Garbage collection (GC) tracks
UObject(s) and its sub-classes, which include AActor and UActorComponent. When creating a new
UObject, UE4 will automatically add them to its internal objects list, so even with improper use, it's not easy to have memory leaks, but it is easy to cause crashes.
UObjects should never be created with
new, but only with the default creation methods (
Objects are primarily kept alive in 3 ways:
By having a strong reference (UPROPERTY) to them (from objects that are also referenced)
UObject::AddReferencedObjects(from objects that are also referenced)
By adding them to the root set with
When objects do not fulfill any of the above conditions, on the next GC cycle they will be marked as unreachable and garbage collected (destroyed).
Passing an object as an
Outer to another object does not automatically mean the other object will be kept alive, the same goes for default subobjects.
To force the destruction of objects that are still reachable, you can call
MarkAsGarbage on them, and it will force their destruction on the next GC cycle (you generally want to avoid doing this, as that is what garbage collection is for, and some classes, like
UActorComponent do not directly support it).
The destruction of an object doesn't necessarily all happen in the same frame, when garbage collection starts on it, it will first call
BeginDestroy (do not call this yourself), then, when ready
The GC runs in the game thread so you can trust that it won't clean up an object within the lifetime of a function.
While the most common way of keeping objects alive is through a
UPROPERTY, actors and components work differently:
Levels reference their actors and actors reference their components. Both work by overriding the
UObject::AddReferencedObjects implementation and collecting what they do not want to be garbage collected. This means that even if there are no strong references to level actors and components, they won't be garbage collected until manually destroyed, or their level is unloaded.
The garbage collector will automatically clear the following references to garbage collected objects1:
Raw pointers declared with
UPROPERTY- will be set to nullptr
Raw pointers in
UObjectcompatible containers declared with
TMap) - the affected elements will be set as nullptr but not removed
Whenever code references an
AActor or a
UActorComponent, it has to deal with the possibility that
AActor::Destroy could be called on the actor or
UActorComponent::DestroyComponent could be called on the component. These functions will mark them for pending kill, thus triggering their garbage collection at the first opportunity (note that destroying an actor also destroys all its components). Since the garbage collector automatically nulls out
UPROPERTY pointers when it actually gets to destroy them, null-checking an actor or component pointer is sufficient to know it's safe to use, though you might also want to check
IsPendingKill on them (through
IsValid) to avoid accessing them after they have been marked for destruction (
TWeakObjectPtr already checks for this when retrieving the raw pointer).
bool IsValid(const UObject* Test)
is a global function that automatically checks if an object pointer is non-null and not pending kill.
Should not be used for Garbage Collection checks, as on
UPROPERTY pointers it will always return true, while on raw pointer it will return true or false depending whether the object had already been destroyed, but in the latter case it's also likely to also crash the application as the pointed memory could have been overwritten.
If you write your own non-garbage classes that references garbage collected objects, you may want to sub-class FGCObject.
Unreal Smart Pointer Library
The Unreal Smart Pointer Library (
TWeakPtr) is for code that is not based on the UObject system. It is similar in function to the C++11 standard library smart pointers. Unreal Smart Pointers cannot be used to reference UObjects, because the garbage collector isn't aware of smart pointers2.
Objects - Explanations of the basic gameplay elements, Actors and Objects
Unreal Object Handling - Overview of the features of the UObject system
Unreal Smart Pointer Library - UE4 implementation of smart pointers, including weak pointers and non-nullable shared references