TMap as UPROPERTY() seems to be fatally bugged in GarbageCollector.cpp

I posted this problem on a fatal crash that I’d get every time I tried running a packaged game. I finally discovered the reason why I kept getting this stack:
[FONT=Helvetica Neue]GarbageCollection.cpp [Line: 1894] Unknown token type (8) when trying to add ARO token.

Type 8 of EGCReferenceType i[FONT=Helvetica Neue]s GCRT_AddTMapReferencedObjects
This made me realize that I recently added UPROPERTY to one of my TMap variables.

The one in question that caused the issue was:
TMap<UWidget*, UBaseNavigableMenuStructure*> NavigationMap;

UBaseNavigableMenuStructure has parent UObject.

I wanted to post here for double coverage as answerhub is a bit of a black hole. This seems like a legit bug. In fact… it was too legit to quit, until I removed UPROPERTY().

To my latest knowledge TMap isn’t supported as a UPROPERTY. To prevent TMap entries from being garbage collected, I believe an often used work around is to have a UPROPERTY TArray that stores a copy of the TMap entries.

4.8 announced initial support for TMaps as UPROPERTY:

Oh, thats a great idea! I hope it gets streamlined over time. :slight_smile:

Replying to this thread for visibility, as well as the AnswerHub link above:

These two commits should help TMap property support in the GC and reference chain:

Both of these fixes should be in 4.9 if you don’t want to integrate the source changes yourself.


Thanks Steve!

Cheers Steve. I have a quick question relating to this.

Am I right in assuming that TMap now supports UObject referencing for UObject* keys as well as UObject* values?
If so, what are the ramifications (if any) of using a UObject*-keyed map as a UPROPERTY, given that the keys would presumably be set to null if the pointed-to object was garbage collected? In my experience map keys need to be unique and immutable. How is this dealt with?

I wondered the same thing pre-4.8 about a TMap< TWeakObjectPtr< UObject >, FSomeType >, which I had come across at places in the engine code.

Oh my god, I’ve been banging my head for days. I also use TMap in my UI navigation and my widgets is being garbage collected and crashes the game. Until now apparently is still a problem.

You shouldn’t have to, but couldn’t you add these widgets to the root? And then you remove them “manually” when you remove entries from the TMap.