build: add EnTT library

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