Passing function pointer with or without &

Question

I'm currently learning game development with c++ in Unreal Engine and I came across the function that takes a function pointer as an input:

InputHandle->BindAction("Grab",IE_Pressed, this, &UGrabber::Grab);

From basic C++ I know that in passing a function pointer as an attribute (UGrabber::Grab) - & is optional, however UEngine complains with the following error code if I omit the &:

error C3867: 'UGrabber::Grab': non-standard syntax; use '&' to create a pointer to member

Could someone explain why?

BindAction function declaration looks like this:

FInputActionBinding& BindAction( const FName ActionName, const EInputEvent KeyEvent, UserClass* Object, typename FInputActionHandlerSignature::TUObjectMethodDelegate< UserClass >::FMethodPtr Func )

Answer 1

The BindAction function makes use of a Dynamic Multicast Delegate.

They are one of Unreal's ways of having callback functions. In this case, they rely not just on calling a function, but calling a specific object's function. This is why you need to bass the third parameter (in this example, the parameter is this).

What it's saying is, when the input action is IE_Pressed, call the UGrabber function Grab on object this (this has to be a UGrabber instance of course). This is why it's a pointer to the method. It actually utilizes Unreal's reflection system to find the method on the object. So the this object needs to be UObject, otherwise you can't call a funciton on an object by name in C++.

For more info on this, search for "unreal delegates" and "unreal reflection" in your search engine of choice. Using them is quite easy, and it's not necessary to understand the reflection system to reliably use them. Just don't forget to bind and unbind at the appropriate times.

p.s. You can get quite in depth in this subject of callbacks you want. There are other delegate types that don't rely on reflection, for example non-dynamic delegates, that can bind to lambda functions, and or a more familiar if you're coming from a pure C++ background, where commonly a void* opaque is used, expected to be cast to the needed class pointer.