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Should I get functionality by inheritance or composition?

Inheritance should be reserved only for cases where a class is a more specific variety of the base class. For example, you might have a Person class, and then you might have Employee and Customer classes inherit common attributes and methods from Person. This is frequently called an “is-a” relationship, as in “A User is a Person.” If your proposed inheritance relationship makes sense phrased that way, it might well be a good candidate for inheritance.

Composition is when you use a class as an attribute. To extend our example, you might have an Address class. You might be tempted to have Person inherit from Address. But a Person is not an Address. Try it: “A Person is an Address.” Nope. Instead, you should just have an instance of the Address class as an attribute of Person. Such a relationship is often called a “has-a” relationship, as in “A Person has an Address.” If the relationship makes sense phrased that way, it is a good candidate for composition. Another way to recognize that you’ve wrongly used inheri- tance is if you end up having a radically different class inherit from the same base class. For example, suppose you have a class, Building. Would it make sense for Building and Person to inherit from Address? Are Buildings and Persons more specific instances of the same general type of thing? No, they are not. Building and Person should get Address functionality by composition.

12.6 Essential and Nonessential 285