Data Types and Expressions(Chapter 4 of Programming in Objective-C 2.0)

　　The id data type is used to store an object of any type. In a sense, it is a generic object type. For example, this line declares number to be a variable of type id:

　　id number;

　　Methods can be declared to return values of type id, like so:

　　-(id) newObject: (int) type;

　　This declares an instance method called newObject that takes a single integer argument called type and returns a value of type id. Note that id is the default type for return and argument type declarations. So, the following declares a class method that returns a value of type id:

　　+allocInit;