Now that I’ve revealed that classes are objects and can be sent messages, you might be wondering why there need to be instances at all. Why doesn’t the mere existence of classes as objects suffice for object-based programming? Why would you ever bother to instantiate any of the classes? Why wouldn’t you write all your code as class methods, have the program send messages from one class object to another, and be done with it?
The answer is that instances have a feature that classes do not: instance variables. An instance variable is just what the name suggests: it’s a variable belonging to an instance. Like instance methods, instance variables are defined as part of the class. But the value of an instance variable is set as the program runs and belongs to one instance alone. In other words, different instances can have different values for the same instance variable.
For example, suppose we have a Dog class and we decide that it might be a good idea for every dog to have a name. Just as you can learn a real-world dog’s name by reading the tag on its collar, we want to be able to assign every dog instance a name and, subsequently, to learn what that name is. So, in designing the Dog class, we declare that this class has an instance variable called
name, whose value is a string (probably an NSString, as we’re using Objective-C). Now when our program runs we can instantiate Dog and assign the resulting dog instance a name (that is, we can assign its
name instance variable ...