C++-style Singletons in Unreal Engine

Coming from other C++-based game dev, you might be used to having singletons that are globally-accessible. With Unreal it requires a bit more work.

How to set up a vanilla C++ singleton

Don't.

Seriously in Unreal you will cause more problems than you solve.

Easy to leave instances hanging around between using Run Game in the Editor
Easy to create instances from the Class Default Object being loaded, or viewing a Blueprint subclass in-editor
Nightmare for automated testing.

No but seriously, how do I do it?

Have you considered some of the many alternatives?

Add your code to some of the easily-accessible classes like AGameState, APlayerState, and use UGameplayStatics helper functions to get access to them.

You could use subsystems, a more Unreal but decoupled way of making something globally-accessible.

What about making a plugin of your code, and accessing the code through that?

Hey kid, over here….

OK so you really want to know? Here's the gross hacky way to do it.

UBUIMyEvilSingleton* UBUIMyEvilSingleton::Instance = nullptr;

UBUIMyEvilSingleton::UBUIMyEvilSingleton(const FObjectInitializer& ObjectInitializer)
	: Super(ObjectInitializer)
{
	// Don't set up Singleton instance in CDO constructor
	// or other Unreal-style constructors
	if (!HasAnyFlags(RF_ClassDefaultObject
		| RF_NeedLoad
		| RF_NeedPostLoad
		| RF_NeedPostLoadSubobjects))
	{
		ensureMsgf(pInstance == nullptr, TEXT("Uh-oh two singletons!"));
		pInstance = this;
	}
}

UBUIMyEvilSingleton::BeginDestroy()
{
	if (pSingleton != nullptr)
	{
		if (ensureMsgf(pSingleton == this, TEXT("We have a singleton that isn't this!"))
		{
			pInstance = nullptr;
		}
	}
}

🌈C++