I always read that composition is to be preferred over inheritance. A blog post on unlike kinds, for example, advocates using composition over inheritance, but I can't see how polymorphism is achieved.

But I have a feeling that when people say prefer composition, they really mean prefer a combination of composition and interface implementation. How are you going to get polymorphism without inheritance?

Here is a concrete example where I use inheritance. How would this be changed to use composition, and what would I gain?

Class Shape
{
    string name;
  public:
    void getName();
    virtual void draw()=0;
}

Class Circle: public Shape
{
    void draw(/*draw circle*/);
}


Polymorphism does not necessarily imply inheritance.  Often inheritance is used as an easy means to implement Polymorphic behaviour, because it is convenient to classify similar behaving objects as having entirely common root structure and behaviour.  Think of all those car and dog code examples you've seen over the years.

But what about objects that aren't the same.  Modelling a car and a planet would be very different, and yet both might want to implement Move() behaviour.

Actually, you basically answered your own question when you said "But I have a feeling that when people say prefer composition, they really mean prefer a combination of composition and interface implementation.".  Common behaviour can be provided through interfaces and a behavioural composite.  

As to which is better, the answer is somewhat subjective, and really comes down to how you want your system to work, what makes sense both contextually and architecturally, and how easy it will be to test and maintain.
