LazySingleton.h

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// Copyright Epic Games, Inc. All Rights Reserved.
 
#pragma once
 
#include "Misc/AssertionMacros.h"
 
class FLazySingleton
{
protected:
    template<class T> static void Construct(void* Place)    { ::new (Place) T; }
    template<class T> static void Destruct(T* Instance)     { Instance->~T(); }
};
 
template<class T>
class TLazySingleton final : public FLazySingleton
{
public:
    static T& Get()
    {
        return GetLazy(Construct<T>).GetValue();
    }
 
    static void TearDown()
    {
        return GetLazy(nullptr).Reset();
    }
 
    static T* TryGet()
    {
        return GetLazy(Construct<T>).TryGetValue();
    }
 
private:
    static TLazySingleton& GetLazy(void(*Constructor)(void*))
    {
        static TLazySingleton Singleton(Constructor);
        return Singleton;
    }
 
    alignas(T) unsigned char Data[sizeof(T)];
    T* Ptr;
 
    TLazySingleton(void(*Constructor)(void*))
    {
        if (Constructor)
        {
            Constructor(Data);
        }
 
        Ptr = Constructor ? (T*)Data : nullptr;
    }
 
#if (!defined(DISABLE_LAZY_SINGLETON_DESTRUCTION) || !DISABLE_LAZY_SINGLETON_DESTRUCTION)
    ~TLazySingleton()
    {
        Reset();
    }
#endif
 
    T* TryGetValue()
    {
        return Ptr;
    }
 
    T& GetValue()
    {
        check(Ptr);
        return *Ptr;
    }
 
    void Reset()
    {
        if (Ptr)
        {
            Destruct(Ptr);
            Ptr = nullptr;
        }
    }
};