Question

Let’s say that I have class, that uses some functionality of dict. I used to composite a dict object inside and provide some access from the outside, but recently thought about simply inheriting dict and adding some attributes and methods that I might require. Is it a good way to go, or should I stick to composition?

Answer 1

Inheritance is very often abused. Unless your class is meant to be used as a generic dictionary with extra functionality, I would say composition is the way to go.

Saving forwarding calls is usually not a good enough reason for choosing inheritance.

From the Design Pattern book:

Favor object composition over class inheritance

Ideally you shouldn’t have to create
new components to achieve reuse. You
should be able to get all the
functionality you need by assembling
existing components through object
composition. But this is rarely the
case, because the set of available
components is never quite rich enough
in practice. Reuse by inheritance
makes it easier to make new components
that can be composed with old ones.
Inheritance and object composition
thus work together.

Nevertheless, our experience is that
designers overuse inheritance as a
reuse technique and designs are often
made more reusable (and simpler) by
depending more on object composition.”

The entire text is here:
http://blog.platinumsolutions.com/node/129