Delegation vs. Inheritance

  There are certain similarities between delegation and inheritance, and in
  fact inheritance can be implemented using delegation. However, delegation
  can be more flexible than inheritance, in that an object being delegated
  to can be replaced by some other object at runtime. The replacement object
  could be another object of the same class as the original delegatee, or it
  could even be an object of a different class. Using inheritance,
  equivalent operations are either not possible or practical.

  Changing the target of delegation in this way occurs at runtime and
  affects individual instances, making it possible for different instances
  of the same class to behave very differently. It is this that makes
  delegation potentially more powerful than inheritance in the right
  circumstances.