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Garbage collection Wiki

Garbage Man / November 15, 2016

Unreal's game-level memory management system uses reflection to implement garbage collection. Working effectively in Unreal requires some understanding of how these two systems interact.

Read this for a great introduction to Unreal's reflection system and using it:

Reflection allows the engine to determine if objects are still referenced by other objects, making garbage collection a viable strategy for managing memory.

One of the most important tasks within a game engine is managing memory. Unreal’s approach to solve this problem is the usage of garbage collection. In this approach, the engine will automatically delete objects when they are no longer needed. An object is no longer needed when it is no longer referenced by any other object. More general information on garbage collection can be found here:

Unreal uses the reflection system to drive garbage collection. Because the engine knows about your objects and properties, it can recognize when an object is no longer needed and automatically delete it.

While this automatic memory management does significantly reduce the mental workload to working in the engine, it is important to understand at a high level how it works, as it can only work well when you follow the “rules.” IMPORTANT: There are always exceptions to the rules, but be sure to know what you are doing if you break the rules!

Every member of a class should be declared as a UPROPERTY If an member is left “naked, ” unreal will not know about it. So, an object you are pointing at could get deleted out from under you! It is safe to leave value types such as an int or a bool “naked” although they could not be saved, replicated, or appear in the editor. Member pointers should only point at UObject or UObject-derived objects The garbage collector is only smart enough to recognize relationships to an object, so the object could get deleted out from under your pointer. Any non-UObject pointer must be pointing to something “global” in the engine, or something within its own UObject The garbage collector could delete the object that owns what you are pointing at. The only container that is safe to have UObject or UObject-derived pointers in is a TArray

NOTE: UObject-derived pointers/objects would be any object, component, or actor.

Remember, the garbage collector relies on reflection data!

Structs are intended to be used as “value” types. They are best used for small bits of data that are to be reused within objects and actors. For example, FVector, FRotator, FQuat. They are not garbage collected, so they must always exist within a UObject.

Source: wiki.unrealengine.com