ofSingleton: tracking upstream (more symmetry/readability) (#7669)

#changelog #core

Author: artificiel

libs/openFrameworks/utils/ofRandomEngine.h

@@ -4,7 +4,7 @@
 #include <random>
 #include <glm/glm.hpp>
 
-#include "ofSingleton.hpp"
+#include "ofSingleton.h"
 #include "ofMath.h"
 
 namespace of::random

libs/openFrameworks/utils/ofSingleton.h

@@ -0,0 +1,68 @@
+#ifndef OF_SINGLETON_H_
+#define OF_SINGLETON_H_
+
+// atomic C++17 DCLP CRTP singleton adapted by burton@artificiel.org from
+// https://github.com/jimmy-park/singleton/blob/main/include/singleton_dclp.hpp (as of df5e4a2)
+
+#include <atomic>
+#include <cassert>
+#include <shared_mutex>
+#include <utility>
+
+namespace of::utils {
+
+template <typename Derived>
+class Singleton {
+public:
+	template <typename... Args>
+	static void construct(Args &&... args) {
+		struct Dummy : public Derived {
+			using Derived::Derived;
+			void prohibit_construct_from_derived() const override { }
+		};
+		
+		using Instance = Dummy;
+		if (!instance_.load(std::memory_order_acquire)) {
+			std::lock_guard lock { mutex_ };
+			if (!instance_.load(std::memory_order_relaxed)) {
+				instance_.store(new Instance { std::forward<Args>(args)... }, std::memory_order_release);
+			}
+		}
+	}
+
+	static void destruct() {
+		if (instance_.load(std::memory_order_acquire)) {
+			std::lock_guard lock { mutex_ };
+			if (auto * the_instance = instance_.load(std::memory_order_relaxed); the_instance) {
+				delete the_instance;
+				instance_.store(nullptr, std::memory_order_release);
+			}
+		}
+	}
+
+	static Derived * instance() {
+		auto * the_instance = instance_.load(std::memory_order_acquire);
+		if (!the_instance) {
+			std::shared_lock lock { mutex_ };
+			the_instance = instance_.load(std::memory_order_relaxed);
+			assert(the_instance);
+		}
+		return the_instance;
+	}
+
+protected:
+	Singleton() = default;
+	Singleton(const Singleton &) = delete;
+	Singleton(Singleton &&) noexcept = delete;
+	Singleton & operator=(const Singleton &) = delete;
+	Singleton & operator=(Singleton &&) noexcept = delete;
+	virtual ~Singleton() = default;
+
+private:
+	virtual void prohibit_construct_from_derived() const = 0;
+	inline static std::atomic<Derived *> instance_ { nullptr };
+	inline static std::shared_mutex mutex_;
+};
+
+} // end namespace of::utils
+#endif // OF_SINGLETON_H_