...if you press Open you get memory leaks since you don't destroy f. When I put a
Nope! I just ommited the rest of code, there exist delete f after all processing with file-dialog object.
So I don't really see the point of that reference.
I know there is no point regard to design of program, but only to make program code a quite bit shorter (similar to "With" as in VB )

What I ommited from your side is message-map from derived class.