Composition vs Inheritance and GUI toolkits

Question

It's said that composition is preferred over inheritance. Every single open source GUI toolkit however uses inheritance for the drawn widgets (windows, labels, frames, buttons, etc). I checked Qt, wxWidgets, and GTK+. Is there an example of a GUI toolkit (written in any language) that uses composition instead of inheritance to separate the various widgets?

Answer 1

What is the problem that you're facing with those GUI Toolkits ?

I think, They derive from a common Widget base class so that they all expose a minimum interface. So you could write code that can work with any SpecializedWidgetType. I think this is a good use of inheritance..

Prefer composition to inheritance.. but do not shoehorn composition where inheritance is the right answer. There are always exceptions to a rule/guideline.