[gtkmm] general design question

[Stephan Zaubzer <s zaubzer gmx at>]

I am relatively new to programming with gtkmm. I often saw in code 
examples that instead of using a standard widget, you derive a widged 
and add your custom "things". If I want to make an application, should I 
derive an application window which contains as members everything what I 
want to show? So the application window would be my only "public" data, 
which contains all other widgets. Can somebody explain some design 
principles for creating applications? Like: Should a widget, that is 
contained in another widget also be a member of its parent widget?
So the only public widget would be my application window?
regards
Stephan