(UE4) Save a lambda function in the header

Question

i am currently defining a lamda function which i want to save in my header file.

void FURealisticGraspingEditorModule::OnPreviewCreation(const TSharedRef<IPersonaPreviewScene>& InPreviewScene)
{
    TSharedRef<IPersonaToolkit> PersonaToolKitRef = InPreviewScene.Get().GetPersonaToolkit();
    auto lambda = [PersonaToolKitRef]() { return PersonaToolKitRef.Get(); };
    DebugMeshComponent = PersonaToolKitRef.Get().GetPreviewMeshComponent();
}

The lamda variable should get saved in the header. I did not manage to do this yet and now i am curious if this is even possible. I tried auto and TFunctionRef. Maybe there is a hint you guys can give me to achive this or even another way to save this call in a variable.

Answer 1

In order to declare a lambda variable separately from initialising it you need to store it in std::function . The type of a lambda is only known at the point you create it so its not possible to declare a variable to store it separately.

For example:

auto a = []{ return 1; }; // OK

auto b; // invalid, can't declare as auto as the type can't be deduced
b = []{ return 1; };

std::function< bool() > c;
c = []{ return 1; }; // OK, we can store a lambda in std::function

Answer 2

You can define (and initialise) your lambda in your header file if you compile for C++17 and declare it inline .

At least, I think it works for lambdas, try it.

Answer 3

Lambdas can be stored in function pointers without too much effort like that:

In header: double (*lambda)(double);

In the .cpp file: lambda = [&](double a) { return 2*a; }; lambda = [&](double a) { return 2*a; };

The trouble with this approach is that the defined type of lambda cannot be dynamic. What I mean by that is the assigned lambda needs to match the type defined in the header, so it must have the same return type and, type-wise, exactly the same list of arguments.

And one more little thing. In C++ you are not allowed to re-declare neither functions nor variables in the same scope. If you define a variable directly in a header file and happen to include it into more than one source file, you will get an unpleasant surprise in form of crazy errors. In a header file, define your variable using extern and then define it normally in ONE source file in order to avoid trouble.