
+
+/**
+ * @brief User defined literal for hashed strings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed string.
+ */
+[[nodiscard]] ENTT_CONSTEVAL hashed_string operator""_hs(const char *str, stl::size_t) noexcept {
+    return hashed_string{str};
+}
+
+/**
+ * @brief User defined literal for hashed wstrings.
+ * @param str The literal without its suffix.
+ * @return A properly initialized hashed wstring.
+ */
+[[nodiscard]] ENTT_CONSTEVAL hashed_wstring operator""_hws(const wchar_t *str, stl::size_t) noexcept {
+    return hashed_wstring{str};
+}
+
+} // namespace literals
+
+} // namespace entt
+
+#endif

include/entt/core/ident.hpp

@@ -0,0 +1,35 @@
+#ifndef ENTT_CORE_IDENT_HPP
+#define ENTT_CORE_IDENT_HPP
+
+#include "../stl/cstddef.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @brief Type integral identifiers.
+ * @tparam Type List of types for which to generate identifiers.
+ */
+template<typename... Type>
+class ident {
+    template<typename Curr, stl::size_t... Index>
+    [[nodiscard]] static ENTT_CONSTEVAL id_type get(stl::index_sequence<Index...>) noexcept {
+        return (0 + ... + (stl::is_same_v<Curr, type_list_element_t<Index, type_list<stl::decay_t<Type>...>>> ? id_type{Index} : id_type{}));
+    }
+
+public:
+    /*! @brief Unsigned integer type. */
+    using value_type = id_type;
+
+    /*! @brief Statically generated unique identifier for the given type. */
+    template<typename Curr>
+    requires (stl::is_same_v<stl::remove_cvref_t<Curr>, Type> || ...)
+    static constexpr value_type value = get<stl::remove_cvref_t<Curr>>(stl::index_sequence_for<Type...>{});
+};
+
+} // namespace entt
+
+#endif

include/entt/core/iterator.hpp

@@ -0,0 +1,181 @@
+#ifndef ENTT_CORE_ITERATOR_HPP
+#define ENTT_CORE_ITERATOR_HPP
+
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+
+namespace entt {
+
+/**
+ * @brief Helper type to use as pointer with input iterators.
+ * @tparam Type of wrapped value.
+ */
+template<typename Type>
+struct input_iterator_pointer final {
+    /*! @brief Value type. */
+    using value_type = Type;
+    /*! @brief Pointer type. */
+    using pointer = Type *;
+    /*! @brief Reference type. */
+    using reference = Type &;
+
+    /**
+     * @brief Constructs a proxy object by move.
+     * @param val Value to use to initialize the proxy object.
+     */
+    constexpr input_iterator_pointer(value_type &&val) noexcept(stl::is_nothrow_move_constructible_v<value_type>)
+        : value{stl::move(val)} {}
+
+    /**
+     * @brief Access operator for accessing wrapped values.
+     * @return A pointer to the wrapped value.
+     */
+    [[nodiscard]] constexpr pointer operator->() noexcept {
+        return stl::addressof(value);
+    }
+
+    /**
+     * @brief Dereference operator for accessing wrapped values.
+     * @return A reference to the wrapped value.
+     */
+    [[nodiscard]] constexpr reference operator*() noexcept {
+        return value;
+    }
+
+private:
+    Type value;
+};
+
+/**
+ * @brief Plain iota iterator (waiting for C++20).
+ * @tparam Type Value type.
+ */
+template<stl::integral Type>
+struct iota_iterator final {
+    /*! @brief Value type, likely an integral one. */
+    using value_type = Type;
+    /*! @brief Invalid pointer type. */
+    using pointer = void;
+    /*! @brief Non-reference type, same as value type. */
+    using reference = value_type;
+    /*! @brief Difference type. */
+    using difference_type = stl::ptrdiff_t;
+    /*! @brief Iterator category. */
+    using iterator_category = stl::input_iterator_tag;
+
+    /*! @brief Default constructor. */
+    constexpr iota_iterator() noexcept
+        : current{} {}
+
+    /**
+     * @brief Constructs an iota iterator from a given value.
+     * @param init The initial value assigned to the iota iterator.
+     */
+    constexpr iota_iterator(const value_type init) noexcept
+        : current{init} {}
+
+    /**
+     * @brief Pre-increment operator.
+     * @return This iota iterator.
+     */
+    constexpr iota_iterator &operator++() noexcept {
+        return ++current, *this;
+    }
+
+    /**
+     * @brief Post-increment operator.
+     * @return This iota iterator.
+     */
+    constexpr iota_iterator operator++(int) noexcept {
+        const iota_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    /**
+     * @brief Dereference operator.
+     * @return The underlying value.
+     */
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return current;
+    }
+
+    /**
+     * @brief Comparison operator.
+     * @param other A properly initialized iota iterator.
+     * @return True if the two iterators are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const iota_iterator &other) const noexcept {
+        return current == other.current;
+    }
+
+private:
+    value_type current;
+};
+
+/**
+ * @brief Utility class to create an iterable object from a pair of iterators.
+ * @tparam It Type of iterator.
+ * @tparam Sentinel Type of sentinel.
+ */
+template<stl::input_or_output_iterator It, stl::sentinel_for<It> Sentinel = It>
+struct iterable_adaptor final {
+    /*! @brief Value type. */
+    using value_type = stl::iterator_traits<It>::value_type;
+    /*! @brief Iterator type. */
+    using iterator = It;
+    /*! @brief Sentinel type. */
+    using sentinel = Sentinel;
+
+    /*! @brief Default constructor. */
+    constexpr iterable_adaptor() noexcept(stl::is_nothrow_default_constructible_v<iterator> && stl::is_nothrow_default_constructible_v<sentinel>)
+        : first{},
+          last{} {}
+
+    /**
+     * @brief Creates an iterable object from a pair of iterators.
+     * @param from Begin iterator.
+     * @param to End iterator.
+     */
+    constexpr iterable_adaptor(iterator from, sentinel to) noexcept(stl::is_nothrow_move_constructible_v<iterator> && stl::is_nothrow_move_constructible_v<sentinel>)
+        : first{stl::move(from)},
+          last{stl::move(to)} {}
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     * @return An iterator to the first element of the range.
+     */
+    [[nodiscard]] constexpr iterator begin() const noexcept {
+        return first;
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last element of the
+     * range.
+     */
+    [[nodiscard]] constexpr sentinel end() const noexcept {
+        return last;
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] constexpr iterator cbegin() const noexcept {
+        return begin();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] constexpr sentinel cend() const noexcept {
+        return end();
+    }
+
+private:
+    It first;
+    Sentinel last;
+};
+
+} // namespace entt
+
+#endif

include/entt/core/memory.hpp

@@ -0,0 +1,225 @@
+#ifndef ENTT_CORE_MEMORY_HPP
+#define ENTT_CORE_MEMORY_HPP
+
+#include "../config/config.h"
+#include "../stl/cstddef.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/tuple.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+
+namespace entt {
+
+/**
+ * @brief Utility function to design allocation-aware containers.
+ * @tparam Allocator Type of allocator.
+ * @param lhs A valid allocator.
+ * @param rhs Another valid allocator.
+ */
+template<typename Allocator>
+constexpr void propagate_on_container_copy_assignment([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) noexcept {
+    if constexpr(stl::allocator_traits<Allocator>::propagate_on_container_copy_assignment::value) {
+        lhs = rhs;
+    }
+}
+
+/**
+ * @brief Utility function to design allocation-aware containers.
+ * @tparam Allocator Type of allocator.
+ * @param lhs A valid allocator.
+ * @param rhs Another valid allocator.
+ */
+template<typename Allocator>
+constexpr void propagate_on_container_move_assignment([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) noexcept {
+    if constexpr(stl::allocator_traits<Allocator>::propagate_on_container_move_assignment::value) {
+        lhs = stl::move(rhs);
+    }
+}
+
+/**
+ * @brief Utility function to design allocation-aware containers.
+ * @tparam Allocator Type of allocator.
+ * @param lhs A valid allocator.
+ * @param rhs Another valid allocator.
+ */
+template<typename Allocator>
+constexpr void propagate_on_container_swap([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) noexcept {
+    if constexpr(stl::allocator_traits<Allocator>::propagate_on_container_swap::value) {
+        using stl::swap;
+        swap(lhs, rhs);
+    } else {
+        ENTT_ASSERT_CONSTEXPR(lhs == rhs, "Cannot swap the containers");
+    }
+}
+
+/**
+ * @brief Deleter for allocator-aware unique pointers (waiting for C++20).
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Allocator>
+struct allocation_deleter: private Allocator {
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Pointer type. */
+    using pointer = stl::allocator_traits<Allocator>::pointer;
+
+    /**
+     * @brief Inherited constructors.
+     * @param alloc The allocator to use.
+     */
+    constexpr allocation_deleter(const allocator_type &alloc) noexcept(stl::is_nothrow_copy_constructible_v<allocator_type>)
+        : Allocator{alloc} {}
+
+    /**
+     * @brief Destroys the pointed object and deallocates its memory.
+     * @param ptr A valid pointer to an object of the given type.
+     */
+    constexpr void operator()(pointer ptr) noexcept(stl::is_nothrow_destructible_v<typename allocator_type::value_type>) {
+        using alloc_traits = stl::allocator_traits<Allocator>;
+        alloc_traits::destroy(*this, stl::to_address(ptr));
+        alloc_traits::deallocate(*this, ptr, 1u);
+    }
+};
+
+/**
+ * @brief Allows `stl::unique_ptr` to use allocators (waiting for C++20).
+ * @tparam Type Type of object to allocate for and to construct.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ * @tparam Args Types of arguments to use to construct the object.
+ * @param allocator The allocator to use.
+ * @param args Parameters to use to construct the object.
+ * @return A properly initialized unique pointer with a custom deleter.
+ */
+template<typename Type, typename Allocator, typename... Args>
+constexpr auto allocate_unique(Allocator &allocator, Args &&...args) {
+    static_assert(!stl::is_array_v<Type>, "Array types are not supported");
+
+    using alloc_traits = stl::allocator_traits<Allocator>::template rebind_traits<Type>;
+    using allocator_type = alloc_traits::allocator_type;
+
+    allocator_type alloc{allocator};
+    auto ptr = alloc_traits::allocate(alloc, 1u);
+
+    ENTT_TRY {
+        alloc_traits::construct(alloc, stl::to_address(ptr), stl::forward<Args>(args)...);
+    }
+    ENTT_CATCH {
+        alloc_traits::deallocate(alloc, ptr, 1u);
+        ENTT_THROW;
+    }
+
+    return stl::unique_ptr<Type, allocation_deleter<allocator_type>>{ptr, alloc};
+}
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename Type>
+struct uses_allocator_construction {
+    template<typename Allocator, typename... Params>
+    static constexpr auto args([[maybe_unused]] const Allocator &allocator, Params &&...params) noexcept {
+        if constexpr(!stl::uses_allocator_v<Type, Allocator> && stl::is_constructible_v<Type, Params...>) {
+            return stl::forward_as_tuple(stl::forward<Params>(params)...);
+        } else {
+            static_assert(stl::uses_allocator_v<Type, Allocator>, "Ill-formed request");
+
+            if constexpr(stl::is_constructible_v<Type, stl::allocator_arg_t, const Allocator &, Params...>) {
+                return stl::tuple<stl::allocator_arg_t, const Allocator &, Params &&...>{stl::allocator_arg, allocator, stl::forward<Params>(params)...};
+            } else {
+                static_assert(stl::is_constructible_v<Type, Params..., const Allocator &>, "Ill-formed request");
+                return stl::forward_as_tuple(stl::forward<Params>(params)..., allocator);
+            }
+        }
+    }
+};
+
+template<typename Type, typename Other>
+struct uses_allocator_construction<stl::pair<Type, Other>> {
+    using type = stl::pair<Type, Other>;
+
+    template<typename First, typename Second>
+    static constexpr auto args(const auto &allocator, stl::piecewise_construct_t, First &&first, Second &&second) noexcept {
+        return stl::make_tuple(
+            stl::piecewise_construct,
+            stl::apply([&allocator](auto &&...curr) { return uses_allocator_construction<Type>::args(allocator, stl::forward<decltype(curr)>(curr)...); }, stl::forward<First>(first)),
+            stl::apply([&allocator](auto &&...curr) { return uses_allocator_construction<Other>::args(allocator, stl::forward<decltype(curr)>(curr)...); }, stl::forward<Second>(second)));
+    }
+
+    static constexpr auto args(const auto &allocator) noexcept {
+        return uses_allocator_construction<type>::args(allocator, stl::piecewise_construct, stl::tuple<>{}, stl::tuple<>{});
+    }
+
+    template<typename First, typename Second>
+    static constexpr auto args(const auto &allocator, First &&first, Second &&second) noexcept {
+        return uses_allocator_construction<type>::args(allocator, stl::piecewise_construct, stl::forward_as_tuple(stl::forward<First>(first)), stl::forward_as_tuple(stl::forward<Second>(second)));
+    }
+
+    template<typename First, typename Second>
+    static constexpr auto args(const auto &allocator, const stl::pair<First, Second> &value) noexcept {
+        return uses_allocator_construction<type>::args(allocator, stl::piecewise_construct, stl::forward_as_tuple(value.first), stl::forward_as_tuple(value.second));
+    }
+
+    template<typename First, typename Second>
+    static constexpr auto args(const auto &allocator, stl::pair<First, Second> &&value) noexcept {
+        return uses_allocator_construction<type>::args(allocator, stl::piecewise_construct, stl::forward_as_tuple(stl::move(value.first)), stl::forward_as_tuple(stl::move(value.second)));
+    }
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Uses-allocator construction utility (waiting for C++20).
+ *
+ * Primarily intended for internal use. Prepares the argument list needed to
+ * create an object of a given type by means of uses-allocator construction.
+ *
+ * @tparam Type Type to return arguments for.
+ * @tparam Args Types of arguments to use to construct the object.
+ * @param allocator The allocator to use.
+ * @param args Parameters to use to construct the object.
+ * @return The arguments needed to create an object of the given type.
+ */
+template<typename Type, typename... Args>
+constexpr auto uses_allocator_construction_args(const auto &allocator, Args &&...args) noexcept {
+    return internal::uses_allocator_construction<Type>::args(allocator, stl::forward<Args>(args)...);
+}
+
+/**
+ * @brief Uses-allocator construction utility (waiting for C++20).
+ *
+ * Primarily intended for internal use. Creates an object of a given type by
+ * means of uses-allocator construction.
+ *
+ * @tparam Type Type of object to create.
+ * @tparam Args Types of arguments to use to construct the object.
+ * @param allocator The allocator to use.
+ * @param args Parameters to use to construct the object.
+ * @return A newly created object of the given type.
+ */
+template<typename Type, typename... Args>
+constexpr Type make_obj_using_allocator(const auto &allocator, Args &&...args) {
+    return stl::make_from_tuple<Type>(internal::uses_allocator_construction<Type>::args(allocator, stl::forward<Args>(args)...));
+}
+
+/**
+ * @brief Uses-allocator construction utility (waiting for C++20).
+ *
+ * Primarily intended for internal use. Creates an object of a given type by
+ * means of uses-allocator construction at an uninitialized memory location.
+ *
+ * @tparam Type Type of object to create.
+ * @tparam Args Types of arguments to use to construct the object.
+ * @param value Memory location in which to place the object.
+ * @param allocator The allocator to use.
+ * @param args Parameters to use to construct the object.
+ * @return A pointer to the newly created object of the given type.
+ */
+template<typename Type, typename... Args>
+constexpr Type *uninitialized_construct_using_allocator(Type *value, const auto &allocator, Args &&...args) {
+    return stl::apply([value](auto &&...curr) { return ::new(value) Type(stl::forward<decltype(curr)>(curr)...); }, internal::uses_allocator_construction<Type>::args(allocator, stl::forward<Args>(args)...));
+}
+
+} // namespace entt
+
+#endif

include/entt/core/monostate.hpp

@@ -0,0 +1,60 @@
+#ifndef ENTT_CORE_MONOSTATE_HPP
+#define ENTT_CORE_MONOSTATE_HPP
+
+#include "../config/config.h"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Minimal implementation of the monostate pattern.
+ *
+ * A minimal, yet complete configuration system built on top of the monostate
+ * pattern. Thread safe by design, it works only with basic types like `int`s or
+ * `bool`s.<br/>
+ * Multiple types and therefore more than one value can be associated with a
+ * single key. Because of this, users must pay attention to use the same type
+ * both during an assignment and when they try to read back their data.
+ * Otherwise, they can incur in unexpected results.
+ */
+template<id_type>
+struct monostate {
+    /**
+     * @brief Assigns a value of a specific type to a given key.
+     * @tparam Type Type of the value to assign.
+     * @param val User data to assign to the given key.
+     * @return This monostate object.
+     */
+    template<typename Type>
+    monostate &operator=(Type val) noexcept {
+        value<Type> = val;
+        return *this;
+    }
+
+    /**
+     * @brief Gets a value of a specific type for a given key.
+     * @tparam Type Type of the value to get.
+     * @return Stored value, if any.
+     */
+    template<typename Type>
+    operator Type() const noexcept {
+        return value<Type>;
+    }
+
+private:
+    template<typename Type>
+    // NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
+    inline static ENTT_MAYBE_ATOMIC(Type) value{};
+};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Value Value used to differentiate between different variables.
+ */
+template<id_type Value>
+// NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
+inline monostate<Value> monostate_v{};
+
+} // namespace entt
+
+#endif

include/entt/core/ranges.hpp

@@ -0,0 +1,22 @@
+#ifndef ENTT_CORE_RANGES_HPP
+#define ENTT_CORE_RANGES_HPP
+
+#include <version>
+
+#if defined(__cpp_lib_ranges)
+#    include <ranges>
+#    include "iterator.hpp"
+
+namespace std::ranges {
+
+template<class... Args>
+inline constexpr bool enable_borrowed_range<entt::iterable_adaptor<Args...>>{true};
+
+template<class... Args>
+inline constexpr bool enable_view<entt::iterable_adaptor<Args...>>{true};
+
+} // namespace std::ranges
+
+#endif
+
+#endif

include/entt/core/tuple.hpp

@@ -0,0 +1,90 @@
+#ifndef ENTT_CORE_TUPLE_HPP
+#define ENTT_CORE_TUPLE_HPP
+
+#include "../stl/tuple.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+
+namespace entt {
+
+/**
+ * @brief Provides the member constant `value` equal to true if a given type is
+ * a tuple, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+struct is_tuple: stl::false_type {};
+
+/**
+ * @copybrief is_tuple
+ * @tparam Args Tuple template arguments.
+ */
+template<typename... Args>
+struct is_tuple<stl::tuple<Args...>>: stl::true_type {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_tuple_v = is_tuple<Type>::value;
+
+/**
+ * @brief Utility function to unwrap tuples of a single element.
+ * @tparam Type Tuple type of any sizes.
+ * @param value A tuple object of the given type.
+ * @return The tuple itself if it contains more than one element, the first
+ * element otherwise.
+ */
+template<typename Type>
+constexpr decltype(auto) unwrap_tuple(Type &&value) noexcept {
+    if constexpr(stl::tuple_size_v<stl::remove_reference_t<Type>> == 1u) {
+        return stl::get<0>(stl::forward<Type>(value));
+    } else {
+        return stl::forward<Type>(value);
+    }
+}
+
+/**
+ * @brief Utility class to forward-and-apply tuple objects.
+ * @tparam Func Type of underlying invocable object.
+ */
+template<typename Func>
+struct forward_apply: private Func {
+    /**
+     * @brief Constructs a forward-and-apply object.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename... Args>
+    constexpr forward_apply(Args &&...args) noexcept(stl::is_nothrow_constructible_v<Func, Args...>)
+        : Func{stl::forward<Args>(args)...} {}
+
+    /**
+     * @brief Forwards and applies the arguments with the underlying function.
+     * @tparam Type Tuple-like type to forward to the underlying function.
+     * @param args Parameters to forward to the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template<typename Type>
+    constexpr decltype(auto) operator()(Type &&args) noexcept(noexcept(stl::apply(stl::declval<Func &>(), args))) {
+        return stl::apply(static_cast<Func &>(*this), stl::forward<Type>(args));
+    }
+
+    /*! @copydoc operator()() */
+    template<typename Type>
+    constexpr decltype(auto) operator()(Type &&args) const noexcept(noexcept(stl::apply(stl::declval<const Func &>(), args))) {
+        return stl::apply(static_cast<const Func &>(*this), stl::forward<Type>(args));
+    }
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Type of underlying invocable object.
+ */
+template<typename Func>
+forward_apply(Func) -> forward_apply<stl::remove_cvref_t<Func>>;
+
+} // namespace entt
+
+#endif

include/entt/core/type_info.hpp

@@ -0,0 +1,232 @@
+#ifndef ENTT_CORE_TYPE_INFO_HPP
+#define ENTT_CORE_TYPE_INFO_HPP
+
+#include <compare>
+#include "../config/config.h"
+#include "../stl/string_view.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+#include "hashed_string.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+struct ENTT_API type_index final {
+    [[nodiscard]] static id_type next() noexcept {
+        static ENTT_MAYBE_ATOMIC(id_type) value{};
+        return value++;
+    }
+};
+
+template<typename Type>
+[[nodiscard]] constexpr const char *pretty_function() noexcept {
+#if defined ENTT_PRETTY_FUNCTION
+    return static_cast<const char *>(ENTT_PRETTY_FUNCTION);
+#else
+    return "";
+#endif
+}
+
+template<typename Type>
+[[nodiscard]] constexpr auto stripped_type_name() noexcept {
+#if defined ENTT_PRETTY_FUNCTION
+    const stl::string_view full_name{pretty_function<Type>()};
+    auto first = full_name.find_first_not_of(' ', full_name.find_first_of(ENTT_PRETTY_FUNCTION_PREFIX) + 1);
+    auto value = full_name.substr(first, full_name.find_last_of(ENTT_PRETTY_FUNCTION_SUFFIX) - first);
+    return value;
+#else
+    return stl::string_view{};
+#endif
+}
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] ENTT_CONSTEVAL stl::string_view type_name(int) noexcept {
+    constexpr auto value = stripped_type_name<Type>();
+    return value;
+}
+
+template<typename Type>
+[[nodiscard]] stl::string_view type_name(char) noexcept {
+    static const auto value = stripped_type_name<Type>();
+    return value;
+}
+
+template<typename Type, auto = stripped_type_name<Type>().find_first_of('.')>
+[[nodiscard]] ENTT_CONSTEVAL id_type type_hash(int) noexcept {
+    constexpr auto stripped = stripped_type_name<Type>();
+    constexpr auto value = hashed_string::value(stripped.data(), stripped.size());
+    return value;
+}
+
+template<typename Type>
+[[nodiscard]] id_type type_hash(char) noexcept {
+    static const auto value = [](const auto stripped) {
+        return hashed_string::value(stripped.data(), stripped.size());
+    }(stripped_type_name<Type>());
+    return value;
+}
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Type sequential identifier.
+ * @tparam Type Type for which to generate a sequential identifier.
+ */
+template<typename Type>
+struct ENTT_API type_index final {
+    /**
+     * @brief Returns the sequential identifier of a given type.
+     * @return The sequential identifier of a given type.
+     */
+    [[nodiscard]] static id_type value() noexcept {
+        static const id_type value = internal::type_index::next();
+        return value;
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const noexcept {
+        return value();
+    }
+};
+
+/**
+ * @brief Type hash.
+ * @tparam Type Type for which to generate a hash value.
+ */
+template<typename Type>
+struct type_hash final {
+    /**
+     * @brief Returns the numeric representation of a given type.
+     * @return The numeric representation of the given type.
+     */
+#if defined ENTT_PRETTY_FUNCTION
+    [[nodiscard]] static constexpr id_type value() noexcept {
+        return internal::type_hash<Type>(0);
+#else
+    [[nodiscard]] static constexpr id_type value() noexcept {
+        return type_index<Type>::value();
+#endif
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator id_type() const noexcept {
+        return value();
+    }
+};
+
+/**
+ * @brief Type name.
+ * @tparam Type Type for which to generate a name.
+ */
+template<typename Type>
+struct type_name final {
+    /**
+     * @brief Returns the name of a given type.
+     * @return The name of the given type.
+     */
+    [[nodiscard]] static constexpr stl::string_view value() noexcept {
+        return internal::type_name<Type>(0);
+    }
+
+    /*! @copydoc value */
+    [[nodiscard]] constexpr operator stl::string_view() const noexcept {
+        return value();
+    }
+};
+
+/*! @brief Implementation specific information about a type. */
+struct type_info final {
+    /**
+     * @brief Constructs a type info object for a given type.
+     * @tparam Type Type for which to construct a type info object.
+     */
+    template<typename Type>
+    // NOLINTBEGIN(modernize-use-transparent-functors)
+    constexpr type_info(stl::in_place_type_t<Type>) noexcept
+        : seq{type_index<stl::remove_cvref_t<Type>>::value()},
+          identifier{type_hash<stl::remove_cvref_t<Type>>::value()},
+          alias{type_name<stl::remove_cvref_t<Type>>::value()} {}
+    // NOLINTEND(modernize-use-transparent-functors)
+
+    /**
+     * @brief Type index.
+     * @return Type index.
+     */
+    [[nodiscard]] constexpr id_type index() const noexcept {
+        return seq;
+    }
+
+    /**
+     * @brief Type hash.
+     * @return Type hash.
+     */
+    [[nodiscard]] constexpr id_type hash() const noexcept {
+        return identifier;
+    }
+
+    /**
+     * @brief Type name.
+     * @return Type name.
+     */
+    [[nodiscard]] constexpr stl::string_view name() const noexcept {
+        return alias;
+    }
+
+    /**
+     * @brief Compares two type info objects.
+     * @param other A type info object.
+     * @return True if the two type info objects are identical, false otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const type_info &other) const noexcept {
+        return identifier == other.identifier;
+    }
+
+    /**
+     * @brief Lexicographically compares two type info objects.
+     * @param other A type info object.
+     * @return The relative order between the two type info objects.
+     */
+    [[nodiscard]] constexpr auto operator<=>(const type_info &other) const noexcept {
+        return seq <=> other.seq;
+    }
+
+private:
+    id_type seq;
+    id_type identifier;
+    stl::string_view alias;
+};
+
+/**
+ * @brief Returns the type info object associated to a given type.
+ *
+ * The returned element refers to an object with static storage duration.<br/>
+ * The type doesn't need to be a complete type. If the type is a reference, the
+ * result refers to the referenced type. In all cases, top-level cv-qualifiers
+ * are ignored.
+ *
+ * @tparam Type Type for which to generate a type info object.
+ * @return A reference to a properly initialized type info object.
+ */
+template<typename Type>
+[[nodiscard]] const type_info &type_id() noexcept {
+    if constexpr(stl::is_same_v<Type, stl::remove_cvref_t<Type>>) {
+        static const type_info instance{stl::in_place_type<Type>};
+        return instance;
+    } else {
+        return type_id<stl::remove_cvref_t<Type>>();
+    }
+}
+
+/*! @copydoc type_id */
+template<typename Type>
+[[nodiscard]] const type_info &type_id(const Type &) noexcept {
+    return type_id<stl::remove_cvref_t<Type>>();
+}
+
+} // namespace entt
+
+#endif

include/entt/core/type_traits.hpp

@@ -0,0 +1,909 @@
+#ifndef ENTT_CORE_TYPE_TRAITS_HPP
+#define ENTT_CORE_TYPE_TRAITS_HPP
+
+#include "../config/config.h"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/tuple.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Utility class to disambiguate overloaded functions.
+ * @tparam N Number of choices available.
+ */
+template<stl::size_t N>
+struct choice_t
+    // unfortunately, doxygen cannot parse such a construct
+    : /*! @cond ENTT_INTERNAL */ choice_t<N - 1> /*! @endcond */
+{};
+
+/*! @copybrief choice_t */
+template<>
+struct choice_t<0> {};
+
+/**
+ * @brief Variable template for the choice trick.
+ * @tparam N Number of choices available.
+ */
+template<stl::size_t N>
+inline constexpr choice_t<N> choice{};
+
+/**
+ * @brief A type-only `sizeof` wrapper that returns 0 where `sizeof` complains.
+ * @tparam Type The type of which to return the size.
+ */
+template<typename Type>
+struct size_of: stl::integral_constant<stl::size_t, 0u> {};
+
+/*! @copydoc size_of */
+template<typename Type>
+requires requires { sizeof(Type); }
+struct size_of<Type>
+    // NOLINTNEXTLINE(bugprone-sizeof-expression)
+    : stl::integral_constant<stl::size_t, sizeof(Type)> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type of which to return the size.
+ */
+template<typename Type>
+inline constexpr stl::size_t size_of_v = size_of<Type>::value;
+
+/**
+ * @brief Using declaration to be used to _repeat_ the same type a number of
+ * times equal to the size of a given parameter pack.
+ * @tparam Type A type to repeat.
+ */
+template<typename Type, typename>
+using unpack_as_type = Type;
+
+/**
+ * @brief Helper variable template to be used to _repeat_ the same value a
+ * number of times equal to the size of a given parameter pack.
+ * @tparam Value A value to repeat.
+ */
+template<auto Value, typename>
+inline constexpr auto unpack_as_value = Value;
+
+/**
+ * @brief Wraps a static constant.
+ * @tparam Value A static constant.
+ */
+template<auto Value>
+using integral_constant = stl::integral_constant<decltype(Value), Value>;
+
+/**
+ * @brief Alias template to facilitate the creation of named values.
+ * @tparam Value A constant value at least convertible to `id_type`.
+ */
+template<id_type Value>
+using tag = integral_constant<Value>;
+
+/**
+ * @brief A class to use to push around lists of types, nothing more.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list {
+    /*! @brief Type list type. */
+    using type = type_list;
+    /*! @brief Compile-time number of elements in the type list. */
+    static constexpr auto size = sizeof...(Type);
+};
+
+/*! @brief Primary template isn't defined on purpose. */
+template<stl::size_t, typename>
+struct type_list_element;
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Index Index of the type to return.
+ * @tparam First First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<stl::size_t Index, typename First, typename... Other>
+struct type_list_element<Index, type_list<First, Other...>>
+    : type_list_element<Index - 1u, type_list<Other...>> {};
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam First First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename First, typename... Other>
+struct type_list_element<0u, type_list<First, Other...>> {
+    /*! @brief Searched type. */
+    using type = First;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Type list to search into.
+ */
+template<stl::size_t Index, typename List>
+using type_list_element_t = type_list_element<Index, List>::type;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename, typename>
+struct type_list_index;
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ * @tparam First First type provided by the type list.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename First, typename... Other>
+struct type_list_index<Type, type_list<First, Other...>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = stl::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 1u + type_list_index<Type, type_list<Other...>>::value;
+};
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ * @tparam Other Other types provided by the type list.
+ */
+template<typename Type, typename... Other>
+requires (type_list_index<Type, type_list<Other...>>::value == sizeof...(Other))
+struct type_list_index<Type, type_list<Type, Other...>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = stl::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Provides compile-time type access to the types of a type list.
+ * @tparam Type Type to look for and for which to return the index.
+ */
+template<typename Type>
+struct type_list_index<Type, type_list<>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = stl::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for and for which to return the index.
+ */
+template<typename Type, typename List>
+inline constexpr stl::size_t type_list_index_v = type_list_index<Type, List>::value;
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @return A type list composed by the types of both the type lists.
+ */
+template<typename... Type, typename... Other>
+ENTT_CONSTEVAL type_list<Type..., Other...> operator+(type_list<Type...>, type_list<Other...>) {
+    return {};
+}
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_cat;
+
+/*! @brief Concatenates multiple type lists. */
+template<>
+struct type_list_cat<> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<>;
+};
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ * @tparam List Other type lists, if any.
+ */
+template<typename... Type, typename... Other, typename... List>
+struct type_list_cat<type_list<Type...>, type_list<Other...>, List...> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list_cat<type_list<Type..., Other...>, List...>::type;
+};
+
+/**
+ * @brief Concatenates multiple type lists.
+ * @tparam Type Types provided by the type list.
+ */
+template<typename... Type>
+struct type_list_cat<type_list<Type...>> {
+    /*! @brief A type list composed by the types of all the type lists. */
+    using type = type_list<Type...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists to concatenate.
+ */
+template<typename... List>
+using type_list_cat_t = type_list_cat<List...>::type;
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename...>
+struct type_list_unique;
+
+template<typename First, typename... Other, typename... Type>
+struct type_list_unique<type_list<First, Other...>, Type...>
+    : stl::conditional_t<(stl::is_same_v<First, Type> || ...), type_list_unique<type_list<Other...>, Type...>, type_list_unique<type_list<Other...>, Type..., First>> {};
+
+template<typename... Type>
+struct type_list_unique<type_list<>, Type...> {
+    using type = type_list<Type...>;
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Removes duplicates types from a type list.
+ * @tparam List Type list.
+ */
+template<typename List>
+struct type_list_unique {
+    /*! @brief A type list without duplicate types. */
+    using type = internal::type_list_unique<List>::type;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Type list.
+ */
+template<typename List>
+using type_list_unique_t = type_list_unique<List>::type;
+
+/**
+ * @brief Provides the member constant `value` equal to true if a type list
+ * contains a given type, false otherwise.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+struct type_list_contains;
+
+/**
+ * @copybrief type_list_contains
+ * @tparam Type Types provided by the type list.
+ * @tparam Other Type to look for.
+ */
+template<typename... Type, typename Other>
+struct type_list_contains<type_list<Type...>, Other>
+    : stl::bool_constant<(stl::is_same_v<Type, Other> || ...)> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Type list.
+ * @tparam Type Type to look for.
+ */
+template<typename List, typename Type>
+inline constexpr bool type_list_contains_v = type_list_contains<List, Type>::value;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct type_list_diff;
+
+/**
+ * @brief Computes the difference between two type lists.
+ * @tparam Type Types provided by the first type list.
+ * @tparam Other Types provided by the second type list.
+ */
+template<typename... Type, typename... Other>
+struct type_list_diff<type_list<Type...>, type_list<Other...>> {
+    /*! @brief A type list that is the difference between the two type lists. */
+    using type = type_list_cat_t<stl::conditional_t<type_list_contains_v<type_list<Other...>, Type>, type_list<>, type_list<Type>>...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Type lists between which to compute the difference.
+ */
+template<typename... List>
+using type_list_diff_t = type_list_diff<List...>::type;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename, template<typename...> class>
+struct type_list_transform;
+
+/**
+ * @brief Applies a given _function_ to a type list and generates a new list.
+ * @tparam Type Types provided by the type list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename... Type, template<typename...> class Op>
+struct type_list_transform<type_list<Type...>, Op> {
+    /*! @brief Resulting type list after applying the transform function. */
+    // NOLINTNEXTLINE(modernize-type-traits)
+    using type = type_list<typename Op<Type>::type...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Type list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename List, template<typename...> class Op>
+using type_list_transform_t = type_list_transform<List, Op>::type;
+
+/**
+ * @brief A class to use to push around lists of constant values, nothing more.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list {
+    /*! @brief Value list type. */
+    using type = value_list;
+    /*! @brief Compile-time number of elements in the value list. */
+    static constexpr auto size = sizeof...(Value);
+};
+
+/*! @brief Primary template isn't defined on purpose. */
+template<stl::size_t, typename>
+struct value_list_element;
+
+/**
+ * @brief Provides compile-time indexed access to the values of a value list.
+ * @tparam Index Index of the value to return.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<stl::size_t Index, auto Value, auto... Other>
+struct value_list_element<Index, value_list<Value, Other...>>
+    : value_list_element<Index - 1u, value_list<Other...>> {};
+
+/**
+ * @brief Provides compile-time indexed access to the types of a type list.
+ * @tparam Value First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_element<0u, value_list<Value, Other...>> {
+    /*! @brief Searched type. */
+    using type = decltype(Value);
+    /*! @brief Searched value. */
+    static constexpr auto value = Value;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the type to return.
+ * @tparam List Value list to search into.
+ */
+template<stl::size_t Index, typename List>
+using value_list_element_t = value_list_element<Index, List>::type;
+
+/**
+ * @brief Helper type.
+ * @tparam Index Index of the value to return.
+ * @tparam List Value list to search into.
+ */
+template<stl::size_t Index, typename List>
+inline constexpr auto value_list_element_v = value_list_element<Index, List>::value;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<auto, typename>
+struct value_list_index;
+
+/**
+ * @brief Provides compile-time type access to the values of a value list.
+ * @tparam Value Value to look for and for which to return the index.
+ * @tparam First First value provided by the value list.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto First, auto... Other>
+struct value_list_index<Value, value_list<First, Other...>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = stl::size_t;
+    /*! @brief Compile-time position of the given value in the sublist. */
+    static constexpr value_type value = 1u + value_list_index<Value, value_list<Other...>>::value;
+};
+
+/**
+ * @brief Provides compile-time type access to the values of a value list.
+ * @tparam Value Value to look for and for which to return the index.
+ * @tparam Other Other values provided by the value list.
+ */
+template<auto Value, auto... Other>
+requires (value_list_index<Value, value_list<Other...>>::value == sizeof...(Other))
+struct value_list_index<Value, value_list<Value, Other...>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = stl::size_t;
+    /*! @brief Compile-time position of the given value in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Provides compile-time type access to the values of a value list.
+ * @tparam Value Value to look for and for which to return the index.
+ */
+template<auto Value>
+struct value_list_index<Value, value_list<>> {
+    /*! @brief Unsigned integer type. */
+    using value_type = stl::size_t;
+    /*! @brief Compile-time position of the given type in the sublist. */
+    static constexpr value_type value = 0u;
+};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Value list.
+ * @tparam Value Value to look for and for which to return the index.
+ */
+template<auto Value, typename List>
+inline constexpr stl::size_t value_list_index_v = value_list_index<Value, List>::value;
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @return A value list composed by the values of both the value lists.
+ */
+template<auto... Value, auto... Other>
+ENTT_CONSTEVAL value_list<Value..., Other...> operator+(value_list<Value...>, value_list<Other...>) {
+    return {};
+}
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_cat;
+
+/*! @brief Concatenates multiple value lists. */
+template<>
+struct value_list_cat<> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<>;
+};
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ * @tparam List Other value lists, if any.
+ */
+template<auto... Value, auto... Other, typename... List>
+struct value_list_cat<value_list<Value...>, value_list<Other...>, List...> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list_cat<value_list<Value..., Other...>, List...>::type;
+};
+
+/**
+ * @brief Concatenates multiple value lists.
+ * @tparam Value Values provided by the value list.
+ */
+template<auto... Value>
+struct value_list_cat<value_list<Value...>> {
+    /*! @brief A value list composed by the values of all the value lists. */
+    using type = value_list<Value...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists to concatenate.
+ */
+template<typename... List>
+using value_list_cat_t = value_list_cat<List...>::type;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+struct value_list_unique;
+
+/**
+ * @brief Removes duplicates values from a value list.
+ * @tparam Value One of the values provided by the given value list.
+ * @tparam Other The other values provided by the given value list.
+ */
+template<auto Value, auto... Other>
+struct value_list_unique<value_list<Value, Other...>> {
+    /*! @brief A value list without duplicate types. */
+    using type = stl::conditional_t<
+        ((Value == Other) || ...),
+        typename value_list_unique<value_list<Other...>>::type,
+        value_list_cat_t<value_list<Value>, typename value_list_unique<value_list<Other...>>::type>>;
+};
+
+/*! @brief Removes duplicates values from a value list. */
+template<>
+struct value_list_unique<value_list<>> {
+    /*! @brief A value list without duplicate types. */
+    using type = value_list<>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Type A value list.
+ */
+template<typename Type>
+using value_list_unique_t = value_list_unique<Type>::type;
+
+/**
+ * @brief Provides the member constant `value` equal to true if a value list
+ * contains a given value, false otherwise.
+ * @tparam List Value list.
+ * @tparam Value Value to look for.
+ */
+template<typename List, auto Value>
+struct value_list_contains;
+
+/**
+ * @copybrief value_list_contains
+ * @tparam Value Values provided by the value list.
+ * @tparam Other Value to look for.
+ */
+template<auto... Value, auto Other>
+struct value_list_contains<value_list<Value...>, Other>
+    : stl::bool_constant<((Value == Other) || ...)> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam List Value list.
+ * @tparam Value Value to look for.
+ */
+template<typename List, auto Value>
+inline constexpr bool value_list_contains_v = value_list_contains<List, Value>::value;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct value_list_diff;
+
+/**
+ * @brief Computes the difference between two value lists.
+ * @tparam Value Values provided by the first value list.
+ * @tparam Other Values provided by the second value list.
+ */
+template<auto... Value, auto... Other>
+struct value_list_diff<value_list<Value...>, value_list<Other...>> {
+    /*! @brief A value list that is the difference between the two lists. */
+    using type = value_list_cat_t<stl::conditional_t<value_list_contains_v<value_list<Other...>, Value>, value_list<>, value_list<Value>>...>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam List Value lists between which to compute the difference.
+ */
+template<typename... List>
+using value_list_diff_t = value_list_diff<List...>::type;
+
+/*! @brief Same as stl::is_invocable, but with tuples. */
+template<typename, typename>
+struct is_applicable: stl::false_type {};
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, Tuple<Args...>>: stl::is_invocable<Func, Args...> {};
+
+/**
+ * @copybrief is_applicable
+ * @tparam Func A valid function type.
+ * @tparam Tuple Tuple-like type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, template<typename...> class Tuple, typename... Args>
+struct is_applicable<Func, const Tuple<Args...>>: stl::is_invocable<Func, Args...> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Func, typename Args>
+inline constexpr bool is_applicable_v = is_applicable<Func, Args>::value;
+
+/*! @brief Same as stl::is_invocable_r, but with tuples for arguments. */
+template<typename, typename, typename>
+struct is_applicable_r: stl::false_type {};
+
+/**
+ * @copybrief is_applicable_r
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename... Args>
+struct is_applicable_r<Ret, Func, stl::tuple<Args...>>: stl::is_invocable_r<Ret, Func, Args...> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Ret The type to which the return type of the function should be
+ * convertible.
+ * @tparam Func A valid function type.
+ * @tparam Args The list of arguments to use to probe the function type.
+ */
+template<typename Ret, typename Func, typename Args>
+inline constexpr bool is_applicable_r_v = is_applicable_r<Ret, Func, Args>::value;
+
+/**
+ * @brief Provides the member constant `value` equal to true if a given type is
+ * complete, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+struct is_complete: stl::false_type {};
+
+/*! @copydoc is_complete */
+template<typename Type>
+requires requires { sizeof(Type); }
+struct is_complete<Type>: stl::true_type {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_complete_v = is_complete<Type>::value;
+
+/**
+ * @brief Provides the member constant `value` equal to true if a given type is
+ * an iterator, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+struct is_iterator: stl::false_type {};
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename>
+struct has_iterator_category: stl::false_type {};
+
+template<typename Type>
+requires requires { typename stl::iterator_traits<Type>::iterator_category; }
+struct has_iterator_category<Type>: stl::true_type {};
+
+} // namespace internal
+/*! @endcond */
+
+/*! @copydoc is_iterator */
+template<typename Type>
+requires (!stl::is_void_v<stl::remove_const_t<stl::remove_pointer_t<Type>>>)
+struct is_iterator<Type>: internal::has_iterator_category<Type> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_iterator_v = is_iterator<Type>::value;
+
+/**
+ * @brief Provides the member constant `value` equal to true if a given type is
+ * both an empty and non-final class, false otherwise.
+ * @tparam Type The type to test
+ */
+template<typename Type>
+struct is_ebco_eligible: stl::bool_constant<stl::is_empty_v<Type> && !stl::is_final_v<Type>> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_ebco_eligible_v = is_ebco_eligible<Type>::value;
+
+/**
+ * @brief Provides the member constant `value` equal to true if
+ * `Type::is_transparent` is valid and denotes a type, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+struct is_transparent: stl::false_type {};
+
+/*! @copydoc is_transparent */
+template<typename Type>
+requires requires { typename Type::is_transparent; }
+struct is_transparent<Type>: stl::true_type {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_transparent_v = is_transparent<Type>::value;
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename>
+struct has_tuple_size_value: stl::false_type {};
+
+template<typename Type>
+requires is_complete_v<stl::tuple_size<const Type>>
+struct has_tuple_size_value<Type>: stl::true_type {};
+
+template<typename>
+struct has_value_type: stl::false_type {};
+
+template<typename Type>
+requires requires { typename Type::value_type; }
+struct has_value_type<Type>: stl::true_type {};
+
+template<typename>
+[[nodiscard]] ENTT_CONSTEVAL bool dispatch_is_equality_comparable();
+
+template<typename Type, stl::size_t... Index>
+[[nodiscard]] ENTT_CONSTEVAL bool unpack_maybe_equality_comparable(stl::index_sequence<Index...>) {
+    return (dispatch_is_equality_comparable<stl::tuple_element_t<Index, Type>>() && ...);
+}
+
+template<typename>
+[[nodiscard]] ENTT_CONSTEVAL bool maybe_equality_comparable(char) {
+    return false;
+}
+
+template<typename Type>
+[[nodiscard]] ENTT_CONSTEVAL auto maybe_equality_comparable(int) -> decltype(stl::declval<Type>() == stl::declval<Type>()) {
+    return true;
+}
+
+template<typename Type>
+[[nodiscard]] ENTT_CONSTEVAL bool dispatch_is_equality_comparable() {
+    // NOLINTBEGIN(modernize-use-transparent-functors)
+    if constexpr(stl::is_array_v<Type>) {
+        return false;
+    } else if constexpr(is_complete_v<stl::tuple_size<stl::remove_const_t<Type>>>) {
+        if constexpr(has_tuple_size_value<Type>::value) {
+            return maybe_equality_comparable<Type>(0) && unpack_maybe_equality_comparable<Type>(stl::make_index_sequence<stl::tuple_size<Type>::value>{});
+        } else {
+            return maybe_equality_comparable<Type>(0);
+        }
+    } else if constexpr(has_value_type<Type>::value) {
+        if constexpr(is_iterator_v<Type> || stl::is_same_v<typename Type::value_type, Type> || dispatch_is_equality_comparable<typename Type::value_type>()) {
+            return maybe_equality_comparable<Type>(0);
+        } else {
+            return false;
+        }
+    } else {
+        return maybe_equality_comparable<Type>(0);
+    }
+    // NOLINTEND(modernize-use-transparent-functors)
+}
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Provides the member constant `value` equal to true if a given type is
+ * equality comparable, false otherwise.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+struct is_equality_comparable: stl::bool_constant<internal::dispatch_is_equality_comparable<Type>()> {};
+
+/*! @copydoc is_equality_comparable */
+template<typename Type>
+struct is_equality_comparable<const Type>: is_equality_comparable<Type> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type The type to test.
+ */
+template<typename Type>
+inline constexpr bool is_equality_comparable_v = is_equality_comparable<Type>::value;
+
+/**
+ * @brief Transcribes the constness of a type to another type.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+struct constness_as {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = stl::remove_const_t<To>;
+};
+
+/*! @copydoc constness_as */
+template<typename To, typename From>
+struct constness_as<To, const From> {
+    /*! @brief The type resulting from the transcription of the constness. */
+    using type = const To;
+};
+
+/**
+ * @brief Alias template to facilitate the transcription of the constness.
+ * @tparam To The type to which to transcribe the constness.
+ * @tparam From The type from which to transcribe the constness.
+ */
+template<typename To, typename From>
+using constness_as_t = constness_as<To, From>::type;
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename>
+class member_class;
+
+/**
+ * @brief Extracts the class of a non-static member object or function.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+requires stl::is_member_pointer_v<Member>
+class member_class<Member> {
+    template<typename Class, typename Ret, typename... Args>
+    static Class *clazz(Ret (Class::*)(Args...));
+
+    template<typename Class, typename Ret, typename... Args>
+    static Class *clazz(Ret (Class::*)(Args...) const);
+
+    template<typename Class, typename Type>
+    static Class *clazz(Type Class::*);
+
+public:
+    /*! @brief The class of the given non-static member object or function. */
+    using type = stl::remove_pointer_t<decltype(clazz(stl::declval<Member>()))>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Member A pointer to a non-static member object or function.
+ */
+template<typename Member>
+using member_class_t = member_class<Member>::type;
+
+/**
+ * @brief Extracts the n-th argument of a _callable_ type.
+ * @tparam Index The index of the argument to extract.
+ * @tparam Candidate A valid _callable_ type.
+ */
+template<stl::size_t Index, typename Candidate>
+class nth_argument {
+    template<typename Ret, typename... Args>
+    static ENTT_CONSTEVAL type_list<Args...> pick_up(Ret (*)(Args...));
+
+    template<typename Ret, typename Class, typename... Args>
+    static ENTT_CONSTEVAL type_list<Args...> pick_up(Ret (Class ::*)(Args...));
+
+    template<typename Ret, typename Class, typename... Args>
+    static ENTT_CONSTEVAL type_list<Args...> pick_up(Ret (Class ::*)(Args...) const);
+
+    template<typename Type, typename Class>
+    static ENTT_CONSTEVAL type_list<Type> pick_up(Type Class ::*);
+
+    template<typename Type>
+    static ENTT_CONSTEVAL decltype(pick_up(&Type::operator())) pick_up(Type &&);
+
+public:
+    /*! @brief N-th argument of the _callable_ type. */
+    using type = type_list_element_t<Index, decltype(pick_up(stl::declval<Candidate>()))>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Index The index of the argument to extract.
+ * @tparam Candidate A valid function, member function or data member type.
+ */
+template<stl::size_t Index, typename Candidate>
+using nth_argument_t = nth_argument<Index, Candidate>::type;
+
+} // namespace entt
+
+template<typename... Type>
+struct entt::stl::tuple_size<entt::type_list<Type...>>: entt::stl::integral_constant<entt::stl::size_t, entt::type_list<Type...>::size> {};
+
+template<entt::stl::size_t Index, typename... Type>
+struct entt::stl::tuple_element<Index, entt::type_list<Type...>>: entt::type_list_element<Index, entt::type_list<Type...>> {};
+
+template<auto... Value>
+struct entt::stl::tuple_size<entt::value_list<Value...>>: entt::stl::integral_constant<entt::stl::size_t, entt::value_list<Value...>::size> {};
+
+template<entt::stl::size_t Index, auto... Value>
+struct entt::stl::tuple_element<Index, entt::value_list<Value...>>: entt::value_list_element<Index, entt::value_list<Value...>> {};
+
+#endif

include/entt/core/utility.hpp

@@ -0,0 +1,84 @@
+#ifndef ENTT_CORE_UTILITY_HPP
+#define ENTT_CORE_UTILITY_HPP
+
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+
+namespace entt {
+
+/**
+ * @brief Constant utility to disambiguate overloaded members of a class.
+ * @tparam Type Type of the desired overload.
+ * @tparam Class Type of class to which the member belongs.
+ * @param member A valid pointer to a member.
+ * @return Pointer to the member.
+ */
+template<typename Type, typename Class>
+[[nodiscard]] constexpr auto overload(Type Class::*member) noexcept {
+    return member;
+}
+
+/**
+ * @brief Constant utility to disambiguate overloaded functions.
+ * @tparam Func Function type of the desired overload.
+ * @param func A valid pointer to a function.
+ * @return Pointer to the function.
+ */
+template<typename Func>
+[[nodiscard]] constexpr auto overload(Func *func) noexcept {
+    return func;
+}
+
+/**
+ * @brief Helper type for visitors.
+ * @tparam Func Types of function objects.
+ */
+template<typename... Func>
+struct overloaded: Func... {
+    using Func::operator()...;
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Func Types of function objects.
+ */
+template<typename... Func>
+overloaded(Func...) -> overloaded<Func...>;
+
+/**
+ * @brief Basic implementation of a y-combinator.
+ * @tparam Func Type of a potentially recursive function.
+ */
+template<typename Func>
+struct y_combinator {
+    /**
+     * @brief Constructs a y-combinator from a given function.
+     * @param recursive A potentially recursive function.
+     */
+    constexpr y_combinator(Func recursive) noexcept(stl::is_nothrow_move_constructible_v<Func>)
+        : func{stl::move(recursive)} {}
+
+    /**
+     * @brief Invokes a y-combinator and therefore its underlying function.
+     * @tparam Args Types of arguments to use to invoke the underlying function.
+     * @param args Parameters to use to invoke the underlying function.
+     * @return Return value of the underlying function, if any.
+     */
+    template<typename... Args>
+    constexpr decltype(auto) operator()(Args &&...args) const noexcept(stl::is_nothrow_invocable_v<Func, const y_combinator &, Args...>) {
+        return func(*this, stl::forward<Args>(args)...);
+    }
+
+    /*! @copydoc operator()() */
+    template<typename... Args>
+    constexpr decltype(auto) operator()(Args &&...args) noexcept(stl::is_nothrow_invocable_v<Func, y_combinator &, Args...>) {
+        return func(*this, stl::forward<Args>(args)...);
+    }
+
+private:
+    Func func;
+};
+
+} // namespace entt
+
+#endif

include/entt/entity/component.hpp

@@ -0,0 +1,59 @@
+#ifndef ENTT_ENTITY_COMPONENT_HPP
+#define ENTT_ENTITY_COMPONENT_HPP
+
+#include "../config/config.h"
+#include "../core/concepts.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/type_traits.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename Type>
+struct in_place_delete: stl::bool_constant<!(stl::is_move_constructible_v<Type> && stl::is_move_assignable_v<Type>)> {};
+
+template<>
+struct in_place_delete<void>: stl::false_type {};
+
+template<typename Type>
+requires Type::in_place_delete
+struct in_place_delete<Type>: stl::true_type {};
+
+template<typename Type>
+struct page_size: stl::integral_constant<stl::size_t, !stl::is_empty_v<ENTT_ETO_TYPE(Type)> * ENTT_PACKED_PAGE> {};
+
+template<>
+struct page_size<void>: stl::integral_constant<stl::size_t, 0u> {};
+
+template<typename Type>
+requires stl::is_convertible_v<decltype(Type::page_size), stl::size_t>
+struct page_size<Type>: stl::integral_constant<stl::size_t, Type::page_size> {};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Common way to access various properties of components.
+ * @tparam Type Element type.
+ * @tparam Entity A valid entity type.
+ */
+template<cvref_unqualified Type, typename Entity>
+struct component_traits {
+    /*! @brief Element type. */
+    using element_type = Type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Entity;
+
+    /*! @brief Pointer stability, default is `false`. */
+    static constexpr bool in_place_delete = internal::in_place_delete<Type>::value;
+    /*! @brief Page size, default is `ENTT_PACKED_PAGE` for non-empty types. */
+    static constexpr stl::size_t page_size = internal::page_size<Type>::value;
+};
+
+} // namespace entt
+
+#endif

include/entt/entity/entity.hpp

@@ -0,0 +1,312 @@
+#ifndef ENTT_ENTITY_ENTITY_HPP
+#define ENTT_ENTITY_ENTITY_HPP
+
+#include "../config/config.h"
+#include "../core/bit.hpp"
+#include "../stl/bit.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/cstdint.hpp"
+#include "../stl/type_traits.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename>
+struct entt_traits;
+
+template<typename Type>
+requires requires {
+    requires stl::is_enum_v<Type>;
+    typename internal::entt_traits<stl::underlying_type_t<Type>>::value_type;
+}
+struct entt_traits<Type>: entt_traits<stl::underlying_type_t<Type>> {
+    using value_type = Type;
+};
+
+template<typename Type>
+requires requires { typename Type::entity_type; }
+struct entt_traits<Type>
+    : entt_traits<typename Type::entity_type> {
+    using value_type = Type;
+};
+
+template<>
+struct entt_traits<stl::uint32_t> {
+    using value_type = stl::uint32_t;
+
+    using entity_type = stl::uint32_t;
+    using version_type = stl::uint16_t;
+
+    static constexpr entity_type entity_mask = 0xFFFFF;
+    static constexpr entity_type version_mask = 0xFFF;
+};
+
+template<>
+struct entt_traits<stl::uint64_t> {
+    using value_type = stl::uint64_t;
+
+    using entity_type = stl::uint64_t;
+    using version_type = stl::uint32_t;
+
+    static constexpr entity_type entity_mask = 0xFFFFFFFF;
+    static constexpr entity_type version_mask = 0xFFFFFFFF;
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Specifies that a type is an entity-like type.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+concept entity_like = requires {
+    typename internal::entt_traits<Type>::value_type;
+};
+
+/**
+ * @brief Common basic entity traits implementation.
+ * @tparam Traits Actual entity traits to use.
+ */
+template<typename Traits>
+class basic_entt_traits {
+    static constexpr auto length = stl::popcount(Traits::entity_mask);
+
+public:
+    /*! @brief Value type. */
+    using value_type = Traits::value_type;
+    /*! @brief Underlying entity type. */
+    using entity_type = Traits::entity_type;
+    /*! @brief Underlying version type. */
+    using version_type = Traits::version_type;
+
+    /*! @brief Entity mask size. */
+    static constexpr entity_type entity_mask = Traits::entity_mask;
+    /*! @brief Version mask size */
+    static constexpr entity_type version_mask = Traits::version_mask;
+
+    /**
+     * @brief Converts an entity to its underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the given value.
+     */
+    [[nodiscard]] static constexpr entity_type to_integral(const value_type value) noexcept {
+        return static_cast<entity_type>(value);
+    }
+
+    /**
+     * @brief Returns the entity part once converted to the underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the entity part.
+     */
+    [[nodiscard]] static constexpr entity_type to_entity(const value_type value) noexcept {
+        static_assert(Traits::entity_mask && ((Traits::entity_mask & (Traits::entity_mask + 1)) == 0), "Invalid entity mask");
+        return (to_integral(value) & entity_mask);
+    }
+
+    /**
+     * @brief Returns the version part once converted to the underlying type.
+     * @param value The value to convert.
+     * @return The integral representation of the version part.
+     */
+    [[nodiscard]] static constexpr version_type to_version(const value_type value) noexcept {
+        if constexpr(Traits::version_mask == 0u) {
+            return version_type{};
+        } else {
+            static_assert((Traits::version_mask & (Traits::version_mask + 1)) == 0, "Invalid version mask");
+            return (static_cast<version_type>(to_integral(value) >> length) & version_mask);
+        }
+    }
+
+    /**
+     * @brief Returns the successor of a given identifier.
+     * @param value The identifier of which to return the successor.
+     * @return The successor of the given identifier.
+     */
+    [[nodiscard]] static constexpr value_type next(const value_type value) noexcept {
+        const auto vers = to_version(value) + 1;
+        return construct(to_integral(value), static_cast<version_type>(vers + (vers == version_mask)));
+    }
+
+    /**
+     * @brief Constructs an identifier from its parts.
+     *
+     * If the version part is not provided, a tombstone is returned.<br/>
+     * If the entity part is not provided, a null identifier is returned.
+     *
+     * @param entity The entity part of the identifier.
+     * @param version The version part of the identifier.
+     * @return A properly constructed identifier.
+     */
+    [[nodiscard]] static constexpr value_type construct(const entity_type entity, const version_type version) noexcept {
+        if constexpr(Traits::version_mask == 0u) {
+            return value_type{entity & entity_mask};
+        } else {
+            return value_type{(entity & entity_mask) | (static_cast<entity_type>(version & version_mask) << length)};
+        }
+    }
+
+    /**
+     * @brief Combines two identifiers in a single one.
+     *
+     * The returned identifier is a copy of the first element except for its
+     * version, which is taken from the second element.
+     *
+     * @param lhs The identifier from which to take the entity part.
+     * @param rhs The identifier from which to take the version part.
+     * @return A properly constructed identifier.
+     */
+    [[nodiscard]] static constexpr value_type combine(const entity_type lhs, const entity_type rhs) noexcept {
+        if constexpr(Traits::version_mask == 0u) {
+            return value_type{lhs & entity_mask};
+        } else {
+            return value_type{(lhs & entity_mask) | (rhs & (version_mask << length))};
+        }
+    }
+};
+
+/**
+ * @brief Entity traits.
+ * @tparam Type Type of identifier.
+ */
+template<entity_like Type>
+struct entt_traits: basic_entt_traits<internal::entt_traits<Type>> {
+    /*! @brief Base type. */
+    using base_type = basic_entt_traits<internal::entt_traits<Type>>;
+    /*! @brief Page size, default is `ENTT_SPARSE_PAGE`. */
+    static constexpr stl::size_t page_size = ENTT_SPARSE_PAGE;
+};
+
+/**
+ * @brief Converts an entity to its underlying type.
+ * @tparam Entity The value type.
+ * @param value The value to convert.
+ * @return The integral representation of the given value.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr entt_traits<Entity>::entity_type to_integral(const Entity value) noexcept {
+    return entt_traits<Entity>::to_integral(value);
+}
+
+/**
+ * @brief Returns the entity part once converted to the underlying type.
+ * @tparam Entity The value type.
+ * @param value The value to convert.
+ * @return The integral representation of the entity part.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr entt_traits<Entity>::entity_type to_entity(const Entity value) noexcept {
+    return entt_traits<Entity>::to_entity(value);
+}
+
+/**
+ * @brief Returns the version part once converted to the underlying type.
+ * @tparam Entity The value type.
+ * @param value The value to convert.
+ * @return The integral representation of the version part.
+ */
+template<typename Entity>
+[[nodiscard]] constexpr entt_traits<Entity>::version_type to_version(const Entity value) noexcept {
+    return entt_traits<Entity>::to_version(value);
+}
+
+/*! @brief Null object for all identifiers.  */
+struct null_t {
+    /**
+     * @brief Converts the null object to identifiers of any type.
+     * @tparam Entity Type of identifier.
+     * @return The null representation for the given type.
+     */
+    template<entity_like Entity>
+    [[nodiscard]] constexpr operator Entity() const noexcept {
+        using traits_type = entt_traits<Entity>;
+        return traits_type::construct(traits_type::entity_mask, traits_type::version_mask);
+    }
+
+    /**
+     * @brief Compares two null objects.
+     * @param other A null object.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const null_t other) const noexcept {
+        return true;
+    }
+
+    /**
+     * @brief Compares a null object and an identifier of any type.
+     * @tparam Entity Type of identifier.
+     * @param entity Identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
+    template<entity_like Entity>
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const noexcept {
+        using traits_type = entt_traits<Entity>;
+        return traits_type::to_entity(entity) == traits_type::to_entity(*this);
+    }
+};
+
+/*! @brief Tombstone object for all identifiers.  */
+struct tombstone_t {
+    /**
+     * @brief Converts the tombstone object to identifiers of any type.
+     * @tparam Entity Type of identifier.
+     * @return The tombstone representation for the given type.
+     */
+    template<entity_like Entity>
+    [[nodiscard]] constexpr operator Entity() const noexcept {
+        using traits_type = entt_traits<Entity>;
+        return traits_type::construct(traits_type::entity_mask, traits_type::version_mask);
+    }
+
+    /**
+     * @brief Compares two tombstone objects.
+     * @param other A tombstone object.
+     * @return True in all cases.
+     */
+    [[nodiscard]] constexpr bool operator==([[maybe_unused]] const tombstone_t other) const noexcept {
+        return true;
+    }
+
+    /**
+     * @brief Compares a tombstone object and an identifier of any type.
+     * @tparam Entity Type of identifier.
+     * @param entity Identifier with which to compare.
+     * @return False if the two elements differ, true otherwise.
+     */
+    template<entity_like Entity>
+    [[nodiscard]] constexpr bool operator==(const Entity entity) const noexcept {
+        using traits_type = entt_traits<Entity>;
+
+        if constexpr(traits_type::version_mask == 0u) {
+            return false;
+        } else {
+            return (traits_type::to_version(entity) == traits_type::to_version(*this));
+        }
+    }
+};
+
+/**
+ * @brief Compile-time constant for null entities.
+ *
+ * There exist implicit conversions from this variable to identifiers of any
+ * allowed type. Similarly, there exist comparison operators between the null
+ * entity and any other identifier.
+ */
+inline constexpr null_t null{};
+
+/**
+ * @brief Compile-time constant for tombstone entities.
+ *
+ * There exist implicit conversions from this variable to identifiers of any
+ * allowed type. Similarly, there exist comparison operators between the
+ * tombstone entity and any other identifier.
+ */
+inline constexpr tombstone_t tombstone{};
+
+} // namespace entt
+
+#endif

include/entt/entity/fwd.hpp

@@ -0,0 +1,291 @@
+#ifndef ENTT_ENTITY_FWD_HPP
+#define ENTT_ENTITY_FWD_HPP
+
+#include "../config/config.h"
+#include "../core/concepts.hpp"
+#include "../core/fwd.hpp"
+#include "../core/type_traits.hpp"
+#include "../stl/cstdint.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+
+namespace entt {
+
+/*! @brief Default entity identifier. */
+enum class entity : id_type {};
+
+/*! @brief Storage deletion policy. */
+enum class deletion_policy : stl::uint8_t {
+    /*! @brief Swap-and-pop deletion policy. */
+    swap_and_pop = 0u,
+    /*! @brief In-place deletion policy. */
+    in_place = 1u,
+    /*! @brief Swap-only deletion policy. */
+    swap_only = 2u,
+    /*! @brief Unspecified deletion policy. */
+    unspecified = swap_and_pop
+};
+
+template<cvref_unqualified Type, typename Entity = entity>
+struct component_traits;
+
+template<typename Entity = entity, typename = stl::allocator<Entity>>
+class basic_sparse_set;
+
+template<typename Type, typename = entity, typename = stl::allocator<Type>>
+class basic_storage;
+
+template<typename, typename>
+class basic_sigh_mixin;
+
+template<typename, typename>
+class basic_reactive_mixin;
+
+template<typename Entity = entity, typename = stl::allocator<Entity>>
+class basic_registry;
+
+template<typename, typename>
+class basic_view;
+
+template<typename Type, typename = stl::allocator<Type *>>
+class basic_runtime_view;
+
+template<typename, typename, typename>
+class basic_group;
+
+template<typename>
+class basic_organizer;
+
+template<typename, typename...>
+class basic_handle;
+
+template<typename>
+class basic_snapshot;
+
+template<typename>
+class basic_snapshot_loader;
+
+template<typename>
+class basic_continuous_loader;
+
+/*! @brief Alias declaration for the most common use case. */
+using sparse_set = basic_sparse_set<>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+using storage = basic_storage<Type>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Type Underlying storage type.
+ */
+template<typename Type>
+using sigh_mixin = basic_sigh_mixin<Type, basic_registry<typename Type::entity_type, typename Type::base_type::allocator_type>>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Type Underlying storage type.
+ */
+template<typename Type>
+using reactive_mixin = basic_reactive_mixin<Type, basic_registry<typename Type::entity_type, typename Type::base_type::allocator_type>>;
+
+/*! @brief Alias declaration for the most common use case. */
+using registry = basic_registry<>;
+
+/*! @brief Alias declaration for the most common use case. */
+using organizer = basic_organizer<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using handle = basic_handle<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using const_handle = basic_handle<const registry>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using handle_view = basic_handle<registry, Args...>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Args Other template parameters.
+ */
+template<typename... Args>
+using const_handle_view = basic_handle<const registry, Args...>;
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot = basic_snapshot<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using snapshot_loader = basic_snapshot_loader<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using continuous_loader = basic_continuous_loader<registry>;
+
+/*! @brief Alias declaration for the most common use case. */
+using runtime_view = basic_runtime_view<sparse_set>;
+
+/*! @brief Alias declaration for the most common use case. */
+using const_runtime_view = basic_runtime_view<const sparse_set>;
+
+/**
+ * @brief Alias for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct exclude_t final: type_list<Type...> {
+    /*! @brief Default constructor. */
+    explicit ENTT_CONSTEVAL exclude_t() = default;
+};
+
+/**
+ * @brief Variable template for exclusion lists.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+inline constexpr exclude_t<Type...> exclude{};
+
+/**
+ * @brief Alias for lists of observed elements.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct get_t final: type_list<Type...> {
+    /*! @brief Default constructor. */
+    explicit ENTT_CONSTEVAL get_t() = default;
+};
+
+/**
+ * @brief Variable template for lists of observed elements.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+inline constexpr get_t<Type...> get{};
+
+/**
+ * @brief Alias for lists of owned elements.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+struct owned_t final: type_list<Type...> {
+    /*! @brief Default constructor. */
+    explicit ENTT_CONSTEVAL owned_t() = default;
+};
+
+/**
+ * @brief Variable template for lists of owned elements.
+ * @tparam Type List of types.
+ */
+template<typename... Type>
+inline constexpr owned_t<Type...> owned{};
+
+/**
+ * @brief Applies a given _function_ to a get list and generate a new list.
+ * @tparam Type Types provided by the get list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename... Type, template<typename...> class Op>
+struct type_list_transform<get_t<Type...>, Op> {
+    /*! @brief Resulting get list after applying the transform function. */
+    using type = get_t<typename Op<Type>::type...>;
+};
+
+/**
+ * @brief Applies a given _function_ to an exclude list and generate a new list.
+ * @tparam Type Types provided by the exclude list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename... Type, template<typename...> class Op>
+struct type_list_transform<exclude_t<Type...>, Op> {
+    /*! @brief Resulting exclude list after applying the transform function. */
+    using type = exclude_t<typename Op<Type>::type...>;
+};
+
+/**
+ * @brief Applies a given _function_ to an owned list and generate a new list.
+ * @tparam Type Types provided by the owned list.
+ * @tparam Op Unary operation as template class with a type member named `type`.
+ */
+template<typename... Type, template<typename...> class Op>
+struct type_list_transform<owned_t<Type...>, Op> {
+    /*! @brief Resulting owned list after applying the transform function. */
+    using type = owned_t<typename Op<Type>::type...>;
+};
+
+/**
+ * @brief Provides a common way to define storage types.
+ * @tparam Type Storage value type.
+ * @tparam Entity A valid entity type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Entity = entity, typename Allocator = stl::allocator<Type>>
+struct storage_type {
+    /*! @brief Type-to-storage conversion result. */
+    using type = ENTT_STORAGE(sigh_mixin, basic_storage<Type, Entity, Allocator>);
+};
+
+/*! @brief Empty value type for reactive storage types. */
+struct reactive final {};
+
+/**
+ * @ brief Partial specialization for reactive storage types.
+ * @tparam Entity A valid entity type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Entity, typename Allocator>
+struct storage_type<reactive, Entity, Allocator> {
+    /*! @brief Type-to-storage conversion result. */
+    using type = ENTT_STORAGE(reactive_mixin, basic_storage<reactive, Entity, Allocator>);
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Args Arguments to forward.
+ */
+template<typename... Args>
+using storage_type_t = storage_type<Args...>::type;
+
+/**
+ * Type-to-storage conversion utility that preserves constness.
+ * @tparam Type Storage value type, eventually const.
+ * @tparam Entity A valid entity type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Entity = entity, typename Allocator = stl::allocator<stl::remove_const_t<Type>>>
+struct storage_for {
+    /*! @brief Type-to-storage conversion result. */
+    using type = constness_as_t<storage_type_t<stl::remove_const_t<Type>, Entity, Allocator>, Type>;
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Args Arguments to forward.
+ */
+template<typename... Args>
+using storage_for_t = storage_for<Args...>::type;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Get Types of storage iterated by the view.
+ * @tparam Exclude Types of storage used to filter the view.
+ */
+template<typename Get, typename Exclude = exclude_t<>>
+using view = basic_view<type_list_transform_t<Get, storage_for>, type_list_transform_t<Exclude, storage_for>>;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Owned Types of storage _owned_ by the group.
+ * @tparam Get Types of storage _observed_ by the group.
+ * @tparam Exclude Types of storage used to filter the group.
+ */
+template<typename Owned, typename Get = get_t<>, typename Exclude = exclude_t<>>
+using group = basic_group<type_list_transform_t<Owned, storage_for>, type_list_transform_t<Get, storage_for>, type_list_transform_t<Exclude, storage_for>>;
+
+} // namespace entt
+
+#endif

include/entt/entity/handle.hpp

@@ -0,0 +1,368 @@
+#ifndef ENTT_ENTITY_HANDLE_HPP
+#define ENTT_ENTITY_HANDLE_HPP
+
+#include "../config/config.h"
+#include "../core/iterator.hpp"
+#include "../core/type_traits.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/tuple.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename It>
+class handle_storage_iterator final {
+    template<typename>
+    friend class handle_storage_iterator;
+
+    using underlying_type = stl::remove_reference_t<typename It::value_type::second_type>;
+    using entity_type = underlying_type::entity_type;
+
+public:
+    using value_type = stl::iterator_traits<It>::value_type;
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using difference_type = stl::ptrdiff_t;
+    using iterator_category = stl::input_iterator_tag;
+    using iterator_concept = stl::forward_iterator_tag;
+
+    constexpr handle_storage_iterator() noexcept
+        : entt{null},
+          it{},
+          last{} {}
+
+    constexpr handle_storage_iterator(entity_type value, It from, It to) noexcept
+        : entt{value},
+          it{from},
+          last{to} {
+        while(it != last && !it->second.contains(entt)) {
+            ++it;
+        }
+    }
+
+    constexpr handle_storage_iterator &operator++() noexcept {
+        for(++it; it != last && !it->second.contains(entt); ++it) {}
+        return *this;
+    }
+
+    constexpr handle_storage_iterator operator++(int) noexcept {
+        const handle_storage_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return *it;
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return operator*();
+    }
+
+    template<typename Other>
+    [[nodiscard]] constexpr bool operator==(const handle_storage_iterator<Other> &other) const noexcept {
+        return it == other.it;
+    }
+
+private:
+    entity_type entt;
+    It it;
+    It last;
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Non-owning handle to an entity.
+ *
+ * Tiny wrapper around a registry and an entity.
+ *
+ * @tparam Registry Basic registry type.
+ * @tparam Scope Types to which to restrict the scope of a handle.
+ */
+template<typename Registry, typename... Scope>
+class basic_handle {
+    using traits_type = entt_traits<typename Registry::entity_type>;
+
+    [[nodiscard]] auto &owner_or_assert() const noexcept {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        return static_cast<Registry &>(*owner);
+    }
+
+public:
+    /*! @brief Type of registry accepted by the handle. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = traits_type::value_type;
+    /*! @brief Underlying version type. */
+    using version_type = traits_type::version_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+    /*! @brief Iterable handle type. */
+    using iterable = iterable_adaptor<internal::handle_storage_iterator<typename decltype(stl::declval<registry_type>().storage())::iterator>>;
+
+    /*! @brief Constructs an invalid handle. */
+    basic_handle() noexcept
+        : owner{},
+          entt{null} {}
+
+    /**
+     * @brief Constructs a handle from a given registry and entity.
+     * @param ref An instance of the registry class.
+     * @param value A valid identifier.
+     */
+    basic_handle(registry_type &ref, entity_type value) noexcept
+        : owner{&ref},
+          entt{value} {}
+
+    /**
+     * @brief Returns an iterable object to use to _visit_ a handle.
+     *
+     * The iterable object returns a pair that contains the name and a reference
+     * to the current storage.<br/>
+     * Returned storage are those that contain the entity associated with the
+     * handle.
+     *
+     * @return An iterable object to use to _visit_ the handle.
+     */
+    [[nodiscard]] iterable storage() const noexcept {
+        auto underlying = owner_or_assert().storage();
+        return iterable{{entt, underlying.begin(), underlying.end()}, {entt, underlying.end(), underlying.end()}};
+    }
+
+    /*! @copydoc valid */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return owner && owner->valid(entt);
+    }
+
+    /**
+     * @brief Checks if a handle refers to a valid registry and entity.
+     * @return True if the handle refers to a valid registry and entity, false
+     * otherwise.
+     */
+    [[nodiscard]] bool valid() const {
+        return static_cast<bool>(*this);
+    }
+
+    /**
+     * @brief Returns a pointer to the underlying registry, if any.
+     * @return A pointer to the underlying registry, if any.
+     */
+    [[nodiscard]] registry_type *registry() const noexcept {
+        return owner;
+    }
+
+    /**
+     * @brief Returns the entity associated with a handle.
+     * @return The entity associated with the handle.
+     */
+    [[nodiscard]] entity_type entity() const noexcept {
+        return entt;
+    }
+
+    /*! @copydoc entity */
+    [[nodiscard]] operator entity_type() const noexcept {
+        return entity();
+    }
+
+    /*! @brief Destroys the entity associated with a handle. */
+    void destroy() {
+        owner_or_assert().destroy(stl::exchange(entt, null));
+    }
+
+    /**
+     * @brief Destroys the entity associated with a handle.
+     * @param version A desired version upon destruction.
+     */
+    void destroy(const version_type version) {
+        owner_or_assert().destroy(stl::exchange(entt, null), version);
+    }
+
+    /**
+     * @brief Assigns the given element to a handle.
+     * @tparam Type Type of element to create.
+     * @tparam Args Types of arguments to use to construct the element.
+     * @param args Parameters to use to initialize the element.
+     * @return A reference to the newly created element.
+     */
+    template<typename Type, typename... Args>
+    // NOLINTNEXTLINE(modernize-use-nodiscard)
+    decltype(auto) emplace(Args &&...args) const {
+        static_assert(((sizeof...(Scope) == 0) || ... || stl::is_same_v<Type, Scope>), "Invalid type");
+        return owner_or_assert().template emplace<Type>(entt, stl::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Assigns or replaces the given element for a handle.
+     * @tparam Type Type of element to assign or replace.
+     * @tparam Args Types of arguments to use to construct the element.
+     * @param args Parameters to use to initialize the element.
+     * @return A reference to the newly created element.
+     */
+    template<typename Type, typename... Args>
+    decltype(auto) emplace_or_replace(Args &&...args) const {
+        static_assert(((sizeof...(Scope) == 0) || ... || stl::is_same_v<Type, Scope>), "Invalid type");
+        return owner_or_assert().template emplace_or_replace<Type>(entt, stl::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Patches the given element for a handle.
+     * @tparam Type Type of element to patch.
+     * @tparam Func Types of the function objects to invoke.
+     * @param func Valid function objects.
+     * @return A reference to the patched element.
+     */
+    template<typename Type, typename... Func>
+    decltype(auto) patch(Func &&...func) const {
+        static_assert(((sizeof...(Scope) == 0) || ... || stl::is_same_v<Type, Scope>), "Invalid type");
+        return owner_or_assert().template patch<Type>(entt, stl::forward<Func>(func)...);
+    }
+
+    /**
+     * @brief Replaces the given element for a handle.
+     * @tparam Type Type of element to replace.
+     * @tparam Args Types of arguments to use to construct the element.
+     * @param args Parameters to use to initialize the element.
+     * @return A reference to the element being replaced.
+     */
+    template<typename Type, typename... Args>
+    decltype(auto) replace(Args &&...args) const {
+        static_assert(((sizeof...(Scope) == 0) || ... || stl::is_same_v<Type, Scope>), "Invalid type");
+        return owner_or_assert().template replace<Type>(entt, stl::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Removes the given elements from a handle.
+     * @tparam Type Types of elements to remove.
+     * @return The number of elements actually removed.
+     */
+    template<typename... Type>
+    // NOLINTNEXTLINE(modernize-use-nodiscard)
+    size_type remove() const {
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Type> && ...), "Invalid type");
+        return owner_or_assert().template remove<Type...>(entt);
+    }
+
+    /**
+     * @brief Erases the given elements from a handle.
+     * @tparam Type Types of elements to erase.
+     */
+    template<typename... Type>
+    void erase() const {
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Type> && ...), "Invalid type");
+        owner_or_assert().template erase<Type...>(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has all the given elements.
+     * @tparam Type Elements for which to perform the check.
+     * @return True if the handle has all the elements, false otherwise.
+     */
+    template<typename... Type>
+    [[nodiscard]] decltype(auto) all_of() const {
+        return owner_or_assert().template all_of<Type...>(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has at least one of the given elements.
+     * @tparam Type Elements for which to perform the check.
+     * @return True if the handle has at least one of the given elements,
+     * false otherwise.
+     */
+    template<typename... Type>
+    [[nodiscard]] decltype(auto) any_of() const {
+        return owner_or_assert().template any_of<Type...>(entt);
+    }
+
+    /**
+     * @brief Returns references to the given elements for a handle.
+     * @tparam Type Types of elements to get.
+     * @return References to the elements owned by the handle.
+     */
+    template<typename... Type>
+    [[nodiscard]] decltype(auto) get() const {
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Type> && ...), "Invalid type");
+        return owner_or_assert().template get<Type...>(entt);
+    }
+
+    /**
+     * @brief Returns a reference to the given element for a handle.
+     * @tparam Type Type of element to get.
+     * @tparam Args Types of arguments to use to construct the element.
+     * @param args Parameters to use to initialize the element.
+     * @return Reference to the element owned by the handle.
+     */
+    template<typename Type, typename... Args>
+    [[nodiscard]] decltype(auto) get_or_emplace(Args &&...args) const {
+        static_assert(((sizeof...(Scope) == 0) || ... || stl::is_same_v<Type, Scope>), "Invalid type");
+        return owner_or_assert().template get_or_emplace<Type>(entt, stl::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Returns pointers to the given elements for a handle.
+     * @tparam Type Types of elements to get.
+     * @return Pointers to the elements owned by the handle.
+     */
+    template<typename... Type>
+    [[nodiscard]] auto try_get() const {
+        static_assert(sizeof...(Scope) == 0 || (type_list_contains_v<type_list<Scope...>, Type> && ...), "Invalid type");
+        return owner_or_assert().template try_get<Type...>(entt);
+    }
+
+    /**
+     * @brief Checks if a handle has elements assigned.
+     * @return True if the handle has no elements assigned, false otherwise.
+     */
+    [[nodiscard]] bool orphan() const {
+        return owner_or_assert().orphan(entt);
+    }
+
+    /**
+     * @brief Compares two handles.
+     * @tparam Other Scope of the other handle.
+     * @param other A valid handle.
+     * @return True if both handles refer to the same registry and the same
+     * entity, false otherwise.
+     */
+    template<typename... Other>
+    [[nodiscard]] bool operator==(const basic_handle<Other...> &other) const noexcept {
+        return owner == other.registry() && entt == other.entity();
+    }
+
+    /**
+     * @brief Compares a handle with the null object.
+     * @param other A null object yet to be converted.
+     * @return False if the two elements differ, true otherwise.
+     */
+    [[nodiscard]] constexpr bool operator==(const null_t other) const noexcept {
+        return (entt == other);
+    }
+
+    /**
+     * @brief Returns a const handle from a non-const one.
+     * @tparam Other A valid entity type.
+     * @tparam Args Scope of the handle to construct.
+     * @return A const handle referring to the same registry and the same
+     * entity.
+     */
+    template<typename Other, typename... Args>
+    operator basic_handle<Other, Args...>() const noexcept {
+        static_assert(stl::is_same_v<Other, Registry> || stl::is_same_v<stl::remove_const_t<Other>, Registry>, "Invalid conversion between different handles");
+        static_assert((sizeof...(Scope) == 0 || ((sizeof...(Args) != 0 && sizeof...(Args) <= sizeof...(Scope)) && ... && (type_list_contains_v<type_list<Scope...>, Args>))), "Invalid conversion between different handles");
+        return owner ? basic_handle<Other, Args...>{*owner, entt} : basic_handle<Other, Args...>{};
+    }
+
+private:
+    registry_type *owner;
+    entity_type entt;
+};
+
+} // namespace entt
+
+#endif

include/entt/entity/helper.hpp

@@ -0,0 +1,256 @@
+#ifndef ENTT_ENTITY_HELPER_HPP
+#define ENTT_ENTITY_HELPER_HPP
+
+#include "../core/fwd.hpp"
+#include "../core/type_traits.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "component.hpp"
+#include "fwd.hpp"
+#include "group.hpp"
+#include "storage.hpp"
+#include "view.hpp"
+
+namespace entt {
+
+/**
+ * @brief Converts a registry to a view.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class as_view {
+    template<typename... Get, typename... Exclude>
+    [[nodiscard]] auto dispatch(get_t<Get...>, exclude_t<Exclude...>) const {
+        return reg->template view<constness_as_t<typename Get::element_type, Get>...>(exclude_t<constness_as_t<typename Exclude::element_type, Exclude>...>{});
+    }
+
+public:
+    /*! @brief Type of registry to convert. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = registry_type::entity_type;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_view(registry_type &source) noexcept
+        : reg{&source} {}
+
+    /**
+     * @brief Conversion function from a registry to a view.
+     * @tparam Get Type of storage used to construct the view.
+     * @tparam Exclude Types of storage used to filter the view.
+     * @return A newly created view.
+     */
+    template<typename Get, typename Exclude>
+    operator basic_view<Get, Exclude>() const {
+        return dispatch(Get{}, Exclude{});
+    }
+
+private:
+    registry_type *reg;
+};
+
+/**
+ * @brief Converts a registry to a group.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class as_group {
+    template<typename... Owned, typename... Get, typename... Exclude>
+    [[nodiscard]] auto dispatch(owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>) const {
+        if constexpr(stl::is_const_v<registry_type>) {
+            return reg->template group_if_exists<typename Owned::element_type...>(get_t<typename Get::element_type...>{}, exclude_t<typename Exclude::element_type...>{});
+        } else {
+            return reg->template group<constness_as_t<typename Owned::element_type, Owned>...>(get_t<constness_as_t<typename Get::element_type, Get>...>{}, exclude_t<constness_as_t<typename Exclude::element_type, Exclude>...>{});
+        }
+    }
+
+public:
+    /*! @brief Type of registry to convert. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = registry_type::entity_type;
+
+    /**
+     * @brief Constructs a converter for a given registry.
+     * @param source A valid reference to a registry.
+     */
+    as_group(registry_type &source) noexcept
+        : reg{&source} {}
+
+    /**
+     * @brief Conversion function from a registry to a group.
+     * @tparam Owned Types of _owned_ by the group.
+     * @tparam Get Types of storage _observed_ by the group.
+     * @tparam Exclude Types of storage used to filter the group.
+     * @return A newly created group.
+     */
+    template<typename Owned, typename Get, typename Exclude>
+    operator basic_group<Owned, Get, Exclude>() const {
+        return dispatch(Owned{}, Get{}, Exclude{});
+    }
+
+private:
+    registry_type *reg;
+};
+
+/**
+ * @brief Helper to create a listener that directly invokes a member function.
+ * @tparam Member Member function to invoke on an element of the given type.
+ * @tparam Registry Basic registry type.
+ * @param reg A registry that contains the given entity and its elements.
+ * @param entt Entity from which to get the element.
+ */
+template<auto Member, typename Registry = stl::decay_t<nth_argument_t<0u, decltype(Member)>>>
+void invoke(Registry &reg, const typename Registry::entity_type entt) {
+    static_assert(stl::is_member_function_pointer_v<decltype(Member)>, "Invalid pointer to non-static member function");
+    (reg.template get<member_class_t<decltype(Member)>>(entt).*Member)(reg, entt);
+}
+
+/**
+ * @brief Returns the entity associated with a given element.
+ *
+ * @warning
+ * Currently, this function only works correctly with the default storage as it
+ * makes assumptions about how the elements are laid out.
+ *
+ * @tparam Args Storage type template parameters.
+ * @param storage A storage that contains the given element.
+ * @param instance A valid element instance.
+ * @return The entity associated with the given element.
+ */
+template<typename... Args>
+basic_storage<Args...>::entity_type to_entity(const basic_storage<Args...> &storage, const typename basic_storage<Args...>::value_type &instance) {
+    using traits_type = component_traits<typename basic_storage<Args...>::value_type, typename basic_storage<Args...>::entity_type>;
+    static_assert(traits_type::page_size != 0u, "Unexpected page size");
+    const auto *page = storage.raw();
+
+    // NOLINTBEGIN(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+    for(stl::size_t pos{}, count = storage.size(); pos < count; pos += traits_type::page_size, ++page) {
+        if(const auto dist = (stl::addressof(instance) - *page); dist >= 0 && dist < static_cast<decltype(dist)>(traits_type::page_size)) {
+            return *(static_cast<const basic_storage<Args...>::base_type &>(storage).rbegin() + static_cast<decltype(dist)>(pos) + dist);
+        }
+    }
+    // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+
+    return null;
+}
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename...>
+struct sigh_helper;
+
+/**
+ * @brief Signal connection helper for registries.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+struct sigh_helper<Registry> {
+    /*! @brief Registry type. */
+    using registry_type = Registry;
+
+    /**
+     * @brief Constructs a helper for a given registry.
+     * @param ref A valid reference to a registry.
+     */
+    sigh_helper(registry_type &ref)
+        : bucket{&ref} {}
+
+    /**
+     * @brief Binds a properly initialized helper to a given signal type.
+     * @tparam Type Type of signal to bind the helper to.
+     * @param id Optional name for the underlying storage to use.
+     * @return A helper for a given registry and signal type.
+     */
+    template<typename Type>
+    auto with(const id_type id = type_hash<Type>::value()) noexcept {
+        return sigh_helper<registry_type, Type>{*bucket, id};
+    }
+
+    /**
+     * @brief Returns a reference to the underlying registry.
+     * @return A reference to the underlying registry.
+     */
+    [[nodiscard]] registry_type &registry() noexcept {
+        return *bucket;
+    }
+
+private:
+    registry_type *bucket;
+};
+
+/**
+ * @brief Signal connection helper for registries.
+ * @tparam Registry Basic registry type.
+ * @tparam Type Type of signal to connect listeners to.
+ */
+template<typename Registry, typename Type>
+struct sigh_helper<Registry, Type> final: sigh_helper<Registry> {
+    /*! @brief Registry type. */
+    using registry_type = Registry;
+
+    /**
+     * @brief Constructs a helper for a given registry.
+     * @param ref A valid reference to a registry.
+     * @param id Optional name for the underlying storage to use.
+     */
+    sigh_helper(registry_type &ref, const id_type id = type_hash<Type>::value())
+        : sigh_helper<Registry>{ref},
+          name{id} {}
+
+    /**
+     * @brief Forwards the call to `on_construct` on the underlying storage.
+     * @tparam Candidate Function or member to connect.
+     * @tparam Args Type of class or type of payload, if any.
+     * @param args A valid object that fits the purpose, if any.
+     * @return This helper.
+     */
+    template<auto Candidate, typename... Args>
+    auto on_construct(Args &&...args) {
+        this->registry().template on_construct<Type>(name).template connect<Candidate>(stl::forward<Args>(args)...);
+        return *this;
+    }
+
+    /**
+     * @brief Forwards the call to `on_update` on the underlying storage.
+     * @tparam Candidate Function or member to connect.
+     * @tparam Args Type of class or type of payload, if any.
+     * @param args A valid object that fits the purpose, if any.
+     * @return This helper.
+     */
+    template<auto Candidate, typename... Args>
+    auto on_update(Args &&...args) {
+        this->registry().template on_update<Type>(name).template connect<Candidate>(stl::forward<Args>(args)...);
+        return *this;
+    }
+
+    /**
+     * @brief Forwards the call to `on_destroy` on the underlying storage.
+     * @tparam Candidate Function or member to connect.
+     * @tparam Args Type of class or type of payload, if any.
+     * @param args A valid object that fits the purpose, if any.
+     * @return This helper.
+     */
+    template<auto Candidate, typename... Args>
+    auto on_destroy(Args &&...args) {
+        this->registry().template on_destroy<Type>(name).template connect<Candidate>(stl::forward<Args>(args)...);
+        return *this;
+    }
+
+private:
+    id_type name;
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+sigh_helper(Registry &) -> sigh_helper<Registry>;
+
+} // namespace entt
+
+#endif

include/entt/entity/mixin.hpp

@@ -0,0 +1,593 @@
+#ifndef ENTT_ENTITY_MIXIN_HPP
+#define ENTT_ENTITY_MIXIN_HPP
+
+#include "../config/config.h"
+#include "../core/any.hpp"
+#include "../core/type_info.hpp"
+#include "../signal/sigh.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename, typename>
+struct has_on_construct final: stl::false_type {};
+
+template<typename Type, typename Registry>
+requires stl::invocable<decltype(&Type::on_construct), Registry &, typename Registry::entity_type>
+struct has_on_construct<Type, Registry>: stl::true_type {};
+
+template<typename, typename>
+struct has_on_update final: stl::false_type {};
+
+template<typename Type, typename Registry>
+requires stl::invocable<decltype(&Type::on_update), Registry &, typename Registry::entity_type>
+struct has_on_update<Type, Registry>: stl::true_type {};
+
+template<typename, typename>
+struct has_on_destroy final: stl::false_type {};
+
+template<typename Type, typename Registry>
+requires stl::invocable<decltype(&Type::on_destroy), Registry &, typename Registry::entity_type>
+struct has_on_destroy<Type, Registry>: stl::true_type {};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Mixin type used to add signal support to storage types.
+ *
+ * The function type of a listener is equivalent to:
+ *
+ * @code{.cpp}
+ * void(basic_registry<entity_type> &, entity_type);
+ * @endcode
+ *
+ * This applies to all signals made available.
+ *
+ * @tparam Type Underlying storage type.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Type, typename Registry>
+class basic_sigh_mixin final: public Type {
+    using underlying_type = Type;
+    using owner_type = Registry;
+
+    using basic_registry_type = basic_registry<typename owner_type::entity_type, typename owner_type::allocator_type>;
+    using sigh_type = sigh<void(owner_type &, const typename underlying_type::entity_type), typename underlying_type::allocator_type>;
+    using underlying_iterator = underlying_type::base_type::basic_iterator;
+
+    static_assert(stl::is_base_of_v<basic_registry_type, owner_type>, "Invalid registry type");
+
+    [[nodiscard]] auto &owner_or_assert() const noexcept {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        return static_cast<owner_type &>(*owner);
+    }
+
+private:
+    void pop(underlying_iterator first, underlying_iterator last) final {
+        if(auto &reg = owner_or_assert(); destruction.empty()) {
+            underlying_type::pop(first, last);
+        } else {
+            for(; first != last; ++first) {
+                const auto entt = *first;
+                destruction.publish(reg, entt);
+                const auto it = underlying_type::find(entt);
+                underlying_type::pop(it, it + 1u);
+            }
+        }
+    }
+
+    void pop_all() final {
+        if(auto &reg = owner_or_assert(); !destruction.empty()) {
+            if constexpr(stl::is_same_v<typename underlying_type::element_type, entity_type>) {
+                for(typename underlying_type::size_type pos{}, last = underlying_type::free_list(); pos < last; ++pos) {
+                    destruction.publish(reg, underlying_type::base_type::operator[](pos));
+                }
+            } else {
+                for(auto entt: static_cast<underlying_type::base_type &>(*this)) {
+                    if constexpr(underlying_type::storage_policy == deletion_policy::in_place) {
+                        if(entt != tombstone) {
+                            destruction.publish(reg, entt);
+                        }
+                    } else {
+                        destruction.publish(reg, entt);
+                    }
+                }
+            }
+        }
+
+        underlying_type::pop_all();
+    }
+
+    underlying_iterator try_emplace(const underlying_type::entity_type entt, const bool force_back, const void *value) final {
+        const auto it = underlying_type::try_emplace(entt, force_back, value);
+
+        if(auto &reg = owner_or_assert(); it != underlying_type::base_type::end()) {
+            construction.publish(reg, *it);
+        }
+
+        return it;
+    }
+
+    void bind_any(any value) noexcept final {
+        owner = any_cast<basic_registry_type>(&value);
+
+        if constexpr(!stl::is_same_v<registry_type, basic_registry_type>) {
+            if(owner == nullptr) {
+                owner = any_cast<registry_type>(&value);
+            }
+        }
+
+        underlying_type::bind_any(stl::move(value));
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = underlying_type::allocator_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = underlying_type::entity_type;
+    /*! @brief Expected registry type. */
+    using registry_type = owner_type;
+
+    /*! @brief Default constructor. */
+    basic_sigh_mixin()
+        : basic_sigh_mixin{allocator_type{}} {}
+
+    /**
+     * @brief Constructs an empty storage with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_sigh_mixin(const allocator_type &allocator)
+        : underlying_type{allocator},
+          owner{},
+          construction{allocator},
+          destruction{allocator},
+          update{allocator} {
+        if constexpr(internal::has_on_construct<typename underlying_type::element_type, Registry>::value) {
+            sink{construction}.template connect<&underlying_type::element_type::on_construct>();
+        }
+
+        if constexpr(internal::has_on_update<typename underlying_type::element_type, Registry>::value) {
+            sink{update}.template connect<&underlying_type::element_type::on_update>();
+        }
+
+        if constexpr(internal::has_on_destroy<typename underlying_type::element_type, Registry>::value) {
+            sink{destruction}.template connect<&underlying_type::element_type::on_destroy>();
+        }
+    }
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_sigh_mixin(const basic_sigh_mixin &) = delete;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_sigh_mixin(basic_sigh_mixin &&other) noexcept
+        : underlying_type{static_cast<underlying_type &&>(other)},
+          owner{other.owner},
+          construction{stl::move(other.construction)},
+          destruction{stl::move(other.destruction)},
+          update{stl::move(other.update)} {}
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_sigh_mixin(basic_sigh_mixin &&other, const allocator_type &allocator)
+        : underlying_type{static_cast<underlying_type &&>(other), allocator},
+          owner{other.owner},
+          construction{stl::move(other.construction), allocator},
+          destruction{stl::move(other.destruction), allocator},
+          update{stl::move(other.update), allocator} {}
+
+    /*! @brief Default destructor. */
+    ~basic_sigh_mixin() override = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This mixin.
+     */
+    basic_sigh_mixin &operator=(const basic_sigh_mixin &) = delete;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This mixin.
+     */
+    basic_sigh_mixin &operator=(basic_sigh_mixin &&other) noexcept {
+        swap(other);
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given storage.
+     * @param other Storage to exchange the content with.
+     */
+    void swap(basic_sigh_mixin &other) noexcept {
+        using stl::swap;
+        swap(owner, other.owner);
+        swap(construction, other.construction);
+        swap(destruction, other.destruction);
+        swap(update, other.update);
+        underlying_type::swap(other);
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever a new instance is created and assigned to an entity.<br/>
+     * Listeners are invoked after the object has been assigned to the entity.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_construct() noexcept {
+        return sink{construction};
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance is explicitly updated.<br/>
+     * Listeners are invoked after the object has been updated.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_update() noexcept {
+        return sink{update};
+    }
+
+    /**
+     * @brief Returns a sink object.
+     *
+     * The sink returned by this function can be used to receive notifications
+     * whenever an instance is removed from an entity and thus destroyed.<br/>
+     * Listeners are invoked before the object has been removed from the entity.
+     *
+     * @sa sink
+     *
+     * @return A temporary sink object.
+     */
+    [[nodiscard]] auto on_destroy() noexcept {
+        return sink{destruction};
+    }
+
+    /**
+     * @brief Checks if a mixin refers to a valid registry.
+     * @return True if the mixin refers to a valid registry, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return (owner != nullptr);
+    }
+
+    /**
+     * @brief Returns a pointer to the underlying registry, if any.
+     * @return A pointer to the underlying registry, if any.
+     */
+    [[nodiscard]] const registry_type &registry() const noexcept {
+        return owner_or_assert();
+    }
+
+    /*! @copydoc registry */
+    [[nodiscard]] registry_type &registry() noexcept {
+        return owner_or_assert();
+    }
+
+    /**
+     * @brief Creates a new identifier or recycles a destroyed one.
+     * @return A valid identifier.
+     */
+    auto generate() {
+        const auto entt = underlying_type::generate();
+        construction.publish(owner_or_assert(), entt);
+        return entt;
+    }
+
+    /**
+     * @brief Creates a new identifier or recycles a destroyed one.
+     * @param hint Required identifier.
+     * @return A valid identifier.
+     */
+    entity_type generate(const entity_type hint) {
+        const auto entt = underlying_type::generate(hint);
+        construction.publish(owner_or_assert(), entt);
+        return entt;
+    }
+
+    /**
+     * @brief Assigns each element in a range an identifier.
+     * @tparam It Type of output iterator.
+     * @param first An iterator to the first element of the range to generate.
+     * @param last An iterator past the last element of the range to generate.
+     */
+    template<stl::output_iterator<entity_type> It>
+    void generate(It first, It last) {
+        underlying_type::generate(first, last);
+
+        if(auto &reg = owner_or_assert(); !construction.empty()) {
+            for(; first != last; ++first) {
+                construction.publish(reg, *first);
+            }
+        }
+    }
+
+    /**
+     * @brief Assigns an entity to a storage and constructs its object.
+     * @tparam Args Types of arguments to forward to the underlying storage.
+     * @param entt A valid identifier.
+     * @param args Parameters to forward to the underlying storage.
+     * @return A reference to the newly created object.
+     */
+    template<typename... Args>
+    decltype(auto) emplace(const entity_type entt, Args &&...args) {
+        underlying_type::emplace(entt, stl::forward<Args>(args)...);
+        construction.publish(owner_or_assert(), entt);
+        return this->get(entt);
+    }
+
+    /**
+     * @brief Updates the instance assigned to a given entity in-place.
+     * @tparam Func Types of the function objects to invoke.
+     * @param entt A valid identifier.
+     * @param func Valid function objects.
+     * @return A reference to the patched instance.
+     */
+    template<typename... Func>
+    decltype(auto) patch(const entity_type entt, Func &&...func) {
+        underlying_type::patch(entt, stl::forward<Func>(func)...);
+        update.publish(owner_or_assert(), entt);
+        return this->get(entt);
+    }
+
+    /**
+     * @brief Assigns one or more entities to a storage and constructs their
+     * objects from a given instance.
+     * @tparam Args Types of arguments to forward to the underlying storage.
+     * @param first An iterator to the first element of the range of entities.
+     * @param last An iterator past the last element of the range of entities.
+     * @param args Parameters to use to forward to the underlying storage.
+     */
+    template<typename... Args>
+    void insert(stl::input_iterator auto first, stl::input_iterator auto last, Args &&...args) {
+        auto from = underlying_type::size();
+        underlying_type::insert(first, last, stl::forward<Args>(args)...);
+
+        if(auto &reg = owner_or_assert(); !construction.empty()) {
+            // fine as long as insert passes force_back true to try_emplace
+            for(const auto to = underlying_type::size(); from != to; ++from) {
+                construction.publish(reg, underlying_type::operator[](from));
+            }
+        }
+    }
+
+private:
+    basic_registry_type *owner;
+    sigh_type construction;
+    sigh_type destruction;
+    sigh_type update;
+};
+
+/**
+ * @brief Mixin type used to add _reactive_ support to storage types.
+ * @tparam Type Underlying storage type.
+ * @tparam Registry Basic registry type.
+ */
+template<typename Type, typename Registry>
+class basic_reactive_mixin final: public Type {
+    using underlying_type = Type;
+    using owner_type = Registry;
+
+    using alloc_traits = stl::allocator_traits<typename underlying_type::allocator_type>;
+    using basic_registry_type = basic_registry<typename owner_type::entity_type, typename owner_type::allocator_type>;
+    using container_type = stl::vector<connection, typename alloc_traits::template rebind_alloc<connection>>;
+
+    static_assert(stl::is_base_of_v<basic_registry_type, owner_type>, "Invalid registry type");
+
+    [[nodiscard]] auto &owner_or_assert() const noexcept {
+        ENTT_ASSERT(owner != nullptr, "Invalid pointer to registry");
+        return static_cast<owner_type &>(*owner);
+    }
+
+    void emplace_element(const Registry &, underlying_type::entity_type entity) {
+        if(!underlying_type::contains(entity)) {
+            underlying_type::emplace(entity);
+        }
+    }
+
+private:
+    void bind_any(any value) noexcept final {
+        owner = any_cast<basic_registry_type>(&value);
+
+        if constexpr(!stl::is_same_v<registry_type, basic_registry_type>) {
+            if(owner == nullptr) {
+                owner = any_cast<registry_type>(&value);
+            }
+        }
+
+        underlying_type::bind_any(stl::move(value));
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = underlying_type::allocator_type;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = underlying_type::entity_type;
+    /*! @brief Expected registry type. */
+    using registry_type = owner_type;
+
+    /*! @brief Default constructor. */
+    basic_reactive_mixin()
+        : basic_reactive_mixin{allocator_type{}} {}
+
+    /**
+     * @brief Constructs an empty storage with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_reactive_mixin(const allocator_type &allocator)
+        : underlying_type{allocator},
+          owner{},
+          conn{allocator} {
+    }
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_reactive_mixin(const basic_reactive_mixin &) = delete;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_reactive_mixin(basic_reactive_mixin &&other) noexcept
+        : underlying_type{static_cast<underlying_type &&>(other)},
+          owner{other.owner},
+          conn{stl::move(other.conn)} {
+    }
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_reactive_mixin(basic_reactive_mixin &&other, const allocator_type &allocator)
+        : underlying_type{static_cast<underlying_type &&>(other), allocator},
+          owner{other.owner},
+          conn{stl::move(other.conn), allocator} {
+    }
+
+    /*! @brief Default destructor. */
+    ~basic_reactive_mixin() override = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This mixin.
+     */
+    basic_reactive_mixin &operator=(const basic_reactive_mixin &) = delete;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This mixin.
+     */
+    basic_reactive_mixin &operator=(basic_reactive_mixin &&other) noexcept {
+        underlying_type::swap(other);
+        return *this;
+    }
+
+    /**
+     * @brief Makes storage _react_ to creation of objects of the given type.
+     * @tparam Clazz Type of element to _react_ to.
+     * @tparam Candidate Function to use to _react_ to the event.
+     * @param id Optional name used to map the storage within the registry.
+     * @return This mixin.
+     */
+    template<typename Clazz, auto Candidate = &basic_reactive_mixin::emplace_element>
+    basic_reactive_mixin &on_construct(const id_type id = type_hash<Clazz>::value()) {
+        auto curr = owner_or_assert().template storage<Clazz>(id).on_construct().template connect<Candidate>(*this);
+        conn.push_back(stl::move(curr));
+        return *this;
+    }
+
+    /**
+     * @brief Makes storage _react_ to update of objects of the given type.
+     * @tparam Clazz Type of element to _react_ to.
+     * @tparam Candidate Function to use to _react_ to the event.
+     * @param id Optional name used to map the storage within the registry.
+     * @return This mixin.
+     */
+    template<typename Clazz, auto Candidate = &basic_reactive_mixin::emplace_element>
+    basic_reactive_mixin &on_update(const id_type id = type_hash<Clazz>::value()) {
+        auto curr = owner_or_assert().template storage<Clazz>(id).on_update().template connect<Candidate>(*this);
+        conn.push_back(stl::move(curr));
+        return *this;
+    }
+
+    /**
+     * @brief Makes storage _react_ to destruction of objects of the given type.
+     * @tparam Clazz Type of element to _react_ to.
+     * @tparam Candidate Function to use to _react_ to the event.
+     * @param id Optional name used to map the storage within the registry.
+     * @return This mixin.
+     */
+    template<typename Clazz, auto Candidate = &basic_reactive_mixin::emplace_element>
+    basic_reactive_mixin &on_destroy(const id_type id = type_hash<Clazz>::value()) {
+        auto curr = owner_or_assert().template storage<Clazz>(id).on_destroy().template connect<Candidate>(*this);
+        conn.push_back(stl::move(curr));
+        return *this;
+    }
+
+    /**
+     * @brief Checks if a mixin refers to a valid registry.
+     * @return True if the mixin refers to a valid registry, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return (owner != nullptr);
+    }
+
+    /**
+     * @brief Returns a pointer to the underlying registry, if any.
+     * @return A pointer to the underlying registry, if any.
+     */
+    [[nodiscard]] const registry_type &registry() const noexcept {
+        return owner_or_assert();
+    }
+
+    /*! @copydoc registry */
+    [[nodiscard]] registry_type &registry() noexcept {
+        return owner_or_assert();
+    }
+
+    /**
+     * @brief Returns a view that is filtered by the underlying storage.
+     * @tparam Get Types of elements used to construct the view.
+     * @tparam Exclude Types of elements used to filter the view.
+     * @return A newly created view.
+     */
+    template<typename... Get, typename... Exclude>
+    [[nodiscard]] basic_view<get_t<const basic_reactive_mixin, typename basic_registry_type::template storage_for_type<const Get>...>, exclude_t<typename basic_registry_type::template storage_for_type<const Exclude>...>>
+    view(exclude_t<Exclude...> = exclude_t{}) const {
+        const owner_type &parent = owner_or_assert();
+        basic_view<get_t<const basic_reactive_mixin, typename basic_registry_type::template storage_for_type<const Get>...>, exclude_t<typename basic_registry_type::template storage_for_type<const Exclude>...>> elem{};
+        [&elem](const auto *...curr) { ((curr ? elem.storage(*curr) : void()), ...); }(parent.template storage<stl::remove_const_t<Exclude>>()..., parent.template storage<stl::remove_const_t<Get>>()..., this);
+        return elem;
+    }
+
+    /*! @copydoc view */
+    template<typename... Get, typename... Exclude>
+    [[nodiscard]] basic_view<get_t<const basic_reactive_mixin, typename basic_registry_type::template storage_for_type<Get>...>, exclude_t<typename basic_registry_type::template storage_for_type<Exclude>...>>
+    view(exclude_t<Exclude...> = exclude_t{}) {
+        stl::conditional_t<((stl::is_const_v<Get> && ...) && (stl::is_const_v<Exclude> && ...)), const owner_type, owner_type> &parent = owner_or_assert();
+        return {*this, parent.template storage<stl::remove_const_t<Get>>()..., parent.template storage<stl::remove_const_t<Exclude>>()...};
+    }
+
+    /*! @brief Releases all connections to the underlying registry, if any. */
+    void reset() {
+        for(auto &&curr: conn) {
+            curr.release();
+        }
+
+        conn.clear();
+    }
+
+private:
+    basic_registry_type *owner;
+    container_type conn;
+};
+
+} // namespace entt
+
+#endif

include/entt/entity/organizer.hpp

@@ -0,0 +1,437 @@
+#ifndef ENTT_ENTITY_ORGANIZER_HPP
+#define ENTT_ENTITY_ORGANIZER_HPP
+
+#include "../core/type_info.hpp"
+#include "../core/type_traits.hpp"
+#include "../core/utility.hpp"
+#include "../graph/adjacency_matrix.hpp"
+#include "../graph/flow.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+#include "fwd.hpp"
+#include "helper.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename>
+struct is_view: stl::false_type {};
+
+template<typename... Args>
+struct is_view<basic_view<Args...>>: stl::true_type {};
+
+template<typename Type>
+inline constexpr bool is_view_v = is_view<Type>::value;
+
+template<typename>
+struct is_group: stl::false_type {};
+
+template<typename... Args>
+struct is_group<basic_group<Args...>>: stl::true_type {};
+
+template<typename Type>
+inline constexpr bool is_group_v = is_group<Type>::value;
+
+template<typename Type, typename Override>
+struct unpack_type {
+    using ro = stl::conditional_t<
+        type_list_contains_v<Override, const Type> || (stl::is_const_v<Type> && !type_list_contains_v<Override, stl::remove_const_t<Type>>),
+        type_list<stl::remove_const_t<Type>>,
+        type_list<>>;
+
+    using rw = stl::conditional_t<
+        type_list_contains_v<Override, stl::remove_const_t<Type>> || (!stl::is_const_v<Type> && !type_list_contains_v<Override, const Type>),
+        type_list<Type>,
+        type_list<>>;
+};
+
+template<typename... Args, typename... Override>
+struct unpack_type<basic_registry<Args...>, type_list<Override...>> {
+    using ro = type_list<>;
+    using rw = type_list<>;
+};
+
+template<typename... Args, typename... Override>
+struct unpack_type<const basic_registry<Args...>, type_list<Override...>>
+    : unpack_type<basic_registry<Args...>, type_list<Override...>> {};
+
+template<typename... Get, typename... Exclude, typename... Override>
+struct unpack_type<basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {
+    using ro = type_list_cat_t<type_list<typename Exclude::element_type...>, typename unpack_type<constness_as_t<typename Get::element_type, Get>, type_list<Override...>>::ro...>;
+    using rw = type_list_cat_t<typename unpack_type<constness_as_t<typename Get::element_type, Get>, type_list<Override...>>::rw...>;
+};
+
+template<typename... Get, typename... Exclude, typename... Override>
+struct unpack_type<const basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>>
+    : unpack_type<basic_view<get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {};
+
+template<typename... Owned, typename... Get, typename... Exclude, typename... Override>
+struct unpack_type<basic_group<owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {
+    using ro = type_list_cat_t<type_list<typename Exclude::element_type...>, typename unpack_type<constness_as_t<typename Get::element_type, Get>, type_list<Override...>>::ro..., typename unpack_type<constness_as_t<typename Owned::element_type, Owned>, type_list<Override...>>::ro...>;
+    using rw = type_list_cat_t<typename unpack_type<constness_as_t<typename Get::element_type, Get>, type_list<Override...>>::rw..., typename unpack_type<constness_as_t<typename Owned::element_type, Owned>, type_list<Override...>>::rw...>;
+};
+
+template<typename... Owned, typename... Get, typename... Exclude, typename... Override>
+struct unpack_type<const basic_group<owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>>
+    : unpack_type<basic_group<owned_t<Owned...>, get_t<Get...>, exclude_t<Exclude...>>, type_list<Override...>> {};
+
+template<typename, typename, typename>
+struct resource_traits;
+
+template<typename Registry, typename... Args, typename... Req>
+struct resource_traits<Registry, type_list<Args...>, type_list<Req...>> {
+    using args = type_list<stl::remove_const_t<Args>...>;
+    using ro = type_list_cat_t<typename unpack_type<Args, type_list<Req...>>::ro..., typename unpack_type<Req, type_list<>>::ro...>;
+    using rw = type_list_cat_t<typename unpack_type<Args, type_list<Req...>>::rw..., typename unpack_type<Req, type_list<>>::rw...>;
+    static constexpr auto sync_point = (stl::is_same_v<Args, Registry> || ...);
+};
+
+template<typename Registry, typename... Req, typename Ret, typename... Args>
+resource_traits<Registry, type_list<stl::remove_reference_t<Args>...>, type_list<Req...>> free_function_to_resource_traits(Ret (*)(Args...));
+
+template<typename Registry, typename... Req, typename Ret, typename Type, typename... Args>
+resource_traits<Registry, type_list<stl::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (*)(Type &, Args...));
+
+template<typename Registry, typename... Req, typename Ret, typename Class, typename... Args>
+resource_traits<Registry, type_list<stl::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (Class::*)(Args...));
+
+template<typename Registry, typename... Req, typename Ret, typename Class, typename... Args>
+resource_traits<Registry, type_list<stl::remove_reference_t<Args>...>, type_list<Req...>> constrained_function_to_resource_traits(Ret (Class::*)(Args...) const);
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Utility class for creating a static task graph.
+ *
+ * This class offers minimal support (but sufficient in many cases) for creating
+ * an execution graph from functions and their requirements on resources.<br/>
+ * Note that the resulting tasks aren't executed in any case. This isn't the
+ * goal of the tool. Instead, they are returned to the user in the form of a
+ * graph that allows for safe execution.
+ *
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class basic_organizer final {
+    using callback_type = void(const void *, Registry &);
+    using prepare_type = void(Registry &);
+    using dependency_type = stl::size_t(const bool, const type_info **, const stl::size_t);
+
+    struct vertex_data final {
+        stl::size_t ro_count{};
+        stl::size_t rw_count{};
+        const char *name{};
+        const void *payload{};
+        callback_type *callback{};
+        dependency_type *dependency{};
+        prepare_type *prepare{};
+        const type_info *info{};
+    };
+
+    template<typename Type>
+    [[nodiscard]] static decltype(auto) extract(Registry &reg) {
+        if constexpr(stl::is_same_v<Type, Registry>) {
+            return reg;
+        } else if constexpr(internal::is_view_v<Type>) {
+            return static_cast<Type>(as_view{reg});
+        } else if constexpr(internal::is_group_v<Type>) {
+            return static_cast<Type>(as_group{reg});
+        } else {
+            return reg.ctx().template emplace<stl::remove_reference_t<Type>>();
+        }
+    }
+
+    template<typename... Args>
+    [[nodiscard]] static auto to_args(Registry &reg, type_list<Args...>) {
+        return stl::tuple<decltype(extract<Args>(reg))...>(extract<Args>(reg)...);
+    }
+
+    template<typename... Type>
+    [[nodiscard]] static stl::size_t fill_dependencies(type_list<Type...>, [[maybe_unused]] const type_info **buffer, [[maybe_unused]] const stl::size_t count) {
+        if constexpr(sizeof...(Type) == 0u) {
+            return {};
+        } else {
+            // NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+            const type_info *info[]{&type_id<Type>()...};
+            const auto length = count < sizeof...(Type) ? count : sizeof...(Type);
+
+            for(stl::size_t pos{}; pos < length; ++pos) {
+                // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+                buffer[pos] = info[pos];
+            }
+
+            return length;
+        }
+    }
+
+    template<typename... RO, typename... RW>
+    void track_dependencies(stl::size_t index, const bool sync_point, type_list<RO...>, type_list<RW...>) {
+        builder.bind(static_cast<id_type>(index));
+        builder.set(type_hash<Registry>::value(), sync_point || (sizeof...(RO) + sizeof...(RW) == 0u));
+        (builder.ro(type_hash<RO>::value()), ...);
+        (builder.rw(type_hash<RW>::value()), ...);
+    }
+
+public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = registry_type::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+    /*! @brief Raw task function type. */
+    using function_type = callback_type;
+
+    /*! @brief Vertex type of a task graph defined as an adjacency list. */
+    struct vertex {
+        /**
+         * @brief Constructs a vertex of the task graph.
+         * @param data The data associated with the vertex.
+         * @param from List of in-edges of the vertex.
+         * @param to List of out-edges of the vertex.
+         */
+        vertex(vertex_data data, stl::vector<stl::size_t> from, stl::vector<stl::size_t> to)
+            : node{stl::move(data)},
+              in{stl::move(from)},
+              out{stl::move(to)} {}
+
+        /**
+         * @brief Fills a buffer with the type info objects for the writable
+         * resources of a vertex.
+         * @param buffer A buffer pre-allocated by the user.
+         * @param length The length of the user-supplied buffer.
+         * @return The number of type info objects written to the buffer.
+         */
+        [[nodiscard]] size_type ro_dependency(const type_info **buffer, const stl::size_t length) const noexcept {
+            return node.dependency(false, buffer, length);
+        }
+
+        /**
+         * @brief Fills a buffer with the type info objects for the read-only
+         * resources of a vertex.
+         * @param buffer A buffer pre-allocated by the user.
+         * @param length The length of the user-supplied buffer.
+         * @return The number of type info objects written to the buffer.
+         */
+        [[nodiscard]] size_type rw_dependency(const type_info **buffer, const stl::size_t length) const noexcept {
+            return node.dependency(true, buffer, length);
+        }
+
+        /**
+         * @brief Returns the number of read-only resources of a vertex.
+         * @return The number of read-only resources of the vertex.
+         */
+        [[nodiscard]] size_type ro_count() const noexcept {
+            return node.ro_count;
+        }
+
+        /**
+         * @brief Returns the number of writable resources of a vertex.
+         * @return The number of writable resources of the vertex.
+         */
+        [[nodiscard]] size_type rw_count() const noexcept {
+            return node.rw_count;
+        }
+
+        /**
+         * @brief Checks if a vertex is also a top-level one.
+         * @return True if the vertex is a top-level one, false otherwise.
+         */
+        [[nodiscard]] bool top_level() const noexcept {
+            return in.empty();
+        }
+
+        /**
+         * @brief Returns a type info object associated with a vertex.
+         * @return A properly initialized type info object.
+         */
+        [[nodiscard]] const type_info &info() const noexcept {
+            return *node.info;
+        }
+
+        /**
+         * @brief Returns a user defined name associated with a vertex, if any.
+         * @return The user defined name associated with the vertex, if any.
+         */
+        [[nodiscard]] const char *name() const noexcept {
+            return node.name;
+        }
+
+        /**
+         * @brief Returns the function associated with a vertex.
+         * @return The function associated with the vertex.
+         */
+        [[nodiscard]] function_type *callback() const noexcept {
+            return node.callback;
+        }
+
+        /**
+         * @brief Returns the payload associated with a vertex, if any.
+         * @return The payload associated with the vertex, if any.
+         */
+        [[nodiscard]] const void *data() const noexcept {
+            return node.payload;
+        }
+
+        /**
+         * @brief Returns the list of in-edges of a vertex.
+         * @return The list of in-edges of a vertex.
+         */
+        [[nodiscard]] const stl::vector<stl::size_t> &in_edges() const noexcept {
+            return in;
+        }
+
+        /**
+         * @brief Returns the list of out-edges of a vertex.
+         * @return The list of out-edges of a vertex.
+         */
+        [[nodiscard]] const stl::vector<stl::size_t> &out_edges() const noexcept {
+            return out;
+        }
+
+        /**
+         * @brief Prepares a registry and assures that all required resources
+         * are properly instantiated before using them.
+         * @param reg A valid registry.
+         */
+        void prepare(registry_type &reg) const {
+            node.prepare ? node.prepare(reg) : void();
+        }
+
+    private:
+        vertex_data node;
+        stl::vector<stl::size_t> in;
+        stl::vector<stl::size_t> out;
+    };
+
+    /**
+     * @brief Adds a free function to the task list.
+     * @tparam Candidate Function to add to the task list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @param name Optional name to associate with the task.
+     */
+    template<auto Candidate, typename... Req>
+    void emplace(const char *name = nullptr) {
+        using resource_type = decltype(internal::free_function_to_resource_traits<registry_type, Req...>(Candidate));
+
+        callback_type *callback = +[](const void *, registry_type &reg) {
+            stl::apply(Candidate, to_args(reg, typename resource_type::args{}));
+        };
+
+        vertex_data vdata{
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            nullptr,
+            callback,
+            +[](const bool rw, const type_info **buffer, const stl::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
+            +[](registry_type &reg) { void(to_args(reg, typename resource_type::args{})); },
+            &type_id<stl::integral_constant<decltype(Candidate), Candidate>>()};
+
+        track_dependencies(vertices.size(), resource_type::sync_point, typename resource_type::ro{}, typename resource_type::rw{});
+        vertices.push_back(stl::move(vdata));
+    }
+
+    /**
+     * @brief Adds a free function with payload or a member function with an
+     * instance to the task list.
+     * @tparam Candidate Function or member to add to the task list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     * @param name Optional name to associate with the task.
+     */
+    template<auto Candidate, typename... Req, typename Type>
+    void emplace(Type &value_or_instance, const char *name = nullptr) {
+        using resource_type = decltype(internal::constrained_function_to_resource_traits<registry_type, Req...>(Candidate));
+
+        callback_type *callback = +[](const void *payload, registry_type &reg) {
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            stl::apply(Candidate, stl::tuple_cat(stl::forward_as_tuple(*curr), to_args(reg, typename resource_type::args{})));
+        };
+
+        vertex_data vdata{
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            &value_or_instance,
+            callback,
+            +[](const bool rw, const type_info **buffer, const stl::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
+            +[](registry_type &reg) { void(to_args(reg, typename resource_type::args{})); },
+            &type_id<stl::integral_constant<decltype(Candidate), Candidate>>()};
+
+        track_dependencies(vertices.size(), resource_type::sync_point, typename resource_type::ro{}, typename resource_type::rw{});
+        vertices.push_back(stl::move(vdata));
+    }
+
+    /**
+     * @brief Adds an user defined function with optional payload to the task
+     * list.
+     * @tparam Req Additional requirements and/or override resource access mode.
+     * @param func Function to add to the task list.
+     * @param payload User defined arbitrary data.
+     * @param name Optional name to associate with the task.
+     */
+    template<typename... Req>
+    void emplace(function_type *func, const void *payload = nullptr, const char *name = nullptr) {
+        using resource_type = internal::resource_traits<registry_type, type_list<>, type_list<Req...>>;
+        track_dependencies(vertices.size(), true, typename resource_type::ro{}, typename resource_type::rw{});
+
+        vertex_data vdata{
+            resource_type::ro::size,
+            resource_type::rw::size,
+            name,
+            payload,
+            func,
+            +[](const bool rw, const type_info **buffer, const stl::size_t length) { return rw ? fill_dependencies(typename resource_type::rw{}, buffer, length) : fill_dependencies(typename resource_type::ro{}, buffer, length); },
+            nullptr,
+            &type_id<void>()};
+
+        vertices.push_back(stl::move(vdata));
+    }
+
+    /**
+     * @brief Generates a task graph for the current content.
+     * @return The adjacency list of the task graph.
+     */
+    [[nodiscard]] stl::vector<vertex> graph() const {
+        stl::vector<vertex> adjacency_list{};
+        adjacency_list.reserve(vertices.size());
+
+        for(auto adjacency_matrix = builder.graph(); auto curr: adjacency_matrix.vertices()) {
+            stl::vector<stl::size_t> in{};
+            stl::vector<stl::size_t> out{};
+
+            for(auto &&edge: adjacency_matrix.in_edges(curr)) {
+                in.push_back(edge.first);
+            }
+
+            for(auto &&edge: adjacency_matrix.out_edges(curr)) {
+                out.push_back(edge.second);
+            }
+
+            adjacency_list.emplace_back(vertices[curr], stl::move(in), stl::move(out));
+        }
+
+        return adjacency_list;
+    }
+
+    /*! @brief Erases all elements from a container. */
+    void clear() {
+        builder.clear();
+        vertices.clear();
+    }
+
+private:
+    stl::vector<vertex_data> vertices;
+    flow builder;
+};
+
+} // namespace entt
+
+#endif

include/entt/entity/ranges.hpp

@@ -0,0 +1,28 @@
+#ifndef ENTT_ENTITY_RANGES_HPP
+#define ENTT_ENTITY_RANGES_HPP
+
+#include <version>
+
+#if defined(__cpp_lib_ranges)
+#    include <ranges>
+#    include "fwd.hpp"
+
+namespace std::ranges {
+
+template<class... Args>
+inline constexpr bool enable_borrowed_range<entt::basic_view<Args...>>{true};
+
+template<class... Args>
+inline constexpr bool enable_borrowed_range<entt::basic_group<Args...>>{true};
+
+template<class... Args>
+inline constexpr bool enable_view<entt::basic_view<Args...>>{true};
+
+template<class... Args>
+inline constexpr bool enable_view<entt::basic_group<Args...>>{true};
+
+} // namespace std::ranges
+
+#endif
+
+#endif

include/entt/entity/runtime_view.hpp

@@ -0,0 +1,322 @@
+#ifndef ENTT_ENTITY_RUNTIME_VIEW_HPP
+#define ENTT_ENTITY_RUNTIME_VIEW_HPP
+
+#include "../stl/algorithm.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename Set>
+class runtime_view_iterator final {
+    using iterator_type = Set::iterator;
+    using iterator_traits = stl::iterator_traits<iterator_type>;
+
+    [[nodiscard]] bool valid() const {
+        return (!tombstone_check || *it != tombstone)
+               && stl::all_of(++pools->begin(), pools->end(), [entt = *it](const auto *curr) { return curr->contains(entt); })
+               && stl::none_of(filter->cbegin(), filter->cend(), [entt = *it](const auto *curr) { return curr && curr->contains(entt); });
+    }
+
+public:
+    using value_type = iterator_traits::value_type;
+    using pointer = iterator_traits::pointer;
+    using reference = iterator_traits::reference;
+    using difference_type = iterator_traits::difference_type;
+    using iterator_category = stl::bidirectional_iterator_tag;
+
+    constexpr runtime_view_iterator() noexcept
+        : pools{},
+          filter{},
+          it{},
+          tombstone_check{} {}
+
+    runtime_view_iterator(const stl::vector<Set *> &cpools, iterator_type curr, const stl::vector<Set *> &ignore) noexcept
+        : pools{&cpools},
+          filter{&ignore},
+          it{curr},
+          tombstone_check{pools->size() == 1u && (*pools)[0u]->policy() == deletion_policy::in_place} {
+        if(it != (*pools)[0]->end() && !valid()) {
+            ++(*this);
+        }
+    }
+
+    runtime_view_iterator &operator++() {
+        ++it;
+        for(const auto last = (*pools)[0]->end(); it != last && !valid(); ++it) {}
+        return *this;
+    }
+
+    runtime_view_iterator operator++(int) {
+        const runtime_view_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    runtime_view_iterator &operator--() {
+        --it;
+        for(const auto first = (*pools)[0]->begin(); it != first && !valid(); --it) {}
+        return *this;
+    }
+
+    runtime_view_iterator operator--(int) {
+        const runtime_view_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    [[nodiscard]] pointer operator->() const noexcept {
+        return it.operator->();
+    }
+
+    [[nodiscard]] reference operator*() const noexcept {
+        return *operator->();
+    }
+
+    [[nodiscard]] constexpr bool operator==(const runtime_view_iterator &other) const noexcept {
+        return it == other.it;
+    }
+
+private:
+    const stl::vector<Set *> *pools;
+    const stl::vector<Set *> *filter;
+    iterator_type it;
+    bool tombstone_check;
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Generic runtime view.
+ *
+ * Runtime views iterate over those entities that are at least in the given
+ * storage. During initialization, a runtime view looks at the number of
+ * entities available for each element and uses the smallest set in order to get
+ * a performance boost when iterating.
+ *
+ * @b Important
+ *
+ * Iterators aren't invalidated if:
+ *
+ * * New elements are added to the storage.
+ * * The entity currently pointed is modified (for example, elements are added
+ *   or removed from it).
+ * * The entity currently pointed is destroyed.
+ *
+ * In all other cases, modifying the storage iterated by the view in any way
+ * invalidates all the iterators.
+ *
+ * @tparam Type Common base type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Allocator>
+class basic_runtime_view {
+    using alloc_traits = stl::allocator_traits<Allocator>;
+    static_assert(stl::is_same_v<typename alloc_traits::value_type, Type *>, "Invalid value type");
+    using container_type = stl::vector<Type *, Allocator>;
+
+    [[nodiscard]] auto offset() const noexcept {
+        ENTT_ASSERT(!pools.empty(), "Invalid view");
+        const auto &leading = *pools.front();
+        return (leading.policy() == deletion_policy::swap_only) ? leading.free_list() : leading.size();
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = Type::entity_type;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+    /*! @brief Signed integer type. */
+    using difference_type = stl::ptrdiff_t;
+    /*! @brief Common type among all storage types. */
+    using common_type = Type;
+    /*! @brief Bidirectional iterator type. */
+    using iterator = internal::runtime_view_iterator<common_type>;
+
+    /*! @brief Default constructor to use to create empty, invalid views. */
+    basic_runtime_view() noexcept
+        : basic_runtime_view{allocator_type{}} {}
+
+    /**
+     * @brief Constructs an empty, invalid view with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_runtime_view(const allocator_type &allocator)
+        : pools{allocator},
+          filter{allocator} {}
+
+    /*! @brief Default copy constructor. */
+    basic_runtime_view(const basic_runtime_view &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    basic_runtime_view(const basic_runtime_view &other, const allocator_type &allocator)
+        : pools{other.pools, allocator},
+          filter{other.filter, allocator} {}
+
+    /*! @brief Default move constructor. */
+    basic_runtime_view(basic_runtime_view &&) noexcept = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_runtime_view(basic_runtime_view &&other, const allocator_type &allocator)
+        : pools{stl::move(other.pools), allocator},
+          filter{stl::move(other.filter), allocator} {}
+
+    /*! @brief Default destructor. */
+    ~basic_runtime_view() = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This runtime view.
+     */
+    basic_runtime_view &operator=(const basic_runtime_view &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This runtime view.
+     */
+    basic_runtime_view &operator=(basic_runtime_view &&) noexcept = default;
+
+    /**
+     * @brief Exchanges the contents with those of a given view.
+     * @param other View to exchange the content with.
+     */
+    void swap(basic_runtime_view &other) noexcept {
+        using stl::swap;
+        swap(pools, other.pools);
+        swap(filter, other.filter);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return pools.get_allocator();
+    }
+
+    /*! @brief Clears the view. */
+    void clear() {
+        pools.clear();
+        filter.clear();
+    }
+
+    /**
+     * @brief Appends an opaque storage object to a runtime view.
+     * @param base An opaque reference to a storage object.
+     * @return This runtime view.
+     */
+    basic_runtime_view &iterate(common_type &base) {
+        if(pools.empty() || !(base.size() < pools.front()->size())) {
+            pools.push_back(&base);
+        } else {
+            pools.push_back(stl::exchange(pools.front(), &base));
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Adds an opaque storage object as a filter of a runtime view.
+     * @param base An opaque reference to a storage object.
+     * @return This runtime view.
+     */
+    basic_runtime_view &exclude(common_type &base) {
+        filter.push_back(&base);
+        return *this;
+    }
+
+    /**
+     * @brief Estimates the number of entities iterated by the view.
+     * @return Estimated number of entities iterated by the view.
+     */
+    [[nodiscard]] size_type size_hint() const {
+        return pools.empty() ? size_type{} : offset();
+    }
+
+    /**
+     * @brief Returns an iterator to the first entity that has the given
+     * elements.
+     *
+     * If the view is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first entity that has the given elements.
+     */
+    [[nodiscard]] iterator begin() const {
+        return pools.empty() ? iterator{} : iterator{pools, pools.front()->end() - static_cast<difference_type>(offset()), filter};
+    }
+
+    /**
+     * @brief Returns an iterator that is past the last entity that has the
+     * given elements.
+     * @return An iterator to the entity following the last entity that has the
+     * given elements.
+     */
+    [[nodiscard]] iterator end() const {
+        return pools.empty() ? iterator{} : iterator{pools, pools.front()->end(), filter};
+    }
+
+    /**
+     * @brief Checks whether a view is initialized or not.
+     * @return True if the view is initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return !(pools.empty() && filter.empty());
+    }
+
+    /**
+     * @brief Checks if a view contains an entity.
+     * @param entt A valid identifier.
+     * @return True if the view contains the given entity, false otherwise.
+     */
+    [[nodiscard]] bool contains(const entity_type entt) const {
+        return !pools.empty()
+               && stl::all_of(pools.cbegin(), pools.cend(), [entt](const auto *curr) { return curr->contains(entt); })
+               && stl::none_of(filter.cbegin(), filter.cend(), [entt](const auto *curr) { return curr && curr->contains(entt); })
+               && pools.front()->index(entt) < offset();
+    }
+
+    /**
+     * @brief Iterates entities and applies the given function object to them.
+     *
+     * The function object is invoked for each entity. It is provided only with
+     * the entity itself.<br/>
+     * The signature of the function should be equivalent to the following:
+     *
+     * @code{.cpp}
+     * void(const entity_type);
+     * @endcode
+     *
+     * @tparam Func Type of the function object to invoke.
+     * @param func A valid function object.
+     */
+    template<typename Func>
+    void each(Func func) const {
+        for(const auto entity: *this) {
+            func(entity);
+        }
+    }
+
+private:
+    container_type pools;
+    container_type filter;
+};
+
+} // namespace entt
+
+#endif

include/entt/entity/snapshot.hpp

@@ -0,0 +1,509 @@
+#ifndef ENTT_ENTITY_SNAPSHOT_HPP
+#define ENTT_ENTITY_SNAPSHOT_HPP
+
+#include "../config/config.h"
+#include "../container/dense_map.hpp"
+#include "../core/type_traits.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/tuple.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "entity.hpp"
+#include "fwd.hpp"
+#include "view.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename Registry>
+void orphans(Registry &registry) {
+    for(auto &storage = registry.template storage<typename Registry::entity_type>(); auto entt: storage) {
+        if(registry.orphan(entt)) {
+            storage.erase(entt);
+        }
+    }
+}
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Utility class to create snapshots from a registry.
+ *
+ * A _snapshot_ can be either a dump of the entire registry or a narrower
+ * selection of elements of interest.<br/>
+ * This type can be used in both cases if provided with a correctly configured
+ * output archive.
+ *
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class basic_snapshot {
+    static_assert(!stl::is_const_v<Registry>, "Non-const registry type required");
+    using traits_type = entt_traits<typename Registry::entity_type>;
+
+public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = registry_type::entity_type;
+
+    /**
+     * @brief Constructs an instance that is bound to a given registry.
+     * @param source A valid reference to a registry.
+     */
+    basic_snapshot(const registry_type &source) noexcept
+        : reg{&source} {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_snapshot(const basic_snapshot &) = delete;
+
+    /*! @brief Default move constructor. */
+    basic_snapshot(basic_snapshot &&) noexcept = default;
+
+    /*! @brief Default destructor. */
+    ~basic_snapshot() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This snapshot.
+     */
+    basic_snapshot &operator=(const basic_snapshot &) = delete;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This snapshot.
+     */
+    basic_snapshot &operator=(basic_snapshot &&) noexcept = default;
+
+    /**
+     * @brief Serializes all elements of a type with associated identifiers.
+     * @tparam Type Type of elements to serialize.
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @param id Optional name used to map the storage within the registry.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename Type, typename Archive>
+    const basic_snapshot &get(Archive &archive, const id_type id = type_hash<Type>::value()) const {
+        if(const auto *storage = reg->template storage<Type>(id); storage) {
+            const typename registry_type::common_type &base = *storage;
+
+            archive(static_cast<traits_type::entity_type>(storage->size()));
+
+            if constexpr(stl::is_same_v<Type, entity_type>) {
+                archive(static_cast<traits_type::entity_type>(storage->free_list()));
+
+                for(auto first = base.rbegin(), last = base.rend(); first != last; ++first) {
+                    archive(*first);
+                }
+            } else if constexpr(registry_type::template storage_for_type<Type>::storage_policy == deletion_policy::in_place) {
+                for(auto it = base.rbegin(), last = base.rend(); it != last; ++it) {
+                    if(const auto entt = *it; entt == tombstone) {
+                        archive(static_cast<entity_type>(null));
+                    } else {
+                        archive(entt);
+                        stl::apply([&archive](auto &&...args) { (archive(stl::forward<decltype(args)>(args)), ...); }, storage->get_as_tuple(entt));
+                    }
+                }
+            } else {
+                for(auto elem: storage->reach()) {
+                    stl::apply([&archive](auto &&...args) { (archive(stl::forward<decltype(args)>(args)), ...); }, elem);
+                }
+            }
+        } else {
+            archive(typename traits_type::entity_type{});
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Serializes all elements of a type with associated identifiers for
+     * the entities in a range.
+     * @tparam Type Type of elements to serialize.
+     * @tparam Archive Type of output archive.
+     * @param archive A valid reference to an output archive.
+     * @param first An iterator to the first element of the range to serialize.
+     * @param last An iterator past the last element of the range to serialize.
+     * @param id Optional name used to map the storage within the registry.
+     * @return An object of this type to continue creating the snapshot.
+     */
+    template<typename Type, typename Archive>
+    const basic_snapshot &get(Archive &archive, stl::input_iterator auto first, stl::input_iterator auto last, const id_type id = type_hash<Type>::value()) const {
+        static_assert(!stl::is_same_v<Type, entity_type>, "Entity types not supported");
+
+        if(const auto *storage = reg->template storage<Type>(id); storage && !storage->empty()) {
+            archive(static_cast<traits_type::entity_type>(stl::distance(first, last)));
+
+            for(; first != last; ++first) {
+                if(const auto entt = *first; storage->contains(entt)) {
+                    archive(entt);
+                    stl::apply([&archive](auto &&...args) { (archive(stl::forward<decltype(args)>(args)), ...); }, storage->get_as_tuple(entt));
+                } else {
+                    archive(static_cast<entity_type>(null));
+                }
+            }
+        } else {
+            archive(typename traits_type::entity_type{});
+        }
+
+        return *this;
+    }
+
+private:
+    const registry_type *reg;
+};
+
+/**
+ * @brief Utility class to restore a snapshot as a whole.
+ *
+ * A snapshot loader requires that the destination registry be empty and loads
+ * all the data at once while keeping intact the identifiers that the entities
+ * originally had.<br/>
+ * An example of use is the implementation of a save/restore utility.
+ *
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class basic_snapshot_loader {
+    static_assert(!stl::is_const_v<Registry>, "Non-const registry type required");
+    using traits_type = entt_traits<typename Registry::entity_type>;
+
+public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = registry_type::entity_type;
+
+    /**
+     * @brief Constructs an instance that is bound to a given registry.
+     * @param source A valid reference to a registry.
+     */
+    basic_snapshot_loader(registry_type &source) noexcept
+        : reg{&source} {
+        // restoring a snapshot as a whole requires a clean registry
+        ENTT_ASSERT(reg->template storage<entity_type>().free_list() == 0u, "Registry must be empty");
+    }
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_snapshot_loader(const basic_snapshot_loader &) = delete;
+
+    /*! @brief Default move constructor. */
+    basic_snapshot_loader(basic_snapshot_loader &&) noexcept = default;
+
+    /*! @brief Default destructor. */
+    ~basic_snapshot_loader() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This loader.
+     */
+    basic_snapshot_loader &operator=(const basic_snapshot_loader &) = delete;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This loader.
+     */
+    basic_snapshot_loader &operator=(basic_snapshot_loader &&) noexcept = default;
+
+    /**
+     * @brief Restores all elements of a type with associated identifiers.
+     * @tparam Type Type of elements to restore.
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @param id Optional name used to map the storage within the registry.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename Type, typename Archive>
+    basic_snapshot_loader &get(Archive &archive, const id_type id = type_hash<Type>::value()) {
+        auto &storage = reg->template storage<Type>(id);
+        typename traits_type::entity_type length{};
+
+        archive(length);
+
+        if constexpr(stl::is_same_v<Type, entity_type>) {
+            typename traits_type::entity_type count{};
+            entity_type placeholder{};
+
+            storage.reserve(length);
+            archive(count);
+
+            for(entity_type entity = null; length; --length) {
+                archive(entity);
+                storage.generate(entity);
+                placeholder = (entity > placeholder) ? entity : placeholder;
+            }
+
+            storage.start_from(traits_type::next(placeholder));
+            storage.free_list(count);
+        } else {
+            auto &other = reg->template storage<entity_type>();
+            entity_type entt{null};
+
+            while(length--) {
+                if(archive(entt); entt != null) {
+                    const auto entity = other.contains(entt) ? entt : other.generate(entt);
+                    ENTT_ASSERT(entity == entt, "Entity not available for use");
+
+                    if constexpr(stl::tuple_size_v<decltype(storage.get_as_tuple({}))> == 0u) {
+                        storage.emplace(entity);
+                    } else {
+                        Type elem{};
+                        archive(elem);
+                        storage.emplace(entity, stl::move(elem));
+                    }
+                }
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have no elements.
+     *
+     * In case all the entities were serialized but only part of the elements
+     * was saved, it could happen that some of the entities have no elements
+     * once restored.<br/>
+     * This function helps to identify and destroy those entities.
+     *
+     * @return A valid loader to continue restoring data.
+     */
+    basic_snapshot_loader &orphans() {
+        internal::orphans(*reg);
+        return *this;
+    }
+
+private:
+    registry_type *reg;
+};
+
+/**
+ * @brief Utility class for _continuous loading_.
+ *
+ * A _continuous loader_ is designed to load data from a source registry to a
+ * (possibly) non-empty destination. The loader can accommodate in a registry
+ * more than one snapshot in a sort of _continuous loading_ that updates the
+ * destination one step at a time.<br/>
+ * Identifiers that entities originally had are not transferred to the target.
+ * Instead, the loader maps remote identifiers to local ones while restoring a
+ * snapshot.<br/>
+ * An example of use is the implementation of a client-server application with
+ * the requirement of transferring somehow parts of the representation side to
+ * side.
+ *
+ * @tparam Registry Basic registry type.
+ */
+template<typename Registry>
+class basic_continuous_loader {
+    static_assert(!stl::is_const_v<Registry>, "Non-const registry type required");
+    using traits_type = entt_traits<typename Registry::entity_type>;
+
+    void restore(Registry::entity_type entt) {
+        if(const auto entity = to_entity(entt); remloc.contains(entity) && remloc[entity].first == entt) {
+            if(!reg->valid(remloc[entity].second)) {
+                remloc[entity].second = reg->create();
+            }
+        } else {
+            remloc.insert_or_assign(entity, stl::make_pair(entt, reg->create()));
+        }
+    }
+
+    template<typename Container>
+    auto update(int, Container &container) -> decltype(typename Container::mapped_type{}, void()) {
+        // map like container
+        Container other;
+
+        for(auto &&pair: container) {
+            using first_type = stl::remove_const_t<typename stl::decay_t<decltype(pair)>::first_type>;
+            using second_type = stl::decay_t<decltype(pair)>::second_type;
+
+            if constexpr(stl::is_same_v<first_type, entity_type> && stl::is_same_v<second_type, entity_type>) {
+                other.emplace(map(pair.first), map(pair.second));
+            } else if constexpr(stl::is_same_v<first_type, entity_type>) {
+                other.emplace(map(pair.first), stl::move(pair.second));
+            } else {
+                static_assert(stl::is_same_v<second_type, entity_type>, "Neither the key nor the value are of entity type");
+                other.emplace(stl::move(pair.first), map(pair.second));
+            }
+        }
+
+        using stl::swap;
+        swap(container, other);
+    }
+
+    template<typename Container>
+    auto update(char, Container &container) -> decltype(typename Container::value_type{}, void()) {
+        // vector like container
+        static_assert(stl::is_same_v<typename Container::value_type, entity_type>, "Invalid value type");
+
+        for(auto &&entt: container) {
+            entt = map(entt);
+        }
+    }
+
+    template<typename Component, typename Other, typename Member>
+    void update([[maybe_unused]] Component &instance, [[maybe_unused]] Member Other::*member) {
+        if constexpr(!stl::is_same_v<Component, Other>) {
+            return;
+        } else if constexpr(stl::is_same_v<Member, entity_type>) {
+            instance.*member = map(instance.*member);
+        } else {
+            // maybe a container? let's try...
+            update(0, instance.*member);
+        }
+    }
+
+public:
+    /*! Basic registry type. */
+    using registry_type = Registry;
+    /*! @brief Underlying entity identifier. */
+    using entity_type = registry_type::entity_type;
+
+    /**
+     * @brief Constructs an instance that is bound to a given registry.
+     * @param source A valid reference to a registry.
+     */
+    basic_continuous_loader(registry_type &source) noexcept
+        : remloc{source.get_allocator()},
+          reg{&source} {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_continuous_loader(const basic_continuous_loader &) = delete;
+
+    /*! @brief Default move constructor. */
+    basic_continuous_loader(basic_continuous_loader &&) noexcept = default;
+
+    /*! @brief Default destructor. */
+    ~basic_continuous_loader() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This loader.
+     */
+    basic_continuous_loader &operator=(const basic_continuous_loader &) = delete;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This loader.
+     */
+    basic_continuous_loader &operator=(basic_continuous_loader &&) noexcept = default;
+
+    /**
+     * @brief Restores all elements of a type with associated identifiers.
+     *
+     * It creates local counterparts for remote elements as needed.<br/>
+     * Members are either data members of type entity_type or containers of
+     * entities. In both cases, a loader visits them and replaces entities with
+     * their local counterpart.
+     *
+     * @tparam Type Type of elements to restore.
+     * @tparam Archive Type of input archive.
+     * @param archive A valid reference to an input archive.
+     * @param id Optional name used to map the storage within the registry.
+     * @return A valid loader to continue restoring data.
+     */
+    template<typename Type, typename Archive>
+    basic_continuous_loader &get(Archive &archive, const id_type id = type_hash<Type>::value()) {
+        auto &storage = reg->template storage<Type>(id);
+        typename traits_type::entity_type length{};
+        entity_type entt{null};
+
+        archive(length);
+
+        if constexpr(stl::is_same_v<Type, entity_type>) {
+            typename traits_type::entity_type in_use{};
+
+            storage.reserve(length);
+            archive(in_use);
+
+            for(stl::size_t pos{}; pos < in_use; ++pos) {
+                archive(entt);
+                restore(entt);
+            }
+
+            for(stl::size_t pos = in_use; pos < length; ++pos) {
+                archive(entt);
+
+                if(const auto entity = to_entity(entt); remloc.contains(entity)) {
+                    if(reg->valid(remloc[entity].second)) {
+                        reg->destroy(remloc[entity].second);
+                    }
+
+                    remloc.erase(entity);
+                }
+            }
+        } else {
+            for(auto &&ref: remloc) {
+                storage.remove(ref.second.second);
+            }
+
+            while(length--) {
+                if(archive(entt); entt != null) {
+                    restore(entt);
+
+                    if constexpr(stl::tuple_size_v<decltype(storage.get_as_tuple({}))> == 0u) {
+                        storage.emplace(map(entt));
+                    } else {
+                        Type elem{};
+                        archive(elem);
+                        storage.emplace(map(entt), stl::move(elem));
+                    }
+                }
+            }
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Destroys those entities that have no elements.
+     *
+     * In case all the entities were serialized but only part of the elements
+     * was saved, it could happen that some of the entities have no elements
+     * once restored.<br/>
+     * This function helps to identify and destroy those entities.
+     *
+     * @return A non-const reference to this loader.
+     */
+    basic_continuous_loader &orphans() {
+        internal::orphans(*reg);
+        return *this;
+    }
+
+    /**
+     * @brief Tests if a loader knows about a given entity.
+     * @param entt A valid identifier.
+     * @return True if `entity` is managed by the loader, false otherwise.
+     */
+    [[nodiscard]] bool contains(entity_type entt) const noexcept {
+        const auto it = remloc.find(to_entity(entt));
+        return it != remloc.cend() && it->second.first == entt;
+    }
+
+    /**
+     * @brief Returns the identifier to which an entity refers.
+     * @param entt A valid identifier.
+     * @return The local identifier if any, the null entity otherwise.
+     */
+    [[nodiscard]] entity_type map(entity_type entt) const noexcept {
+        if(const auto it = remloc.find(to_entity(entt)); it != remloc.cend() && it->second.first == entt) {
+            return it->second.second;
+        }
+
+        return null;
+    }
+
+private:
+    dense_map<typename traits_type::entity_type, stl::pair<entity_type, entity_type>> remloc;
+    registry_type *reg;
+};
+
+} // namespace entt
+
+#endif

include/entt/entt.hpp

@@ -0,0 +1,93 @@
+/*! @brief `EnTT` default namespace. */
+namespace entt {}
+
+/*! @brief Custom `EnTT` namespace for the standard template library. */
+namespace entt::stl {}
+
+// IWYU pragma: begin_exports
+#include "config/config.h"
+#include "config/macro.h"
+#include "config/version.h"
+#include "container/dense_map.hpp"
+#include "container/dense_set.hpp"
+#include "container/table.hpp"
+#include "core/algorithm.hpp"
+#include "core/any.hpp"
+#include "core/bit.hpp"
+#include "core/compressed_pair.hpp"
+#include "core/concepts.hpp"
+#include "core/enum.hpp"
+#include "core/family.hpp"
+#include "core/hashed_string.hpp"
+#include "core/ident.hpp"
+#include "core/iterator.hpp"
+#include "core/memory.hpp"
+#include "core/monostate.hpp"
+#include "core/ranges.hpp"
+#include "core/tuple.hpp"
+#include "core/type_info.hpp"
+#include "core/type_traits.hpp"
+#include "core/utility.hpp"
+#include "entity/component.hpp"
+#include "entity/entity.hpp"
+#include "entity/group.hpp"
+#include "entity/handle.hpp"
+#include "entity/helper.hpp"
+#include "entity/mixin.hpp"
+#include "entity/organizer.hpp"
+#include "entity/ranges.hpp"
+#include "entity/registry.hpp"
+#include "entity/runtime_view.hpp"
+#include "entity/snapshot.hpp"
+#include "entity/sparse_set.hpp"
+#include "entity/storage.hpp"
+#include "entity/view.hpp"
+#include "graph/adjacency_matrix.hpp"
+#include "graph/dot.hpp"
+#include "graph/flow.hpp"
+#include "locator/locator.hpp"
+#include "meta/adl_pointer.hpp"
+#include "meta/container.hpp"
+#include "meta/context.hpp"
+#include "meta/factory.hpp"
+#include "meta/meta.hpp"
+#include "meta/node.hpp"
+#include "meta/pointer.hpp"
+#include "meta/policy.hpp"
+#include "meta/range.hpp"
+#include "meta/resolve.hpp"
+#include "meta/template.hpp"
+#include "meta/type_traits.hpp"
+#include "meta/utility.hpp"
+#include "poly/poly.hpp"
+#include "process/process.hpp"
+#include "process/scheduler.hpp"
+#include "resource/cache.hpp"
+#include "resource/loader.hpp"
+#include "resource/resource.hpp"
+#include "signal/delegate.hpp"
+#include "signal/dispatcher.hpp"
+#include "signal/emitter.hpp"
+#include "signal/sigh.hpp"
+#include "stl/algorithm.hpp"
+#include "stl/array.hpp"
+#include "stl/atomic.hpp"
+#include "stl/bit.hpp"
+#include "stl/cmath.hpp"
+#include "stl/concepts.hpp"
+#include "stl/cstddef.hpp"
+#include "stl/cstdint.hpp"
+#include "stl/functional.hpp"
+#include "stl/ios.hpp"
+#include "stl/iterator.hpp"
+#include "stl/limits.hpp"
+#include "stl/memory.hpp"
+#include "stl/ostream.hpp"
+#include "stl/sstream.hpp"
+#include "stl/string.hpp"
+#include "stl/string_view.hpp"
+#include "stl/tuple.hpp"
+#include "stl/type_traits.hpp"
+#include "stl/utility.hpp"
+#include "stl/vector.hpp"
+// IWYU pragma: end_exports

include/entt/fwd.hpp

@@ -0,0 +1,11 @@
+// IWYU pragma: begin_exports
+#include "container/fwd.hpp"
+#include "core/fwd.hpp"
+#include "entity/fwd.hpp"
+#include "graph/fwd.hpp"
+#include "meta/fwd.hpp"
+#include "poly/fwd.hpp"
+#include "process/fwd.hpp"
+#include "resource/fwd.hpp"
+#include "signal/fwd.hpp"
+// IWYU pragma: end_exports

include/entt/graph/adjacency_matrix.hpp

@@ -0,0 +1,332 @@
+#ifndef ENTT_GRAPH_ADJACENCY_MATRIX_HPP
+#define ENTT_GRAPH_ADJACENCY_MATRIX_HPP
+
+#include "../config/config.h"
+#include "../core/iterator.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename It>
+class edge_iterator {
+    using size_type = stl::size_t;
+
+    void find_next() noexcept {
+        for(; pos != last && !it[static_cast<It::difference_type>(pos)]; pos += offset) {}
+    }
+
+public:
+    using value_type = stl::pair<size_type, size_type>;
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using difference_type = stl::ptrdiff_t;
+    using iterator_category = stl::input_iterator_tag;
+    using iterator_concept = stl::forward_iterator_tag;
+
+    constexpr edge_iterator() noexcept = default;
+
+    // NOLINTNEXTLINE(bugprone-easily-swappable-parameters)
+    constexpr edge_iterator(It base, const size_type vertices, const size_type from, const size_type to, const size_type step) noexcept
+        : it{stl::move(base)},
+          vert{vertices},
+          pos{from},
+          last{to},
+          offset{step} {
+        find_next();
+    }
+
+    constexpr edge_iterator &operator++() noexcept {
+        pos += offset;
+        find_next();
+        return *this;
+    }
+
+    constexpr edge_iterator operator++(int) noexcept {
+        const edge_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return *operator->();
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return stl::make_pair<size_type>(pos / vert, pos % vert);
+    }
+
+    [[nodiscard]] constexpr bool operator==(const edge_iterator &other) const noexcept {
+        return pos == other.pos;
+    }
+
+private:
+    It it{};
+    size_type vert{};
+    size_type pos{};
+    size_type last{};
+    size_type offset{};
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Basic implementation of a directed adjacency matrix.
+ * @tparam Category Either a directed or undirected category tag.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<stl::derived_from<directed_tag> Category, typename Allocator>
+class adjacency_matrix {
+    using alloc_traits = stl::allocator_traits<Allocator>;
+    static_assert(stl::is_same_v<typename alloc_traits::value_type, stl::size_t>, "Invalid value type");
+    using container_type = stl::vector<stl::size_t, typename alloc_traits::template rebind_alloc<stl::size_t>>;
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+    /*! @brief Vertex type. */
+    using vertex_type = size_type;
+    /*! @brief Edge type. */
+    using edge_type = stl::pair<vertex_type, vertex_type>;
+    /*! @brief Vertex iterator type. */
+    using vertex_iterator = iota_iterator<vertex_type>;
+    /*! @brief Edge iterator type. */
+    using edge_iterator = internal::edge_iterator<typename container_type::const_iterator>;
+    /*! @brief Out-edge iterator type. */
+    using out_edge_iterator = edge_iterator;
+    /*! @brief In-edge iterator type. */
+    using in_edge_iterator = edge_iterator;
+    /*! @brief Graph category tag. */
+    using graph_category = Category;
+
+    /*! @brief Default constructor. */
+    adjacency_matrix() noexcept(noexcept(allocator_type{}))
+        : adjacency_matrix{0u} {
+    }
+
+    /**
+     * @brief Constructs an empty container with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit adjacency_matrix(const allocator_type &allocator) noexcept
+        : adjacency_matrix{0u, allocator} {}
+
+    /**
+     * @brief Constructs an empty container with a given allocator and user
+     * supplied number of vertices.
+     * @param vertices Number of vertices.
+     * @param allocator The allocator to use.
+     */
+    adjacency_matrix(const size_type vertices, const allocator_type &allocator = allocator_type{})
+        : matrix{vertices * vertices, allocator},
+          vert{vertices} {}
+
+    /*! @brief Default copy constructor. */
+    adjacency_matrix(const adjacency_matrix &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    adjacency_matrix(const adjacency_matrix &other, const allocator_type &allocator)
+        : matrix{other.matrix, allocator},
+          vert{other.vert} {}
+
+    /*! @brief Default move constructor. */
+    adjacency_matrix(adjacency_matrix &&) noexcept = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    adjacency_matrix(adjacency_matrix &&other, const allocator_type &allocator)
+        : matrix{stl::move(other.matrix), allocator},
+          vert{other.vert} {}
+
+    /*! @brief Default destructor. */
+    ~adjacency_matrix() = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This container.
+     */
+    adjacency_matrix &operator=(const adjacency_matrix &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This container.
+     */
+    adjacency_matrix &operator=(adjacency_matrix &&) noexcept = default;
+
+    /**
+     * @brief Exchanges the contents with those of a given adjacency matrix.
+     * @param other Adjacency matrix to exchange the content with.
+     */
+    void swap(adjacency_matrix &other) noexcept {
+        using stl::swap;
+        swap(matrix, other.matrix);
+        swap(vert, other.vert);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return matrix.get_allocator();
+    }
+
+    /*! @brief Clears the adjacency matrix. */
+    void clear() noexcept {
+        matrix.clear();
+        vert = {};
+    }
+
+    /**
+     * @brief Returns true if an adjacency matrix is empty, false otherwise.
+     *
+     * @warning
+     * Potentially expensive, try to avoid it on hot paths.
+     *
+     * @return True if the adjacency matrix is empty, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        const auto iterable = edges();
+        return (iterable.begin() == iterable.end());
+    }
+
+    /**
+     * @brief Returns the number of vertices.
+     * @return The number of vertices.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return vert;
+    }
+
+    /**
+     * @brief Returns an iterable object to visit all vertices of a matrix.
+     * @return An iterable object to visit all vertices of a matrix.
+     */
+    [[nodiscard]] iterable_adaptor<vertex_iterator> vertices() const noexcept {
+        return {0u, vert};
+    }
+
+    /**
+     * @brief Returns an iterable object to visit all edges of a matrix.
+     * @return An iterable object to visit all edges of a matrix.
+     */
+    [[nodiscard]] iterable_adaptor<edge_iterator> edges() const noexcept {
+        const auto it = matrix.cbegin();
+        const auto sz = matrix.size();
+        return {{it, vert, 0u, sz, 1u}, {it, vert, sz, sz, 1u}};
+    }
+
+    /**
+     * @brief Returns an iterable object to visit all out-edges of a vertex.
+     * @param vertex The vertex of which to return all out-edges.
+     * @return An iterable object to visit all out-edges of a vertex.
+     */
+    [[nodiscard]] iterable_adaptor<out_edge_iterator> out_edges(const vertex_type vertex) const noexcept {
+        const auto it = matrix.cbegin();
+        const auto from = vertex * vert;
+        const auto to = from + vert;
+        return {{it, vert, from, to, 1u}, {it, vert, to, to, 1u}};
+    }
+
+    /**
+     * @brief Returns an iterable object to visit all in-edges of a vertex.
+     * @param vertex The vertex of which to return all in-edges.
+     * @return An iterable object to visit all in-edges of a vertex.
+     */
+    [[nodiscard]] iterable_adaptor<in_edge_iterator> in_edges(const vertex_type vertex) const noexcept {
+        const auto it = matrix.cbegin();
+        const auto from = vertex;
+        const auto to = vert * vert + from;
+        return {{it, vert, from, to, vert}, {it, vert, to, to, vert}};
+    }
+
+    /**
+     * @brief Resizes an adjacency matrix.
+     * @param vertices The new number of vertices.
+     */
+    void resize(const size_type vertices) {
+        adjacency_matrix other{vertices, get_allocator()};
+
+        for(auto [lhs, rhs]: edges()) {
+            other.insert(lhs, rhs);
+        }
+
+        other.swap(*this);
+    }
+
+    /**
+     * @brief Inserts an edge into the adjacency matrix, if it does not exist.
+     * @param lhs The left hand vertex of the edge.
+     * @param rhs The right hand vertex of the edge.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    stl::pair<edge_iterator, bool> insert(const vertex_type lhs, const vertex_type rhs) {
+        const auto pos = lhs * vert + rhs;
+
+        if constexpr(stl::is_same_v<graph_category, undirected_tag>) {
+            const auto rev = rhs * vert + lhs;
+            ENTT_ASSERT(matrix[pos] == matrix[rev], "Something went really wrong");
+            matrix[rev] = 1u;
+        }
+
+        const auto inserted = !stl::exchange(matrix[pos], 1u);
+        return {edge_iterator{matrix.cbegin(), vert, pos, matrix.size(), 1u}, inserted};
+    }
+
+    /**
+     * @brief Removes the edge associated with a pair of given vertices.
+     * @param lhs The left hand vertex of the edge.
+     * @param rhs The right hand vertex of the edge.
+     * @return Number of elements removed (either 0 or 1).
+     */
+    size_type erase(const vertex_type lhs, const vertex_type rhs) {
+        const auto pos = lhs * vert + rhs;
+
+        if constexpr(stl::is_same_v<graph_category, undirected_tag>) {
+            const auto rev = rhs * vert + lhs;
+            ENTT_ASSERT(matrix[pos] == matrix[rev], "Something went really wrong");
+            matrix[rev] = 0u;
+        }
+
+        return stl::exchange(matrix[pos], 0u);
+    }
+
+    /**
+     * @brief Checks if an adjacency matrix contains a given edge.
+     * @param lhs The left hand vertex of the edge.
+     * @param rhs The right hand vertex of the edge.
+     * @return True if there is such an edge, false otherwise.
+     */
+    [[nodiscard]] bool contains(const vertex_type lhs, const vertex_type rhs) const {
+        const auto pos = lhs * vert + rhs;
+        return pos < matrix.size() && matrix[pos];
+    }
+
+private:
+    container_type matrix;
+    size_type vert;
+};
+
+} // namespace entt
+
+#endif

include/entt/graph/dot.hpp

@@ -0,0 +1,56 @@
+#ifndef ENTT_GRAPH_DOT_HPP
+#define ENTT_GRAPH_DOT_HPP
+
+#include "../stl/concepts.hpp"
+#include "../stl/ostream.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Outputs a graph in dot format.
+ * @tparam Graph Graph type, valid as long as it exposes edges and vertices.
+ * @param out A standard output stream.
+ * @param graph The graph to output.
+ * @param writer Vertex decorator object.
+ */
+template<typename Graph>
+requires stl::derived_from<typename Graph::graph_category, directed_tag>
+void dot(stl::ostream &out, const Graph &graph, stl::invocable<stl::ostream &, typename Graph::vertex_type> auto writer) {
+    if constexpr(stl::same_as<typename Graph::graph_category, undirected_tag>) {
+        out << "graph{";
+    } else {
+        out << "digraph{";
+    }
+
+    for(auto &&vertex: graph.vertices()) {
+        out << vertex << "[";
+        writer(out, vertex);
+        out << "];";
+    }
+
+    for(auto [lhs, rhs]: graph.edges()) {
+        if constexpr(stl::same_as<typename Graph::graph_category, undirected_tag>) {
+            out << lhs << "--" << rhs << ";";
+        } else {
+            out << lhs << "->" << rhs << ";";
+        }
+    }
+
+    out << "}";
+}
+
+/**
+ * @brief Outputs a graph in dot format.
+ * @tparam Graph Graph type, valid as long as it exposes edges and vertices.
+ * @param out A standard output stream.
+ * @param graph The graph to output.
+ */
+template<typename Graph>
+void dot(stl::ostream &out, const Graph &graph) {
+    return dot(out, graph, [](auto &&...) {});
+}
+
+} // namespace entt
+
+#endif

include/entt/graph/flow.hpp

@@ -0,0 +1,345 @@
+#ifndef ENTT_GRAPH_FLOW_HPP
+#define ENTT_GRAPH_FLOW_HPP
+
+#include "../config/config.h"
+#include "../container/dense_map.hpp"
+#include "../container/dense_set.hpp"
+#include "../core/compressed_pair.hpp"
+#include "../core/fwd.hpp"
+#include "../core/iterator.hpp"
+#include "../stl/algorithm.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+#include "adjacency_matrix.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Utility class for creating task graphs.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Allocator>
+class basic_flow {
+    using alloc_traits = stl::allocator_traits<Allocator>;
+    static_assert(stl::is_same_v<typename alloc_traits::value_type, id_type>, "Invalid value type");
+    using task_container_type = dense_set<id_type, stl::identity, stl::equal_to<>, typename alloc_traits::template rebind_alloc<id_type>>;
+    using ro_rw_container_type = stl::vector<stl::pair<stl::size_t, bool>, typename alloc_traits::template rebind_alloc<stl::pair<stl::size_t, bool>>>;
+    using deps_container_type = dense_map<id_type, ro_rw_container_type, stl::identity, stl::equal_to<>, typename alloc_traits::template rebind_alloc<stl::pair<const id_type, ro_rw_container_type>>>;
+    using adjacency_matrix_type = adjacency_matrix<directed_tag, typename alloc_traits::template rebind_alloc<stl::size_t>>;
+
+    void emplace(const id_type res, const bool is_rw) {
+        ENTT_ASSERT(index.first() < vertices.size(), "Invalid node");
+
+        if(!deps.contains(res) && sync_on != vertices.size()) {
+            deps[res].emplace_back(sync_on, true);
+        }
+
+        deps[res].emplace_back(index.first(), is_rw);
+    }
+
+    void setup_graph(adjacency_matrix_type &matrix) const {
+        for(const auto &elem: deps) {
+            const auto last = elem.second.cend();
+            auto it = elem.second.cbegin();
+
+            while(it != last) {
+                if(it->second) {
+                    // rw item
+                    if(auto curr = it++; it != last) {
+                        if(it->second) {
+                            matrix.insert(curr->first, it->first);
+                        } else if(const auto next = stl::find_if(it, last, [](const auto &value) { return value.second; }); next != last) {
+                            for(; it != next; ++it) {
+                                matrix.insert(curr->first, it->first);
+                                matrix.insert(it->first, next->first);
+                            }
+                        } else {
+                            for(; it != next; ++it) {
+                                matrix.insert(curr->first, it->first);
+                            }
+                        }
+                    }
+                } else {
+                    // ro item (first iteration only)
+                    if(const auto next = stl::find_if(it, last, [](const auto &value) { return value.second; }); next != last) {
+                        for(; it != next; ++it) {
+                            matrix.insert(it->first, next->first);
+                        }
+                    } else {
+                        it = last;
+                    }
+                }
+            }
+        }
+    }
+
+    void transitive_closure(adjacency_matrix_type &matrix) const {
+        const auto length = matrix.size();
+
+        for(stl::size_t vk{}; vk < length; ++vk) {
+            for(stl::size_t vi{}; vi < length; ++vi) {
+                for(stl::size_t vj{}; vj < length; ++vj) {
+                    if(matrix.contains(vi, vk) && matrix.contains(vk, vj)) {
+                        matrix.insert(vi, vj);
+                    }
+                }
+            }
+        }
+    }
+
+    void transitive_reduction(adjacency_matrix_type &matrix) const {
+        const auto length = matrix.size();
+
+        for(stl::size_t vert{}; vert < length; ++vert) {
+            matrix.erase(vert, vert);
+        }
+
+        for(stl::size_t vj{}; vj < length; ++vj) {
+            for(stl::size_t vi{}; vi < length; ++vi) {
+                if(matrix.contains(vi, vj)) {
+                    for(stl::size_t vk{}; vk < length; ++vk) {
+                        if(matrix.contains(vj, vk)) {
+                            matrix.erase(vi, vk);
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+    /*! @brief Iterable task list. */
+    using iterable = iterable_adaptor<typename task_container_type::const_iterator>;
+    /*! @brief Adjacency matrix type. */
+    using graph_type = adjacency_matrix_type;
+
+    /*! @brief Default constructor. */
+    basic_flow()
+        : basic_flow{allocator_type{}} {}
+
+    /**
+     * @brief Constructs a flow builder with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_flow(const allocator_type &allocator)
+        : index{0u, allocator},
+          vertices{allocator},
+          deps{allocator} {}
+
+    /*! @brief Default copy constructor. */
+    basic_flow(const basic_flow &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    basic_flow(const basic_flow &other, const allocator_type &allocator)
+        : index{other.index.first(), allocator},
+          vertices{other.vertices, allocator},
+          deps{other.deps, allocator},
+          sync_on{other.sync_on} {}
+
+    /*! @brief Default move constructor. */
+    basic_flow(basic_flow &&) noexcept = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_flow(basic_flow &&other, const allocator_type &allocator)
+        : index{other.index.first(), allocator},
+          vertices{stl::move(other.vertices), allocator},
+          deps{stl::move(other.deps), allocator},
+          sync_on{other.sync_on} {}
+
+    /*! @brief Default destructor. */
+    ~basic_flow() = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This flow builder.
+     */
+    basic_flow &operator=(const basic_flow &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This flow builder.
+     */
+    basic_flow &operator=(basic_flow &&) noexcept = default;
+
+    /**
+     * @brief Exchanges the contents with those of a given flow builder.
+     * @param other Flow builder to exchange the content with.
+     */
+    void swap(basic_flow &other) noexcept {
+        using stl::swap;
+        swap(index, other.index);
+        swap(vertices, other.vertices);
+        swap(deps, other.deps);
+        swap(sync_on, other.sync_on);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return allocator_type{index.second()};
+    }
+
+    /**
+     * @brief Returns the identifier at specified location.
+     * @param pos Position of the identifier to return.
+     * @return The requested identifier.
+     */
+    [[nodiscard]] id_type operator[](const size_type pos) const {
+        return vertices.cbegin()[static_cast<task_container_type::difference_type>(pos)];
+    }
+
+    /*! @brief Clears the flow builder. */
+    void clear() noexcept {
+        index.first() = {};
+        vertices.clear();
+        deps.clear();
+        sync_on = {};
+    }
+
+    /**
+     * @brief Returns true if a flow builder contains no tasks, false otherwise.
+     * @return True if the flow builder contains no tasks, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return vertices.empty();
+    }
+
+    /**
+     * @brief Returns the number of tasks.
+     * @return The number of tasks.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return vertices.size();
+    }
+
+    /**
+     * @brief Binds a task to a flow builder.
+     * @param value Task identifier.
+     * @return This flow builder.
+     */
+    basic_flow &bind(const id_type value) {
+        sync_on += (sync_on == vertices.size());
+        const auto it = vertices.emplace(value).first;
+        index.first() = size_type(it - vertices.begin());
+        return *this;
+    }
+
+    /**
+     * @brief Turns the current task into a sync point.
+     * @return This flow builder.
+     */
+    basic_flow &sync() {
+        ENTT_ASSERT(index.first() < vertices.size(), "Invalid node");
+        sync_on = index.first();
+
+        for(const auto &elem: deps) {
+            elem.second.emplace_back(sync_on, true);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a resource to the current task with a given access mode.
+     * @param res Resource identifier.
+     * @param is_rw Access mode.
+     * @return This flow builder.
+     */
+    basic_flow &set(const id_type res, bool is_rw = false) {
+        emplace(res, is_rw);
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a read-only resource to the current task.
+     * @param res Resource identifier.
+     * @return This flow builder.
+     */
+    basic_flow &ro(const id_type res) {
+        emplace(res, false);
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a range of read-only resources to the current task.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     * @return This flow builder.
+     */
+    basic_flow &ro(stl::input_iterator auto first, stl::input_iterator auto last) {
+        for(; first != last; ++first) {
+            emplace(*first, false);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a writable resource to the current task.
+     * @param res Resource identifier.
+     * @return This flow builder.
+     */
+    basic_flow &rw(const id_type res) {
+        emplace(res, true);
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a range of writable resources to the current task.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     * @return This flow builder.
+     */
+    basic_flow &rw(stl::input_iterator auto first, stl::input_iterator auto last) {
+        for(; first != last; ++first) {
+            emplace(*first, true);
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Generates a task graph for the current content.
+     * @return The adjacency matrix of the task graph.
+     */
+    [[nodiscard]] graph_type graph() const {
+        graph_type matrix{vertices.size(), get_allocator()};
+
+        setup_graph(matrix);
+        transitive_closure(matrix);
+        transitive_reduction(matrix);
+
+        return matrix;
+    }
+
+private:
+    compressed_pair<size_type, allocator_type> index;
+    task_container_type vertices;
+    deps_container_type deps;
+    size_type sync_on{};
+};
+
+} // namespace entt
+
+#endif

include/entt/graph/fwd.hpp

@@ -0,0 +1,28 @@
+#ifndef ENTT_GRAPH_FWD_HPP
+#define ENTT_GRAPH_FWD_HPP
+
+#include "../core/fwd.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/memory.hpp"
+
+namespace entt {
+
+/*! @brief Undirected graph category tag. */
+struct directed_tag {};
+
+/*! @brief Directed graph category tag. */
+struct undirected_tag: directed_tag {};
+
+template<stl::derived_from<directed_tag>, typename = stl::allocator<stl::size_t>>
+class adjacency_matrix;
+
+template<typename = stl::allocator<id_type>>
+class basic_flow;
+
+/*! @brief Alias declaration for the most common use case. */
+using flow = basic_flow<>;
+
+} // namespace entt
+
+#endif

include/entt/locator/locator.hpp

@@ -0,0 +1,161 @@
+#ifndef ENTT_LOCATOR_LOCATOR_HPP
+#define ENTT_LOCATOR_LOCATOR_HPP
+
+#include "../config/config.h"
+#include "../stl/concepts.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/utility.hpp"
+
+namespace entt {
+
+/**
+ * @brief Service locator, nothing more.
+ *
+ * A service locator is used to do what it promises: locate services.<br/>
+ * Usually service locators are tightly bound to the services they expose and
+ * thus it's hard to define a general purpose class to do that. This tiny class
+ * tries to fill the gap and to get rid of the burden of defining a different
+ * specific locator for each application.
+ *
+ * @note
+ * Users shouldn't retain references to a service. The recommended way is to
+ * retrieve the service implementation currently set each and every time the
+ * need for it arises. The risk is to incur in unexpected behaviors otherwise.
+ *
+ * @tparam Service Service type.
+ */
+template<typename Service>
+class locator final {
+    class service_handle {
+        friend class locator<Service>;
+        stl::shared_ptr<Service> value{};
+    };
+
+public:
+    /*! @brief Service type. */
+    using type = Service;
+    /*! @brief Service node type. */
+    using node_type = service_handle;
+
+    /*! @brief Default constructor, deleted on purpose. */
+    locator() = delete;
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    locator(const locator &) = delete;
+
+    /*! @brief Default destructor, deleted on purpose. */
+    ~locator() = delete;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This locator.
+     */
+    locator &operator=(const locator &) = delete;
+
+    /**
+     * @brief Checks whether a service locator contains a value.
+     * @return True if the service locator contains a value, false otherwise.
+     */
+    [[nodiscard]] static bool has_value() noexcept {
+        return (service != nullptr);
+    }
+
+    /**
+     * @brief Returns a reference to a valid service, if any.
+     *
+     * @warning
+     * Invoking this function can result in undefined behavior if the service
+     * hasn't been set yet.
+     *
+     * @return A reference to the service currently set, if any.
+     */
+    [[nodiscard]] static Service &value() noexcept {
+        ENTT_ASSERT(has_value(), "Service not available");
+        return *service;
+    }
+
+    /**
+     * @brief Returns a service if available or sets it from a fallback type.
+     *
+     * Arguments are used only if a service doesn't already exist. In all other
+     * cases, they are discarded.
+     *
+     * @tparam Args Types of arguments to use to construct the fallback service.
+     * @tparam Type Fallback service type.
+     * @param args Parameters to use to construct the fallback service.
+     * @return A reference to a valid service.
+     */
+    template<stl::derived_from<Service> Type = Service, typename... Args>
+    requires stl::constructible_from<Type, Args...>
+    [[nodiscard]] static Service &value_or(Args &&...args) {
+        return service ? *service : emplace<Type>(stl::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Sets or replaces a service.
+     * @tparam Type Service type.
+     * @tparam Args Types of arguments to use to construct the service.
+     * @param args Parameters to use to construct the service.
+     * @return A reference to a valid service.
+     */
+    template<stl::derived_from<Service> Type = Service, typename... Args>
+    requires stl::constructible_from<Type, Args...>
+    static Service &emplace(Args &&...args) {
+        service = stl::make_shared<Type>(stl::forward<Args>(args)...);
+        return *service;
+    }
+
+    /**
+     * @brief Sets or replaces a service using a given allocator.
+     * @tparam Type Service type.
+     * @tparam Args Types of arguments to use to construct the service.
+     * @param alloc The allocator to use.
+     * @param args Parameters to use to construct the service.
+     * @return A reference to a valid service.
+     */
+    template<stl::derived_from<Service> Type = Service, typename... Args>
+    requires stl::constructible_from<Type, Args...>
+    static Service &emplace(stl::allocator_arg_t, auto alloc, Args &&...args) {
+        service = stl::allocate_shared<Type>(alloc, stl::forward<Args>(args)...);
+        return *service;
+    }
+
+    /**
+     * @brief Returns a handle to the underlying service.
+     * @return A handle to the underlying service.
+     */
+    static node_type handle() noexcept {
+        node_type node{};
+        node.value = service;
+        return node;
+    }
+
+    /**
+     * @brief Resets or replaces a service.
+     * @param other Optional handle with which to replace the service.
+     */
+    static void reset(const node_type &other = {}) noexcept {
+        service = other.value;
+    }
+
+    /**
+     * @brief Resets or replaces a service.
+     * @tparam Type Service type.
+     * @tparam Deleter Deleter type.
+     * @param elem A pointer to a service to manage.
+     * @param deleter A deleter to use to destroy the service.
+     */
+    template<stl::derived_from<Service> Type, typename Deleter = stl::default_delete<Type>>
+    static void reset(Type *elem, Deleter deleter = {}) {
+        service = stl::shared_ptr<Service>{elem, stl::move(deleter)};
+    }
+
+private:
+    // stl::shared_ptr because of its type erased allocator which is useful here
+    // NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
+    inline static stl::shared_ptr<Service> service{};
+};
+
+} // namespace entt
+
+#endif

include/entt/meta/adl_pointer.hpp

@@ -0,0 +1,35 @@
+#ifndef ENTT_META_ADL_POINTER_HPP
+#define ENTT_META_ADL_POINTER_HPP
+
+namespace entt {
+
+/**
+ * @brief ADL based lookup function for dereferencing meta pointer-like types.
+ * @tparam Type Element type.
+ * @param value A pointer-like object.
+ * @return The value returned from the dereferenced pointer.
+ */
+template<typename Type>
+decltype(auto) dereference_meta_pointer_like(const Type &value) {
+    return *value;
+}
+
+/**
+ * @brief Fake ADL based lookup function for meta pointer-like types.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+struct adl_meta_pointer_like {
+    /**
+     * @brief Uses the default ADL based lookup method to resolve the call.
+     * @param value A pointer-like object.
+     * @return The value returned from the dereferenced pointer.
+     */
+    static decltype(auto) dereference(const Type &value) {
+        return dereference_meta_pointer_like(value);
+    }
+};
+
+} // namespace entt
+
+#endif

include/entt/meta/container.hpp

@@ -0,0 +1,298 @@
+// IWYU pragma: always_keep
+
+#ifndef ENTT_META_CONTAINER_HPP
+#define ENTT_META_CONTAINER_HPP
+
+#include "../core/concepts.hpp"
+#include "../core/type_traits.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "context.hpp"
+#include "fwd.hpp"
+#include "meta.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename Type>
+struct sequence_container_extent: integral_constant<meta_dynamic_extent> {};
+
+template<typename Type>
+requires is_complete_v<stl::tuple_size<Type>>
+struct sequence_container_extent<Type>: integral_constant<stl::tuple_size_v<Type>> {};
+
+template<typename Type>
+inline constexpr stl::size_t sequence_container_extent_v = sequence_container_extent<Type>::value;
+
+template<typename Type>
+concept meta_sequence_container_like = requires(Type elem) {
+    typename Type::value_type;
+    typename Type::iterator;
+    requires entt::stl::forward_iterator<typename Type::iterator>;
+    { elem.begin() } -> stl::same_as<typename Type::iterator>;
+    { elem.end() } -> stl::same_as<typename Type::iterator>;
+    requires !requires { typename Type::key_type; };
+    requires !requires { elem.substr(); };
+};
+
+template<typename Type>
+concept meta_associative_container_like = requires(Type value) {
+    typename Type::key_type;
+    typename Type::value_type;
+    typename Type::iterator;
+    requires entt::stl::forward_iterator<typename Type::iterator>;
+    { value.begin() } -> stl::same_as<typename Type::iterator>;
+    { value.end() } -> stl::same_as<typename Type::iterator>;
+    value.find(stl::declval<typename Type::key_type>());
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief General purpose implementation of meta sequence container traits.
+ * @tparam Type Type of underlying sequence container.
+ */
+template<cvref_unqualified Type>
+struct basic_meta_sequence_container_traits {
+    /*! @brief Unsigned integer type. */
+    using size_type = meta_sequence_container::size_type;
+    /*! @brief Meta iterator type. */
+    using iterator = meta_sequence_container::iterator;
+
+    /*! @brief Number of elements, or `meta_dynamic_extent` if dynamic. */
+    static constexpr stl::size_t extent = internal::sequence_container_extent_v<Type>;
+
+    /**
+     * @brief Returns the number of elements in a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @return Number of elements.
+     */
+    [[nodiscard]] static size_type size(const void *container) {
+        return static_cast<const Type *>(container)->size();
+    }
+
+    /**
+     * @brief Clears a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool clear([[maybe_unused]] void *container) {
+        if constexpr(requires(Type elem) { elem.clear(); }) {
+            static_cast<Type *>(container)->clear();
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+    /**
+     * @brief Increases the capacity of a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @param sz Desired capacity.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool reserve([[maybe_unused]] void *container, [[maybe_unused]] const size_type sz) {
+        if constexpr(requires(Type elem) { elem.reserve(sz); }) {
+            static_cast<Type *>(container)->reserve(sz);
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+    /**
+     * @brief Resizes a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @param sz The new number of elements.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool resize([[maybe_unused]] void *container, [[maybe_unused]] const size_type sz) {
+        if constexpr(stl::is_default_constructible_v<typename Type::value_type> && requires(Type elem) { elem.resize(sz); }) {
+            static_cast<Type *>(container)->resize(sz);
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+    /**
+     * @brief Returns a possibly const iterator to the beginning or the end.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param as_const Const opaque pointer fallback.
+     * @param end False to get a pointer that is past the last element.
+     * @return An iterator to the first or past the last element of the
+     * container.
+     */
+    static iterator iter(const meta_ctx &area, void *container, const void *as_const, const bool end) {
+        return (container == nullptr)
+                   ? iterator{area, end ? static_cast<const Type *>(as_const)->cend() : static_cast<const Type *>(as_const)->cbegin()}
+                   : iterator{area, end ? static_cast<Type *>(container)->end() : static_cast<Type *>(container)->begin()};
+    }
+
+    /**
+     * @brief Assigns one element to a container and constructs its object from
+     * a given opaque instance.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param value Optional opaque instance of the object to construct (as
+     * value type).
+     * @param cref Optional opaque instance of the object to construct (as
+     * decayed const reference type).
+     * @param it Iterator before which the element will be inserted.
+     * @return A possibly invalid iterator to the inserted element.
+     */
+    [[nodiscard]] static iterator insert([[maybe_unused]] const meta_ctx &area, [[maybe_unused]] void *container, [[maybe_unused]] const void *value, [[maybe_unused]] const void *cref, [[maybe_unused]] const iterator &it) {
+        if constexpr(requires(Type elem, typename Type::const_iterator iter, Type::value_type instance) { elem.insert(iter, instance); }) {
+            auto *const non_const = any_cast<typename Type::iterator>(&it.base());
+            return {area, static_cast<Type *>(container)->insert(
+                              non_const ? *non_const : any_cast<const typename Type::const_iterator &>(it.base()),
+                              (value != nullptr) ? *static_cast<const Type::value_type *>(value) : *static_cast<const stl::remove_reference_t<typename Type::const_reference> *>(cref))};
+        } else {
+            return iterator{};
+        }
+    }
+
+    /**
+     * @brief Erases an element from a container.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param it An opaque iterator to the element to erase.
+     * @return A possibly invalid iterator following the last removed element.
+     */
+    [[nodiscard]] static iterator erase([[maybe_unused]] const meta_ctx &area, [[maybe_unused]] void *container, [[maybe_unused]] const iterator &it) {
+        if constexpr(requires(Type elem, typename Type::const_iterator iter) { elem.erase(iter); }) {
+            auto *const non_const = any_cast<typename Type::iterator>(&it.base());
+            return {area, static_cast<Type *>(container)->erase(non_const ? *non_const : any_cast<const typename Type::const_iterator &>(it.base()))};
+        } else {
+            return iterator{};
+        }
+    }
+};
+
+/**
+ * @brief General purpose implementation of meta associative container traits.
+ * @tparam Type Type of underlying associative container.
+ */
+template<cvref_unqualified Type>
+struct basic_meta_associative_container_traits {
+    /*! @brief Unsigned integer type. */
+    using size_type = meta_associative_container::size_type;
+    /*! @brief Meta iterator type. */
+    using iterator = meta_associative_container::iterator;
+
+    /*! @brief True in case of key-only containers, false otherwise. */
+    static constexpr bool key_only = !requires { typename Type::mapped_type; };
+
+    /**
+     * @brief Returns the number of elements in a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @return Number of elements.
+     */
+    [[nodiscard]] static size_type size(const void *container) {
+        return static_cast<const Type *>(container)->size();
+    }
+
+    /**
+     * @brief Clears a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool clear(void *container) {
+        static_cast<Type *>(container)->clear();
+        return true;
+    }
+
+    /**
+     * @brief Increases the capacity of a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @param sz Desired capacity.
+     * @return True in case of success, false otherwise.
+     */
+    [[nodiscard]] static bool reserve([[maybe_unused]] void *container, [[maybe_unused]] const size_type sz) {
+        if constexpr(requires(Type elem) { elem.reserve(sz); }) {
+            static_cast<Type *>(container)->reserve(sz);
+            return true;
+        } else {
+            return false;
+        }
+    }
+
+    /**
+     * @brief Returns a possibly const iterator to the beginning or the end.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param as_const Const opaque pointer fallback.
+     * @param end False to get a pointer that is past the last element.
+     * @return An iterator to the first or past the last element of the
+     * container.
+     */
+    static iterator iter(const meta_ctx &area, void *container, const void *as_const, const bool end) {
+        return (container == nullptr)
+                   ? iterator{area, stl::bool_constant<key_only>{}, end ? static_cast<const Type *>(as_const)->cend() : static_cast<const Type *>(as_const)->cbegin()}
+                   : iterator{area, stl::bool_constant<key_only>{}, end ? static_cast<Type *>(container)->end() : static_cast<Type *>(container)->begin()};
+    }
+
+    /**
+     * @brief Inserts an element into a container, if the key does not exist.
+     * @param container Opaque pointer to a container of the given type.
+     * @param key An opaque key value of an element to insert.
+     * @param value Optional opaque value to insert (key-value containers).
+     * @return True if the insertion took place, false otherwise.
+     */
+    [[nodiscard]] static bool insert(void *container, const void *key, [[maybe_unused]] const void *value) {
+        if constexpr(key_only) {
+            return static_cast<Type *>(container)->insert(*static_cast<const Type::key_type *>(key)).second;
+        } else {
+            return static_cast<Type *>(container)->emplace(*static_cast<const Type::key_type *>(key), *static_cast<const Type::mapped_type *>(value)).second;
+        }
+    }
+
+    /**
+     * @brief Removes an element from a container.
+     * @param container Opaque pointer to a container of the given type.
+     * @param key An opaque key value of an element to remove.
+     * @return Number of elements removed (either 0 or 1).
+     */
+    [[nodiscard]] static size_type erase(void *container, const void *key) {
+        return static_cast<Type *>(container)->erase(*static_cast<const Type::key_type *>(key));
+    }
+
+    /**
+     * @brief Finds an element with a given key.
+     * @param area The context to pass to the newly created iterator.
+     * @param container Opaque pointer to a container of the given type.
+     * @param as_const Const opaque pointer fallback.
+     * @param key Opaque key value of an element to search for.
+     * @return An iterator to the element with the given key, if any.
+     */
+    static iterator find(const meta_ctx &area, void *container, const void *as_const, const void *key) {
+        return (container != nullptr) ? iterator{area, stl::bool_constant<key_only>{}, static_cast<Type *>(container)->find(*static_cast<const Type::key_type *>(key))}
+                                      : iterator{area, stl::bool_constant<key_only>{}, static_cast<const Type *>(as_const)->find(*static_cast<const Type::key_type *>(key))};
+    }
+};
+
+/**
+ * @brief Traits meta sequence container like types.
+ * @tparam Type Container type to inspect.
+ */
+template<internal::meta_sequence_container_like Type>
+struct meta_sequence_container_traits<Type>: basic_meta_sequence_container_traits<Type> {};
+
+/**
+ * @brief Traits for meta associative container like types.
+ * @tparam Type Container type to inspect.
+ */
+template<internal::meta_associative_container_like Type>
+struct meta_associative_container_traits<Type>: basic_meta_associative_container_traits<Type> {};
+
+} // namespace entt
+
+#endif

include/entt/meta/context.hpp

@@ -0,0 +1,47 @@
+#ifndef ENTT_META_CTX_HPP
+#define ENTT_META_CTX_HPP
+
+#include "../container/dense_map.hpp"
+#include "../core/fwd.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/memory.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+struct meta_type_node;
+
+struct meta_context {
+    using bucket_type = dense_map<id_type, stl::unique_ptr<meta_type_node>, stl::identity>;
+
+    bucket_type bucket;
+
+    [[nodiscard]] inline static meta_context &from(meta_ctx &);
+    [[nodiscard]] inline static const meta_context &from(const meta_ctx &);
+};
+
+} // namespace internal
+/*! @endcond */
+
+/*! @brief Opaque meta context type. */
+struct meta_ctx: private internal::meta_context {
+    // attorney idiom like model to access the base class
+    friend struct internal::meta_context;
+};
+
+/*! @cond ENTT_INTERNAL */
+[[nodiscard]] inline internal::meta_context &internal::meta_context::from(meta_ctx &ctx) {
+    return ctx;
+}
+
+[[nodiscard]] inline const internal::meta_context &internal::meta_context::from(const meta_ctx &ctx) {
+    return ctx;
+}
+/*! @endcond */
+
+} // namespace entt
+
+#endif

include/entt/meta/factory.hpp

@@ -0,0 +1,656 @@
+#ifndef ENTT_META_FACTORY_HPP
+#define ENTT_META_FACTORY_HPP
+
+#include "../config/config.h"
+#include "../core/bit.hpp"
+#include "../core/fwd.hpp"
+#include "../core/hashed_string.hpp"
+#include "../core/type_info.hpp"
+#include "../core/type_traits.hpp"
+#include "../locator/locator.hpp"
+#include "../stl/algorithm.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/cstdint.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "context.hpp"
+#include "fwd.hpp"
+#include "meta.hpp"
+#include "node.hpp"
+#include "policy.hpp"
+#include "range.hpp"
+#include "utility.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+class basic_meta_factory {
+    using invoke_type = stl::remove_pointer_t<decltype(meta_func_node::invoke)>;
+
+    enum class mode {
+        type,
+        data,
+        func
+    };
+
+    [[nodiscard]] auto *find_member_or_assert() {
+        auto *member = find_member(parent->details->data, bucket);
+        ENTT_ASSERT(member != nullptr, "Cannot find member");
+        return member;
+    }
+
+    [[nodiscard]] auto *find_overload_or_assert() {
+        ENTT_ASSERT(invoke != nullptr, "Invoke function not available");
+        auto *overload = find_overload(find_member(parent->details->func, bucket), invoke);
+        ENTT_ASSERT(overload != nullptr, "Cannot find overload");
+        return overload;
+    }
+
+    bool unique_alias(const id_type alias) const noexcept {
+        return (ctx->bucket.find(alias) == ctx->bucket.cend()) && (stl::find_if(ctx->bucket.cbegin(), ctx->bucket.cend(), [alias](const auto &value) { return value.second->alias == alias; }) == ctx->bucket.cend());
+    }
+
+protected:
+    void type(const id_type alias, const char *name) noexcept {
+        state = mode::type;
+        ENTT_ASSERT((parent->alias == alias) || unique_alias(alias), "Duplicate identifier");
+        parent->alias = alias;
+        parent->name = name;
+    }
+
+    template<typename Type>
+    void insert_or_assign(Type node) {
+        state = mode::type;
+
+        if constexpr(stl::is_same_v<Type, meta_base_node>) {
+            auto *member = find_member(parent->details->base, node.id);
+            member ? (*member = node) : parent->details->base.emplace_back(node);
+        } else if constexpr(stl::is_same_v<Type, meta_conv_node>) {
+            auto *member = find_member(parent->details->conv, node.id);
+            member ? (*member = node) : parent->details->conv.emplace_back(node);
+        } else {
+            static_assert(stl::is_same_v<Type, meta_ctor_node>, "Unexpected type");
+            auto *member = find_member(parent->details->ctor, node.id);
+            member ? (*member = node) : parent->details->ctor.emplace_back(node);
+        }
+    }
+
+    void data(meta_data_node node) {
+        state = mode::data;
+        bucket = node.id;
+
+        if(auto *member = find_member(parent->details->data, node.id); member == nullptr) {
+            parent->details->data.emplace_back(stl::move(node));
+        } else if(member->set != node.set || member->get != node.get) {
+            *member = stl::move(node);
+        }
+    }
+
+    void func(meta_func_node node) {
+        state = mode::func;
+        bucket = node.id;
+        invoke = node.invoke;
+
+        if(auto *member = find_member(parent->details->func, node.id); member == nullptr) {
+            parent->details->func.emplace_back(stl::move(node));
+        } else if(auto *overload = find_overload(member, node.invoke); overload == nullptr) {
+            while(member->next != nullptr) { member = member->next.get(); }
+            member->next = stl::make_unique<meta_func_node>(stl::move(node));
+        }
+    }
+
+    void traits(const meta_traits value, const bool unset) {
+        const auto set_or_unset_on = [=](auto &node) {
+            node.traits = (unset ? (node.traits & ~value) : (node.traits | value));
+        };
+
+        switch(state) {
+        case mode::type:
+            set_or_unset_on(*parent);
+            break;
+        case mode::data:
+            set_or_unset_on(*find_member_or_assert());
+            break;
+        case mode::func:
+            set_or_unset_on(*find_overload_or_assert());
+            break;
+        }
+    }
+
+    void custom(meta_custom_node node) {
+        switch(state) {
+        case mode::type:
+            parent->custom = stl::move(node);
+            break;
+        case mode::data:
+            find_member_or_assert()->custom = stl::move(node);
+            break;
+        case mode::func:
+            find_overload_or_assert()->custom = stl::move(node);
+            break;
+        }
+    }
+
+public:
+    basic_meta_factory(meta_ctx &area, meta_type_node node, const id_type id)
+        : ctx{&meta_context::from(area)},
+          bucket{},
+          state{mode::type} {
+        if(const auto it = ctx->bucket.find(id); it == ctx->bucket.cend()) {
+            ENTT_ASSERT(unique_alias(id), "Duplicate identifier");
+            parent = ctx->bucket.emplace(id, stl::make_unique<meta_type_node>(stl::move(node))).first->second.get();
+            parent->details = stl::make_unique<meta_type_descriptor>();
+            parent->alias = id;
+        } else {
+            parent = it->second.get();
+        }
+    }
+
+private:
+    meta_context *ctx{};
+    invoke_type *invoke{};
+    meta_type_node *parent{};
+    id_type bucket{};
+    mode state{};
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Meta factory to be used for reflection purposes.
+ * @tparam Type Type for which the factory was created.
+ */
+template<typename Type>
+class meta_factory: private internal::basic_meta_factory {
+    using base_type = internal::basic_meta_factory;
+
+public:
+    /*! @brief Type of object for which this factory builds a meta type. */
+    using element_type = Type;
+
+    /*! @brief Default constructor. */
+    meta_factory() noexcept
+        : meta_factory{locator<meta_ctx>::value_or()} {}
+
+    /**
+     * @brief Context aware constructor.
+     * @param area The context into which to construct meta types.
+     */
+    meta_factory(meta_ctx &area) noexcept
+        : base_type{area, internal::setup_node_for<element_type>(), type_hash<Type>::value()} {}
+
+    /**
+     * @brief Constructs an unconstrained type assigned to a given identifier.
+     * @param id A custom unique identifier.
+     */
+    meta_factory(const id_type id) noexcept
+        : meta_factory{locator<meta_ctx>::value_or(), id} {}
+
+    /**
+     * @brief Context aware constructor.
+     * @param id A custom unique identifier.
+     * @param area The context into which to construct meta types.
+     */
+    meta_factory(meta_ctx &area, const id_type id) noexcept
+        : base_type{area, internal::setup_node_for<element_type>(), id} {}
+
+    /**
+     * @brief Assigns a custom unique identifier to a meta type.
+     * @param name A custom unique identifier as a **string literal**.
+     * @return A meta factory for the given type.
+     */
+    meta_factory type(const char *name) noexcept {
+        return type(hashed_string::value(name), name);
+    }
+
+    /**
+     * @brief Assigns a custom unique identifier to a meta type.
+     * @param alias A custom unique identifier.
+     * @param name An optional name for the type as a **string literal**.
+     * @return A meta factory for the given type.
+     */
+    meta_factory type(const id_type alias, const char *name = nullptr) noexcept {
+        base_type::type(alias, name);
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta base to a meta type.
+     *
+     * A reflected base class must be a real base class of the reflected type.
+     *
+     * @tparam Base Type of the base class to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename Base>
+    requires stl::derived_from<element_type, Base>
+    meta_factory base() noexcept {
+        if constexpr(!stl::same_as<element_type, Base>) {
+            auto *const op = +[](const void *instance) noexcept { return static_cast<const void *>(static_cast<const Base *>(static_cast<const element_type *>(instance))); };
+
+            base_type::insert_or_assign(
+                internal::meta_base_node{
+                    type_id<Base>().hash(),
+                    &internal::resolve<Base>,
+                    op});
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * Conversion functions can be either free functions or member
+     * functions.<br/>
+     * In case of free functions, they must accept a const reference to an
+     * instance of the parent type as an argument. In case of member functions,
+     * they should have no arguments at all.
+     *
+     * @tparam Candidate The actual function to use for the conversion.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Candidate>
+    auto conv() noexcept {
+        using conv_type = stl::remove_cvref_t<stl::invoke_result_t<decltype(Candidate), element_type &>>;
+        auto *const op = +[](const meta_ctx &area, const void *instance) { return forward_as_meta(area, stl::invoke(Candidate, *static_cast<const element_type *>(instance))); };
+
+        base_type::insert_or_assign(
+            internal::meta_conv_node{
+                type_id<conv_type>().hash(),
+                op});
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta conversion function to a meta type.
+     *
+     * The given type must be such that an instance of the reflected type can be
+     * converted to it.
+     *
+     * @tparam To Type of the conversion function to assign to the meta type.
+     * @return A meta factory for the parent type.
+     */
+    template<typename To>
+    meta_factory conv() noexcept {
+        using conv_type = stl::remove_cvref_t<To>;
+        auto *const op = +[](const meta_ctx &area, const void *instance) { return forward_as_meta(area, static_cast<To>(*static_cast<const element_type *>(instance))); };
+
+        base_type::insert_or_assign(
+            internal::meta_conv_node{
+                type_id<conv_type>().hash(),
+                op});
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * Both member functions and free function can be assigned to meta types in
+     * the role of constructors. All that is required is that they return an
+     * instance of the underlying type.<br/>
+     * From a client's point of view, nothing changes if a constructor of a meta
+     * type is a built-in one or not.
+     *
+     * @tparam Candidate The actual function to use as a constructor.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @return A meta factory for the parent type.
+     */
+    template<auto Candidate, typename Policy = as_value_t>
+    meta_factory ctor() noexcept {
+        using descriptor = meta_function_helper_t<element_type, decltype(Candidate)>;
+        static_assert(Policy::template value<typename descriptor::return_type>, "Invalid return type for the given policy");
+        static_assert(stl::is_same_v<stl::remove_cvref_t<typename descriptor::return_type>, element_type>, "The function doesn't return an object of the required type");
+
+        base_type::insert_or_assign(
+            internal::meta_ctor_node{
+                type_id<typename descriptor::args_type>().hash(),
+                descriptor::args_type::size,
+                &meta_arg<typename descriptor::args_type>,
+                &meta_construct<element_type, Candidate, Policy>});
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta constructor to a meta type.
+     *
+     * A meta constructor is uniquely identified by the types of its arguments
+     * and is such that there exists an actual constructor of the underlying
+     * type that can be invoked with parameters whose types are those given.
+     *
+     * @tparam Args Types of arguments to use to construct an instance.
+     * @return A meta factory for the parent type.
+     */
+    template<typename... Args>
+    meta_factory ctor() noexcept {
+        // default constructor is already implicitly generated, no need for redundancy
+        if constexpr(sizeof...(Args) != 0u) {
+            using descriptor = meta_function_helper_t<element_type, element_type (*)(Args...)>;
+
+            base_type::insert_or_assign(
+                internal::meta_ctor_node{
+                    type_id<typename descriptor::args_type>().hash(),
+                    descriptor::args_type::size,
+                    &meta_arg<typename descriptor::args_type>,
+                    &meta_construct<element_type, Args...>});
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type.
+     * @tparam Data The actual variable to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param name A custom unique identifier as a **string literal**.
+     * @return A meta factory for the given type.
+     */
+    template<auto Data, typename Policy = as_value_t>
+    meta_factory data(const char *name) noexcept {
+        return data<Data, Policy>(hashed_string::value(name), name);
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type.
+     *
+     * Both data members and static and global variables, as well as constants
+     * of any kind, can be assigned to a meta type.<br/>
+     * From a client's point of view, all the variables associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Data The actual variable to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param id Unique identifier.
+     * @param name An optional name for the meta data as a **string literal**.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Data, typename Policy = as_value_t>
+    meta_factory data(const id_type id, const char *name = nullptr) noexcept {
+        if constexpr(stl::is_member_object_pointer_v<decltype(Data)>) {
+            using data_type = stl::invoke_result_t<decltype(Data), element_type &>;
+            static_assert(Policy::template value<data_type>, "Invalid return type for the given policy");
+
+            base_type::data(
+                internal::meta_data_node{
+                    id,
+                    name,
+                    /* this is never static */
+                    stl::is_const_v<stl::remove_reference_t<data_type>> ? internal::meta_traits::is_const : internal::meta_traits::is_none,
+                    1u,
+                    0u,
+                    &meta_arg<type_list<stl::remove_cvref_t<data_type>>>,
+                    &meta_arg<type_list<>>,
+                    &internal::resolve<stl::remove_cvref_t<data_type>>,
+                    &meta_setter<element_type, Data>,
+                    &meta_getter<element_type, Data, Policy>});
+        } else {
+            using data_type = stl::remove_pointer_t<decltype(Data)>;
+
+            if constexpr(stl::is_pointer_v<decltype(Data)>) {
+                static_assert(Policy::template value<decltype(*Data)>, "Invalid return type for the given policy");
+            } else {
+                static_assert(Policy::template value<data_type>, "Invalid return type for the given policy");
+            }
+
+            base_type::data(
+                internal::meta_data_node{
+                    id,
+                    name,
+                    ((!stl::is_pointer_v<decltype(Data)> || stl::is_const_v<data_type>) ? internal::meta_traits::is_const : internal::meta_traits::is_none) | internal::meta_traits::is_static,
+                    1u,
+                    0u,
+                    &meta_arg<type_list<stl::remove_cvref_t<data_type>>>,
+                    &meta_arg<type_list<>>,
+                    &internal::resolve<stl::remove_cvref_t<data_type>>,
+                    &meta_setter<element_type, Data>,
+                    &meta_getter<element_type, Data, Policy>});
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type by means of its setter and
+     * getter.
+     * @tparam Setter The actual function to use as a setter.
+     * @tparam Getter The actual function to use as a getter.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param name A custom unique identifier as a **string literal**.
+     * @return A meta factory for the given type.
+     */
+    template<auto Setter, auto Getter, typename Policy = as_value_t>
+    meta_factory data(const char *name) noexcept {
+        return data<Setter, Getter, Policy>(hashed_string::value(name), name);
+    }
+
+    /**
+     * @brief Assigns a meta data to a meta type by means of its setter and
+     * getter.
+     *
+     * Setters and getters can be either free functions, member functions or a
+     * mix of them.<br/>
+     * In case of free functions, setters and getters must accept a reference to
+     * an instance of the parent type as their first argument. A setter has then
+     * an extra argument of a type convertible to that of the parameter to
+     * set.<br/>
+     * In case of member functions, getters have no arguments at all, while
+     * setters has an argument of a type convertible to that of the parameter to
+     * set.
+     *
+     * @tparam Setter The actual function to use as a setter.
+     * @tparam Getter The actual function to use as a getter.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param id Unique identifier.
+     * @param name An optional name for the meta data as a **string literal**.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Setter, auto Getter, typename Policy = as_value_t>
+    meta_factory data(const id_type id, const char *name = nullptr) noexcept {
+        using getter = meta_function_helper_t<element_type, decltype(Getter)>;
+        static_assert(Policy::template value<typename getter::return_type>, "Invalid return type for the given policy");
+
+        if constexpr(stl::is_same_v<decltype(Setter), stl::nullptr_t>) {
+            base_type::data(
+                internal::meta_data_node{
+                    id,
+                    name,
+                    /* this is never static */
+                    internal::meta_traits::is_const,
+                    0u,
+                    getter::args_type::size,
+                    &meta_arg<type_list<>>,
+                    &meta_arg<typename getter::args_type>,
+                    &internal::resolve<stl::remove_cvref_t<typename getter::return_type>>,
+                    &meta_setter<element_type, Setter>,
+                    &meta_getter<element_type, Getter, Policy>});
+        } else {
+            using setter = meta_function_helper_t<element_type, decltype(Setter)>;
+
+            base_type::data(
+                internal::meta_data_node{
+                    id,
+                    name,
+                    /* this is never static nor const */
+                    internal::meta_traits::is_none,
+                    setter::args_type::size,
+                    getter::args_type::size,
+                    &meta_arg<typename setter::args_type>,
+                    &meta_arg<typename getter::args_type>,
+                    &internal::resolve<stl::remove_cvref_t<typename getter::return_type>>,
+                    &meta_setter<element_type, Setter>,
+                    &meta_getter<element_type, Getter, Policy>});
+        }
+
+        return *this;
+    }
+
+    /**
+     * @brief Assigns a meta function to a meta type.
+     * @tparam Candidate The actual function to attach to the meta function.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param name A custom unique identifier as a **string literal**.
+     * @return A meta factory for the given type.
+     */
+    template<auto Candidate, typename Policy = as_value_t>
+    meta_factory func(const char *name) noexcept {
+        return func<Candidate, Policy>(hashed_string::value(name), name);
+    }
+
+    /**
+     * @brief Assigns a meta function to a meta type.
+     *
+     * Both member functions and free functions can be assigned to a meta
+     * type.<br/>
+     * From a client's point of view, all the functions associated with the
+     * reflected object will appear as if they were part of the type itself.
+     *
+     * @tparam Candidate The actual function to attach to the meta type.
+     * @tparam Policy Optional policy (no policy set by default).
+     * @param id Unique identifier.
+     * @param name An optional name for the function as a **string literal**.
+     * @return A meta factory for the parent type.
+     */
+    template<auto Candidate, typename Policy = as_value_t>
+    meta_factory func(const id_type id, const char *name = nullptr) noexcept {
+        using descriptor = meta_function_helper_t<element_type, decltype(Candidate)>;
+        static_assert(Policy::template value<typename descriptor::return_type>, "Invalid return type for the given policy");
+
+        base_type::func(
+            internal::meta_func_node{
+                id,
+                name,
+                (descriptor::is_const ? internal::meta_traits::is_const : internal::meta_traits::is_none) | (descriptor::is_static ? internal::meta_traits::is_static : internal::meta_traits::is_none),
+                descriptor::args_type::size,
+                &internal::resolve<stl::conditional_t<stl::is_same_v<Policy, as_void_t>, void, stl::remove_cvref_t<typename descriptor::return_type>>>,
+                &meta_arg<typename descriptor::args_type>,
+                &meta_invoke<element_type, Candidate, Policy>});
+
+        return *this;
+    }
+
+    /**
+     * @brief Sets traits on the last created meta object.
+     *
+     * The assigned value must be an enum and intended as a bitmask.
+     *
+     * @tparam Value Type of the traits value.
+     * @param value Traits value.
+     * @param unset True to unset the given traits, false otherwise.
+     * @return A meta factory for the parent type.
+     */
+    template<typename Value>
+    meta_factory traits(const Value value, const bool unset = false) {
+        static_assert(stl::is_enum_v<Value>, "Invalid enum type");
+        base_type::traits(internal::user_to_meta_traits(value), unset);
+        return *this;
+    }
+
+    /**
+     * @brief Sets user defined data that will never be used by the library.
+     * @tparam Value Type of user defined data to store.
+     * @tparam Args Types of arguments to use to construct the user data.
+     * @param args Parameters to use to initialize the user data.
+     * @return A meta factory for the parent type.
+     */
+    template<typename Value, typename... Args>
+    meta_factory custom(Args &&...args) {
+        base_type::custom(internal::meta_custom_node{type_id<Value>().hash(), stl::make_shared<Value>(stl::forward<Args>(args)...)});
+        return *this;
+    }
+};
+
+/**
+ * @brief Resets a type and all its parts.
+ *
+ * Resets a type and all its data members, member functions and properties, as
+ * well as its constructors, destructors and conversion functions if any.<br/>
+ * Base classes aren't reset but the link between the two types is removed.
+ *
+ * The type is also removed from the set of searchable types.
+ *
+ * @param alias Unique identifier.
+ * @param ctx The context from which to reset meta types.
+ */
+inline void meta_reset(meta_ctx &ctx, const id_type alias) noexcept {
+    auto &bucket = internal::meta_context::from(ctx).bucket;
+
+    // fast path for unsearchable and overloaded types
+    if(bucket.erase(alias) == 0u) {
+        if(const auto it = stl::find_if(bucket.cbegin(), bucket.cend(), [alias](const auto &value) { return value.second->alias == alias; }); it != bucket.cend()) {
+            bucket.erase(it);
+        }
+    }
+}
+
+/**
+ * @brief Resets a type and all its parts.
+ *
+ * Resets a type and all its data members, member functions and properties, as
+ * well as its constructors, destructors and conversion functions if any.<br/>
+ * Base classes aren't reset but the link between the two types is removed.
+ *
+ * The type is also removed from the set of searchable types.
+ *
+ * @param alias Unique identifier.
+ */
+inline void meta_reset(const id_type alias) noexcept {
+    meta_reset(locator<meta_ctx>::value_or(), alias);
+}
+
+/**
+ * @brief Resets a type and all its parts.
+ *
+ * @sa meta_reset
+ *
+ * @tparam Type Type to reset.
+ * @param ctx The context from which to reset meta types.
+ */
+template<typename Type>
+void meta_reset(meta_ctx &ctx) noexcept {
+    internal::meta_context::from(ctx).bucket.erase(type_id<Type>().hash());
+}
+
+/**
+ * @brief Resets a type and all its parts.
+ *
+ * @sa meta_reset
+ *
+ * @tparam Type Type to reset.
+ */
+template<typename Type>
+void meta_reset() noexcept {
+    meta_reset<Type>(locator<meta_ctx>::value_or());
+}
+
+/**
+ * @brief Resets all meta types.
+ *
+ * @sa meta_reset
+ *
+ * @param ctx The context from which to reset meta types.
+ */
+inline void meta_reset(meta_ctx &ctx) noexcept {
+    internal::meta_context::from(ctx).bucket.clear();
+}
+
+/**
+ * @brief Resets all meta types.
+ *
+ * @sa meta_reset
+ */
+inline void meta_reset() noexcept {
+    meta_reset(locator<meta_ctx>::value_or());
+}
+
+} // namespace entt
+
+#endif

include/entt/meta/fwd.hpp

@@ -0,0 +1,43 @@
+#ifndef ENTT_META_FWD_HPP
+#define ENTT_META_FWD_HPP
+
+#include "../stl/cstddef.hpp"
+#include "../stl/limits.hpp"
+
+namespace entt {
+
+struct meta_ctx;
+
+class meta_sequence_container;
+
+class meta_associative_container;
+
+class meta_any;
+
+class meta_handle;
+
+struct meta_custom;
+
+struct meta_data;
+
+struct meta_func;
+
+struct meta_base;
+
+class meta_type;
+
+template<typename>
+class meta_factory;
+
+/*! @brief Used to identicate that a sequence container has not a fixed size. */
+inline constexpr stl::size_t meta_dynamic_extent = (stl::numeric_limits<stl::size_t>::max)();
+
+/*! @brief Disambiguation tag for constructors and the like. */
+struct meta_ctx_arg_t final {};
+
+/*! @brief Constant of type meta_context_arg_t used to disambiguate calls. */
+inline constexpr meta_ctx_arg_t meta_ctx_arg{};
+
+} // namespace entt
+
+#endif

include/entt/meta/node.hpp

@@ -0,0 +1,287 @@
+#ifndef ENTT_META_NODE_HPP
+#define ENTT_META_NODE_HPP
+
+#include "../config/config.h"
+#include "../core/bit.hpp"
+#include "../core/concepts.hpp"
+#include "../core/enum.hpp"
+#include "../core/fwd.hpp"
+#include "../core/type_info.hpp"
+#include "../core/type_traits.hpp"
+#include "../core/utility.hpp"
+#include "../stl/array.hpp"
+#include "../stl/bit.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/cstdint.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+#include "context.hpp"
+#include "fwd.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+enum class meta_traits : stl::uint32_t {
+    is_none = 0x0000,
+    is_const = 0x0001,
+    is_static = 0x0002,
+    is_arithmetic = 0x0004,
+    is_integral = 0x0008,
+    is_signed = 0x0010,
+    is_array = 0x0020,
+    is_enum = 0x0040,
+    is_class = 0x0080,
+    is_pointer = 0x0100,
+    is_pointer_like = 0x0200,
+    is_sequence_container = 0x0400,
+    is_associative_container = 0x0800,
+    _user_defined_traits = 0xFFFF,
+    _entt_enum_as_bitmask = 0xFFFF
+};
+
+template<typename Type>
+requires stl::is_enum_v<Type>
+[[nodiscard]] auto meta_to_user_traits(const meta_traits traits) noexcept {
+    constexpr auto shift = stl::popcount(static_cast<stl::underlying_type_t<meta_traits>>(meta_traits::_user_defined_traits));
+    return Type{static_cast<stl::underlying_type_t<Type>>(static_cast<stl::underlying_type_t<meta_traits>>(traits) >> shift)};
+}
+
+template<typename Type>
+requires stl::is_enum_v<Type>
+[[nodiscard]] auto user_to_meta_traits(const Type value) noexcept {
+    constexpr auto shift = stl::popcount(static_cast<stl::underlying_type_t<meta_traits>>(meta_traits::_user_defined_traits));
+    const auto traits = static_cast<stl::underlying_type_t<internal::meta_traits>>(static_cast<stl::underlying_type_t<Type>>(value));
+    ENTT_ASSERT(traits < ((~static_cast<stl::underlying_type_t<meta_traits>>(meta_traits::_user_defined_traits)) >> shift), "Invalid traits");
+    return meta_traits{traits << shift};
+}
+
+struct meta_type_node;
+
+struct meta_custom_node {
+    id_type id{};
+    stl::shared_ptr<void> value{};
+};
+
+struct meta_base_node {
+    id_type id{};
+    const meta_type_node &(*type)(const meta_context &) noexcept {};
+    const void *(*cast)(const void *) noexcept {};
+};
+
+struct meta_conv_node {
+    id_type id{};
+    meta_any (*conv)(const meta_ctx &, const void *){};
+};
+
+struct meta_ctor_node {
+    using size_type = stl::size_t;
+
+    id_type id{};
+    size_type arity{0u};
+    meta_type (*arg)(const meta_ctx &, const size_type) noexcept {};
+    meta_any (*invoke)(const meta_ctx &, meta_any *const){};
+};
+
+struct meta_data_node {
+    using size_type = stl::size_t;
+
+    id_type id{};
+    const char *name{};
+    meta_traits traits{meta_traits::is_none};
+    size_type set_arity{0u};
+    size_type get_arity{0u};
+    meta_type (*set_arg)(const meta_ctx &, const size_type) noexcept {};
+    meta_type (*get_arg)(const meta_ctx &, const size_type) noexcept {};
+    const meta_type_node &(*type)(const meta_context &) noexcept {};
+    bool (*set)(meta_handle, meta_any *const){};
+    meta_any (*get)(meta_handle, meta_any *const){};
+    meta_custom_node custom{};
+};
+
+struct meta_func_node {
+    using size_type = stl::size_t;
+
+    id_type id{};
+    const char *name{};
+    meta_traits traits{meta_traits::is_none};
+    size_type arity{0u};
+    const meta_type_node &(*ret)(const meta_context &) noexcept {};
+    meta_type (*arg)(const meta_ctx &, const size_type) noexcept {};
+    meta_any (*invoke)(meta_handle, meta_any *const){};
+    stl::unique_ptr<meta_func_node> next;
+    meta_custom_node custom{};
+};
+
+struct meta_template_node {
+    using size_type = stl::size_t;
+
+    size_type arity{0u};
+    const meta_type_node &(*resolve)(const meta_context &) noexcept {};
+    const meta_type_node &(*arg)(const meta_context &, const size_type) noexcept {};
+};
+
+struct meta_type_descriptor {
+    stl::vector<meta_ctor_node> ctor{};
+    stl::vector<meta_base_node> base{};
+    stl::vector<meta_conv_node> conv{};
+    stl::vector<meta_data_node> data{};
+    stl::vector<meta_func_node> func{};
+};
+
+struct meta_type_node {
+    using size_type = stl::size_t;
+
+    const type_info *info{};
+    id_type alias{};
+    const char *name{};
+    meta_traits traits{meta_traits::is_none};
+    size_type size_of{0u};
+    const meta_type_node &(*remove_pointer)(const meta_context &) noexcept {};
+    meta_any (*default_constructor)(const meta_ctx &){};
+    double (*conversion_helper)(void *, const void *){};
+    meta_any (*from_void)(const meta_ctx &, void *, const void *){};
+    meta_template_node templ{};
+    meta_custom_node custom{};
+    stl::unique_ptr<meta_type_descriptor> details{};
+};
+
+template<typename Type, typename Value>
+[[nodiscard]] auto *find_member(Type &from, const Value value) {
+    for(auto &&elem: from) {
+        if(elem.id == value) {
+            return &elem;
+        }
+    }
+
+    return static_cast<Type::value_type *>(nullptr);
+}
+
+[[nodiscard]] inline auto *find_overload(meta_func_node *curr, stl::remove_pointer_t<decltype(meta_func_node::invoke)> *const ref) {
+    while((curr != nullptr) && (curr->invoke != ref)) { curr = curr->next.get(); }
+    return curr;
+}
+
+template<auto Member>
+[[nodiscard]] auto *look_for(const meta_context &context, const meta_type_node &node, const id_type id, bool recursive) {
+    using value_type = stl::remove_reference_t<decltype((node.details.get()->*Member))>::value_type;
+
+    if(node.details) {
+        if(auto *member = find_member((node.details.get()->*Member), id); member != nullptr) {
+            return member;
+        }
+
+        if(recursive) {
+            for(auto &&curr: node.details->base) {
+                if(auto *elem = look_for<Member>(context, curr.type(context), id, recursive); elem) {
+                    return elem;
+                }
+            }
+        }
+    }
+
+    return static_cast<value_type *>(nullptr);
+}
+
+template<cvref_unqualified Type>
+const meta_type_node &resolve(const meta_context &) noexcept;
+
+template<typename... Args>
+[[nodiscard]] const meta_type_node &meta_arg_node(const meta_context &context, type_list<Args...>, const stl::size_t index) noexcept {
+    using resolve_type = const meta_type_node &(*)(const meta_context &) noexcept;
+    constexpr stl::array<resolve_type, sizeof...(Args)> list{&resolve<stl::remove_cvref_t<Args>>...};
+    ENTT_ASSERT(index < sizeof...(Args), "Out of bounds");
+    return list[index](context);
+}
+
+[[nodiscard]] inline const void *try_cast(const meta_context &context, const meta_type_node &from, const id_type to, const void *instance) noexcept {
+    if(from.details) {
+        for(auto &&curr: from.details->base) {
+            if(const void *other = curr.cast(instance); curr.id == to) {
+                return other;
+            } else if(const void *elem = try_cast(context, curr.type(context), to, other); elem) {
+                return elem;
+            }
+        }
+    }
+
+    return nullptr;
+}
+
+template<typename Type>
+auto setup_node_for() noexcept {
+    meta_type_node node{
+        &type_id<Type>(),
+        type_id<Type>().hash(),
+        nullptr,
+        (stl::is_arithmetic_v<Type> ? meta_traits::is_arithmetic : meta_traits::is_none)
+            | (stl::is_integral_v<Type> ? meta_traits::is_integral : meta_traits::is_none)
+            | (stl::is_signed_v<Type> ? meta_traits::is_signed : meta_traits::is_none)
+            | (stl::is_array_v<Type> ? meta_traits::is_array : meta_traits::is_none)
+            | (stl::is_enum_v<Type> ? meta_traits::is_enum : meta_traits::is_none)
+            | (stl::is_class_v<Type> ? meta_traits::is_class : meta_traits::is_none)
+            | (stl::is_pointer_v<Type> ? meta_traits::is_pointer : meta_traits::is_none)
+            | (is_meta_pointer_like_v<Type> ? meta_traits::is_pointer_like : meta_traits::is_none)
+            | (is_complete_v<meta_sequence_container_traits<Type>> ? meta_traits::is_sequence_container : meta_traits::is_none)
+            | (is_complete_v<meta_associative_container_traits<Type>> ? meta_traits::is_associative_container : meta_traits::is_none),
+        size_of_v<Type>,
+        &resolve<stl::remove_const_t<stl::remove_pointer_t<Type>>>};
+
+    if constexpr(stl::is_default_constructible_v<Type>) {
+        node.default_constructor = +[](const meta_ctx &ctx) {
+            return meta_any{ctx, stl::in_place_type<Type>};
+        };
+    }
+
+    if constexpr(stl::is_arithmetic_v<Type>) {
+        node.conversion_helper = +[](void *lhs, const void *rhs) {
+            return lhs ? static_cast<double>(*static_cast<Type *>(lhs) = static_cast<Type>(*static_cast<const double *>(rhs))) : static_cast<double>(*static_cast<const Type *>(rhs));
+        };
+    } else if constexpr(stl::is_enum_v<Type>) {
+        node.conversion_helper = +[](void *lhs, const void *rhs) {
+            return lhs ? static_cast<double>(*static_cast<Type *>(lhs) = static_cast<Type>(static_cast<stl::underlying_type_t<Type>>(*static_cast<const double *>(rhs)))) : static_cast<double>(*static_cast<const Type *>(rhs));
+        };
+    }
+
+    if constexpr(!stl::is_void_v<Type> && !stl::is_function_v<Type>) {
+        node.from_void = +[](const meta_ctx &ctx, void *elem, const void *celem) {
+            if(elem && celem) { // ownership construction request
+                return meta_any{ctx, stl::in_place, static_cast<stl::decay_t<Type> *>(elem)};
+            }
+
+            if(elem) { // non-const reference construction request
+                return meta_any{ctx, stl::in_place_type<stl::decay_t<Type> &>, *static_cast<stl::decay_t<Type> *>(elem)};
+            }
+
+            // const reference construction request
+            return meta_any{ctx, stl::in_place_type<const stl::decay_t<Type> &>, *static_cast<const stl::decay_t<Type> *>(celem)};
+        };
+    }
+
+    if constexpr(is_complete_v<meta_template_traits<Type>>) {
+        node.templ = meta_template_node{
+            meta_template_traits<Type>::args_type::size,
+            &resolve<typename meta_template_traits<Type>::class_type>,
+            +[](const meta_context &area, const stl::size_t index) noexcept -> decltype(auto) { return meta_arg_node(area, typename meta_template_traits<Type>::args_type{}, index); }};
+    }
+
+    return node;
+}
+
+template<cvref_unqualified Type>
+[[nodiscard]] const meta_type_node &resolve(const meta_context &context) noexcept {
+    static const meta_type_node node = setup_node_for<Type>();
+    const auto it = context.bucket.find(node.info->hash());
+    return (it == context.bucket.cend()) ? node : *it->second;
+}
+
+} // namespace internal
+/*! @endcond */
+
+} // namespace entt
+
+#endif

include/entt/meta/pointer.hpp

@@ -0,0 +1,42 @@
+// IWYU pragma: always_keep
+
+#ifndef ENTT_META_POINTER_HPP
+#define ENTT_META_POINTER_HPP
+
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "type_traits.hpp"
+
+namespace entt {
+
+/**
+ * @brief Makes `stl::shared_ptr`s of any type pointer-like types for the meta
+ * system.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+struct is_meta_pointer_like<stl::shared_ptr<Type>>
+    : stl::true_type {};
+
+/**
+ * @brief Makes `stl::unique_ptr`s of any type pointer-like types for the meta
+ * system.
+ * @tparam Type Element type.
+ * @tparam Args Other arguments.
+ */
+template<typename Type, typename... Args>
+struct is_meta_pointer_like<stl::unique_ptr<Type, Args...>>
+    : stl::true_type {};
+
+/**
+ * @brief Specialization for self-proclaimed meta pointer like types.
+ * @tparam Type Element type.
+ */
+template<typename Type>
+requires requires { typename Type::is_meta_pointer_like; }
+struct is_meta_pointer_like<Type>
+    : stl::true_type {};
+
+} // namespace entt
+
+#endif

include/entt/meta/policy.hpp

@@ -0,0 +1,81 @@
+#ifndef ENTT_META_POLICY_HPP
+#define ENTT_META_POLICY_HPP
+
+#include "../stl/type_traits.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+struct meta_policy {};
+
+} // namespace internal
+/*! @endcond */
+
+/*! @brief Empty class type used to request the _as-is_ policy. */
+struct as_value_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename>
+    static constexpr bool value = true;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as void_ policy. */
+struct as_void_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename>
+    static constexpr bool value = true;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as ref_ policy. */
+struct as_ref_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename Type>
+    static constexpr bool value = stl::is_reference_v<Type> && !stl::is_const_v<stl::remove_reference_t<Type>>;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as cref_ policy. */
+struct as_cref_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename Type>
+    static constexpr bool value = stl::is_reference_v<Type>;
+    /*! @endcond */
+};
+
+/*! @brief Empty class type used to request the _as auto_ policy. */
+struct as_is_t final: private internal::meta_policy {
+    /*! @cond ENTT_INTERNAL */
+    template<typename>
+    static constexpr bool value = true;
+    /*! @endcond */
+};
+
+/**
+ * @brief Provides the member constant `value` equal to true if a type also is a
+ * meta policy, false otherwise.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+struct is_meta_policy
+    : stl::bool_constant<stl::is_base_of_v<internal::meta_policy, Type>> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+inline constexpr bool is_meta_policy_v = is_meta_policy<Type>::value;
+
+/**
+ * @brief Specifies whether a type is a meta policy.
+ * @tparam Type Type to check.
+ */
+template<typename Type>
+concept meta_policy = is_meta_policy_v<Type>;
+
+} // namespace entt
+
+#endif

include/entt/meta/range.hpp

@@ -0,0 +1,119 @@
+#ifndef ENTT_META_RANGE_HPP
+#define ENTT_META_RANGE_HPP
+
+#include <compare>
+#include "../core/fwd.hpp"
+#include "../core/iterator.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/utility.hpp"
+#include "context.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+struct meta_base_node;
+
+template<typename Type, typename It>
+struct meta_range_iterator final {
+    using value_type = stl::pair<id_type, Type>;
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using difference_type = stl::ptrdiff_t;
+    using iterator_category = stl::input_iterator_tag;
+    using iterator_concept = stl::random_access_iterator_tag;
+
+    constexpr meta_range_iterator() noexcept
+        : it{},
+          ctx{} {}
+
+    constexpr meta_range_iterator(const meta_ctx &area, const It iter) noexcept
+        : it{iter},
+          ctx{&area} {}
+
+    constexpr meta_range_iterator &operator++() noexcept {
+        return ++it, *this;
+    }
+
+    constexpr meta_range_iterator operator++(int) noexcept {
+        const meta_range_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    constexpr meta_range_iterator &operator--() noexcept {
+        return --it, *this;
+    }
+
+    constexpr meta_range_iterator operator--(int) noexcept {
+        const meta_range_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    constexpr meta_range_iterator &operator+=(const difference_type value) noexcept {
+        it += value;
+        return *this;
+    }
+
+    constexpr meta_range_iterator operator+(const difference_type value) const noexcept {
+        meta_range_iterator copy = *this;
+        return (copy += value);
+    }
+
+    constexpr meta_range_iterator &operator-=(const difference_type value) noexcept {
+        return (*this += -value);
+    }
+
+    constexpr meta_range_iterator operator-(const difference_type value) const noexcept {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
+        if constexpr(stl::is_same_v<It, typename meta_context::bucket_type::const_iterator>) {
+            return {it[value].first, Type{*ctx, *it[value].second}};
+        } else {
+            return {it[value].id, Type{*ctx, it[value]}};
+        }
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return operator*();
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return operator[](0);
+    }
+
+    [[nodiscard]] constexpr stl::ptrdiff_t operator-(const meta_range_iterator &other) const noexcept {
+        return it - other.it;
+    }
+
+    [[nodiscard]] constexpr bool operator==(const meta_range_iterator &other) const noexcept {
+        return it == other.it;
+    }
+
+    [[nodiscard]] constexpr auto operator<=>(const meta_range_iterator &other) const noexcept {
+        return it <=> other.it;
+    }
+
+private:
+    It it;
+    const meta_ctx *ctx;
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Iterable range to use to iterate all types of meta objects.
+ * @tparam Type Type of meta objects returned.
+ * @tparam It Type of forward iterator.
+ */
+template<typename Type, stl::forward_iterator It>
+using meta_range = iterable_adaptor<internal::meta_range_iterator<Type, It>>;
+
+} // namespace entt
+
+#endif

include/entt/meta/resolve.hpp

@@ -0,0 +1,109 @@
+#ifndef ENTT_META_RESOLVE_HPP
+#define ENTT_META_RESOLVE_HPP
+
+#include "../core/type_info.hpp"
+#include "../locator/locator.hpp"
+#include "../stl/type_traits.hpp"
+#include "context.hpp"
+#include "meta.hpp"
+#include "node.hpp"
+#include "range.hpp"
+
+namespace entt {
+
+/**
+ * @brief Returns the meta type associated with a given type.
+ * @tparam Type Type to use to search for a meta type.
+ * @param ctx The context from which to search for meta types.
+ * @return The meta type associated with the given type, if any.
+ */
+template<typename Type>
+[[nodiscard]] meta_type resolve(const meta_ctx &ctx) noexcept {
+    const auto &context = internal::meta_context::from(ctx);
+    return {ctx, internal::resolve<stl::remove_cvref_t<Type>>(context)};
+}
+
+/**
+ * @brief Returns the meta type associated with a given type.
+ * @tparam Type Type to use to search for a meta type.
+ * @return The meta type associated with the given type, if any.
+ */
+template<typename Type>
+[[nodiscard]] meta_type resolve() noexcept {
+    return resolve<Type>(locator<meta_ctx>::value_or());
+}
+
+/**
+ * @brief Returns a range to use to visit all meta types.
+ * @param ctx The context from which to search for meta types.
+ * @return An iterable range to use to visit all meta types.
+ */
+[[nodiscard]] inline meta_range<meta_type, typename internal::meta_context::bucket_type::const_iterator> resolve(const meta_ctx &ctx) noexcept {
+    const auto &context = internal::meta_context::from(ctx);
+    return {{ctx, context.bucket.cbegin()}, {ctx, context.bucket.cend()}};
+}
+
+/**
+ * @brief Returns a range to use to visit all meta types.
+ * @return An iterable range to use to visit all meta types.
+ */
+[[nodiscard]] inline meta_range<meta_type, typename internal::meta_context::bucket_type::const_iterator> resolve() noexcept {
+    return resolve(locator<meta_ctx>::value_or());
+}
+
+/**
+ * @brief Returns the meta type associated with a given identifier, if any.
+ * @param ctx The context from which to search for meta types.
+ * @param alias Unique identifier.
+ * @return The meta type associated with the given identifier, if any.
+ */
+[[nodiscard]] inline meta_type resolve(const meta_ctx &ctx, const id_type alias) noexcept {
+    const auto &context = internal::meta_context::from(ctx);
+
+    // fast lookup for unsearchable and overloaded types
+    if(const auto it = context.bucket.find(alias); it != context.bucket.end()) {
+        return meta_type{ctx, *it->second};
+    }
+
+    for(auto &&curr: context.bucket) {
+        if(curr.second->alias == alias) {
+            return meta_type{ctx, *curr.second};
+        }
+    }
+
+    return meta_type{};
+}
+
+/**
+ * @brief Returns the meta type associated with a given identifier, if any.
+ * @param alias Unique identifier.
+ * @return The meta type associated with the given identifier, if any.
+ */
+[[nodiscard]] inline meta_type resolve(const id_type alias) noexcept {
+    return resolve(locator<meta_ctx>::value_or(), alias);
+}
+
+/**
+ * @brief Returns the meta type associated with a given type info object.
+ * @param ctx The context from which to search for meta types.
+ * @param info The type info object of the requested type.
+ * @return The meta type associated with the given type info object, if any.
+ */
+[[nodiscard]] inline meta_type resolve(const meta_ctx &ctx, const type_info &info) noexcept {
+    const auto &context = internal::meta_context::from(ctx);
+    const auto it = context.bucket.find(info.hash());
+    return (it == context.bucket.cend()) ? meta_type{} : meta_type{ctx, *it->second};
+}
+
+/**
+ * @brief Returns the meta type associated with a given type info object.
+ * @param info The type info object of the requested type.
+ * @return The meta type associated with the given type info object, if any.
+ */
+[[nodiscard]] inline meta_type resolve(const type_info &info) noexcept {
+    return resolve(locator<meta_ctx>::value_or(), info);
+}
+
+} // namespace entt
+
+#endif

include/entt/meta/template.hpp

@@ -0,0 +1,29 @@
+// IWYU pragma: always_keep
+
+#ifndef ENTT_META_TEMPLATE_HPP
+#define ENTT_META_TEMPLATE_HPP
+
+#include "../core/type_traits.hpp"
+
+namespace entt {
+
+/*! @brief Utility class to disambiguate class templates. */
+template<template<typename...> class>
+struct meta_class_template_tag {};
+
+/**
+ * @brief General purpose traits class for generating meta template information.
+ * @tparam Clazz Type of class template.
+ * @tparam Args Types of template arguments.
+ */
+template<template<typename...> class Clazz, typename... Args>
+struct meta_template_traits<Clazz<Args...>> {
+    /*! @brief Wrapped class template. */
+    using class_type = meta_class_template_tag<Clazz>;
+    /*! @brief List of template arguments. */
+    using args_type = type_list<Args...>;
+};
+
+} // namespace entt
+
+#endif

include/entt/meta/type_traits.hpp

@@ -0,0 +1,54 @@
+#ifndef ENTT_META_TYPE_TRAITS_HPP
+#define ENTT_META_TYPE_TRAITS_HPP
+
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+
+namespace entt {
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * template information.
+ */
+template<typename>
+struct meta_template_traits;
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * sequence containers.
+ */
+template<typename>
+struct meta_sequence_container_traits;
+
+/**
+ * @brief Traits class template to be specialized to enable support for meta
+ * associative containers.
+ */
+template<typename>
+struct meta_associative_container_traits;
+
+/**
+ * @brief Provides the member constant `value` equal to true if a given type is
+ * a pointer-like type, false otherwise.
+ */
+template<typename>
+struct is_meta_pointer_like: stl::false_type {};
+
+/**
+ * @brief Partial specialization to ensure that const pointer-like types are
+ * also accepted.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename Type>
+struct is_meta_pointer_like<const Type>: is_meta_pointer_like<Type> {};
+
+/**
+ * @brief Helper variable template.
+ * @tparam Type Potentially pointer-like type.
+ */
+template<typename Type>
+inline constexpr auto is_meta_pointer_like_v = is_meta_pointer_like<Type>::value;
+
+} // namespace entt
+
+#endif

include/entt/meta/utility.hpp

@@ -0,0 +1,500 @@
+#ifndef ENTT_META_UTILITY_HPP
+#define ENTT_META_UTILITY_HPP
+
+#include "../core/type_traits.hpp"
+#include "../locator/locator.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "meta.hpp"
+#include "node.hpp"
+#include "policy.hpp"
+
+namespace entt {
+
+/**
+ * @brief Meta function descriptor traits.
+ * @tparam Ret Function return type.
+ * @tparam Args Function arguments.
+ * @tparam Static Function staticness.
+ * @tparam Const Function constness.
+ */
+template<typename Ret, typename Args, bool Static, bool Const>
+struct meta_function_descriptor_traits {
+    /*! @brief Meta function return type. */
+    using return_type = Ret;
+    /*! @brief Meta function arguments. */
+    using args_type = Args;
+
+    /*! @brief True if the meta function is static, false otherwise. */
+    static constexpr bool is_static = Static;
+    /*! @brief True if the meta function is const, false otherwise. */
+    static constexpr bool is_const = Const;
+};
+
+/*! @brief Primary template isn't defined on purpose. */
+template<typename, typename>
+struct meta_function_descriptor;
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam Class Actual owner of the member function.
+ * @tparam Args Function arguments.
+ */
+template<typename Type, typename Ret, typename Class, typename... Args>
+struct meta_function_descriptor<Type, Ret (Class::*)(Args...) const>
+    : meta_function_descriptor_traits<
+          Ret,
+          stl::conditional_t<stl::is_base_of_v<Class, Type>, type_list<Args...>, type_list<const Class &, Args...>>,
+          !stl::is_base_of_v<Class, Type>,
+          true> {};
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam Class Actual owner of the member function.
+ * @tparam Args Function arguments.
+ */
+template<typename Type, typename Ret, typename Class, typename... Args>
+struct meta_function_descriptor<Type, Ret (Class::*)(Args...)>
+    : meta_function_descriptor_traits<
+          Ret,
+          stl::conditional_t<stl::is_base_of_v<Class, Type>, type_list<Args...>, type_list<Class &, Args...>>,
+          !stl::is_base_of_v<Class, Type>,
+          false> {};
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta data is associated.
+ * @tparam Class Actual owner of the data member.
+ * @tparam Ret Data member type.
+ */
+template<typename Type, typename Ret, typename Class>
+struct meta_function_descriptor<Type, Ret Class::*>
+    : meta_function_descriptor_traits<
+          Ret &,
+          stl::conditional_t<stl::is_base_of_v<Class, Type>, type_list<>, type_list<Class &>>,
+          !stl::is_base_of_v<Class, Type>,
+          false> {};
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ * @tparam MaybeType First function argument.
+ * @tparam Args Other function arguments.
+ */
+template<typename Type, typename Ret, typename MaybeType, typename... Args>
+struct meta_function_descriptor<Type, Ret (*)(MaybeType, Args...)>
+    : meta_function_descriptor_traits<
+          Ret,
+          stl::conditional_t<
+              stl::is_same_v<stl::remove_cvref_t<MaybeType>, Type> || stl::is_base_of_v<stl::remove_cvref_t<MaybeType>, Type>,
+              type_list<Args...>,
+              type_list<MaybeType, Args...>>,
+          !(stl::is_same_v<stl::remove_cvref_t<MaybeType>, Type> || stl::is_base_of_v<stl::remove_cvref_t<MaybeType>, Type>),
+          stl::is_const_v<stl::remove_reference_t<MaybeType>> && (stl::is_same_v<stl::remove_cvref_t<MaybeType>, Type> || stl::is_base_of_v<stl::remove_cvref_t<MaybeType>, Type>)> {};
+
+/**
+ * @brief Meta function descriptor.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Ret Function return type.
+ */
+template<typename Type, typename Ret>
+struct meta_function_descriptor<Type, Ret (*)()>
+    : meta_function_descriptor_traits<
+          Ret,
+          type_list<>,
+          true,
+          false> {};
+
+/**
+ * @brief Meta function helper.
+ *
+ * Converts a function type to be associated with a reflected type into its meta
+ * function descriptor.
+ *
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Candidate The actual function to associate with the reflected type.
+ */
+template<typename Type, typename Candidate>
+class meta_function_helper {
+    template<typename Ret, typename... Args, typename Class>
+    static meta_function_descriptor<Type, Ret (Class::*)(Args...) const> get_rid_of_noexcept(Ret (Class::*)(Args...) const);
+
+    template<typename Ret, typename... Args, typename Class>
+    static meta_function_descriptor<Type, Ret (Class::*)(Args...)> get_rid_of_noexcept(Ret (Class::*)(Args...));
+
+    template<typename Ret, typename Class>
+    requires stl::is_member_object_pointer_v<Ret Class::*>
+    static meta_function_descriptor<Type, Ret Class::*> get_rid_of_noexcept(Ret Class::*);
+
+    template<typename Ret, typename... Args>
+    static meta_function_descriptor<Type, Ret (*)(Args...)> get_rid_of_noexcept(Ret (*)(Args...));
+
+    template<typename Class>
+    static meta_function_descriptor<Class, decltype(&Class::operator())> get_rid_of_noexcept(Class);
+
+public:
+    /*! @brief The meta function descriptor of the given function. */
+    using type = decltype(get_rid_of_noexcept(stl::declval<Candidate>()));
+};
+
+/**
+ * @brief Helper type.
+ * @tparam Type Reflected type to which the meta function is associated.
+ * @tparam Candidate The actual function to associate with the reflected type.
+ */
+template<typename Type, typename Candidate>
+using meta_function_helper_t = meta_function_helper<Type, Candidate>::type;
+
+/**
+ * @brief Wraps a value depending on the given policy.
+ *
+ * This function always returns a wrapped value in the requested context.<br/>
+ * Therefore, if the passed value is itself a wrapped object with a different
+ * context, it undergoes a rebinding to the requested context.
+ *
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Type Type of value to wrap.
+ * @param ctx The context from which to search for meta types.
+ * @param value Value to wrap.
+ * @return A meta any containing the returned value, if any.
+ */
+template<meta_policy Policy = as_value_t, typename Type>
+[[nodiscard]] meta_any meta_dispatch(const meta_ctx &ctx, [[maybe_unused]] Type &&value) {
+    if constexpr(stl::is_same_v<Policy, as_cref_t>) {
+        static_assert(stl::is_lvalue_reference_v<Type>, "Invalid type");
+        return meta_any{ctx, stl::in_place_type<const stl::remove_reference_t<Type> &>, stl::as_const(value)};
+    } else if constexpr(stl::is_same_v<Policy, as_ref_t> || (stl::is_same_v<Policy, as_is_t> && stl::is_lvalue_reference_v<Type>)) {
+        return meta_any{ctx, stl::in_place_type<Type>, value};
+    } else if constexpr(stl::is_same_v<Policy, as_void_t>) {
+        return meta_any{ctx, stl::in_place_type<void>};
+    } else {
+        return meta_any{ctx, stl::forward<Type>(value)};
+    }
+}
+
+/**
+ * @brief Wraps a value depending on the given policy.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Type Type of value to wrap.
+ * @param value Value to wrap.
+ * @return A meta any containing the returned value, if any.
+ */
+template<meta_policy Policy = as_value_t, typename Type>
+[[nodiscard]] meta_any meta_dispatch(Type &&value) {
+    return meta_dispatch<Policy, Type>(locator<meta_ctx>::value_or(), stl::forward<Type>(value));
+}
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename Policy, typename Candidate, typename... Args>
+[[nodiscard]] meta_any meta_invoke_with_args(const meta_ctx &ctx, Candidate &&candidate, Args &&...args) {
+    if constexpr(stl::is_void_v<decltype(stl::invoke(stl::forward<Candidate>(candidate), args...))>) {
+        stl::invoke(stl::forward<Candidate>(candidate), args...);
+        return meta_any{ctx, stl::in_place_type<void>};
+    } else {
+        return meta_dispatch<Policy>(ctx, stl::invoke(stl::forward<Candidate>(candidate), args...));
+    }
+}
+
+template<typename Type, typename Policy, typename Candidate, stl::size_t... Index>
+[[nodiscard]] meta_any meta_invoke(meta_any &instance, Candidate &&candidate, [[maybe_unused]] meta_any *const args, stl::index_sequence<Index...>) {
+    using descriptor = meta_function_helper_t<Type, stl::remove_reference_t<Candidate>>;
+
+    // NOLINTBEGIN(cppcoreguidelines-pro-bounds-pointer-arithmetic) - waiting for C++20 (and stl::span)
+    if constexpr(stl::is_invocable_v<stl::remove_reference_t<Candidate>, const Type &, type_list_element_t<Index, typename descriptor::args_type>...>) {
+        if(const auto *const clazz = instance.try_cast<const Type>(); clazz && ((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return meta_invoke_with_args<Policy>(instance.context(), stl::forward<Candidate>(candidate), *clazz, (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    } else if constexpr(stl::is_invocable_v<stl::remove_reference_t<Candidate>, Type &, type_list_element_t<Index, typename descriptor::args_type>...>) {
+        if(auto *const clazz = instance.try_cast<Type>(); clazz && ((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return meta_invoke_with_args<Policy>(instance.context(), stl::forward<Candidate>(candidate), *clazz, (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    } else {
+        if(((args + Index)->allow_cast<type_list_element_t<Index, typename descriptor::args_type>>() && ...)) {
+            return meta_invoke_with_args<Policy>(instance.context(), stl::forward<Candidate>(candidate), (args + Index)->cast<type_list_element_t<Index, typename descriptor::args_type>>()...);
+        }
+    }
+    // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+
+    return meta_any{meta_ctx_arg, instance.context()};
+}
+
+template<typename Type, typename... Args, stl::size_t... Index>
+[[nodiscard]] meta_any meta_construct(const meta_ctx &ctx, meta_any *const args, stl::index_sequence<Index...>) {
+    // NOLINTBEGIN(cppcoreguidelines-pro-bounds-pointer-arithmetic) - waiting for C++20 (and stl::span)
+    if(((args + Index)->allow_cast<Args>() && ...)) {
+        return meta_any{ctx, stl::in_place_type<Type>, (args + Index)->cast<Args>()...};
+    }
+    // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
+
+    return meta_any{meta_ctx_arg, ctx};
+}
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Returns the meta type of the i-th element of a list of arguments.
+ * @tparam Type Type list of the actual types of arguments.
+ * @param ctx The context from which to search for meta types.
+ * @param index The index of the element for which to return the meta type.
+ * @return The meta type of the i-th element of the list of arguments.
+ */
+template<typename Type>
+[[nodiscard]] meta_type meta_arg(const meta_ctx &ctx, const stl::size_t index) noexcept {
+    const auto &context = internal::meta_context::from(ctx);
+    return {ctx, internal::meta_arg_node(context, Type{}, index)};
+}
+
+/**
+ * @brief Returns the meta type of the i-th element of a list of arguments.
+ * @tparam Type Type list of the actual types of arguments.
+ * @param index The index of the element for which to return the meta type.
+ * @return The meta type of the i-th element of the list of arguments.
+ */
+template<typename Type>
+[[nodiscard]] meta_type meta_arg(const stl::size_t index) noexcept {
+    return meta_arg<Type>(locator<meta_ctx>::value_or(), index);
+}
+
+/**
+ * @brief Sets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to set.
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param args Parameters to use to set the variable.
+ * @return True in case of success, false otherwise.
+ */
+template<typename Type, auto Data>
+[[nodiscard]] bool meta_setter([[maybe_unused]] meta_handle instance, [[maybe_unused]] meta_any *const args) {
+    if constexpr(stl::is_member_function_pointer_v<decltype(Data)> || stl::is_function_v<stl::remove_reference_t<stl::remove_pointer_t<decltype(Data)>>>) {
+        return static_cast<bool>(internal::meta_invoke<Type, as_void_t>(*instance.operator->(), Data, args, stl::make_index_sequence<meta_function_helper_t<Type, decltype(Data)>::args_type::size>{}));
+    } else if constexpr(stl::is_member_object_pointer_v<decltype(Data)>) {
+        using data_type = stl::remove_reference_t<typename meta_function_helper_t<Type, decltype(Data)>::return_type>;
+
+        if constexpr(!stl::is_array_v<data_type> && !stl::is_const_v<data_type>) {
+            if(auto *const clazz = instance->try_cast<Type>(); clazz && args->allow_cast<data_type>()) {
+                stl::invoke(Data, *clazz) = args->cast<data_type>();
+                return true;
+            }
+        }
+
+        return false;
+    } else if constexpr(stl::is_pointer_v<decltype(Data)>) {
+        using data_type = stl::remove_reference_t<decltype(*Data)>;
+
+        if constexpr(!stl::is_array_v<data_type> && !stl::is_const_v<data_type>) {
+            if(args->allow_cast<data_type>()) {
+                *Data = args->cast<data_type>();
+                return true;
+            }
+        }
+
+        return false;
+    } else {
+        return false;
+    }
+}
+
+/**
+ * @brief Sets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to set.
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param value Parameter to use to set the variable.
+ * @return True in case of success, false otherwise.
+ */
+template<typename Type, auto Data>
+[[nodiscard]] bool meta_setter(meta_handle instance, meta_any value) {
+    return meta_setter<Type, Data>(*instance.operator->(), &value);
+}
+
+/**
+ * @brief Gets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to get.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param args Parameters to use to set the variable.
+ * @return A meta any containing the value of the underlying variable.
+ */
+template<typename Type, auto Data, meta_policy Policy = as_value_t>
+[[nodiscard]] meta_any meta_getter(meta_handle instance, [[maybe_unused]] meta_any *const args) {
+    if constexpr(stl::is_member_function_pointer_v<decltype(Data)> || stl::is_function_v<stl::remove_reference_t<stl::remove_pointer_t<decltype(Data)>>>) {
+        return internal::meta_invoke<Type, Policy>(*instance.operator->(), Data, args, stl::make_index_sequence<meta_function_helper_t<Type, decltype(Data)>::args_type::size>{});
+    } else if constexpr(stl::is_member_object_pointer_v<decltype(Data)>) {
+        if constexpr(!stl::is_array_v<stl::remove_cvref_t<stl::invoke_result_t<decltype(Data), Type &>>>) {
+            if(auto *clazz = instance->try_cast<Type>(); clazz) {
+                return meta_dispatch<Policy>(instance->context(), stl::invoke(Data, *clazz));
+            } else if(auto *fallback = instance->try_cast<const Type>(); fallback) {
+                return meta_dispatch<Policy>(instance->context(), stl::invoke(Data, *fallback));
+            }
+        }
+
+        return meta_any{meta_ctx_arg, instance->context()};
+    } else if constexpr(stl::is_pointer_v<decltype(Data)>) {
+        if constexpr(stl::is_array_v<stl::remove_pointer_t<decltype(Data)>>) {
+            return meta_any{meta_ctx_arg, instance->context()};
+        } else {
+            return meta_dispatch<Policy>(instance->context(), *Data);
+        }
+    } else {
+        return meta_dispatch<Policy>(instance->context(), Data);
+    }
+}
+
+/**
+ * @brief Gets the value of a given variable.
+ * @tparam Type Reflected type to which the variable is associated.
+ * @tparam Data The actual variable to get.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param instance An opaque instance of the underlying type, if required.
+ * @return A meta any containing the value of the underlying variable.
+ */
+template<typename Type, auto Data, meta_policy Policy = as_value_t>
+[[nodiscard]] meta_any meta_getter(meta_handle instance) {
+    return meta_getter<Type, Data, Policy>(*instance.operator->(), nullptr);
+}
+
+/**
+ * @brief Tries to _invoke_ an object given a list of erased parameters.
+ * @tparam Type Reflected type to which the object to _invoke_ is associated.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Candidate The type of the actual object to _invoke_.
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param candidate The actual object to _invoke_.
+ * @param args Parameters to use to _invoke_ the object.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, meta_policy Policy = as_value_t, typename Candidate>
+[[nodiscard]] meta_any meta_invoke(meta_handle instance, Candidate &&candidate, meta_any *const args) {
+    return internal::meta_invoke<Type, Policy>(*instance.operator->(), stl::forward<Candidate>(candidate), args, stl::make_index_sequence<meta_function_helper_t<Type, stl::remove_reference_t<Candidate>>::args_type::size>{});
+}
+
+/**
+ * @brief Tries to invoke a function given a list of erased parameters.
+ * @tparam Type Reflected type to which the function is associated.
+ * @tparam Candidate The actual function to invoke.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param instance An opaque instance of the underlying type, if required.
+ * @param args Parameters to use to invoke the function.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, auto Candidate, meta_policy Policy = as_value_t>
+[[nodiscard]] meta_any meta_invoke(meta_handle instance, meta_any *const args) {
+    return internal::meta_invoke<Type, Policy>(*instance.operator->(), Candidate, args, stl::make_index_sequence<meta_function_helper_t<Type, stl::remove_reference_t<decltype(Candidate)>>::args_type::size>{});
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ *
+ * @warning
+ * The context provided is used only for the return type.<br/>
+ * It's up to the caller to bind the arguments to the right context(s).
+ *
+ * @tparam Type Actual type of the instance to construct.
+ * @tparam Args Types of arguments expected.
+ * @param ctx The context from which to search for meta types.
+ * @param args Parameters to use to construct the instance.
+ * @return A meta any containing the new instance, if any.
+ */
+template<typename Type, typename... Args>
+[[nodiscard]] meta_any meta_construct(const meta_ctx &ctx, meta_any *const args) {
+    return internal::meta_construct<Type, Args...>(ctx, args, stl::index_sequence_for<Args...>{});
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ * @tparam Type Actual type of the instance to construct.
+ * @tparam Args Types of arguments expected.
+ * @param args Parameters to use to construct the instance.
+ * @return A meta any containing the new instance, if any.
+ */
+template<typename Type, typename... Args>
+[[nodiscard]] meta_any meta_construct(meta_any *const args) {
+    return meta_construct<Type, Args...>(locator<meta_ctx>::value_or(), args);
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ *
+ * @warning
+ * The context provided is used only for the return type.<br/>
+ * It's up to the caller to bind the arguments to the right context(s).
+ *
+ * @tparam Type Reflected type to which the object to _invoke_ is associated.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Candidate The type of the actual object to _invoke_.
+ * @param ctx The context from which to search for meta types.
+ * @param candidate The actual object to _invoke_.
+ * @param args Parameters to use to _invoke_ the object.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, typename Policy = as_value_t, typename Candidate>
+[[nodiscard]] meta_any meta_construct(const meta_ctx &ctx, Candidate &&candidate, meta_any *const args) {
+    if constexpr(meta_function_helper_t<Type, Candidate>::is_static || stl::is_class_v<stl::remove_cvref_t<Candidate>>) {
+        meta_any placeholder{meta_ctx_arg, ctx};
+        return internal::meta_invoke<Type, Policy>(placeholder, stl::forward<Candidate>(candidate), args, stl::make_index_sequence<meta_function_helper_t<Type, stl::remove_reference_t<Candidate>>::args_type::size>{});
+    } else {
+        // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-pointer-arithmetic) - waiting for C++20 (and stl::span)
+        return internal::meta_invoke<Type, Policy>(*args, stl::forward<Candidate>(candidate), args + 1u, stl::make_index_sequence<meta_function_helper_t<Type, stl::remove_reference_t<Candidate>>::args_type::size>{});
+    }
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ * @tparam Type Reflected type to which the object to _invoke_ is associated.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @tparam Candidate The type of the actual object to _invoke_.
+ * @param candidate The actual object to _invoke_.
+ * @param args Parameters to use to _invoke_ the object.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, meta_policy Policy = as_value_t, typename Candidate>
+[[nodiscard]] meta_any meta_construct(Candidate &&candidate, meta_any *const args) {
+    return meta_construct<Type, Policy>(locator<meta_ctx>::value_or(), stl::forward<Candidate>(candidate), args);
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ *
+ * @warning
+ * The context provided is used only for the return type.<br/>
+ * It's up to the caller to bind the arguments to the right context(s).
+ *
+ * @tparam Type Reflected type to which the function is associated.
+ * @tparam Candidate The actual function to invoke.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param ctx The context from which to search for meta types.
+ * @param args Parameters to use to invoke the function.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, auto Candidate, meta_policy Policy = as_value_t>
+[[nodiscard]] meta_any meta_construct(const meta_ctx &ctx, meta_any *const args) {
+    return meta_construct<Type, Policy>(ctx, Candidate, args);
+}
+
+/**
+ * @brief Tries to construct an instance given a list of erased parameters.
+ * @tparam Type Reflected type to which the function is associated.
+ * @tparam Candidate The actual function to invoke.
+ * @tparam Policy Optional policy (no policy set by default).
+ * @param args Parameters to use to invoke the function.
+ * @return A meta any containing the returned value, if any.
+ */
+template<typename Type, auto Candidate, meta_policy Policy = as_value_t>
+[[nodiscard]] meta_any meta_construct(meta_any *const args) {
+    return meta_construct<Type, Candidate, Policy>(locator<meta_ctx>::value_or(), args);
+}
+
+} // namespace entt
+
+#endif

include/entt/natvis/config.natvis

@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+</AutoVisualizer>

include/entt/natvis/container.natvis

@@ -0,0 +1,39 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::dense_map&lt;*&gt;">
+		<Intrinsic Name="size" Expression="packed.first_base::value.size()"/>
+		<Intrinsic Name="bucket_count" Expression="sparse.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">packed.first_base::value.capacity()</Item>
+			<Item Name="[bucket_count]" ExcludeView="simple">bucket_count()</Item>
+			<Item Name="[load_factor]" ExcludeView="simple">(float)size() / (float)bucket_count()</Item>
+			<Item Name="[max_load_factor]" ExcludeView="simple">threshold</Item>
+			<IndexListItems>
+				<Size>size()</Size>
+				<ValueNode>packed.first_base::value[$i].element</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::dense_set&lt;*&gt;">
+		<Intrinsic Name="size" Expression="packed.first_base::value.size()"/>
+		<Intrinsic Name="bucket_count" Expression="sparse.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">packed.first_base::value.capacity()</Item>
+			<Item Name="[bucket_count]" ExcludeView="simple">bucket_count()</Item>
+			<Item Name="[load_factor]" ExcludeView="simple">(float)size() / (float)bucket_count()</Item>
+			<Item Name="[max_load_factor]" ExcludeView="simple">threshold</Item>
+			<IndexListItems>
+				<Size>size()</Size>
+				<ValueNode>packed.first_base::value[$i].second</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_table&lt;*&gt;">
+		<DisplayString>{ payload }</DisplayString>
+		<Expand>
+			<ExpandedItem>payload</ExpandedItem>
+		</Expand>
+	</Type>
+</AutoVisualizer>

include/entt/natvis/core.natvis

@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+    <Type Name="entt::basic_any&lt;*&gt;">
+		<DisplayString>{{ policy={ mode,en } }}</DisplayString>
+    </Type>
+	<Type Name="entt::compressed_pair&lt;*&gt;">
+		<Intrinsic Name="first" Optional="true" Expression="((first_base*)this)->value"/>
+		<Intrinsic Name="first" Optional="true" Expression="*(first_base::base_type*)this"/>
+		<Intrinsic Name="second" Optional="true" Expression="((second_base*)this)->value"/>
+		<Intrinsic Name="second" Optional="true" Expression="*(second_base::base_type*)this"/>
+		<DisplayString>({ first() }, { second() })</DisplayString>
+		<Expand>
+			<Item Name="[first]">first()</Item>
+			<Item Name="[second]">second()</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_hashed_string&lt;*&gt;">
+		<DisplayString Condition="base_type::repr != nullptr">{{ hash={ base_type::hash } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[data]">base_type::repr,na</Item>
+			<Item Name="[length]">base_type::length</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::type_info">
+		<DisplayString>{{ name={ alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[hash]">identifier</Item>
+			<Item Name="[index]">seq</Item>
+		</Expand>
+	</Type>
+</AutoVisualizer>

include/entt/natvis/entity.natvis

@@ -0,0 +1,181 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::basic_registry&lt;*&gt;">
+		<DisplayString>{{ pools={ pools.size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[entities]">entities</Item>
+			<Synthetic Name="[pools]">
+				<DisplayString>{ pools.size() }</DisplayString>
+				<Expand>
+					<CustomListItems>
+						<Variable Name="pos" InitialValue="0" />
+						<Variable Name="last" InitialValue="pools.size()"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<Item Name="[{ pools.packed.first_base::value[pos].element.first }]">
+								*pools.packed.first_base::value[pos].element.second,view(simple)
+							</Item>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+			<Item Name="[groups]">groups.size()</Item>
+			<Synthetic Name="[vars]">
+				<DisplayString>{ vars.ctx.size() }</DisplayString>
+				<Expand>
+					<CustomListItems>
+						<Variable Name="pos" InitialValue="0" />
+						<Variable Name="last" InitialValue="vars.ctx.size()"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<Item Name="[{ vars.ctx.packed.first_base::value[pos].element.first }]">
+								vars.ctx.packed.first_base::value[pos].element.second
+							</Item>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_sparse_set&lt;*&gt;">
+		<Intrinsic Name="is_valid_position" Expression="sparse[page] &amp;&amp; ((*((traits_type::entity_type *)&amp;sparse[page][offset]) &amp; traits_type::entity_mask) != traits_type::entity_mask)">
+			<Parameter Name="page" Type="traits_type::entity_type"/>
+			<Parameter Name="offset" Type="traits_type::entity_type"/>
+		</Intrinsic>
+		<Intrinsic Name="is_valid_entity" Expression="!traits_type::version_mask || (*((traits_type::entity_type *)&amp;entity) &lt; (traits_type::version_mask &lt;&lt; traits_type::length))">
+			<Parameter Name="entity" Type="const traits_type::value_type &amp;"/>
+		</Intrinsic>
+		<DisplayString>{{ size={ packed.size() }, type={ descriptor->alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">packed.capacity()</Item>
+			<Item Name="[policy]" ExcludeView="simple">mode,en</Item>
+			<Item Name="[free_list]" ExcludeView="simple">head</Item>
+			<Synthetic Name="[sparse]">
+				<DisplayString>{ sparse.size() * traits_type::page_size }</DisplayString>
+				<Expand>
+					<CustomListItems>
+						<Variable Name="pos" InitialValue="0"/>
+						<Variable Name="page" InitialValue="0"/>
+						<Variable Name="offset" InitialValue="0"/>
+						<Variable Name="last" InitialValue="sparse.size() * traits_type::page_size"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<Exec>page = pos / traits_type::page_size</Exec>
+							<Exec>offset = pos &amp; (traits_type::page_size - 1)</Exec>
+							<If Condition="is_valid_position(page, offset)">
+								<Item Name="[{ pos }]">*((traits_type::entity_type *)&amp;sparse[page][offset]) &amp; traits_type::entity_mask</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+			<Synthetic Name="[packed]">
+				<DisplayString>{ packed.size() }</DisplayString>
+				<Expand>
+					<CustomListItems>
+						<Variable Name="pos" InitialValue="0"/>
+						<Variable Name="last" InitialValue="packed.size()"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<If Condition="is_valid_entity(packed[pos])">
+								<Item Name="[{ pos }]">packed[pos]</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_storage&lt;*&gt;">
+		<Intrinsic Name="is_valid_entity" Expression="!base_type::traits_type::version_mask || (*((base_type::traits_type::entity_type *)&amp;entity) &lt; (base_type::traits_type::version_mask &lt;&lt; base_type::traits_type::length))">
+			<Parameter Name="entity" Type="const base_type::traits_type::value_type &amp;"/>
+		</Intrinsic>
+		<DisplayString>{{ size={ base_type::packed.size() }, type={ base_type::descriptor->alias,na } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" Optional="true" ExcludeView="simple">payload.capacity() * traits_type::page_size</Item>
+			<Item Name="[page size]" Optional="true" ExcludeView="simple">traits_type::page_size</Item>
+			<Item Name="[placeholder]" Optional="true" ExcludeView="simple">placeholder</Item>
+			<Item Name="[base]" ExcludeView="simple">(base_type*)this,nand</Item>
+			<Item Name="[base]" IncludeView="simple">(base_type*)this,view(simple)nand</Item>
+			<CustomListItems Condition="payload.size() != 0" Optional="true">
+				<Variable Name="pos" InitialValue="0" />
+				<Variable Name="last" InitialValue="base_type::packed.size()"/>
+				<Loop>
+					<Break Condition="pos == last"/>
+					<If Condition="is_valid_entity(base_type::packed[pos])">
+						<Item Name="[{ pos }:{ base_type::packed[pos] }]">payload[pos / traits_type::page_size][pos &amp; (traits_type::page_size - 1)]</Item>
+					</If>
+					<Exec>++pos</Exec>
+				</Loop>
+			</CustomListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_common_view&lt;*,*,*&gt;">
+		<DisplayString Condition="index != $T2">{{ size_hint={ pools[index]->packed.size() } }}</DisplayString>
+		<DisplayString>{{ size_hint=0 }}</DisplayString>
+		<Expand>
+			<Item Name="[pools]">pools,na</Item>
+			<Item Name="[filter]">filter,na</Item>
+			<Item Name="[handle]" Condition="index != $T2">pools[index],na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_storage_view&lt;*&gt;">
+		<DisplayString Condition="leading != nullptr">{{ size={ leading->packed.size() } }}</DisplayString>
+		<DisplayString>{{ size=0 }}</DisplayString>
+		<Expand>
+			<Item Name="[handle]" Condition="leading != nullptr">leading,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_view&lt;*&gt;">
+		<DisplayString>{ *(base_type*)this }</DisplayString>
+		<Expand>
+			<ExpandedItem>*(base_type*)this</ExpandedItem>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_runtime_view&lt;*&gt;">
+		<DisplayString Condition="pools.size() != 0u">{{ size_hint={ pools[0]->packed.size() } }}</DisplayString>
+		<DisplayString>{{ size_hint=0 }}</DisplayString>
+		<Expand>
+			<Item Name="[pools]">pools,na</Item>
+			<Item Name="[filter]">filter,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_handle&lt;*&gt;">
+		<Intrinsic Name="pool_at" Expression="owner->pools.packed.first_base::value[index].element.second._Ptr">
+			<Parameter Name="index" Type="unsigned int"/>
+		</Intrinsic>
+		<DisplayString>{{ entity={ entt } }}</DisplayString>
+		<Expand>
+			<Item Name="[entity]">entt</Item>
+			<Item Name="[registry]" Condition="owner != nullptr">owner,na</Item>
+			<Synthetic Name="[components]" Condition="owner != nullptr">
+				<Expand>
+					<CustomListItems>
+						<Variable Name="entity_mask" InitialValue="traits_type::entity_mask"/>
+						<Variable Name="page" InitialValue="((*((traits_type::entity_type *)&amp;entt)) &amp; entity_mask) / traits_type::page_size"/>
+						<Variable Name="offset" InitialValue="(*((traits_type::entity_type *)&amp;entt)) &amp; (traits_type::page_size - 1u)"/>
+						<Variable Name="last" InitialValue="owner->pools.packed.first_base::value.size()"/>
+						<Variable Name="pos" InitialValue="0u"/>
+						<Loop>
+							<Break Condition="pos == last"/>
+							<If Condition="pool_at(pos)->sparse.size() &gt; page &amp;&amp; pool_at(pos)->sparse[page] != nullptr &amp;&amp; ((*((traits_type::entity_type *)&amp;pool_at(pos)->sparse[page][offset])) &amp; entity_mask) != entity_mask">
+								<Item Name="[{ pool_at(pos)->descriptor->alias,na }:{ ((*((traits_type::entity_type *)&amp;pool_at(pos)->sparse[page][offset])) &amp; entity_mask) != entity_mask }]">pool_at(pos),view(simple)nanr</Item>
+							</If>
+							<Exec>++pos</Exec>
+						</Loop>
+					</CustomListItems>
+				</Expand>
+			</Synthetic>
+		</Expand>
+	</Type>
+	<Type Name="entt::null_t">
+		<DisplayString>&lt;null&gt;</DisplayString>
+	</Type>
+	<Type Name="entt::tombstone_t">
+		<DisplayString>&lt;tombstone&gt;</DisplayString>
+	</Type>
+</AutoVisualizer>

include/entt/natvis/graph.natvis

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+    <Type Name="entt::adjacency_matrix&lt;*&gt;">
+		<DisplayString>{{ size={ vert } }}</DisplayString>
+        <Expand>
+			<CustomListItems>
+				<Variable Name="pos" InitialValue="0" />
+				<Variable Name="last" InitialValue="vert * vert"/>
+				<Loop>
+					<Break Condition="pos == last"/>
+					<If Condition="matrix[pos] != 0u">
+						<Item Name="{pos / vert}">pos % vert</Item>
+					</If>
+					<Exec>++pos</Exec>
+				</Loop>
+			</CustomListItems>
+        </Expand>
+    </Type>
+</AutoVisualizer>

include/entt/natvis/locator.natvis

@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+</AutoVisualizer>

include/entt/natvis/meta.natvis

@@ -0,0 +1,200 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::internal::meta_base_node">
+		<DisplayString Condition="type != nullptr">{{ id={ id } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[id]">id</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_conv_node">
+		<DisplayString Condition="conv != nullptr">{{ id={ id } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[type]">id</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_ctor_node">
+		<DisplayString Condition="invoke != nullptr">{{ id={ id } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[id]">id</Item>
+			<Item Name="[arity]">arity</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_custom_node">
+		<DisplayString Condition="value != nullptr">{{ id={ id } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[id]">id</Item>
+			<Item Name="[value]">value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_data_node">
+		<Intrinsic Name="has_trait" Expression="!!(traits &amp; property)">
+			<Parameter Name="property" Type="int"/>
+		</Intrinsic>
+		<DisplayString Condition="name != nullptr">{{ id={ name,na } }}</DisplayString>
+		<DisplayString Condition="get != nullptr">{{ id={ id } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[id]">id</Item>
+			<Item Name="[name]" Condition="name != nullptr">name,na</Item>
+			<Item Name="[arity]">arity</Item>
+			<Item Name="[is_const]">has_trait(entt::internal::meta_traits::is_const)</Item>
+			<Item Name="[is_static]">has_trait(entt::internal::meta_traits::is_static)</Item>
+			<Item Name="[custom]" Condition="custom.value != nullptr">custom</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_func_node"	>
+		<Intrinsic Name="has_trait" Expression="!!(traits &amp; property)">
+			<Parameter Name="property" Type="int"/>
+		</Intrinsic>
+		<DisplayString Condition="name != nullptr">{{ id={ name,na } }}</DisplayString>
+		<DisplayString Condition="invoke != nullptr">{{ id={ id } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[id]">id</Item>
+			<Item Name="[name]" Condition="name != nullptr">name,na</Item>
+			<Item Name="[arity]">arity</Item>
+			<Item Name="[is_const]">has_trait(entt::internal::meta_traits::is_const)</Item>
+			<Item Name="[is_static]">has_trait(entt::internal::meta_traits::is_static)</Item>
+			<Item Name="[next]" Condition="next != nullptr">*next</Item>
+			<Item Name="[custom]" Condition="custom.value != nullptr">custom</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_template_node">
+		<DisplayString Condition="arity != 0u">{{ arity={ arity } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[arity]">arity</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_type_descriptor">
+		<DisplayString/>
+		<Expand>
+			<Item Name="[ctor]">ctor,view(simple)</Item>
+			<Item Name="[base]">base,view(simple)</Item>
+			<Item Name="[conv]">conv,view(simple)</Item>
+			<Item Name="[data]">data,view(simple)</Item>
+			<Item Name="[func]">func,view(simple)</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::meta_type_node">
+		<Intrinsic Name="has_trait" Expression="!!(traits &amp; property)">
+			<Parameter Name="property" Type="int"/>
+		</Intrinsic>
+		<DisplayString Condition="name != nullptr">{{ type={ name,na } }}</DisplayString>
+		<DisplayString Condition="info != nullptr">{{ type={ info->alias,na } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[alias]">alias</Item>
+			<Item Name="[name]" Condition="name != nullptr">name,na</Item>
+			<Item Name="[sizeof]">size_of</Item>
+			<Item Name="[is_arithmetic]">has_trait(entt::internal::meta_traits::is_arithmetic)</Item>
+			<Item Name="[is_integral]">has_trait(entt::internal::meta_traits::is_integral)</Item>
+			<Item Name="[is_signed]">has_trait(entt::internal::meta_traits::is_signed)</Item>
+			<Item Name="[is_array]">has_trait(entt::internal::meta_traits::is_array)</Item>
+			<Item Name="[is_enum]">has_trait(entt::internal::meta_traits::is_enum)</Item>
+			<Item Name="[is_class]">has_trait(entt::internal::meta_traits::is_class)</Item>
+			<Item Name="[is_pointer]">has_trait(entt::internal::meta_traits::is_pointer)</Item>
+			<Item Name="[is_pointer_like]">has_trait(entt::internal::meta_traits::is_pointer_like)</Item>
+			<Item Name="[is_sequence_container]">has_trait(entt::internal::meta_traits::is_sequence_container)</Item>
+			<Item Name="[is_associative_container]">has_trait(entt::internal::meta_traits::is_associative_container)</Item>
+			<Item Name="[default_constructor]">default_constructor != nullptr</Item>
+			<Item Name="[conversion_helper]">conversion_helper != nullptr</Item>
+			<Item Name="[from_void]">from_void != nullptr</Item>
+			<Item Name="[template_info]" Condition="templ.arity != 0u">templ</Item>
+			<Item Name="[custom]" Condition="custom.value != nullptr">custom</Item>
+			<Item Name="[details]" Condition="details != nullptr">*details</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_any">
+		<DisplayString>{ storage }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node,na</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_handle">
+		<DisplayString>{ any }</DisplayString>
+		<Expand>
+			<ExpandedItem>any</ExpandedItem>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_associative_container">
+		<DisplayString Condition="data != nullptr">{{ const={ const_only } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
+			<Item Name="[const]">const_only</Item>
+			<Item Name="[data]">data</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_sequence_container">
+		<DisplayString Condition="data != nullptr">{{ const={ const_only } }}</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
+			<Item Name="[const]">const_only</Item>
+			<Item Name="[data]">data</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_custom">
+		<DisplayString Condition="node != nullptr">{ node,na }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_data">
+		<DisplayString Condition="node != nullptr">{ node,na }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_func">
+		<DisplayString Condition="node != nullptr">{ node,na }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_base">
+		<DisplayString Condition="node != nullptr">{ node,na }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_type">
+		<DisplayString Condition="node != nullptr">{ node,na }</DisplayString>
+		<DisplayString>{{}}</DisplayString>
+		<Expand>
+			<ExpandedItem Condition="node != nullptr">node</ExpandedItem>
+			<Item Name="[context]" Condition="ctx != nullptr">ctx,na</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::meta_ctx">
+		<Intrinsic Name="element_at" Expression="bucket.packed.first_base::value[pos].element">
+			<Parameter Name="pos" Type="int"/>
+		</Intrinsic>
+		<DisplayString>{ bucket }</DisplayString>
+		<Expand>
+			<CustomListItems>
+				<Variable Name="pos" InitialValue="0"/>
+				<Variable Name="last" InitialValue="bucket.size()"/>
+				<Loop>
+					<Break Condition="pos == last"/>
+					<Item Name="[{ element_at(pos).first }]">element_at(pos).second</Item>
+					<Exec>++pos</Exec>
+				</Loop>
+			</CustomListItems>
+		</Expand>
+	</Type>
+</AutoVisualizer>

include/entt/natvis/poly.natvis

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::basic_poly&lt;*&gt;">
+		<DisplayString>{ storage }</DisplayString>
+	</Type>
+</AutoVisualizer>

include/entt/natvis/process.natvis

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::basic_process&lt;*&gt;">
+		<DisplayString>{{ state={ current,en } }}</DisplayString>
+		<Expand>
+			<Item Name="[state]">current,en</Item>
+			<Item Name="[child]" Condition="next.first_base::value != nullptr">*next.first_base::value</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_scheduler&lt;*&gt;">
+		<Intrinsic Name="size" Expression="handlers.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
+		<Expand>
+			<Item Name="[capacity]" ExcludeView="simple">handlers.first_base::value.capacity()</Item>
+			<IndexListItems>
+				<Size>size()</Size>
+				<ValueNode>*handlers.first_base::value[$i]</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+</AutoVisualizer>

include/entt/natvis/resource.natvis

@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::resource&lt;*&gt;">
+		<DisplayString>{ value }</DisplayString>
+		<Expand>
+			<ExpandedItem>value</ExpandedItem>
+		</Expand>
+	</Type>
+	<Type Name="entt::resource_cache&lt;*&gt;">
+		<Intrinsic Name="size" Expression="pool.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
+		<Expand>
+			<CustomListItems>
+				<Variable Name="pos" InitialValue="0" />
+				<Variable Name="last" InitialValue="size()"/>
+				<Loop>
+					<Break Condition="pos == last"/>
+					<Item Name="[{ pool.first_base::value.packed.first_base::value[pos].element.first }]">
+						*pool.first_base::value.packed.first_base::value[pos].element.second
+					</Item>
+					<Exec>++pos</Exec>
+				</Loop>
+			</CustomListItems>
+		</Expand>
+	</Type>
+</AutoVisualizer>

include/entt/natvis/signal.natvis

@@ -0,0 +1,51 @@
+<?xml version="1.0" encoding="utf-8"?>
+<AutoVisualizer xmlns="http://schemas.microsoft.com/vstudio/debugger/natvis/2010">
+	<Type Name="entt::delegate&lt;*&gt;">
+		<DisplayString>{{ type={ "$T1" } }}</DisplayString>
+		<Expand>
+			<Item Name="[empty]">fn == nullptr</Item>
+			<Item Name="[data]">instance</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::basic_dispatcher&lt;*&gt;">
+		<Intrinsic Name="size" Expression="pools.first_base::value.size()"/>
+		<DisplayString>{{ size={ size() } }}</DisplayString>
+		<Expand>
+			<IndexListItems>
+				<Size>size()</Size>
+				<ValueNode>*pools.first_base::value.packed.first_base::value[$i].element.second</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::internal::dispatcher_handler&lt;*&gt;">
+		<DisplayString>{{ size={ events.size() }, event={ "$T1" } }}</DisplayString>
+		<Expand>
+			<Item Name="[signal]">signal</Item>
+			<Item Name="[events]">events,view(simple)</Item>
+		</Expand>
+	</Type>
+	<Type Name="entt::emitter&lt;*&gt;">
+		<DisplayString>{{ size={ handlers.first_base::value.size() } }}</DisplayString>
+	</Type>
+	<Type Name="entt::connection">
+		<DisplayString>{{ bound={ signal != nullptr } }}</DisplayString>
+	</Type>
+	<Type Name="entt::scoped_connection">
+		<DisplayString>{ conn }</DisplayString>
+	</Type>
+	<Type Name="entt::sigh&lt;*&gt;">
+		<DisplayString>{{ size={ calls.size() }, type={ "$T1" } }}</DisplayString>
+		<Expand>
+			<IndexListItems>
+				<Size>calls.size()</Size>
+				<ValueNode>calls[$i]</ValueNode>
+			</IndexListItems>
+		</Expand>
+	</Type>
+	<Type Name="entt::sink&lt;*&gt;">
+		<DisplayString>{{ type={ "$T1" } }}</DisplayString>
+		<Expand>
+			<Item Name="[signal]">signal,na</Item>
+		</Expand>
+	</Type>
+</AutoVisualizer>

include/entt/poly/fwd.hpp

@@ -0,0 +1,21 @@
+#ifndef ENTT_POLY_FWD_HPP
+#define ENTT_POLY_FWD_HPP
+
+#include "../stl/cstddef.hpp"
+
+namespace entt {
+
+// NOLINTNEXTLINE(cppcoreguidelines-avoid-c-arrays, modernize-avoid-c-arrays)
+template<typename, stl::size_t Len = sizeof(double[2]), stl::size_t = alignof(double[2])>
+class basic_poly;
+
+/**
+ * @brief Alias declaration for the most common use case.
+ * @tparam Concept Concept descriptor.
+ */
+template<typename Concept>
+using poly = basic_poly<Concept>;
+
+} // namespace entt
+
+#endif

include/entt/poly/poly.hpp

@@ -0,0 +1,316 @@
+#ifndef ENTT_POLY_POLY_HPP
+#define ENTT_POLY_POLY_HPP
+
+#include "../core/any.hpp"
+#include "../core/concepts.hpp"
+#include "../core/type_info.hpp"
+#include "../core/type_traits.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/tuple.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @brief Inspector class used to infer the type of the virtual table. */
+struct poly_inspector {
+    /**
+     * @brief Generic conversion operator (definition only).
+     * @tparam Type Type to which conversion is requested.
+     */
+    template<typename Type>
+    operator Type &&() const;
+
+    /**
+     * @brief Dummy invocation function (definition only).
+     * @tparam Member Index of the function to invoke.
+     * @tparam Args Types of arguments to pass to the function.
+     * @param args The arguments to pass to the function.
+     * @return A poly inspector convertible to any type.
+     */
+    template<stl::size_t Member, typename... Args>
+    [[nodiscard]] poly_inspector invoke(Args &&...args) const;
+
+    /*! @copydoc invoke */
+    template<stl::size_t Member, typename... Args>
+    [[nodiscard]] poly_inspector invoke(Args &&...args);
+};
+
+/**
+ * @brief Static virtual table factory.
+ * @tparam Concept Concept descriptor.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Alignment requirement.
+ */
+template<typename Concept, stl::size_t Len, stl::size_t Align>
+class poly_vtable {
+    using inspector = Concept::template type<poly_inspector>;
+
+    template<typename Ret, typename Clazz, typename... Args>
+    requires stl::derived_from<inspector, stl::remove_const_t<Clazz>>
+    static auto vtable_entry(Ret (*)(Clazz &, Args...))
+        -> Ret (*)(constness_as_t<basic_any<Len, Align>, Clazz> &, Args...);
+
+    template<typename Ret, typename... Args>
+    static auto vtable_entry(Ret (*)(Args...))
+        -> Ret (*)(const basic_any<Len, Align> &, Args...);
+
+    template<typename Ret, typename Clazz, typename... Args>
+    requires stl::derived_from<inspector, Clazz>
+    static auto vtable_entry(Ret (Clazz::*)(Args...))
+        -> Ret (*)(basic_any<Len, Align> &, Args...);
+
+    template<typename Ret, typename Clazz, typename... Args>
+    requires stl::derived_from<inspector, Clazz>
+    static auto vtable_entry(Ret (Clazz::*)(Args...) const)
+        -> Ret (*)(const basic_any<Len, Align> &, Args...);
+
+    template<auto... Candidate>
+    static auto make_vtable(value_list<Candidate...>) noexcept
+        -> decltype(stl::make_tuple(vtable_entry(Candidate)...));
+
+    template<typename... Func>
+    [[nodiscard]] static ENTT_CONSTEVAL auto make_vtable(type_list<Func...>) noexcept {
+        if constexpr(sizeof...(Func) == 0u) {
+            return decltype(make_vtable(typename Concept::template impl<inspector>{})){};
+        } else if constexpr((stl::is_function_v<Func> && ...)) {
+            return decltype(stl::make_tuple(vtable_entry(stl::declval<Func inspector::*>())...)){};
+        }
+    }
+
+    template<typename Type, auto Candidate, typename Ret, typename Any, typename... Args>
+    static void fill_vtable_entry(Ret (*&entry)(Any &, Args...)) noexcept {
+        if constexpr(stl::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
+            entry = +[](Any &, Args... args) -> Ret {
+                return stl::invoke(Candidate, stl::forward<Args>(args)...);
+            };
+        } else {
+            entry = +[](Any &instance, Args... args) -> Ret {
+                return static_cast<Ret>(stl::invoke(Candidate, any_cast<constness_as_t<Type, Any> &>(instance), stl::forward<Args>(args)...));
+            };
+        }
+    }
+
+    template<typename Type, auto... Index>
+    [[nodiscard]] static auto fill_vtable(stl::index_sequence<Index...>) noexcept {
+        vtable_type impl{};
+        (fill_vtable_entry<Type, value_list_element_v<Index, typename Concept::template impl<Type>>>(stl::get<Index>(impl)), ...);
+        return impl;
+    }
+
+    using vtable_type = decltype(make_vtable(Concept{}));
+    static constexpr bool is_mono = stl::tuple_size_v<vtable_type> == 1u;
+
+public:
+    /*! @brief Virtual table type. */
+    using type = stl::conditional_t<is_mono, stl::tuple_element_t<0u, vtable_type>, const vtable_type *>;
+
+    /**
+     * @brief Returns a static virtual table for a specific concept and type.
+     * @tparam Type The type for which to generate the virtual table.
+     * @return A static virtual table for the given concept and type.
+     */
+    template<cvref_unqualified Type>
+    [[nodiscard]] static type instance() noexcept {
+        static const vtable_type vtable = fill_vtable<Type>(stl::make_index_sequence<Concept::template impl<Type>::size>{});
+
+        if constexpr(is_mono) {
+            return stl::get<0>(vtable);
+        } else {
+            return &vtable;
+        }
+    }
+};
+
+/**
+ * @brief Poly base class used to inject functionalities into concepts.
+ * @tparam Poly The outermost poly class.
+ */
+template<typename Poly>
+struct poly_base {
+    /**
+     * @brief Invokes a function from the static virtual table.
+     * @tparam Member Index of the function to invoke.
+     * @tparam Args Types of arguments to pass to the function.
+     * @param self A reference to the poly object that made the call.
+     * @param args The arguments to pass to the function.
+     * @return The return value of the invoked function, if any.
+     */
+    template<stl::size_t Member, typename... Args>
+    [[nodiscard]] decltype(auto) invoke(const poly_base &self, Args &&...args) const {
+        const auto &poly = static_cast<const Poly &>(self);
+
+        if constexpr(stl::is_function_v<stl::remove_pointer_t<decltype(poly.vtable)>>) {
+            return poly.vtable(poly.storage, stl::forward<Args>(args)...);
+        } else {
+            return stl::get<Member>(*poly.vtable)(poly.storage, stl::forward<Args>(args)...);
+        }
+    }
+
+    /*! @copydoc invoke */
+    template<stl::size_t Member, typename... Args>
+    [[nodiscard]] decltype(auto) invoke(poly_base &self, Args &&...args) {
+        auto &poly = static_cast<Poly &>(self);
+
+        if constexpr(stl::is_function_v<stl::remove_pointer_t<decltype(poly.vtable)>>) {
+            static_assert(Member == 0u, "Unknown member");
+            return poly.vtable(poly.storage, stl::forward<Args>(args)...);
+        } else {
+            return stl::get<Member>(*poly.vtable)(poly.storage, stl::forward<Args>(args)...);
+        }
+    }
+};
+
+/**
+ * @brief Shortcut for calling `poly_base<Type>::invoke`.
+ * @tparam Member Index of the function to invoke.
+ * @tparam Poly A fully defined poly object.
+ * @tparam Args Types of arguments to pass to the function.
+ * @param self A reference to the poly object that made the call.
+ * @param args The arguments to pass to the function.
+ * @return The return value of the invoked function, if any.
+ */
+template<stl::size_t Member, typename Poly, typename... Args>
+decltype(auto) poly_call(Poly &&self, Args &&...args) {
+    return stl::forward<Poly>(self).template invoke<Member>(self, stl::forward<Args>(args)...);
+}
+
+/**
+ * @brief Static polymorphism made simple and within everyone's reach.
+ *
+ * Static polymorphism is a very powerful tool in C++, albeit sometimes
+ * cumbersome to obtain.<br/>
+ * This class aims to make it simple and easy to use.
+ *
+ * @note
+ * Both deduced and defined static virtual tables are supported.<br/>
+ * Moreover, the `poly` class template also works with unmanaged objects.
+ *
+ * @tparam Concept Concept descriptor.
+ * @tparam Len Size of the storage reserved for the small buffer optimization.
+ * @tparam Align Optional alignment requirement.
+ */
+template<typename Concept, stl::size_t Len, stl::size_t Align>
+class basic_poly: private Concept::template type<poly_base<basic_poly<Concept, Len, Align>>> {
+    friend struct poly_base<basic_poly>;
+
+public:
+    /*! @brief Concept type. */
+    using concept_type = Concept::template type<poly_base<basic_poly>>;
+    /*! @brief Virtual table type. */
+    using vtable_type = poly_vtable<Concept, Len, Align>::type;
+
+    /*! @brief Default constructor. */
+    basic_poly() noexcept = default;
+
+    /**
+     * @brief Constructs a poly by directly initializing the new object.
+     * @tparam Type Type of object to use to initialize the poly.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    explicit basic_poly(stl::in_place_type_t<Type>, Args &&...args)
+        : storage{stl::in_place_type<Type>, stl::forward<Args>(args)...},
+          vtable{poly_vtable<Concept, Len, Align>::template instance<stl::remove_cvref_t<Type>>()} {}
+
+    /**
+     * @brief Constructs a poly from a given value.
+     * @tparam Type Type of object to use to initialize the poly.
+     * @param value An instance of an object to use to initialize the poly.
+     */
+    template<typename Type>
+    requires (!stl::same_as<stl::remove_cvref_t<Type>, basic_poly>)
+    basic_poly(Type &&value) noexcept
+        : basic_poly{stl::in_place_type<stl::remove_cvref_t<Type>>, stl::forward<Type>(value)} {}
+
+    /**
+     * @brief Returns the object type info if any, `type_id<void>()` otherwise.
+     * @return The object type info if any, `type_id<void>()` otherwise.
+     */
+    [[nodiscard]] const type_info &info() const noexcept {
+        return storage.info();
+    }
+
+    /**
+     * @brief Returns an opaque pointer to the contained instance.
+     * @return An opaque pointer the contained instance, if any.
+     */
+    [[nodiscard]] const void *data() const noexcept {
+        return storage.data();
+    }
+
+    /*! @copydoc data */
+    [[nodiscard]] void *data() noexcept {
+        return storage.data();
+    }
+
+    /**
+     * @brief Replaces the contained object by creating a new instance directly.
+     * @tparam Type Type of object to use to initialize the poly.
+     * @tparam Args Types of arguments to use to construct the new instance.
+     * @param args Parameters to use to construct the instance.
+     */
+    template<typename Type, typename... Args>
+    void emplace(Args &&...args) {
+        storage.template emplace<Type>(stl::forward<Args>(args)...);
+        vtable = poly_vtable<Concept, Len, Align>::template instance<stl::remove_cvref_t<Type>>();
+    }
+
+    /*! @brief Destroys contained object */
+    void reset() {
+        storage.reset();
+        vtable = {};
+    }
+
+    /**
+     * @brief Returns false if a poly is empty, true otherwise.
+     * @return False if the poly is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return static_cast<bool>(storage);
+    }
+
+    /**
+     * @brief Returns a pointer to the underlying concept.
+     * @return A pointer to the underlying concept.
+     */
+    [[nodiscard]] concept_type *operator->() noexcept {
+        return this;
+    }
+
+    /*! @copydoc operator-> */
+    [[nodiscard]] const concept_type *operator->() const noexcept {
+        return this;
+    }
+
+    /**
+     * @brief Aliasing constructor.
+     * @return A poly that shares a reference to an unmanaged object.
+     */
+    [[nodiscard]] basic_poly as_ref() noexcept {
+        basic_poly ref{};
+        ref.storage = storage.as_ref();
+        ref.vtable = vtable;
+        return ref;
+    }
+
+    /*! @copydoc as_ref */
+    [[nodiscard]] basic_poly as_ref() const noexcept {
+        basic_poly ref{};
+        ref.storage = storage.as_ref();
+        ref.vtable = vtable;
+        return ref;
+    }
+
+private:
+    basic_any<Len, Align> storage{};
+    vtable_type vtable{};
+};
+
+} // namespace entt
+
+#endif

include/entt/process/fwd.hpp

@@ -0,0 +1,23 @@
+#ifndef ENTT_PROCESS_FWD_HPP
+#define ENTT_PROCESS_FWD_HPP
+
+#include "../stl/cstdint.hpp"
+#include "../stl/memory.hpp"
+
+namespace entt {
+
+template<typename, typename = stl::allocator<void>>
+class basic_process;
+
+/*! @brief Alias declaration for the most common use case. */
+using process = basic_process<stl::uint32_t>;
+
+template<typename, typename = stl::allocator<void>>
+class basic_scheduler;
+
+/*! @brief Alias declaration for the most common use case. */
+using scheduler = basic_scheduler<stl::uint32_t>;
+
+} // namespace entt
+
+#endif

include/entt/process/process.hpp

@@ -0,0 +1,316 @@
+#ifndef ENTT_PROCESS_PROCESS_HPP
+#define ENTT_PROCESS_PROCESS_HPP
+
+#include "../core/compressed_pair.hpp"
+#include "../core/type_traits.hpp"
+#include "../stl/cstdint.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename, typename, typename>
+struct process_adaptor;
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Base class for processes.
+ *
+ * Derived classes must specify what's the intended type for elapsed times.<br/>
+ * A process can implement the following member functions whether required:
+ *
+ * * @code{.cpp}
+ *   void update(Delta, void *) override;
+ *   @endcode
+ *
+ *   It's invoked once per tick until a process is explicitly aborted or it
+ *   terminates either with or without errors. Even though it's not mandatory to
+ *   declare this member function, as a rule of thumb each process should at
+ *   least define it to work properly. The `void *` parameter is an opaque
+ *   pointer to user data (if any) forwarded directly to the process during an
+ *   update.
+ *
+ * * @code{.cpp}
+ *   void succeeded() override;
+ *   @endcode
+ *
+ *   It's invoked in case of success, immediately after an update and during the
+ *   same tick.
+ *
+ * * @code{.cpp}
+ *   void failed() override;
+ *   @endcode
+ *
+ *   It's invoked in case of errors, immediately after an update and during the
+ *   same tick.
+ *
+ * * @code{.cpp}
+ *   void aborted() override;
+ *   @endcode
+ *
+ *   It's invoked only if a process is explicitly aborted. There is no guarantee
+ *   that it executes in the same tick, this depends solely on whether the
+ *   process is aborted immediately or not.
+ *
+ * Derived classes can change the internal state of a process by invoking the
+ * `succeed` and `fail` member functions and even pause or unpause the process
+ * itself.
+ *
+ * @sa scheduler
+ *
+ * @tparam Delta Type to use to provide elapsed time.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Delta, typename Allocator>
+class basic_process: public stl::enable_shared_from_this<basic_process<Delta, Allocator>> {
+    enum class state : stl::uint8_t {
+        idle = 0,
+        running,
+        paused,
+        succeeded,
+        failed,
+        aborted,
+        finished,
+        rejected
+    };
+
+    virtual void update(const Delta, void *) {
+        abort();
+    }
+
+    virtual void succeeded() {}
+    virtual void failed() {}
+    virtual void aborted() {}
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Type used to provide elapsed time. */
+    using delta_type = Delta;
+    /*! @brief Handle type. */
+    using handle_type = stl::shared_ptr<basic_process>;
+
+    /*! @brief Default constructor. */
+    basic_process()
+        : basic_process{allocator_type{}} {}
+
+    /**
+     * @brief Constructs a scheduler with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_process(const allocator_type &allocator)
+        : next{nullptr, allocator},
+          current{state::idle} {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_process(const basic_process &) = delete;
+
+    /*! @brief Default move constructor, deleted on purpose. */
+    basic_process(basic_process &&) = delete;
+
+    /*! @brief Default destructor. */
+    virtual ~basic_process() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This process scheduler.
+     */
+    basic_process &operator=(const basic_process &) = delete;
+
+    /**
+     * @brief Default move assignment operator, deleted on purpose.
+     * @return This process scheduler.
+     */
+    basic_process &operator=(basic_process &&) = delete;
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return next.second();
+    }
+
+    /*! @brief Aborts a process if it's still alive, otherwise does nothing. */
+    void abort() {
+        if(alive()) {
+            current = state::aborted;
+        }
+    }
+
+    /**
+     * @brief Terminates a process with success if it's still alive, otherwise
+     * does nothing.
+     */
+    void succeed() noexcept {
+        if(alive()) {
+            current = state::succeeded;
+        }
+    }
+
+    /**
+     * @brief Terminates a process with errors if it's still alive, otherwise
+     * does nothing.
+     */
+    void fail() noexcept {
+        if(alive()) {
+            current = state::failed;
+        }
+    }
+
+    /*! @brief Stops a process if it's running, otherwise does nothing. */
+    void pause() noexcept {
+        if(alive()) {
+            current = state::paused;
+        }
+    }
+
+    /*! @brief Restarts a process if it's paused, otherwise does nothing. */
+    void unpause() noexcept {
+        if(alive()) {
+            current = state::running;
+        }
+    }
+
+    /**
+     * @brief Returns true if a process is either running or paused.
+     * @return True if the process is still alive, false otherwise.
+     */
+    [[nodiscard]] bool alive() const noexcept {
+        return current == state::running || current == state::paused;
+    }
+
+    /**
+     * @brief Returns true if a process is already terminated.
+     * @return True if the process is terminated, false otherwise.
+     */
+    [[nodiscard]] bool finished() const noexcept {
+        return current == state::finished;
+    }
+
+    /**
+     * @brief Returns true if a process is currently paused.
+     * @return True if the process is paused, false otherwise.
+     */
+    [[nodiscard]] bool paused() const noexcept {
+        return current == state::paused;
+    }
+
+    /**
+     * @brief Returns true if a process terminated with errors.
+     * @return True if the process terminated with errors, false otherwise.
+     */
+    [[nodiscard]] bool rejected() const noexcept {
+        return current == state::rejected;
+    }
+
+    /**
+     * @brief Assigns a child process to run in case of success.
+     * @tparam Type Type of child process to create.
+     * @tparam Args Types of arguments to use to initialize the child process.
+     * @param args Parameters to use to initialize the child process.
+     * @return A reference to the newly created child process.
+     */
+    template<typename Type, typename... Args>
+    basic_process &then(Args &&...args) {
+        const auto &allocator = next.second();
+        return *(next.first() = stl::allocate_shared<Type>(allocator, allocator, stl::forward<Args>(args)...));
+    }
+
+    /**
+     * @brief Assigns a child process to run in case of success.
+     * @tparam Func Type of child process to create.
+     * @param func Either a lambda or a functor to use as a child process.
+     * @return A reference to the newly created child process.
+     */
+    template<typename Func>
+    basic_process &then(Func func) {
+        const auto &allocator = next.second();
+        using process_type = internal::process_adaptor<delta_type, Func, allocator_type>;
+        return *(next.first() = stl::allocate_shared<process_type>(allocator, allocator, stl::move(func)));
+    }
+
+    /**
+     * @brief Returns the child process without releasing ownership, if any.
+     * @return The child process attached to the object, if any.
+     */
+    handle_type peek() {
+        return next.first();
+    }
+
+    /**
+     * @brief Updates a process and its internal state, if required.
+     * @param delta Elapsed time.
+     * @param data Optional data.
+     */
+    void tick(const Delta delta, void *data = nullptr) {
+        switch(current) {
+        case state::idle:
+        case state::running:
+            current = state::running;
+            update(delta, data);
+            break;
+        default:
+            // suppress warnings
+            break;
+        }
+
+        // if it's dead, it must be notified and removed immediately
+        switch(current) {
+        case state::succeeded:
+            succeeded();
+            current = state::finished;
+            break;
+        case state::failed:
+            failed();
+            current = state::rejected;
+            break;
+        case state::aborted:
+            aborted();
+            current = state::rejected;
+            break;
+        default:
+            // suppress warnings
+            break;
+        }
+    }
+
+private:
+    compressed_pair<handle_type, allocator_type> next;
+    state current;
+};
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename Delta, typename Func, typename Allocator>
+struct process_adaptor: public basic_process<Delta, Allocator> {
+    using allocator_type = Allocator;
+    using base_type = basic_process<Delta, Allocator>;
+    using delta_type = base_type::delta_type;
+
+    process_adaptor(const allocator_type &allocator, Func proc)
+        : base_type{allocator},
+          func{stl::move(proc)} {}
+
+    void update(const delta_type delta, void *data) override {
+        func(*this, delta, data);
+    }
+
+private:
+    Func func;
+};
+
+} // namespace internal
+/*! @endcond */
+
+} // namespace entt
+
+#endif

include/entt/process/scheduler.hpp

@@ -0,0 +1,227 @@
+#ifndef ENTT_PROCESS_SCHEDULER_HPP
+#define ENTT_PROCESS_SCHEDULER_HPP
+
+#include "../config/config.h"
+#include "../core/compressed_pair.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+#include "fwd.hpp"
+#include "process.hpp"
+
+namespace entt {
+
+/**
+ * @brief Cooperative scheduler for processes.
+ *
+ * A cooperative scheduler runs processes and helps managing their life cycles.
+ *
+ * Each process is invoked once per tick. If a process terminates, it's
+ * removed automatically from the scheduler and it's never invoked again.<br/>
+ * A process can also have a child. In this case, the process is replaced with
+ * its child when it terminates if it returns with success. In case of errors,
+ * both the process and its child are discarded.
+ *
+ * In order to invoke all scheduled processes, call the `update` member function
+ * passing it the elapsed time to forward to the tasks.
+ *
+ * @sa process
+ *
+ * @tparam Delta Type to use to provide elapsed time.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Delta, typename Allocator>
+class basic_scheduler {
+    using base_type = basic_process<Delta, Allocator>;
+    using alloc_traits = stl::allocator_traits<Allocator>;
+    using container_allocator = alloc_traits::template rebind_alloc<stl::shared_ptr<base_type>>;
+    using container_type = stl::vector<stl::shared_ptr<base_type>, container_allocator>;
+
+public:
+    /*! @brief Process type. */
+    using type = base_type;
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+    /*! @brief Unsigned integer type. */
+    using delta_type = Delta;
+
+    /*! @brief Default constructor. */
+    basic_scheduler()
+        : basic_scheduler{allocator_type{}} {}
+
+    /**
+     * @brief Constructs a scheduler with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_scheduler(const allocator_type &allocator)
+        : handlers{allocator, allocator} {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_scheduler(const basic_scheduler &) = delete;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_scheduler(basic_scheduler &&other) noexcept
+        : handlers{stl::move(other.handlers)} {}
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_scheduler(basic_scheduler &&other, const allocator_type &allocator)
+        : handlers{container_type{stl::move(other.handlers.first()), allocator}, allocator} {
+        ENTT_ASSERT(alloc_traits::is_always_equal::value || get_allocator() == other.get_allocator(), "Copying a scheduler is not allowed");
+    }
+
+    /*! @brief Default destructor. */
+    ~basic_scheduler() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This process scheduler.
+     */
+    basic_scheduler &operator=(const basic_scheduler &) = delete;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This process scheduler.
+     */
+    basic_scheduler &operator=(basic_scheduler &&other) noexcept {
+        ENTT_ASSERT(alloc_traits::is_always_equal::value || get_allocator() == other.get_allocator(), "Copying a scheduler is not allowed");
+        swap(other);
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given scheduler.
+     * @param other Scheduler to exchange the content with.
+     */
+    void swap(basic_scheduler &other) noexcept {
+        using stl::swap;
+        swap(handlers, other.handlers);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return handlers.second();
+    }
+
+    /**
+     * @brief Number of processes currently scheduled.
+     * @return Number of processes currently scheduled.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return handlers.first().size();
+    }
+
+    /**
+     * @brief Returns true if at least a process is currently scheduled.
+     * @return True if there are scheduled processes, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return handlers.first().empty();
+    }
+
+    /**
+     * @brief Discards all scheduled processes.
+     *
+     * Processes aren't aborted. They are discarded along with their children
+     * and never executed again.
+     */
+    void clear() {
+        handlers.first().clear();
+    }
+
+    /**
+     * @brief Schedules a process for the next tick.
+     * @tparam Type Type of process to create.
+     * @tparam Args Types of arguments to use to initialize the process.
+     * @param args Parameters to use to initialize the process.
+     * @return A reference to the newly created process.
+     */
+    template<typename Type, typename... Args>
+    type &attach(Args &&...args) {
+        const auto &allocator = handlers.second();
+        return *handlers.first().emplace_back(stl::allocate_shared<Type>(allocator, allocator, stl::forward<Args>(args)...));
+    }
+
+    /**
+     * @brief Schedules a process for the next tick.
+     * @tparam Func Type of process to create.
+     * @param func Either a lambda or a functor to use as a process.
+     * @return A reference to the newly created process.
+     */
+    template<typename Func>
+    type &attach(Func func) {
+        const auto &allocator = handlers.second();
+        using process_type = internal::process_adaptor<delta_type, Func, allocator_type>;
+        return *handlers.first().emplace_back(stl::allocate_shared<process_type>(allocator, allocator, stl::move(func)));
+    }
+
+    /**
+     * @brief Updates all scheduled processes.
+     *
+     * All scheduled processes are executed in no specific order.<br/>
+     * If a process terminates with success, it's replaced with its child, if
+     * any. Otherwise, if a process terminates with an error, it's removed along
+     * with its child.
+     *
+     * @param delta Elapsed time.
+     * @param data Optional data.
+     */
+    void update(const delta_type delta, void *data = nullptr) {
+        for(auto next = handlers.first().size(); next; --next) {
+            const auto pos = next - 1u;
+            handlers.first()[pos]->tick(delta, data);
+            // updating might spawn/reallocate, cannot hold refs until here
+            auto &elem = handlers.first()[pos];
+
+            if(elem->finished()) {
+                elem = elem->peek();
+            }
+
+            if(!elem || elem->rejected()) {
+                elem = stl::move(handlers.first().back());
+                handlers.first().pop_back();
+            }
+        }
+    }
+
+    /**
+     * @brief Aborts all scheduled processes.
+     *
+     * Unless an immediate operation is requested, the abort is scheduled for
+     * the next tick. Processes won't be executed anymore in any case.<br/>
+     * Once a process is fully aborted and thus finished, it's discarded along
+     * with its child, if any.
+     *
+     * @param immediate Requests an immediate operation.
+     */
+    void abort(const bool immediate = false) {
+        for(auto &&curr: handlers.first()) {
+            curr->abort();
+
+            if(immediate) {
+                curr->tick({});
+            }
+        }
+    }
+
+private:
+    compressed_pair<container_type, allocator_type> handlers;
+};
+
+} // namespace entt
+
+#endif

include/entt/resource/cache.hpp

@@ -0,0 +1,386 @@
+#ifndef ENTT_RESOURCE_RESOURCE_CACHE_HPP
+#define ENTT_RESOURCE_RESOURCE_CACHE_HPP
+
+#include <compare>
+#include "../container/dense_map.hpp"
+#include "../core/compressed_pair.hpp"
+#include "../core/fwd.hpp"
+#include "../core/iterator.hpp"
+#include "../stl/concepts.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/iterator.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/tuple.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+#include "loader.hpp"
+#include "resource.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename Type, typename It>
+class resource_cache_iterator final {
+    template<typename, typename>
+    friend class resource_cache_iterator;
+
+public:
+    using value_type = stl::pair<id_type, resource<Type>>;
+    using pointer = input_iterator_pointer<value_type>;
+    using reference = value_type;
+    using difference_type = stl::ptrdiff_t;
+    using iterator_category = stl::input_iterator_tag;
+    using iterator_concept = stl::random_access_iterator_tag;
+
+    constexpr resource_cache_iterator() noexcept = default;
+
+    constexpr resource_cache_iterator(const It iter) noexcept
+        : it{iter} {}
+
+    template<typename Other>
+    requires (!stl::same_as<It, Other> && stl::constructible_from<It, Other>)
+    constexpr resource_cache_iterator(const resource_cache_iterator<stl::remove_const_t<Type>, Other> &other) noexcept
+        : it{other.it} {}
+
+    constexpr resource_cache_iterator &operator++() noexcept {
+        return ++it, *this;
+    }
+
+    constexpr resource_cache_iterator operator++(int) noexcept {
+        const resource_cache_iterator orig = *this;
+        return ++(*this), orig;
+    }
+
+    constexpr resource_cache_iterator &operator--() noexcept {
+        return --it, *this;
+    }
+
+    constexpr resource_cache_iterator operator--(int) noexcept {
+        const resource_cache_iterator orig = *this;
+        return operator--(), orig;
+    }
+
+    constexpr resource_cache_iterator &operator+=(const difference_type value) noexcept {
+        it += value;
+        return *this;
+    }
+
+    constexpr resource_cache_iterator operator+(const difference_type value) const noexcept {
+        resource_cache_iterator copy = *this;
+        return (copy += value);
+    }
+
+    constexpr resource_cache_iterator &operator-=(const difference_type value) noexcept {
+        return (*this += -value);
+    }
+
+    constexpr resource_cache_iterator operator-(const difference_type value) const noexcept {
+        return (*this + -value);
+    }
+
+    [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
+        return {it[value].first, resource<Type>{it[value].second}};
+    }
+
+    [[nodiscard]] constexpr reference operator*() const noexcept {
+        return operator[](0);
+    }
+
+    [[nodiscard]] constexpr pointer operator->() const noexcept {
+        return operator*();
+    }
+
+    template<typename... Args>
+    [[nodiscard]] constexpr stl::ptrdiff_t operator-(const resource_cache_iterator<Args...> &other) const noexcept {
+        return it - other.it;
+    }
+
+    template<typename... Args>
+    [[nodiscard]] constexpr bool operator==(const resource_cache_iterator<Args...> &other) const noexcept {
+        return it == other.it;
+    }
+
+    template<typename... Args>
+    [[nodiscard]] constexpr auto operator<=>(const resource_cache_iterator<Args...> &other) const noexcept {
+        return it <=> other.it;
+    }
+
+private:
+    It it;
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Basic cache for resources of any type.
+ * @tparam Type Type of resources managed by a cache.
+ * @tparam Loader Type of loader used to create the resources.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Loader, typename Allocator>
+class resource_cache {
+    using alloc_traits = stl::allocator_traits<Allocator>;
+    static_assert(stl::is_same_v<typename alloc_traits::value_type, Type>, "Invalid value type");
+    using container_allocator = alloc_traits::template rebind_alloc<stl::pair<const id_type, typename Loader::result_type>>;
+    using container_type = dense_map<id_type, typename Loader::result_type, stl::identity, stl::equal_to<>, container_allocator>;
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Resource type. */
+    using value_type = Type;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+    /*! @brief Loader type. */
+    using loader_type = Loader;
+    /*! @brief Input iterator type. */
+    using iterator = internal::resource_cache_iterator<Type, typename container_type::iterator>;
+    /*! @brief Constant input iterator type. */
+    using const_iterator = internal::resource_cache_iterator<const Type, typename container_type::const_iterator>;
+
+    /*! @brief Default constructor. */
+    resource_cache()
+        : resource_cache{loader_type{}} {}
+
+    /**
+     * @brief Constructs an empty cache with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit resource_cache(const allocator_type &allocator)
+        : resource_cache{loader_type{}, allocator} {}
+
+    /**
+     * @brief Constructs an empty cache with a given allocator and loader.
+     * @param callable The loader to use.
+     * @param allocator The allocator to use.
+     */
+    explicit resource_cache(const loader_type &callable, const allocator_type &allocator = allocator_type{})
+        : pool{container_type{allocator}, callable} {}
+
+    /*! @brief Default copy constructor. */
+    resource_cache(const resource_cache &) = default;
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    resource_cache(const resource_cache &other, const allocator_type &allocator)
+        : pool{stl::piecewise_construct, stl::forward_as_tuple(other.pool.first(), allocator), stl::forward_as_tuple(other.pool.second())} {}
+
+    /*! @brief Default move constructor. */
+    resource_cache(resource_cache &&) noexcept = default;
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    resource_cache(resource_cache &&other, const allocator_type &allocator)
+        : pool{stl::piecewise_construct, stl::forward_as_tuple(stl::move(other.pool.first()), allocator), stl::forward_as_tuple(stl::move(other.pool.second()))} {}
+
+    /*! @brief Default destructor. */
+    ~resource_cache() = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This cache.
+     */
+    resource_cache &operator=(const resource_cache &) = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This cache.
+     */
+    resource_cache &operator=(resource_cache &&) noexcept = default;
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return pool.first().get_allocator();
+    }
+
+    /**
+     * @brief Returns an iterator to the beginning.
+     *
+     * If the cache is empty, the returned iterator will be equal to `end()`.
+     *
+     * @return An iterator to the first instance of the internal cache.
+     */
+    [[nodiscard]] const_iterator cbegin() const noexcept {
+        return pool.first().begin();
+    }
+
+    /*! @copydoc cbegin */
+    [[nodiscard]] const_iterator begin() const noexcept {
+        return cbegin();
+    }
+
+    /*! @copydoc begin */
+    [[nodiscard]] iterator begin() noexcept {
+        return pool.first().begin();
+    }
+
+    /**
+     * @brief Returns an iterator to the end.
+     * @return An iterator to the element following the last instance of the
+     * internal cache.
+     */
+    [[nodiscard]] const_iterator cend() const noexcept {
+        return pool.first().end();
+    }
+
+    /*! @copydoc cend */
+    [[nodiscard]] const_iterator end() const noexcept {
+        return cend();
+    }
+
+    /*! @copydoc end */
+    [[nodiscard]] iterator end() noexcept {
+        return pool.first().end();
+    }
+
+    /**
+     * @brief Returns true if a cache contains no resources, false otherwise.
+     * @return True if the cache contains no resources, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return pool.first().empty();
+    }
+
+    /**
+     * @brief Number of resources managed by a cache.
+     * @return Number of resources currently stored.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return pool.first().size();
+    }
+
+    /*! @brief Clears a cache. */
+    void clear() noexcept {
+        pool.first().clear();
+    }
+
+    /**
+     * @brief Loads a resource, if its identifier does not exist.
+     *
+     * Arguments are forwarded directly to the loader and _consumed_ only if the
+     * resource doesn't already exist.
+     *
+     * @warning
+     * If the resource isn't loaded correctly, the returned handle could be
+     * invalid and any use of it will result in undefined behavior.
+     *
+     * @tparam Args Types of arguments to use to load the resource if required.
+     * @param id Unique resource identifier.
+     * @param args Arguments to use to load the resource if required.
+     * @return A pair consisting of an iterator to the inserted element (or to
+     * the element that prevented the insertion) and a bool denoting whether the
+     * insertion took place.
+     */
+    template<typename... Args>
+    stl::pair<iterator, bool> load(const id_type id, Args &&...args) {
+        if(auto it = pool.first().find(id); it != pool.first().end()) {
+            return {it, false};
+        }
+
+        return pool.first().emplace(id, pool.second()(stl::forward<Args>(args)...));
+    }
+
+    /**
+     * @brief Force loads a resource, even if its identifier already exists.
+     * @copydetails load
+     */
+    template<typename... Args>
+    stl::pair<iterator, bool> force_load(const id_type id, Args &&...args) {
+        return {pool.first().insert_or_assign(id, pool.second()(stl::forward<Args>(args)...)).first, true};
+    }
+
+    /**
+     * @brief Returns a handle for a given resource identifier.
+     *
+     * @warning
+     * There is no guarantee that the returned handle is valid.<br/>
+     * If it is not, any use will result in undefined behavior.
+     *
+     * @param id Unique resource identifier.
+     * @return A handle for the given resource.
+     */
+    [[nodiscard]] resource<const value_type> operator[](const id_type id) const {
+        if(auto it = pool.first().find(id); it != pool.first().cend()) {
+            return resource<const value_type>{it->second};
+        }
+
+        return {};
+    }
+
+    /*! @copydoc operator[] */
+    [[nodiscard]] resource<value_type> operator[](const id_type id) {
+        if(auto it = pool.first().find(id); it != pool.first().end()) {
+            return resource<value_type>{it->second};
+        }
+
+        return {};
+    }
+
+    /**
+     * @brief Checks if a cache contains a given identifier.
+     * @param id Unique resource identifier.
+     * @return True if the cache contains the resource, false otherwise.
+     */
+    [[nodiscard]] bool contains(const id_type id) const {
+        return pool.first().contains(id);
+    }
+
+    /**
+     * @brief Removes an element from a given position.
+     * @param pos An iterator to the element to remove.
+     * @return An iterator following the removed element.
+     */
+    iterator erase(const_iterator pos) {
+        const auto it = pool.first().begin();
+        return pool.first().erase(it + (pos - const_iterator{it}));
+    }
+
+    /**
+     * @brief Removes the given elements from a cache.
+     * @param first An iterator to the first element of the range of elements.
+     * @param last An iterator past the last element of the range of elements.
+     * @return An iterator following the last removed element.
+     */
+    iterator erase(const_iterator first, const_iterator last) {
+        const auto it = pool.first().begin();
+        return pool.first().erase(it + (first - const_iterator{it}), it + (last - const_iterator{it}));
+    }
+
+    /**
+     * @brief Removes the given elements from a cache.
+     * @param id Unique resource identifier.
+     * @return Number of resources erased (either 0 or 1).
+     */
+    size_type erase(const id_type id) {
+        return pool.first().erase(id);
+    }
+
+    /**
+     * @brief Returns the loader used to create resources.
+     * @return The loader used to create resources.
+     */
+    [[nodiscard]] loader_type loader() const {
+        return pool.second();
+    }
+
+private:
+    compressed_pair<container_type, loader_type> pool;
+};
+
+} // namespace entt
+
+#endif

include/entt/resource/fwd.hpp

@@ -0,0 +1,19 @@
+#ifndef ENTT_RESOURCE_FWD_HPP
+#define ENTT_RESOURCE_FWD_HPP
+
+#include "../stl/memory.hpp"
+
+namespace entt {
+
+template<typename>
+struct resource_loader;
+
+template<typename Type, typename = resource_loader<Type>, typename = stl::allocator<Type>>
+class resource_cache;
+
+template<typename>
+class resource;
+
+} // namespace entt
+
+#endif

include/entt/resource/loader.hpp

@@ -0,0 +1,33 @@
+#ifndef ENTT_RESOURCE_LOADER_HPP
+#define ENTT_RESOURCE_LOADER_HPP
+
+#include "../stl/memory.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Transparent loader for shared resources.
+ * @tparam Type Type of resources created by the loader.
+ */
+template<typename Type>
+struct resource_loader {
+    /*! @brief Result type. */
+    using result_type = stl::shared_ptr<Type>;
+
+    /**
+     * @brief Constructs a shared pointer to a resource from its arguments.
+     * @tparam Args Types of arguments to use to construct the resource.
+     * @param args Parameters to use to construct the resource.
+     * @return A shared pointer to a resource of the given type.
+     */
+    template<typename... Args>
+    result_type operator()(Args &&...args) const {
+        return stl::make_shared<Type>(stl::forward<Args>(args)...);
+    }
+};
+
+} // namespace entt
+
+#endif

include/entt/resource/resource.hpp

@@ -0,0 +1,212 @@
+#ifndef ENTT_RESOURCE_RESOURCE_HPP
+#define ENTT_RESOURCE_RESOURCE_HPP
+
+#include <compare>
+#include "../stl/concepts.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Basic resource handle.
+ *
+ * A handle wraps a resource and extends its lifetime. It also shares the same
+ * resource with all other handles constructed from the same element.<br/>
+ * As a rule of thumb, resources should never be copied nor moved. Handles are
+ * the way to go to push references around.
+ *
+ * @tparam Type Type of resource managed by a handle.
+ */
+template<typename Type>
+class resource {
+    template<typename>
+    friend class resource;
+
+public:
+    /*! @brief Resource type. */
+    using element_type = Type;
+    /*! @brief Handle type. */
+    using handle_type = stl::shared_ptr<element_type>;
+
+    /*! @brief Default constructor. */
+    resource() noexcept
+        : value{} {}
+
+    /**
+     * @brief Creates a new resource handle.
+     * @param res A handle to a resource.
+     */
+    explicit resource(handle_type res) noexcept
+        : value{stl::move(res)} {}
+
+    /*! @brief Default copy constructor. */
+    resource(const resource &) noexcept = default;
+
+    /*! @brief Default move constructor. */
+    resource(resource &&) noexcept = default;
+
+    /**
+     * @brief Aliasing constructor.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle with which to share ownership information.
+     * @param res Unrelated and unmanaged resources.
+     */
+    template<typename Other>
+    resource(const resource<Other> &other, element_type &res) noexcept
+        : value{other.value, stl::addressof(res)} {}
+
+    /**
+     * @brief Copy constructs a handle which shares ownership of the resource.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to copy from.
+     */
+    template<typename Other>
+    requires (!stl::same_as<Type, Other> && stl::constructible_from<Type &, Other &>)
+    resource(const resource<Other> &other) noexcept
+        : value{other.value} {}
+
+    /**
+     * @brief Move constructs a handle which takes ownership of the resource.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to move from.
+     */
+    template<typename Other>
+    requires (!stl::same_as<Type, Other> && stl::constructible_from<Type &, Other &>)
+    resource(resource<Other> &&other) noexcept
+        : value{stl::move(other.value)} {}
+
+    /*! @brief Default destructor. */
+    ~resource() = default;
+
+    /**
+     * @brief Default copy assignment operator.
+     * @return This resource handle.
+     */
+    resource &operator=(const resource &) noexcept = default;
+
+    /**
+     * @brief Default move assignment operator.
+     * @return This resource handle.
+     */
+    resource &operator=(resource &&) noexcept = default;
+
+    /**
+     * @brief Copy assignment operator from foreign handle.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to copy from.
+     * @return This resource handle.
+     */
+    template<typename Other>
+    requires (!stl::same_as<Type, Other> && stl::constructible_from<Type &, Other &>)
+    resource &operator=(const resource<Other> &other) noexcept {
+        value = other.value;
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator from foreign handle.
+     * @tparam Other Type of resource managed by the received handle.
+     * @param other The handle to move from.
+     * @return This resource handle.
+     */
+    template<typename Other>
+    requires (!stl::same_as<Type, Other> && stl::constructible_from<Type &, Other &>)
+    resource &operator=(resource<Other> &&other) noexcept {
+        value = stl::move(other.value);
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the content with that of a given resource.
+     * @param other Resource to exchange the content with.
+     */
+    void swap(resource &other) noexcept {
+        using stl::swap;
+        swap(value, other.value);
+    }
+
+    /**
+     * @brief Returns a reference to the managed resource.
+     *
+     * @warning
+     * The behavior is undefined if the handle doesn't contain a resource.
+     *
+     * @return A reference to the managed resource.
+     */
+    [[nodiscard]] element_type &operator*() const noexcept {
+        return *value;
+    }
+
+    /*! @copydoc operator* */
+    [[nodiscard]] operator element_type &() const noexcept {
+        return *value;
+    }
+
+    /**
+     * @brief Returns a pointer to the managed resource.
+     * @return A pointer to the managed resource.
+     */
+    [[nodiscard]] element_type *operator->() const noexcept {
+        return value.get();
+    }
+
+    /**
+     * @brief Returns true if a handle contains a resource, false otherwise.
+     * @return True if the handle contains a resource, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return static_cast<bool>(value);
+    }
+
+    /**
+     * @brief Compares two handles.
+     * @tparam Other Type of resource managed by the other handle.
+     * @param other A valid handle.
+     * @return True if both handles refer to the same resource, false otherwise.
+     */
+    template<typename Other>
+    [[nodiscard]] bool operator==(const resource<Other> &other) const noexcept {
+        return (value == other.value);
+    }
+
+    /**
+     * @brief Lexicographically compares two handles.
+     * @tparam Other Type of resource managed by the other handle.
+     * @param other A valid handle.
+     * @return The relative order between the two handles.
+     */
+    template<typename Other>
+    [[nodiscard]] auto operator<=>(const resource<Other> &other) const noexcept {
+        return (value <=> other.value);
+    }
+
+    /*! @brief Releases the ownership of the managed resource. */
+    void reset() {
+        value.reset();
+    }
+
+    /**
+     * @brief Replaces the managed resource.
+     * @param other A handle to a resource.
+     */
+    void reset(handle_type other) {
+        value = stl::move(other);
+    }
+
+    /**
+     * @brief Returns the underlying resource handle.
+     * @return The underlying resource handle.
+     */
+    [[nodiscard]] handle_type handle() const noexcept {
+        return value;
+    }
+
+private:
+    handle_type value;
+};
+
+} // namespace entt
+
+#endif

include/entt/signal/delegate.hpp

@@ -0,0 +1,314 @@
+#ifndef ENTT_SIGNAL_DELEGATE_HPP
+#define ENTT_SIGNAL_DELEGATE_HPP
+
+#include "../config/config.h"
+#include "../core/type_traits.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/tuple.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+template<typename Ret, typename... Args>
+auto function_pointer(Ret (*)(Args...)) -> Ret (*)(Args...);
+
+template<typename Ret, typename Type, typename... Args, typename Other>
+auto function_pointer(Ret (*)(Type, Args...), Other &&) -> Ret (*)(Args...);
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret (Class::*)(Args...), Other &&...) -> Ret (*)(Args...);
+
+template<typename Class, typename Ret, typename... Args, typename... Other>
+auto function_pointer(Ret (Class::*)(Args...) const, Other &&...) -> Ret (*)(Args...);
+
+template<typename Class, typename Type, typename... Other>
+requires stl::is_member_object_pointer_v<Type Class::*>
+auto function_pointer(Type Class::*, Other &&...) -> Type (*)();
+
+template<typename... Type>
+using function_pointer_t = decltype(function_pointer(stl::declval<Type>()...));
+
+template<typename... Class, typename Ret, typename... Args>
+[[nodiscard]] ENTT_CONSTEVAL auto index_sequence_for(Ret (*)(Args...)) {
+    return stl::index_sequence_for<Class..., Args...>{};
+}
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Basic delegate implementation.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ */
+template<typename>
+class delegate;
+
+/**
+ * @brief Utility class to use to send around functions and members.
+ *
+ * Unmanaged delegate for function pointers and members. Users of this class are
+ * in charge of disconnecting instances before deleting them.
+ *
+ * A delegate can be used as a general purpose invoker without memory overhead
+ * for free functions possibly with payloads and bound or unbound members.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+class delegate<Ret(Args...)> {
+    using return_type = stl::remove_const_t<Ret>;
+    using delegate_type = return_type(const void *, Args...);
+
+    template<auto Candidate, stl::size_t... Index>
+    [[nodiscard]] auto wrap(stl::index_sequence<Index...>) noexcept {
+        return [](const void *, Args... args) -> return_type {
+            [[maybe_unused]] const auto arguments = stl::forward_as_tuple(stl::forward<Args>(args)...);
+            [[maybe_unused]] constexpr auto offset = !stl::is_invocable_r_v<Ret, decltype(Candidate), type_list_element_t<Index, type_list<Args...>>...> * (sizeof...(Args) - sizeof...(Index));
+            return static_cast<Ret>(stl::invoke(Candidate, stl::forward<type_list_element_t<Index + offset, type_list<Args...>>>(stl::get<Index + offset>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, stl::size_t... Index>
+    [[nodiscard]] auto wrap(Type &, stl::index_sequence<Index...>) noexcept {
+        return [](const void *payload, Args... args) -> return_type {
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            [[maybe_unused]] const auto arguments = stl::forward_as_tuple(stl::forward<Args>(args)...);
+            [[maybe_unused]] constexpr auto offset = !stl::is_invocable_r_v<Ret, decltype(Candidate), Type &, type_list_element_t<Index, type_list<Args...>>...> * (sizeof...(Args) - sizeof...(Index));
+            return static_cast<Ret>(stl::invoke(Candidate, *curr, stl::forward<type_list_element_t<Index + offset, type_list<Args...>>>(stl::get<Index + offset>(arguments))...));
+        };
+    }
+
+    template<auto Candidate, typename Type, stl::size_t... Index>
+    [[nodiscard]] auto wrap(Type *, stl::index_sequence<Index...>) noexcept {
+        return [](const void *payload, Args... args) -> return_type {
+            Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+            [[maybe_unused]] const auto arguments = stl::forward_as_tuple(stl::forward<Args>(args)...);
+            [[maybe_unused]] constexpr auto offset = !stl::is_invocable_r_v<Ret, decltype(Candidate), Type *, type_list_element_t<Index, type_list<Args...>>...> * (sizeof...(Args) - sizeof...(Index));
+            return static_cast<Ret>(stl::invoke(Candidate, curr, stl::forward<type_list_element_t<Index + offset, type_list<Args...>>>(stl::get<Index + offset>(arguments))...));
+        };
+    }
+
+public:
+    /*! @brief Function type of the contained target. */
+    using function_type = Ret(const void *, Args...);
+    /*! @brief Function type of the delegate. */
+    using type = Ret(Args...);
+    /*! @brief Return type of the delegate. */
+    using result_type = Ret;
+
+    /*! @brief Default constructor. */
+    delegate() noexcept = default;
+
+    /**
+     * @brief Constructs a delegate with a given object or payload, if any.
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload, if any.
+     * @param value_or_instance Optional valid object that fits the purpose.
+     */
+    template<auto Candidate, typename... Type>
+    delegate(connect_arg_t<Candidate>, Type &&...value_or_instance) noexcept {
+        connect<Candidate>(stl::forward<Type>(value_or_instance)...);
+    }
+
+    /**
+     * @brief Constructs a delegate and connects an user defined function with
+     * optional payload.
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    delegate(function_type *function, const void *payload = nullptr) noexcept {
+        connect(function, payload);
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a delegate.
+     * @tparam Candidate Function or member to connect to the delegate.
+     */
+    template<auto Candidate>
+    void connect() noexcept {
+        instance = nullptr;
+
+        if constexpr(stl::is_invocable_r_v<Ret, decltype(Candidate), Args...>) {
+            fn = [](const void *, Args... args) -> return_type {
+                return Ret(stl::invoke(Candidate, stl::forward<Args>(args)...));
+            };
+        } else if constexpr(stl::is_member_pointer_v<decltype(Candidate)>) {
+            fn = wrap<Candidate>(internal::index_sequence_for<type_list_element_t<0, type_list<Args...>>>(internal::function_pointer_t<decltype(Candidate)>{}));
+        } else {
+            fn = wrap<Candidate>(internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate)>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the delegate.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the delegate itself.
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type &value_or_instance) noexcept {
+        instance = &value_or_instance;
+
+        if constexpr(stl::is_invocable_r_v<Ret, decltype(Candidate), Type &, Args...>) {
+            fn = [](const void *payload, Args... args) -> return_type {
+                Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+                return Ret(stl::invoke(Candidate, *curr, stl::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * delegate.
+     *
+     * @sa connect(Type &)
+     *
+     * @tparam Candidate Function or member to connect to the delegate.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void connect(Type *value_or_instance) noexcept {
+        instance = value_or_instance;
+
+        if constexpr(stl::is_invocable_r_v<Ret, decltype(Candidate), Type *, Args...>) {
+            fn = [](const void *payload, Args... args) -> return_type {
+                Type *curr = static_cast<Type *>(const_cast<constness_as_t<void, Type> *>(payload));
+                return Ret(stl::invoke(Candidate, curr, stl::forward<Args>(args)...));
+            };
+        } else {
+            fn = wrap<Candidate>(value_or_instance, internal::index_sequence_for(internal::function_pointer_t<decltype(Candidate), Type>{}));
+        }
+    }
+
+    /**
+     * @brief Connects an user defined function with optional payload to a
+     * delegate.
+     *
+     * The delegate isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of an instance overcomes
+     * the one of the delegate.<br/>
+     * The payload is returned as the first argument to the target function in
+     * all cases.
+     *
+     * @param function Function to connect to the delegate.
+     * @param payload User defined arbitrary data.
+     */
+    void connect(function_type *function, const void *payload = nullptr) noexcept {
+        ENTT_ASSERT(function != nullptr, "Uninitialized function pointer");
+        instance = payload;
+        fn = function;
+    }
+
+    /**
+     * @brief Resets a delegate.
+     *
+     * After a reset, a delegate cannot be invoked anymore.
+     */
+    void reset() noexcept {
+        instance = nullptr;
+        fn = nullptr;
+    }
+
+    /**
+     * @brief Returns a pointer to the stored callable function target, if any.
+     * @return An opaque pointer to the stored callable function target.
+     */
+    [[nodiscard]] function_type *target() const noexcept {
+        return fn;
+    }
+
+    /**
+     * @brief Returns the instance or the payload linked to a delegate, if any.
+     * @return An opaque pointer to the underlying data.
+     */
+    [[nodiscard]] const void *data() const noexcept {
+        return instance;
+    }
+
+    /**
+     * @brief Triggers a delegate.
+     *
+     * The delegate invokes the underlying function and returns the result.
+     *
+     * @warning
+     * Attempting to trigger an invalid delegate results in undefined
+     * behavior.
+     *
+     * @param args Arguments to use to invoke the underlying function.
+     * @return The value returned by the underlying function.
+     */
+    Ret operator()(Args... args) const {
+        ENTT_ASSERT(static_cast<bool>(*this), "Uninitialized delegate");
+        return fn(instance, stl::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Checks whether a delegate actually stores a listener.
+     * @return False if the delegate is empty, true otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        // no need to also test instance
+        return !(fn == nullptr);
+    }
+
+    /**
+     * @brief Compares the contents of two delegates.
+     * @param other Delegate with which to compare.
+     * @return False if the two contents differ, true otherwise.
+     */
+    [[nodiscard]] bool operator==(const delegate<Ret(Args...)> &other) const noexcept {
+        return fn == other.fn && instance == other.instance;
+    }
+
+private:
+    const void *instance{};
+    delegate_type *fn{};
+};
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ */
+template<auto Candidate>
+delegate(connect_arg_t<Candidate>) -> delegate<stl::remove_pointer_t<internal::function_pointer_t<decltype(Candidate)>>>;
+
+/**
+ * @brief Deduction guide.
+ * @tparam Candidate Function or member to connect to the delegate.
+ * @tparam Type Type of class or type of payload.
+ */
+template<auto Candidate, typename Type>
+delegate(connect_arg_t<Candidate>, Type &&) -> delegate<stl::remove_pointer_t<internal::function_pointer_t<decltype(Candidate), Type>>>;
+
+/**
+ * @brief Deduction guide.
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ */
+template<typename Ret, typename... Args>
+delegate(Ret (*)(const void *, Args...), const void * = nullptr) -> delegate<Ret(Args...)>;
+
+} // namespace entt
+
+#endif

include/entt/signal/dispatcher.hpp

@@ -0,0 +1,391 @@
+#ifndef ENTT_SIGNAL_DISPATCHER_HPP
+#define ENTT_SIGNAL_DISPATCHER_HPP
+
+#include "../container/dense_map.hpp"
+#include "../core/compressed_pair.hpp"
+#include "../core/concepts.hpp"
+#include "../core/fwd.hpp"
+#include "../core/type_info.hpp"
+#include "../stl/cstddef.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+#include "fwd.hpp"
+#include "sigh.hpp"
+
+namespace entt {
+
+/*! @cond ENTT_INTERNAL */
+namespace internal {
+
+struct basic_dispatcher_handler {
+    virtual ~basic_dispatcher_handler() = default;
+    virtual void publish() = 0;
+    virtual void disconnect(void *) = 0;
+    virtual void clear() noexcept = 0;
+    [[nodiscard]] virtual stl::size_t size() const noexcept = 0;
+};
+
+template<cvref_unqualified Type, typename Allocator>
+class dispatcher_handler final: public basic_dispatcher_handler {
+    using alloc_traits = stl::allocator_traits<Allocator>;
+    using signal_type = sigh<void(Type &), Allocator>;
+    using container_type = stl::vector<Type, typename alloc_traits::template rebind_alloc<Type>>;
+
+public:
+    using allocator_type = Allocator;
+
+    dispatcher_handler(const allocator_type &allocator)
+        : signal{allocator},
+          events{allocator} {}
+
+    void publish() override {
+        container_type other{};
+        other.swap(events);
+
+        for(auto &&elem: other) {
+            signal.publish(elem);
+        }
+    }
+
+    void disconnect(void *instance) override {
+        bucket().disconnect(instance);
+    }
+
+    void clear() noexcept override {
+        events.clear();
+    }
+
+    [[nodiscard]] auto bucket() noexcept {
+        return typename signal_type::sink_type{signal};
+    }
+
+    void trigger(Type &event) {
+        signal.publish(event);
+    }
+
+    template<typename... Args>
+    void enqueue(Args &&...args) {
+        if constexpr(stl::is_aggregate_v<Type> && (sizeof...(Args) != 0u || !stl::is_default_constructible_v<Type>)) {
+            events.push_back(Type{stl::forward<Args>(args)...});
+        } else {
+            events.emplace_back(stl::forward<Args>(args)...);
+        }
+    }
+
+    [[nodiscard]] stl::size_t size() const noexcept override {
+        return events.size();
+    }
+
+private:
+    signal_type signal;
+    container_type events;
+};
+
+} // namespace internal
+/*! @endcond */
+
+/**
+ * @brief Basic dispatcher implementation.
+ *
+ * A dispatcher can be used either to trigger an immediate event or to enqueue
+ * events to be published all together once per tick.<br/>
+ * Listeners are provided in the form of member functions. For each event of
+ * type `Type`, listeners are such that they can be invoked with an argument of
+ * type `Type &`, no matter what the return type is.
+ *
+ * The dispatcher creates instances of the `sigh` class internally. Refer to the
+ * documentation of the latter for more details.
+ *
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Allocator>
+class basic_dispatcher {
+    template<typename Type>
+    using handler_type = internal::dispatcher_handler<Type, Allocator>;
+
+    using key_type = id_type;
+    // stl::shared_ptr because of its type erased allocator which is useful here
+    using mapped_type = stl::shared_ptr<internal::basic_dispatcher_handler>;
+
+    using alloc_traits = stl::allocator_traits<Allocator>;
+    using container_allocator = alloc_traits::template rebind_alloc<stl::pair<const key_type, mapped_type>>;
+    using container_type = dense_map<key_type, mapped_type, stl::identity, stl::equal_to<>, container_allocator>;
+
+    template<cvref_unqualified Type>
+    [[nodiscard]] handler_type<Type> &assure(const id_type id) {
+        auto &&ptr = pools.first()[id];
+
+        if(!ptr) {
+            const auto &allocator = get_allocator();
+            ptr = stl::allocate_shared<handler_type<Type>>(allocator, allocator);
+        }
+
+        return static_cast<handler_type<Type> &>(*ptr);
+    }
+
+    template<cvref_unqualified Type>
+    [[nodiscard]] const handler_type<Type> *assure(const id_type id) const {
+        if(auto it = pools.first().find(id); it != pools.first().cend()) {
+            return static_cast<const handler_type<Type> *>(it->second.get());
+        }
+
+        return nullptr;
+    }
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+
+    /*! @brief Default constructor. */
+    basic_dispatcher()
+        : basic_dispatcher{allocator_type{}} {}
+
+    /**
+     * @brief Constructs a dispatcher with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit basic_dispatcher(const allocator_type &allocator)
+        : pools{allocator, allocator} {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    basic_dispatcher(const basic_dispatcher &) = delete;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    basic_dispatcher(basic_dispatcher &&other) noexcept
+        : pools{stl::move(other.pools)} {}
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    basic_dispatcher(basic_dispatcher &&other, const allocator_type &allocator)
+        : pools{container_type{stl::move(other.pools.first()), allocator}, allocator} {
+        ENTT_ASSERT(alloc_traits::is_always_equal::value || get_allocator() == other.get_allocator(), "Copying a dispatcher is not allowed");
+    }
+
+    /*! @brief Default destructor. */
+    ~basic_dispatcher() = default;
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This dispatcher.
+     */
+    basic_dispatcher &operator=(const basic_dispatcher &) = delete;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This dispatcher.
+     */
+    basic_dispatcher &operator=(basic_dispatcher &&other) noexcept {
+        ENTT_ASSERT(alloc_traits::is_always_equal::value || get_allocator() == other.get_allocator(), "Copying a dispatcher is not allowed");
+        swap(other);
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given dispatcher.
+     * @param other Dispatcher to exchange the content with.
+     */
+    void swap(basic_dispatcher &other) noexcept {
+        using stl::swap;
+        swap(pools, other.pools);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return pools.second();
+    }
+
+    /**
+     * @brief Returns the number of pending events for a given type.
+     * @tparam Type Type of event for which to return the count.
+     * @param id Name used to map the event queue within the dispatcher.
+     * @return The number of pending events for the given type.
+     */
+    template<typename Type>
+    [[nodiscard]] size_type size(const id_type id = type_hash<Type>::value()) const noexcept {
+        const auto *cpool = assure<stl::decay_t<Type>>(id);
+        return cpool ? cpool->size() : 0u;
+    }
+
+    /**
+     * @brief Returns the total number of pending events.
+     * @return The total number of pending events.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        size_type count{};
+
+        for(auto &&cpool: pools.first()) {
+            count += cpool.second->size();
+        }
+
+        return count;
+    }
+
+    /**
+     * @brief Returns a sink object for the given event and queue.
+     *
+     * A sink is an opaque object used to connect listeners to events.
+     *
+     * The function type for a listener is _compatible_ with:
+     *
+     * @code{.cpp}
+     * void(Type &);
+     * @endcode
+     *
+     * The order of invocation of the listeners isn't guaranteed.
+     *
+     * @sa sink
+     *
+     * @tparam Type Type of event of which to get the sink.
+     * @param id Name used to map the event queue within the dispatcher.
+     * @return A temporary sink object.
+     */
+    template<typename Type>
+    [[nodiscard]] auto sink(const id_type id = type_hash<Type>::value()) {
+        return assure<Type>(id).bucket();
+    }
+
+    /**
+     * @brief Triggers an immediate event of a given type.
+     * @tparam Type Type of event to trigger.
+     * @param value An instance of the given type of event.
+     */
+    template<typename Type>
+    void trigger(Type value) {
+        trigger(type_hash<stl::decay_t<Type>>::value(), value);
+    }
+
+    /**
+     * @brief Triggers an immediate event on a queue of a given type.
+     * @tparam Type Type of event to trigger.
+     * @param value An instance of the given type of event.
+     * @param id Name used to map the event queue within the dispatcher.
+     */
+    template<typename Type>
+    void trigger(const id_type id, Type value) {
+        assure<stl::decay_t<Type>>(id).trigger(value);
+    }
+
+    /**
+     * @brief Enqueues an event of the given type.
+     * @tparam Type Type of event to enqueue.
+     * @tparam Args Types of arguments to use to construct the event.
+     * @param args Arguments to use to construct the event.
+     */
+    template<typename Type, typename... Args>
+    void enqueue(Args &&...args) {
+        enqueue_hint<Type>(type_hash<Type>::value(), stl::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Enqueues an event of the given type.
+     * @tparam Type Type of event to enqueue.
+     * @param value An instance of the given type of event.
+     */
+    template<typename Type>
+    void enqueue(Type &&value) {
+        enqueue_hint(type_hash<stl::decay_t<Type>>::value(), stl::forward<Type>(value));
+    }
+
+    /**
+     * @brief Enqueues an event of the given type.
+     * @tparam Type Type of event to enqueue.
+     * @tparam Args Types of arguments to use to construct the event.
+     * @param id Name used to map the event queue within the dispatcher.
+     * @param args Arguments to use to construct the event.
+     */
+    template<typename Type, typename... Args>
+    void enqueue_hint(const id_type id, Args &&...args) {
+        assure<Type>(id).enqueue(stl::forward<Args>(args)...);
+    }
+
+    /**
+     * @brief Enqueues an event of the given type.
+     * @tparam Type Type of event to enqueue.
+     * @param id Name used to map the event queue within the dispatcher.
+     * @param value An instance of the given type of event.
+     */
+    template<typename Type>
+    void enqueue_hint(const id_type id, Type &&value) {
+        assure<stl::decay_t<Type>>(id).enqueue(stl::forward<Type>(value));
+    }
+
+    /**
+     * @brief Utility function to disconnect everything related to a given value
+     * or instance from a dispatcher.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type &value_or_instance) {
+        disconnect(&value_or_instance);
+    }
+
+    /**
+     * @brief Utility function to disconnect everything related to a given value
+     * or instance from a dispatcher.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    template<typename Type>
+    void disconnect(Type *value_or_instance) {
+        for(auto &&cpool: pools.first()) {
+            cpool.second->disconnect(value_or_instance);
+        }
+    }
+
+    /**
+     * @brief Discards all the events stored so far in a given queue.
+     * @tparam Type Type of event to discard.
+     * @param id Name used to map the event queue within the dispatcher.
+     */
+    template<typename Type>
+    void clear(const id_type id = type_hash<Type>::value()) {
+        assure<Type>(id).clear();
+    }
+
+    /*! @brief Discards all the events queued so far. */
+    void clear() noexcept {
+        for(auto &&cpool: pools.first()) {
+            cpool.second->clear();
+        }
+    }
+
+    /**
+     * @brief Delivers all the pending events of a given queue.
+     * @tparam Type Type of event to send.
+     * @param id Name used to map the event queue within the dispatcher.
+     */
+    template<typename Type>
+    void update(const id_type id = type_hash<Type>::value()) {
+        assure<Type>(id).publish();
+    }
+
+    /*! @brief Delivers all the pending events. */
+    void update() const {
+        for(auto &&cpool: pools.first()) {
+            cpool.second->publish();
+        }
+    }
+
+private:
+    compressed_pair<container_type, allocator_type> pools;
+};
+
+} // namespace entt
+
+#endif

include/entt/signal/emitter.hpp

@@ -0,0 +1,181 @@
+#ifndef ENTT_SIGNAL_EMITTER_HPP
+#define ENTT_SIGNAL_EMITTER_HPP
+
+#include "../container/dense_map.hpp"
+#include "../core/compressed_pair.hpp"
+#include "../core/fwd.hpp"
+#include "../core/type_info.hpp"
+#include "../stl/functional.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief General purpose event emitter.
+ *
+ * To create an emitter type, derived classes must inherit from the base as:
+ *
+ * @code{.cpp}
+ * struct my_emitter: emitter<my_emitter> {
+ *     // ...
+ * }
+ * @endcode
+ *
+ * Handlers for the different events are created internally on the fly. It's not
+ * required to specify in advance the full list of accepted events.<br/>
+ * Moreover, whenever an event is published, an emitter also passes a reference
+ * to itself to its listeners.
+ *
+ * @tparam Derived Emitter type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Derived, typename Allocator>
+class emitter {
+    using key_type = id_type;
+    using mapped_type = stl::function<void(void *)>;
+
+    using alloc_traits = stl::allocator_traits<Allocator>;
+    using container_allocator = alloc_traits::template rebind_alloc<stl::pair<const key_type, mapped_type>>;
+    using container_type = dense_map<key_type, mapped_type, stl::identity, stl::equal_to<>, container_allocator>;
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+
+    /*! @brief Default constructor. */
+    emitter()
+        : emitter{allocator_type{}} {}
+
+    /**
+     * @brief Constructs an emitter with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit emitter(const allocator_type &allocator)
+        : handlers{allocator, allocator} {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    emitter(const emitter &) = delete;
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    emitter(emitter &&other) noexcept
+        : handlers{stl::move(other.handlers)} {}
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    emitter(emitter &&other, const allocator_type &allocator)
+        : handlers{container_type{stl::move(other.handlers.first()), allocator}, allocator} {
+        ENTT_ASSERT(alloc_traits::is_always_equal::value || handlers.second() == other.handlers.second(), "Copying an emitter is not allowed");
+    }
+
+    /*! @brief Default destructor. */
+    virtual ~emitter() {
+        static_assert(stl::is_base_of_v<emitter<Derived, Allocator>, Derived>, "Invalid emitter type");
+    }
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This emitter.
+     */
+    emitter &operator=(const emitter &) = delete;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This emitter.
+     */
+    emitter &operator=(emitter &&other) noexcept {
+        ENTT_ASSERT(alloc_traits::is_always_equal::value || handlers.second() == other.handlers.second(), "Copying an emitter is not allowed");
+        swap(other);
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given emitter.
+     * @param other Emitter to exchange the content with.
+     */
+    void swap(emitter &other) noexcept {
+        using stl::swap;
+        swap(handlers, other.handlers);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return handlers.second();
+    }
+
+    /**
+     * @brief Publishes a given event.
+     * @tparam Type Type of event to trigger.
+     * @param value An instance of the given type of event.
+     */
+    template<typename Type>
+    void publish(Type value) {
+        if(const auto id = type_id<Type>().hash(); handlers.first().contains(id)) {
+            handlers.first()[id](&value);
+        }
+    }
+
+    /**
+     * @brief Registers a listener with the event emitter.
+     * @tparam Type Type of event to which to connect the listener.
+     * @param func The listener to register.
+     */
+    template<typename Type>
+    void on(stl::function<void(Type &, Derived &)> func) {
+        handlers.first().insert_or_assign(type_id<Type>().hash(), [func = stl::move(func), this](void *value) {
+            func(*static_cast<Type *>(value), static_cast<Derived &>(*this));
+        });
+    }
+
+    /**
+     * @brief Disconnects a listener from the event emitter.
+     * @tparam Type Type of event of the listener.
+     */
+    template<typename Type>
+    void erase() {
+        handlers.first().erase(type_hash<stl::remove_cvref_t<Type>>::value());
+    }
+
+    /*! @brief Disconnects all the listeners. */
+    void clear() noexcept {
+        handlers.first().clear();
+    }
+
+    /**
+     * @brief Checks if there are listeners registered for the specific event.
+     * @tparam Type Type of event to test.
+     * @return True if there are no listeners registered, false otherwise.
+     */
+    template<typename Type>
+    [[nodiscard]] bool contains() const {
+        return handlers.first().contains(type_hash<stl::remove_cvref_t<Type>>::value());
+    }
+
+    /**
+     * @brief Checks if there are listeners registered with the event emitter.
+     * @return True if there are no listeners registered, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return handlers.first().empty();
+    }
+
+private:
+    compressed_pair<container_type, allocator_type> handlers;
+};
+
+} // namespace entt
+
+#endif

include/entt/signal/fwd.hpp

@@ -0,0 +1,46 @@
+#ifndef ENTT_SIGNAL_FWD_HPP
+#define ENTT_SIGNAL_FWD_HPP
+
+#include "../stl/memory.hpp"
+
+namespace entt {
+
+template<typename>
+class delegate;
+
+template<typename = stl::allocator<void>>
+class basic_dispatcher;
+
+template<typename, typename = stl::allocator<void>>
+class emitter;
+
+class connection;
+
+struct scoped_connection;
+
+template<typename>
+class sink;
+
+template<typename Type, typename = stl::allocator<void>>
+class sigh;
+
+/*! @brief Alias declaration for the most common use case. */
+using dispatcher = basic_dispatcher<>;
+
+/*! @brief Disambiguation tag for constructors and the like. */
+template<auto>
+struct connect_arg_t {
+    /*! @brief Default constructor. */
+    explicit connect_arg_t() = default;
+};
+
+/**
+ * @brief Constant of type connect_arg_t used to disambiguate calls.
+ * @tparam Candidate Element to connect (likely a free or member function).
+ */
+template<auto Candidate>
+inline constexpr connect_arg_t<Candidate> connect_arg{};
+
+} // namespace entt
+
+#endif

include/entt/signal/sigh.hpp

@@ -0,0 +1,573 @@
+#ifndef ENTT_SIGNAL_SIGH_HPP
+#define ENTT_SIGNAL_SIGH_HPP
+
+#include "../stl/cstddef.hpp"
+#include "../stl/memory.hpp"
+#include "../stl/type_traits.hpp"
+#include "../stl/utility.hpp"
+#include "../stl/vector.hpp"
+#include "delegate.hpp"
+#include "fwd.hpp"
+
+namespace entt {
+
+/**
+ * @brief Sink class.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ *
+ * @tparam Type A valid signal handler type.
+ */
+template<typename Type>
+class sink;
+
+/**
+ * @brief Unmanaged signal handler.
+ *
+ * Primary template isn't defined on purpose. All the specializations give a
+ * compile-time error unless the template parameter is a function type.
+ *
+ * @tparam Type A valid function type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Type, typename Allocator>
+class sigh;
+
+/**
+ * @brief Unmanaged signal handler.
+ *
+ * It works directly with references to classes and pointers to member functions
+ * as well as pointers to free functions. Users of this class are in charge of
+ * disconnecting instances before deleting them.
+ *
+ * This class serves mainly two purposes:
+ *
+ * * Creating signals to use later to notify a bunch of listeners.
+ * * Collecting results from a set of functions like in a voting system.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Ret, typename... Args, typename Allocator>
+class sigh<Ret(Args...), Allocator> {
+    friend class sink<sigh<Ret(Args...), Allocator>>;
+
+    using alloc_traits = stl::allocator_traits<Allocator>;
+    using delegate_type = delegate<Ret(Args...)>;
+    using container_type = stl::vector<delegate_type, typename alloc_traits::template rebind_alloc<delegate_type>>;
+
+public:
+    /*! @brief Allocator type. */
+    using allocator_type = Allocator;
+    /*! @brief Unsigned integer type. */
+    using size_type = stl::size_t;
+    /*! @brief Sink type. */
+    using sink_type = sink<sigh<Ret(Args...), Allocator>>;
+
+    /*! @brief Default constructor. */
+    sigh() noexcept(noexcept(allocator_type{}))
+        : sigh{allocator_type{}} {}
+
+    /**
+     * @brief Constructs a signal handler with a given allocator.
+     * @param allocator The allocator to use.
+     */
+    explicit sigh(const allocator_type &allocator) noexcept
+        : calls{allocator} {}
+
+    /**
+     * @brief Copy constructor.
+     * @param other The instance to copy from.
+     */
+    sigh(const sigh &other)
+        : calls{other.calls} {}
+
+    /**
+     * @brief Allocator-extended copy constructor.
+     * @param other The instance to copy from.
+     * @param allocator The allocator to use.
+     */
+    sigh(const sigh &other, const allocator_type &allocator)
+        : calls{other.calls, allocator} {}
+
+    /**
+     * @brief Move constructor.
+     * @param other The instance to move from.
+     */
+    sigh(sigh &&other) noexcept
+        : calls{stl::move(other.calls)} {}
+
+    /**
+     * @brief Allocator-extended move constructor.
+     * @param other The instance to move from.
+     * @param allocator The allocator to use.
+     */
+    sigh(sigh &&other, const allocator_type &allocator)
+        : calls{stl::move(other.calls), allocator} {}
+
+    /*! @brief Default destructor. */
+    ~sigh() = default;
+
+    /**
+     * @brief Copy assignment operator.
+     * @param other The instance to copy from.
+     * @return This signal handler.
+     */
+    sigh &operator=(const sigh &other) {
+        calls = other.calls;
+        return *this;
+    }
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The instance to move from.
+     * @return This signal handler.
+     */
+    sigh &operator=(sigh &&other) noexcept {
+        swap(other);
+        return *this;
+    }
+
+    /**
+     * @brief Exchanges the contents with those of a given signal handler.
+     * @param other Signal handler to exchange the content with.
+     */
+    void swap(sigh &other) noexcept {
+        using stl::swap;
+        swap(calls, other.calls);
+    }
+
+    /**
+     * @brief Returns the associated allocator.
+     * @return The associated allocator.
+     */
+    [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
+        return calls.get_allocator();
+    }
+
+    /**
+     * @brief Number of listeners connected to the signal.
+     * @return Number of listeners currently connected.
+     */
+    [[nodiscard]] size_type size() const noexcept {
+        return calls.size();
+    }
+
+    /**
+     * @brief Returns false if at least a listener is connected to the signal.
+     * @return True if the signal has no listeners connected, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return calls.empty();
+    }
+
+    /**
+     * @brief Triggers a signal.
+     *
+     * All the listeners are notified. Order isn't guaranteed.
+     *
+     * @param args Arguments to use to invoke listeners.
+     */
+    void publish(Args... args) const {
+        for(auto pos = calls.size(); pos; --pos) {
+            calls[pos - 1u](args...);
+        }
+    }
+
+    /**
+     * @brief Collects return values from the listeners.
+     *
+     * The collector must expose a call operator with the following properties:
+     *
+     * * The return type is either `void` or such that it's convertible to
+     *   `bool`. In the second case, a true value will stop the iteration.
+     * * The list of parameters is empty if `Ret` is `void`, otherwise it
+     *   contains a single element such that `Ret` is convertible to it.
+     *
+     * @tparam Func Type of collector to use, if any.
+     * @param func A valid function object.
+     * @param args Arguments to use to invoke listeners.
+     */
+    template<typename Func>
+    void collect(Func func, Args... args) const {
+        for(auto pos = calls.size(); pos; --pos) {
+            if constexpr(stl::is_void_v<Ret> || !stl::is_invocable_v<Func, Ret>) {
+                calls[pos - 1u](args...);
+
+                if constexpr(stl::is_invocable_r_v<bool, Func>) {
+                    if(func()) {
+                        break;
+                    }
+                } else {
+                    func();
+                }
+            } else if constexpr(stl::is_invocable_r_v<bool, Func, Ret>) {
+                if(func(calls[pos - 1u](args...))) {
+                    break;
+                }
+            } else {
+                func(calls[pos - 1u](args...));
+            }
+        }
+    }
+
+private:
+    container_type calls;
+};
+
+/**
+ * @brief Connection class.
+ *
+ * Opaque object the aim of which is to allow users to release an already
+ * established connection without having to keep a reference to the signal or
+ * the sink that generated it.
+ */
+class connection {
+    template<typename>
+    friend class sink;
+
+    connection(delegate<void(void *)> fn, void *ref)
+        : disconnect{fn}, signal{ref} {}
+
+public:
+    /*! @brief Default constructor. */
+    connection()
+        : signal{} {}
+
+    /**
+     * @brief Checks whether a connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return static_cast<bool>(disconnect);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        if(disconnect) {
+            disconnect(signal);
+            disconnect.reset();
+        }
+    }
+
+private:
+    delegate<void(void *)> disconnect;
+    void *signal;
+};
+
+/**
+ * @brief Scoped connection class.
+ *
+ * Opaque object the aim of which is to allow users to release an already
+ * established connection without having to keep a reference to the signal or
+ * the sink that generated it.<br/>
+ * A scoped connection automatically breaks the link between the two objects
+ * when it goes out of scope.
+ */
+struct scoped_connection {
+    /*! @brief Default constructor. */
+    scoped_connection() = default;
+
+    /**
+     * @brief Constructs a scoped connection from a basic connection.
+     * @param other A valid connection object.
+     */
+    scoped_connection(const connection &other)
+        : conn{other} {}
+
+    /*! @brief Default copy constructor, deleted on purpose. */
+    scoped_connection(const scoped_connection &) = delete;
+
+    /**
+     * @brief Move constructor.
+     * @param other The scoped connection to move from.
+     */
+    scoped_connection(scoped_connection &&other) noexcept
+        : conn{stl::exchange(other.conn, {})} {}
+
+    /*! @brief Automatically breaks the link on destruction. */
+    ~scoped_connection() {
+        conn.release();
+    }
+
+    /**
+     * @brief Default copy assignment operator, deleted on purpose.
+     * @return This scoped connection.
+     */
+    scoped_connection &operator=(const scoped_connection &) = delete;
+
+    /**
+     * @brief Move assignment operator.
+     * @param other The scoped connection to move from.
+     * @return This scoped connection.
+     */
+    scoped_connection &operator=(scoped_connection &&other) noexcept {
+        conn = stl::exchange(other.conn, {});
+        return *this;
+    }
+
+    /**
+     * @brief Acquires a connection.
+     * @param other The connection object to acquire.
+     * @return This scoped connection.
+     */
+    scoped_connection &operator=(connection other) {
+        conn = other;
+        return *this;
+    }
+
+    /**
+     * @brief Checks whether a scoped connection is properly initialized.
+     * @return True if the connection is properly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return static_cast<bool>(conn);
+    }
+
+    /*! @brief Breaks the connection. */
+    void release() {
+        conn.release();
+    }
+
+private:
+    connection conn;
+};
+
+/**
+ * @brief Sink class.
+ *
+ * A sink is used to connect listeners to signals and to disconnect them.<br/>
+ * The function type for a listener is the one of the signal to which it
+ * belongs.
+ *
+ * The clear separation between a signal and a sink permits to store the former
+ * as private data member without exposing the publish functionality to the
+ * users of the class.
+ *
+ * @warning
+ * Lifetime of a sink must not overcome that of the signal to which it refers.
+ * In any other case, attempting to use a sink results in undefined behavior.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Ret, typename... Args, typename Allocator>
+class sink<sigh<Ret(Args...), Allocator>> {
+    using signal_type = sigh<Ret(Args...), Allocator>;
+    using delegate_type = signal_type::delegate_type;
+    using difference_type = signal_type::container_type::difference_type;
+
+    template<auto Candidate, typename Type>
+    static void release(Type value_or_instance, void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>(value_or_instance);
+    }
+
+    template<auto Candidate>
+    static void release(void *signal) {
+        sink{*static_cast<signal_type *>(signal)}.disconnect<Candidate>();
+    }
+
+    template<typename Func>
+    void disconnect_if(Func callback) {
+        auto &ref = signal_or_assert();
+
+        for(auto pos = ref.calls.size(); pos; --pos) {
+            if(auto &elem = ref.calls[pos - 1u]; callback(elem)) {
+                elem = stl::move(ref.calls.back());
+                ref.calls.pop_back();
+            }
+        }
+    }
+
+    [[nodiscard]] auto &signal_or_assert() const noexcept {
+        ENTT_ASSERT(signal != nullptr, "Invalid pointer to signal");
+        return *signal;
+    }
+
+public:
+    /*! @brief Constructs an invalid sink. */
+    sink() noexcept
+        : signal{} {}
+
+    /**
+     * @brief Constructs a sink that is allowed to modify a given signal.
+     * @param ref A valid reference to a signal object.
+     */
+    sink(sigh<Ret(Args...), Allocator> &ref) noexcept
+        : signal{&ref} {}
+
+    /**
+     * @brief Returns false if at least a listener is connected to the sink.
+     * @return True if the sink has no listeners connected, false otherwise.
+     */
+    [[nodiscard]] bool empty() const noexcept {
+        return signal_or_assert().calls.empty();
+    }
+
+    /**
+     * @brief Connects a free function or an unbound member to a signal.
+     * @tparam Candidate Function or member to connect to the signal.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate>
+    connection connect() {
+        disconnect<Candidate>();
+
+        delegate_type call{};
+        call.template connect<Candidate>();
+        signal_or_assert().calls.push_back(stl::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate>>();
+        return {conn, signal};
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * signal.
+     *
+     * The signal isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the signal.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the signal itself.
+     *
+     * @tparam Candidate Function or member to connect to the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid reference that fits the purpose.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate, typename Type>
+    connection connect(Type &value_or_instance) {
+        disconnect<Candidate>(value_or_instance);
+
+        delegate_type call{};
+        call.template connect<Candidate>(value_or_instance);
+        signal_or_assert().calls.push_back(stl::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate, Type &>>(value_or_instance);
+        return {conn, signal};
+    }
+
+    /**
+     * @brief Connects a free function with payload or a bound member to a
+     * signal.
+     *
+     * @sa connect(Type &)
+     *
+     * @tparam Candidate Function or member to connect to the signal.
+     * @tparam Type Type of class or type of payload.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     * @return A properly initialized connection object.
+     */
+    template<auto Candidate, typename Type>
+    connection connect(Type *value_or_instance) {
+        disconnect<Candidate>(value_or_instance);
+
+        delegate_type call{};
+        call.template connect<Candidate>(value_or_instance);
+        signal_or_assert().calls.push_back(stl::move(call));
+
+        delegate<void(void *)> conn{};
+        conn.template connect<&release<Candidate, Type *>>(value_or_instance);
+        return {conn, signal};
+    }
+
+    /**
+     * @brief Disconnects a free function or an unbound member from a signal.
+     * @tparam Candidate Function or member to disconnect from the signal.
+     */
+    template<auto Candidate>
+    void disconnect() {
+        delegate_type call{};
+        call.template connect<Candidate>();
+        disconnect_if([&call](const auto &elem) { return elem == call; });
+    }
+
+    /**
+     * @brief Disconnects a free function with payload or a bound member from a
+     * signal.
+     *
+     * The signal isn't responsible for the connected object or the payload.
+     * Users must always guarantee that the lifetime of the instance overcomes
+     * the one of the signal.<br/>
+     * When used to connect a free function with payload, its signature must be
+     * such that the instance is the first argument before the ones used to
+     * define the signal itself.
+     *
+     * @tparam Candidate Function or member to disconnect from the signal.
+     * @tparam Type Type of class or type of payload, if any.
+     * @param value_or_instance A valid reference that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void disconnect(Type &value_or_instance) {
+        delegate_type call{};
+        call.template connect<Candidate>(value_or_instance);
+        disconnect_if([&call](const auto &elem) { return elem == call; });
+    }
+
+    /**
+     * @brief Disconnects a free function with payload or a bound member from a
+     * signal.
+     *
+     * @sa disconnect(Type &)
+     *
+     * @tparam Candidate Function or member to disconnect from the signal.
+     * @tparam Type Type of class or type of payload, if any.
+     * @param value_or_instance A valid pointer that fits the purpose.
+     */
+    template<auto Candidate, typename Type>
+    void disconnect(Type *value_or_instance) {
+        delegate_type call{};
+        call.template connect<Candidate>(value_or_instance);
+        disconnect_if([&call](const auto &elem) { return elem == call; });
+    }
+
+    /**
+     * @brief Disconnects free functions with payload or bound members from a
+     * signal.
+     * @param value_or_instance A valid object that fits the purpose.
+     */
+    void disconnect(const void *value_or_instance) {
+        ENTT_ASSERT(value_or_instance != nullptr, "Invalid value or instance");
+        disconnect_if([value_or_instance](const auto &elem) { return elem.data() == value_or_instance; });
+    }
+
+    /*! @brief Disconnects all the listeners from a signal. */
+    void disconnect() {
+        signal_or_assert().calls.clear();
+    }
+
+    /**
+     * @brief Returns true if a sink is correctly initialized, false otherwise.
+     * @return True if a sink is correctly initialized, false otherwise.
+     */
+    [[nodiscard]] explicit operator bool() const noexcept {
+        return signal != nullptr;
+    }
+
+private:
+    signal_type *signal;
+};
+
+/**
+ * @brief Deduction guide.
+ *
+ * It allows to deduce the signal handler type of a sink directly from the
+ * signal it refers to.
+ *
+ * @tparam Ret Return type of a function type.
+ * @tparam Args Types of arguments of a function type.
+ * @tparam Allocator Type of allocator used to manage memory and elements.
+ */
+template<typename Ret, typename... Args, typename Allocator>
+sink(sigh<Ret(Args...), Allocator> &) -> sink<sigh<Ret(Args...), Allocator>>;
+
+} // namespace entt
+
+#endif

include/entt/stl/algorithm.hpp

@@ -0,0 +1,22 @@
+#ifndef ENTT_STL_ALGORITHM_HPP
+#define ENTT_STL_ALGORITHM_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/algorithm.hpp>)
+#    include <entt/ext/stl/algorithm.hpp>
+#else
+#    include <algorithm>
+
+namespace entt::stl {
+
+using std::all_of;
+using std::any_of;
+using std::find_if;
+using std::none_of;
+using std::sort;
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#endif

include/entt/stl/array.hpp

@@ -0,0 +1,19 @@
+#ifndef ENTT_STL_ARRAY_HPP
+#define ENTT_STL_ARRAY_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/array.hpp>)
+#    include <entt/ext/stl/array.hpp>
+#else
+#    include <array>
+
+namespace entt::stl {
+
+using std::array;
+using std::get;
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#endif

include/entt/stl/atomic.hpp

@@ -0,0 +1,18 @@
+#ifndef ENTT_STL_ATOMIC_HPP
+#define ENTT_STL_ATOMIC_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/atomic.hpp>)
+#    include <entt/ext/stl/atomic.hpp>
+#else
+#    include <atomic>
+
+namespace entt::stl {
+
+using std::atomic;
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#endif

include/entt/stl/bit.hpp

@@ -0,0 +1,20 @@
+#ifndef ENTT_STL_BIT_HPP
+#define ENTT_STL_BIT_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/bit.hpp>)
+#    include <entt/ext/stl/bit.hpp>
+#else
+#    include <bit>
+
+namespace entt::stl {
+
+using std::bit_ceil;
+using std::has_single_bit;
+using std::popcount;
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#endif

include/entt/stl/cmath.hpp

@@ -0,0 +1,18 @@
+#ifndef ENTT_STL_CMATH_HPP
+#define ENTT_STL_CMATH_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/cmath.hpp>)
+#    include <entt/ext/stl/cmath.hpp>
+#else
+#    include <cmath>
+
+namespace entt::stl {
+
+using std::ceil;
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#endif

include/entt/stl/concepts.hpp

@@ -0,0 +1,24 @@
+#ifndef ENTT_STL_CONCEPTS_HPP
+#define ENTT_STL_CONCEPTS_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/concepts.hpp>)
+#    include <entt/ext/stl/concepts.hpp>
+#else
+#    include <concepts>
+
+namespace entt::stl {
+
+using std::constructible_from;
+using std::default_initializable;
+using std::derived_from;
+using std::integral;
+using std::invocable;
+using std::same_as;
+using std::unsigned_integral;
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#endif

include/entt/stl/cstddef.hpp

@@ -0,0 +1,21 @@
+#ifndef ENTT_STL_CSTDDEF_HPP
+#define ENTT_STL_CSTDDEF_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/cstddef.hpp>)
+#    include <entt/ext/stl/cstddef.hpp>
+#else
+#    include <cstddef>
+
+namespace entt::stl {
+
+using std::byte;
+using std::nullptr_t;
+using std::ptrdiff_t;
+using std::size_t;
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#endif

include/entt/stl/cstdint.hpp

@@ -0,0 +1,21 @@
+#ifndef ENTT_STL_CSTDINT_HPP
+#define ENTT_STL_CSTDINT_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/cstdint.hpp>)
+#    include <entt/ext/stl/cstdint.hpp>
+#else
+#    include <cstdint>
+
+namespace entt::stl {
+
+using std::uint16_t;
+using std::uint32_t;
+using std::uint64_t;
+using std::uint8_t;
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#endif

include/entt/stl/functional.hpp

@@ -0,0 +1,55 @@
+#ifndef ENTT_STL_FUNCTIONAL_HPP
+#define ENTT_STL_FUNCTIONAL_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/functional.hpp>)
+#    include <entt/ext/stl/functional.hpp>
+#else
+#    include <functional>
+#    include <version>
+#    include "../config/config.h"
+
+namespace entt::stl {
+
+using std::equal_to;
+using std::function;
+using std::hash;
+using std::invoke;
+using std::less;
+
+} // namespace entt::stl
+
+#    ifndef ENTT_FORCE_STL
+#        if defined(__cpp_lib_ranges)
+#            define ENTT_HAS_IDENTITY
+namespace entt::stl {
+
+using std::identity;
+
+} // namespace entt::stl
+#        endif
+#    endif
+
+#    ifndef ENTT_HAS_IDENTITY
+#        include <utility>
+
+namespace entt::stl {
+
+struct identity {
+    using is_transparent = void;
+
+    template<typename Type>
+    [[nodiscard]] constexpr Type &&operator()(Type &&value) const noexcept {
+        return std::forward<Type>(value);
+    }
+};
+
+} // namespace entt::stl
+#    endif
+
+#endif
+/*! @endcond */
+
+#undef ENTT_HAS_IDENTITY
+
+#endif

include/entt/stl/ios.hpp

@@ -0,0 +1,18 @@
+#ifndef ENTT_STL_IOS_HPP
+#define ENTT_STL_IOS_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/ios.hpp>)
+#    include <entt/ext/stl/ios.hpp>
+#else
+#    include <ios>
+
+namespace entt::stl {
+
+using std::boolalpha;
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#endif

include/entt/stl/iterator.hpp

@@ -0,0 +1,110 @@
+#ifndef ENTT_STL_ITERATOR_HPP
+#define ENTT_STL_ITERATOR_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/iterator.hpp>)
+#    include <entt/ext/stl/iterator.hpp>
+#else
+#    include <iterator>
+#    include <version>
+#    include "../config/config.h"
+
+namespace entt::stl {
+
+using std::advance;
+using std::bidirectional_iterator_tag;
+using std::distance;
+using std::forward_iterator_tag;
+using std::input_iterator_tag;
+using std::iterator_traits;
+using std::make_reverse_iterator;
+using std::random_access_iterator_tag;
+using std::reverse_iterator;
+
+#    ifndef ENTT_FORCE_STL
+#        if defined(__cpp_lib_ranges)
+#            define ENTT_HAS_ITERATOR_CONCEPTS
+
+using std::bidirectional_iterator;
+using std::forward_iterator;
+using std::input_iterator;
+using std::input_or_output_iterator;
+using std::output_iterator;
+using std::random_access_iterator;
+using std::sentinel_for;
+
+#        endif
+#    endif
+
+#    ifndef ENTT_HAS_ITERATOR_CONCEPTS
+#        include <concepts>
+#        include <utility>
+
+namespace internal {
+
+template<typename It>
+concept has_iterator_category = requires {
+    typename std::iterator_traits<It>::iterator_category;
+};
+
+template<typename It>
+concept has_iterator_concept = has_iterator_category<It> && requires {
+    typename It::iterator_concept;
+};
+
+template<has_iterator_category It>
+struct iterator_tag {
+    using type = typename std::iterator_traits<It>::iterator_category;
+};
+
+template<has_iterator_concept It>
+struct iterator_tag<It> {
+    using type = typename It::iterator_concept;
+};
+
+template<typename It, typename Tag>
+concept has_iterator_tag = std::derived_from<typename iterator_tag<It>::type, Tag>;
+
+} // namespace internal
+
+// Bare minimum definitions to support broken platforms like PS4.
+// EnTT does not provide full featured definitions for iterator concepts.
+
+template<typename It>
+concept input_or_output_iterator = requires(It it) {
+    *it;
+    { ++it } -> std::same_as<It &>;
+    it++;
+};
+
+template<typename It>
+concept input_iterator = input_or_output_iterator<It> && internal::has_iterator_tag<It, std::input_iterator_tag>;
+
+template<typename It, typename Type>
+concept output_iterator = input_or_output_iterator<It> && requires(It it, Type &&value) {
+    *it++ = std::forward<Type>(value);
+};
+
+template<typename It>
+concept forward_iterator = input_iterator<It> && internal::has_iterator_tag<It, std::forward_iterator_tag>;
+
+template<typename It>
+concept bidirectional_iterator = forward_iterator<It> && internal::has_iterator_tag<It, std::bidirectional_iterator_tag>;
+
+template<typename It>
+concept random_access_iterator = bidirectional_iterator<It> && internal::has_iterator_tag<It, std::random_access_iterator_tag>;
+
+template<class Sentinel, typename It>
+concept sentinel_for = input_or_output_iterator<It> && requires(Sentinel sentinel, It it) {
+    { it == sentinel } -> std::same_as<bool>;
+};
+
+#    endif
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#undef ENTT_HAS_ITERATOR_CONCEPTS
+
+#endif

include/entt/stl/limits.hpp

@@ -0,0 +1,18 @@
+#ifndef ENTT_STL_LIMITS_HPP
+#define ENTT_STL_LIMITS_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/limits.hpp>)
+#    include <entt/ext/stl/limits.hpp>
+#else
+#    include <limits>
+
+namespace entt::stl {
+
+using std::numeric_limits;
+
+} // namespace entt::stl
+#endif
+/*! @endcond */
+
+#endif

include/entt/stl/memory.hpp

@@ -0,0 +1,74 @@
+#ifndef ENTT_STL_MEMORY_HPP
+#define ENTT_STL_MEMORY_HPP
+
+/*! @cond ENTT_INTERNAL */
+#if __has_include(<entt/ext/stl/memory.hpp>)
+#    include <entt/ext/stl/memory.hpp>
+#else
+#    include <memory>
+#    include <version>
+#    include "../config/config.h"
+
+namespace entt::stl {
+
+using std::addressof;
+using std::allocate_shared;
+using std::allocator;
+using std::allocator_arg;
+using std::allocator_arg_t;
+using std::allocator_traits;
+using std::default_delete;
+using std::destroy;
+using std::enable_shared_from_this;
+using std::make_shared;
+using std::make_unique;
+using std::pointer_traits;
+using std::shared_ptr;
+using std::static_pointer_cast;
+using std::uninitialized_fill;
+using std::unique_ptr;
+using std::uses_allocator_v;
+
+} // namespace entt::stl
+
+#    ifndef ENTT_FORCE_STL
+#        if defined(__cpp_lib_to_address)
+#            define ENTT_HAS_TO_ADDRESS
+
+namespace entt::stl {
+
+using std::to_address;
+
+} // namespace entt::stl
+
+#        endif
+#    endif
+
+#    ifndef ENTT_HAS_TO_ADDRESS
+#        include <type_traits>
+
+namespace entt::stl {
+
+template<typename Type>
+constexpr Type *to_address(Type *ptr) noexcept {
+    static_assert(!std::is_function_v<Type>, "Invalid type");
+    return ptr;
+}
+
+template<typename Type>
+constexpr auto to_address(const Type &ptr) noexcept {
+    if constexpr(requires { std::pointer_traits<Type>::to_address(ptr); }) {
+        return std::pointer_traits<Type>::to_address(ptr);
+    } else {
+        return to_address(ptr.operator->());
+    }
+}
+
+} // namespace entt::stl
+#    endif
+#endif
+/*! @endcond */
+
+#undef ENTT_HAS_TO_ADDRESS
+
+#endif