SingletonHolder.h

// Copyright(c) 2019 - 2020, #Momo
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#pragma once

#include "CreationPolicy.h"
#include "LifetimePolicy.h"
#include "Utilities/Threading/ThreadingModel.h"
#include "Core/Macro/Macro.h"

namespace MxEngine
{
    template<
        typename T,
        template<typename> class CreationPolicy = CreateWithNew,
        template<typename> class LifetimePolicy = DefaultLifetime,
        template<typename> class ThreadingModel = SingleThreaded
    >
    class SingletonHolder
    {
        using InstanceType = typename ThreadingModel<T>::VolatileType;

        inline static InstanceType* instance = nullptr;
        inline static bool destroyed = false;

        static inline void DestroySingleton()
        {
            MX_ASSERT(instance != nullptr);
            CreationPolicy<T>::Destroy(instance);
            instance = nullptr;
            destroyed = true;
        }
    public:
        SingletonHolder() = delete;
        SingletonHolder(const SingletonHolder&) = delete;
        SingletonHolder& operator=(const SingletonHolder&) = delete;

        static inline T& Instance()
        {
            if (instance == nullptr)
            {
                typename ThreadingModel<T>::Lock guard;
                guard;
                if (destroyed)
                {
                    LifetimePolicy<T>::OnDeadReference();
                    destroyed = false;
                }
                instance = CreationPolicy<T>::Create();
                LifetimePolicy<T>::ScheduleDestruction(instance, &DestroySingleton);
            }
            return *instance;
        }
    };
}