Programming iOS 6, 3rd Edition

Class Methods

Up to now I’ve been keeping something back, and if you’ve been paying close attention, you may have caught me at it, because it looks as though I’ve contradicted myself. I said that nothing happens in a program unless a message is sent to an object. But I also said that there are no instances until they are created as the program runs. The contradiction is that if messages can be sent only to instances, it appears that no instances can ever be created (because, when the program starts up, there are no instances to which you can send the message asking for an instance to be created).

The truth that I’ve been keeping back, which complicates things only a little, is that classes are themselves objects and can be sent messages. This revelation solves the contradiction completely. No instances exist as the program starts up, but the classes do. The classes may live off in a world of Platonic Forms, but they can still be sent messages. And one of the most important things you can ask a class to do by sending it a message is to instantiate itself.

You cannot, however, ask an instance to instantiate itself. It thus begins to look as if there must be two kinds of message: messages that you are allowed to send to a class (such as telling the Dog class to instantiate itself) and messages that you are allowed to send to an instance (such as telling an individual dog to bark). That is exactly true. More precisely, all code lives as a method in a class, but methods are of two kinds: ...