c++ coroutine - winrt::fire_and_forget, What does this do?

Question

I' m porting C++/CX application to C++/WinRT Core Application.

and I found a useful sample code (Simple3DGameDX) at this link.

https://github.com/microsoft/Windows-universal-samples/tree/master/Samples/Simple3DGameDX/cppwinrt

and its Suspending method's return type is winrt::fire_and_forget. but anther example in C++/CX, its Suspending method's return type is void.

why this return type is not void in C++/WinRT? and What does this do?

C++/WinRT

winrt::fire_and_forget OnSuspending(IInspectable const& /* sender */, SuspendingEventArgs const& args)

C++/CX

void OnSuspending(Object^ Sender, SuspendingEventArgs^ Args)

Answer 1

why this return type is not void in C++/WinRT? and What does this do?

Apart from documentation for Fire and forget, note the following: the function itself uses co_await operator in its body.

This requires that the function itself is coroutine friendly and could be compiled into "stackless" form, for asynchronous execution. void return type does not work out, but fire_and_forget struct is okay because C++/WinRT defines coroutine handling for it, as explained by the documentation.

Think of this as void which can be asynchronous and has no need to be waited on.