Programming iOS 6, 3rd Edition

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, there is a hierarchical tree of subclasses, each branching from its superclass, and so on, with a single class, the base class, at the top. Indeed, Cocoa itself consists of just such a tree (a huge tree!) of hierarchically arranged classes, even before you write a single line of code or create any classes of your own. We can imagine diagramming this tree as an outline, with a single ultimate superclass at the top, 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. And in fact Xcode will show you this outline: choose View → Navigators → Show Symbol Navigator and click Hierarchical, with the first and third icons in the filter bar darkened (Figure 4-1).

Figure 4-1. Browsing ...