Editing Class (computer programming) (section)

=== Compositional ===

Classes can be composed of other classes, thereby establishing a compositional relationship between the enclosing class and its embedded classes. Compositional relationship between classes is also commonly known as a ''[[has-a]]'' relationship.{{sfn|Booch|1994|p=180}} For example, a class "Car" could be composed of and contain a class "Engine". Therefore, a Car ''has an'' Engine. One aspect of composition is containment, which is the enclosure of component instances by the instance that has them. If an enclosing object contains component instances by value, the components and their enclosing object have a similar [[Object lifetime|lifetime]]. If the components are contained by reference, they may not have a similar lifetime.{{sfn|Booch|1994|p=128-129}} For example, in Objective-C 2.0:

<syntaxhighlight lang="objc">
@interface Car : NSObject

@property NSString *name;
@property Engine *engine
@property NSArray *tires;

@end
</syntaxhighlight>

This {{Mono|Car}} class ''has'' an instance of {{Mono|NSString}} (a [[string (computer science)|string]] object), {{Mono|Engine}}, and {{Mono|NSArray}} (an array object).