When you define a member function in a class Interface in Qt creator you can right click on the function prototype and select Refactor->Add Definition in YouClass.cpp
As proposed by @Robin Karlsson:
You can press Alt+Enter to open a context menu that contains refactoring actions available in the current cursor position.
As mentioned by @Dmitry Volosnykh, the above appears in the documentation (more concretely at Applying Refactoring Actions )
You can press Alt+Enter to open a context menu that contains refactoring actions, which are available at the current cursor position.
This is also mentioned in the documentation under Refactoring C++ Code
Maybe a solution with a macro or something in that direction exists as well.