Detection of unitialised UClass properties

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1

Hello,

I see that the function FindUninitializedScriptStructMembers finds and reports uninitialized UStruct properties during the engine initialization. I see no equivalent for properties in UClass and I am wondering why it does not exist. Is it an oversight? Or maybe there is a technical reason behind it?

Thank you,

Clement

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2

Hi,

UObjects can only be created via a CoreUObject function like NewObject or StaticConstructObject, and these functions zero the target memory of the UObject before running its constructor. This causes uninitialised members without a default constructor to have a zero bit pattern, which in almost all cases (*) is the desired behaviour: pointers become null, bools become false, ints and floats become zero.

This also includes uninitialised UStruct properties which are properties of UStructs, so would not be a problem in that case, but the function you mention exists because a common thing to do is construct a UStruct on the stack - which is not zeroed by default - then copy it into your UObject property, like:

USTRUCT()
struct FMyStruct
{
    GENERATED_BODY()
 
    UPROPERTY()
    int32 X = 0;
 
    UPROPERTY()
    int32 Y = 0;
 
    UPROPERTY()
    int32 Z; // oops
};
 
UCLASS()
class UMyObject : public UObject
{
    GENERATED_BODY()
 
public:
    UMyObject();
 
    UPROPERTY()
    FMyStruct MyStruct;
};
 
UMyObject::UMyObject()
{
    // Override some values
    FMyStruct LocalStruct;
    LocalStruct.Y = 5;
    // LocalStruct.X is initialised to zero, but LocalStruct.Z is uninitialised
 
    MyStruct = LocalStruct; // copy uninitialised junk back into object
}

This is why UStructs in particular are checked.

Hope this is clear,

Steve

(*) One instance where this isn’t the case is pointer-to-data-member types, where a zero bit pattern is *not* typically a null pointer, because they’re not really pointers. But as these are pretty much never used as UObject members, it’s not really an issue. See the difference here: Compiler Explorer

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3

It is very clear, thank you for the thorough answer.

Clement

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