Cocoa in a Nutshell

Messaging

Objects in Objective-C are largely autonomous, self-contained, opaque entities within the scope of a program. They are not passive containers for state behavior, nor data and a collection of functions that can be applied to that data. The Objective-C language reinforces this concept by allowing any message— a request to perform a particular action—to be passed to any object. The object is then expected to respond at runtime with appropriate behavior. In object-oriented terminology, this is called dynamic binding .

When an object receives a message at runtime, it can do one of three things:

• Perform the functionality requested, if it knows how.

• Forward the message to some other object that might know how to perform the action.

• Emit a warning (usually stopping program execution), stating that it doesn’t know how to respond to the message.

A key feature here is that an object can forward messages that it doesn’t know how to deal with to other objects. This feature is one of the significant differences between Objective-C and other object-oriented languages such as Java and C++.

Dynamic binding, as implemented in Objective-C, is different than the late binding provided by Java and C++. While the late binding provided by those languages does provide flexibility, it comes with strict compile-time constraints and is enforced at link time. In Objective-C, binding is performed as messages are resolved to methods and is free from constraints until that time.

Structure of a Message

Message ...