Add referenced object for map of UStructs

Development Programming & Scripting C++
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1

Hi,

I understood a few things about Unreal garbage collector but here I’m stuck.
I have a UObject derived class holding a TMap to UStruct by value:

USTRUCT()
struct FSomeStruct
{
	GENERATED_USTRUCT_BODY()
  float value1;
  float value2;
}

UCLASS()
class USomeCollectionManager : public UObject
{
	GENERATED_UCLASS_BODY()
private:
	TMap<SomeKey, FSomeStruct> Collection;
}

The “Collection” member is regularly wiped out by the GC (which I can verify by explicitly calling it using “obj gc”).

The issue is that:

  • I cannot make the map a UPROPERTY since they do not handle maps
  • I cannot use AddReferencedObjects since neither the map nor the FSomeStruct is a UObject

What would be the best thing to do here, in order to force the map to be referenced by its owning class and prevent it to be wiped out?

2

Indeed I just figured that out, but somehow deleted the question instead of replying it…Thanks!

3

Hi,

There is no problem with the USomeCollectionManager as written. ‘Collection’ does not need to be a property. It sounds like the GC thinks your USomeCollectionManager object is not being referenced, and so it’s being collected, and so the Collection member is being destroyed.

You need to root a pointer to USomeCollectionManager somehow, so that’s known to the GC, probably by having:

UPROPERTY()
USomeCollectionManager* Manager;

… in your module class, or by calling

Manager->AddToRoot();

… just after construction (and ->RemoveFromRoot() if you want it to be GC’d again).

Steve