1.1 Objects

The base unit of activity in all object-oriented languages is the object—an entity that associates data with operations that can be performed on that data. Objective-C provides a distinct data type, id, defined as a pointer to an object's data that allows you to work with objects. An object may be declared in code as follows:

id anObject;

For all object-oriented constructs of Objective-C, including method return values, id replaces the default C int as the default return data type.

1.1.1 Dynamic Typing

The id type is completely nonrestrictive. It says very little about an object, indicating only that it is an entity in the system that can respond to messages and be queried for its behavior. This type of behavior, known as dynamic typing, allows the system to find the class to which the object belongs and resolve messages into method calls.

1.1.2 Static Typing

Objective-C also supports static typing, in which you declare a variable using a pointer to its class type instead of id, for example:

NSObject *object;

This declaration will turn on some degree of compile time checking to generate warnings when a type mismatch is made, as well as when you use methods not implemented by a class. Static typing can also clarify your intentions to other developers who have access to your source code. However, unlike other languages' use of the term, static typing in Objective-C is used only at compile time. At runtime, all objects are treated as type id to preserve dynamism in the system.

There are no class-cast exceptions like those present in more strongly typed languages, such as Java. If a variable declared as a Dog turns out to be a Cat, but responds to the messages called on it at runtime, then the runtime won't complain.