CS 4448 - Fall 1998
Object-Oriented Programming and Design
Talk 10.4
by
Derek Reiger

Inheritance versus composition: Which one should you choose?

by

Bill Venners

• Inheritance - a new class is created from and existing class and inherits the properties of that existing class. It models an is-a relationship.
  
  
• Composition - a front-end class holds a reference to a back-end class in one of its instance variables. It models a has-a relationship.
  
  
• The goal of this paper is to influence the reader to choose the best relationship model for their design, specifically with regard to code reuse.
  
  
• Inheritance may not be the best relationship model if the goal is code reuse because changes to a superclass interface can break code that uses the superclass or any of its subclasses.
  
  
• Composition might be a better solution because changes might be limited to referenced classes, but code that uses those classes should be safe.
  
  
• Guidelines to go by:
  1. Make sure inheritance actually models the is-a relationship.
  2. If the goal is specifically code reuse, composition offers more maneagable changes.