TSubclassOf sometimes fails to compile for forward declared classes.

Development Programming & Scripting C++
1

I’ve seen code written by Epic themselves like this:



UPROPERTY(Stuff goes here)
TSubclassOf<class ASomeActorClass> SomeClass;

And sometimes it works for me too.

Other times I get errors like this:



2>F:\Code\UnrealEngine\Engine\Source\Runtime\CoreUObject\Public\UObject\Class.h(2829): error C2027: use of undefined type 'AWR2InventoryPhysical'
2>  F:\Code\UnrealEngineProj\Wraith\Source\Wraith\Public\Gameplay2\Characters\Inventory\WR2PickupInventory.h(6): note: see declaration of 'AWR2InventoryPhysical'
2>  F:\Code\UnrealEngine\Engine\Source\Runtime\CoreUObject\Public\UObject\Class.h(2828): note: while compiling class template member function 'UClass *TSubclassOf<AWR2InventoryPhysical>::operator *(void) const'
2>  F:\Code\UnrealEngine\Engine\Source\Runtime\CoreUObject\Public\UObject\ObjectBase.h(1740): note: see reference to function template instantiation 'UClass *TSubclassOf<AWR2InventoryPhysical>::operator *(void) const' being compiled
2>  F:\Code\UnrealEngineProj\Wraith\Source\Wraith\Public\Gameplay2\Characters\Inventory\WR2PickupInventory.h(18): note: see reference to class template instantiation 'TSubclassOf<AWR2InventoryPhysical>' being compiled
2>F:\Code\UnrealEngine\Engine\Source\Runtime\CoreUObject\Public\UObject\Class.h(2829): error C3861: 'StaticClass': identifier not found

Doesn’t matter if I forward declare it at the top of the header file or inline.

2

The forward declaration is fine. You’re getting an error because somewhere in your code you’re trying to get the UClass pointer (operator*) and that requires the full definition for the type. You need to find the line causing that and ensure you’ve #included the type’s header before that point.

If it’d caused by a line in the same header then you’ll need to move that line to a cpp file if you want to stick with a forward declaration.

3 Likes
3

I noticed in one case it was a UProperty in a struct that was causing the compiler error.

Another time it was a Key in a TMap.