Unreal-style Singletons with Subsystems

The Singleton design pattern has a lot of baggage, but its utility can’t be ignored. Luckily Unreal provides a way to get the benefits of a Singleton, with less of the drawbacks.

The Bad Way: C++ Static Singleton

class UMySingleton : public UObject
{
public:
	static UMySingleton* GetInstance() { return Instance; }

private:
	static UMySingleton* Instance;
};

Benefits and drawbacks of C++ static class singletons:

• Interface programmers are used to
• Interacts badly with the editor: Without work, instances are preserved between running game through the editor multiple times.
• Interacts badly class Class Default Objects: Without work, instances are created when CDOs are created.
• Lifetime unclear: Requires careful programming and clear intentions to manage the lifetime of singletons.

Unreal Subsystems

Unreal has something it calls Subsystems, that can be used to create globally-accessible modules that have explicitly-defined lifecycles.

Your subsystem’s lifetime will match the lifetime of its parent. There are 5 different parent classes to choose from (see below). More info on their lifecycle can be found in the documentation.

Subsystem	Parent Class	Lifetime
Engine	UEngineSubsystem	Both in editor and in-game, I think.
Editor	UEditorSubsystem	When the Editor starts.
GameInstance	UGameInstanceSubsystem	As soon as your game starts, stays alive until the game is closed.
LocalPlayer	ULocalPlayerSubsystem	Matches the lifetime of its parent `ULocalPlayer, can move between levels.”
World	UWorldSubsystem	Matches its parent UWorld, is effectively per-level.

Benefits over vanilla C++ singletons:

• Lifetime is automatically managed: Subclassing the correct Subsystem ensures that the instance will be created and destroyed for you.
• Desired lifetime is made explicit: It is clear that a Subsystem that inherits from `UWorldSubsystem` will only exist as long as a World.
• Cleaner Blueprint access.
• Accessible in Python scripts.
• Requires some understanding of the lifecycles of a few Unreal classes.

Accessing Subsystems from C++

UGameInstance* GameInstance = ...;
UMyGameSubsystem* MySubsystem = GameInstance->GetSubsystem<UMyGameSubsystem>();

ULocalPlayer* MyLocalPlayer = ...;
UMyPlayerSubsystem* MySubsystem = LocalPlayer->GetSubsystem<UMyPlayerSubsystem>();

Example Usage

Imagine that we want to save player telemetry as they progress through the game, from the main menu to in-game. We could create a subclass of UGameInstanceSubsystem called UTelemetrySubsystem. Our telemetry class would be instantiated as soon as the game starts, and All logic related to telemetry would be stored within that subsystem.

Subsystem Base Class

UCLASS(Abstract)
class ENGINE_API USubsystem : public UObject
{
	GENERATED_BODY()

public:
	USubsystem();

	/** Override to control if the Subsystem should be created at all.
	 * For example you could only have your system created on servers.
	 * It's important to note that if using this is becomes very important to null check whenever getting the Subsystem.
	 *
	 * Note: This function is called on the CDO prior to instances being created!
	 */
	virtual bool ShouldCreateSubsystem(UObject* Outer) const { return true; }

	/** Implement this for initialization of instances of the system */
	virtual void Initialize(FSubsystemCollectionBase& Collection) {}

	/** Implement this for deinitialization of instances of the system */
	virtual void Deinitialize() {}

private:
	friend class FSubsystemCollectionBase;
	FSubsystemCollectionBase* InternalOwningSubsystem;

};