build: add EnTT library

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