Programming in Objective-C, Sixth Edition

Composite Objects

You’ve learned several ways to extend the definition of a class through techniques such as subclassing and using categories. Another technique involves defining a class that consists of one or more objects from other classes. An object from this new class is known as a composite object because it is composed of other objects.

For example, consider the Square class you defined in Chapter 8, “Inheritance.” You defined this as a subclass of a Rectangle because you recognized that a square was just a rectangle with equal sides. When you define a subclass, it inherits all the instance variables and methods of the parent class. In some cases, this is undesirable (for example, some of the methods defined in the parent class might ...