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. Inheritance is closely related to the topics of dynamic binding and polymorphism. Dynamic binding refers to a language's support for virtual methods, allowing the determination of the actual method invoked on a particular object to be determined at runtime. Polymorphism refers to the ability to store a reference to a derived class within a variable declared as a base class type.

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. A class which contains other classes is called a composite class, while a class being contained is called a 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.

Arguments against Inheritance

Inheritance breaks encapsulation

• Critics of inheritance argue that the public and protected access modifiers, when used for attributes, break encapsulation by letting a derived class peer inside of its base class [3]
• The point about breaking encapsulation also comes up when talking specifically about implementation inheritance. Implementation inheritance refers to the practice of inheriting from an actual class as opposed to an interface. The idea behind implementation inheritance is to re-use code from the base class and add or customize some desired behavior. The argument against implementation inheritance is that it breaks encapsulation by making the functionality of the subclass dependent on the implementation of the base class [4]. Interface inheritance, on the other hand, is when a class either derived from a pure virtual class in c++, or implements an interface in Java. Interface inheritance does not break encapsulation, and is the idea behind the GoF's design principle to program to an interface

Inheritance increases coupling

• Critics of inheritance argue that inheritance can lead to brittle code due to coupling between base and derived classes [5]. When using delegation there is a single, explicit dependency between the composite and composed classes; the interface of the composed class. 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 and runtime behavior that can be difficult to predict.
• The coupling introduced by inheritance introduces compile time dependencies that can lead to increased build times. [6]. In C++, the header file containing the base class declaration must be pulled in when compiling a derived class header. There is the obvious penalty of reading the file from disk, but the bigger problem is that the changes high up in a class hierarchy can cause a large number of objects to be rebuilt. This can be contrasted to delegation, where a composited class can be declared with a forward declaration to avoid the compile time penalty.

Inheritance is static and determined at compile time

• [7]

The Diamond of Death

• [8]

Arguments against delegation

Bloated Code

• [9]

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