build: add EnTT library

include/entt/locator/locator.hpp

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