When to use Class Inheritance in Programming

Last week I listened to a friend of mine complain about some straight-out-of-college programmers he had recently hired: "All of their code has 10 layers of inheritance. It's crazy trying to maintain their code!".

Unfortunately my friend is right: inheritance, although powerful - when used correctly, naturally tends to increase complexity.

Since your primary purpose as a programmer is to manage and reduce complexity, you should favors solutions that don't use inheritance.

At this point you're probably asking yourself: When am I supposed to use inheritance? How do I know if I'm using it correctly?Fortunately, the great Steve McConnel, has written 4 clear cut rules on when to use inheritance and when to use containment:

1. If multiple classes share common data but not behavior, create a common object that those classes contain.

2. If multiple classes share common behavior but not data, derive them from a common bases class that defines the common routines.

3. If multiple clses share commond data and behavior, inherit from a common base class that defines the common data and routines.

4. Inherit when you want the base class to control your itnerface; contain when you want to control the interface.

In summary, only inherit if the new class truly is-a more specialized version of the base class.

Oh, and one more piece of advice on inheritance: avoid multiple inheritance like the plague.