CSC/ECE 517 Summer 2008/wiki2 8 jb
This wiki will explore the age old debate of inheritance vs. delegation, showing the strengths and weakness of each approach, and where each approach is preferred.
Background
An in-depth definition of Inheritance and Delegation is out of the scope of this wiki, but a brief discussion of each approach will help get us started.
Inheritance
Inheritance is one of the fundamental tenets of object oriented programming. Inheritance refers to the ability to model hierarchies of classes that are related to each other through the is-a relationship. It is commonly agreed upon that inheritance done correctly must conform to the Liskov substitution principle.
Delegation
Delegation, sometimes referred to as aggregation, is the concept that one class may contain an instance of another class, and delegate some responsibility to that class. This is also referred to as the has-a relationship. Aggregation is closely related to composition. Both aggregation and composition are used to describe one object containing another object, but composition implies ownership [1]. Aggregation is more general and doesn't imply any responsibilities for memory management. The class which contains other classes is called the composite class, while a class being contained is called the composited or composed class [2].
The Debate
Much of the debate is focussed on arguments against using inheritance. The most notable and widely accepted statement on the subject came from the original Gof or Gang of Four, in their book titled Design Patterns: Elements of Reusable Object-Oriented Software. The book stated to "Favor object composition over class inheritance" as design principle. This position is backed up by many strong arguments.
Inheritance breaks encapsulation
- Critics of inheritance argue that the protected access modifier breaks encapsulation by letting a derived class peer inside of its base class [3]
- interface inheritance
Inheritance increases coupling
- Critics of inheritance argue that inheritance can lead to brittle code due to coupling between base and derived classes [4]. When using delegation there are limited and explicit dependencies between the composite and composed classes. The interface of the composed class is the entire contract between the two classes. With inheritance the dependencies become blurred with multiple factors leading to close coupling. Access to base class members, unanticipated consequences of executing virtual methods, etc... all contribute to a more brittle relationship
Inheritance is static and determined at compile time
Links
Wikipedia: Inheritance
Wikipedia: Delegation
Wikipedia: Composition
Replace Inheritance With Delegation design pattern
Dr. Dobbs: Composition vs. Inheritance
Good debate
Great article covering delgation and composition
http://www.artima.com/designtechniques/compoinh.html
http://www.artima.com/lejava/articles/designprinciples4.html