Nested Containers & UBT Bug with Compiler Warnings

General Feedback & Requests
1

I’ve just discovered that it’s not possible to have nested containers, such as 2D TArrays<> for example (weird that I’ve never tried to use them before now).

While I’m okay with this, there is a bug with UBT in regards to them. If you create a 2D TArray of FVectors, and DON’T mark the variable as a UPROPERTY, the code will compile and execute without problems, but the FVectors will all read as zero even if they’re not. For example:



--- .h ---
TArray<TArray<FVector>> GridBuffer;

--- .cpp ---
FVector UMyClass::GetPosAtGrid(const int32 ColumnX, const int32 RowY)
{
	return GridBuffer[ColumnX][RowY];
}

Will compile and return zeroed vectors, any accesses and writes will appear to work fine though. If you mark it as a UPROPERTY, you get the warning in UBT. This is probably a bug and UBT shouldn’t allow this to compile, unless it’s being changed for the future?

2

A TArray of TArrays is perfectly valid C++ and it should work just fine except for as a property. The property system isn’t currently set up to handle nested containers, but that is a problem with the property system and nothing to do with the actual containers themselves.

3

Ahhh okay interesting. Good to know!

4

Have you tried wrapping second level TArray into UStruct? I’d expect that kind of thing to work.

5

Yeah I saw that on the Wiki but it felt a bit ‘hacky’, but I guess in reality it doesn’t make a difference. In the end I just used a 1D array and wrote a function to ‘fake’ it being 2D (pass in two indices, get one item back kind of thing). Probably faster to access this way anyway.

6

Wouldn’t call that hacky. TArray is not an UCLASS/USTRUCT, so when you nest them, property system won’t be able to pickup 2nd level TArray. So, wrapping things into UStruct seems like a fairly obvious choice.

Oh, and representing 2d array as 1d is a good idea. Array of Array of X (or vector of vector of X) only really makes when your rows can have different lengths. You could also add blueprint accessor to that, if you wanted.