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 . But this will be really handy If you can set a shortcut for it. I couldn't find it in options->shortcut section.
For example in Visual Assist you can press ALT+SHIFT+Q.
You can Alt+return click on the selected function and then press the menu item that appears, there might be another solution with a macro or something
As proposed by @Robin Karlsson:
As mentioned by @Dmitry Volosnykh, the above appears in the documentation (more concretely at Applying Refactoring Actions )
(I'd have added this as a comment, but not enough reputation yet)