Objective-C Programming: The Big Nerd Ranch Guide

Inheritance hierarchy

All objects inherit (either directly or indirectly) from NSObject:

Figure 20.3  Inheritance diagram of some classes you know

NSObject has many methods but only one instance variable: the isa pointer. Every object’s isa pointer points at the class that created it. (Get it? When you have a BNRPerson instance, that object “is a” BNRPerson. When you have an NSString instance, that object “is a[n]” NSString.)

Figure 20.4  Every object knows which class created it

When you send a message to an object, ...