Programming iOS 5, 2nd Edition

Chapter 4. Objective-C Classes

This chapter describes some linguistic and structural features of Objective-C having to do with classes; in the next chapter, we’ll do the same for instances.

Class and Superclass

In Objective-C, as in many other object-oriented languages, a mechanism is provided for specifying a relationship between two classes: they can be subclass and superclass of one another. For example, we might have a class Quadruped and a class Dog and make Quadruped the superclass of Dog. A class may have many subclasses, but a class can have only one immediate superclass. (I say “immediate” because that superclass might itself have a superclass, and so on in a rising chain, until we get to the ultimate superclass, called the base class, or root class.)

Because a class can have many subclasses but only one superclass, we can imagine all classes in a program as being arranged in a tree that splits into branches, such that each branch splits into smaller branches, each smaller branch splits into even smaller branches, and so on. Or we can imagine all the classes arranged in a hierarchy, such as might be displayed in an outline, with a single ultimate superclass, then all of its immediate subclasses in the next level below that, then each of their immediate subclasses in the next level below that, and so on. Indeed, before you write a line of your own code, Cocoa already consists of exactly such a vast repertoire of classes arranged in exactly such a hierarchical relationship. ...