Class method (original) (raw)
A class method is a method that operates on class objects rather than instances of the class. In Objective-C, a class method is denoted by a plus (+) sign at the beginning of the method declaration and implementation:
To send a message to a class, you put the name of the class as the receiver in the message expression:
Subclasses
You can send class messages to subclasses of the class that declared the method. For example, NSArray declares the class method array that returns a new instance of an array object. You can also use the method with NSMutableArray, which is a subclass of NSArray:
NSMutableArray *aMutableArray = [NSMutableArray array];
In this case, the new object is an instance of NSMutableArray, not NSArray.
Instance Variables
Class methods can’t refer directly to instance variables. For example, given the following class declaration:
| @interface MyClass : NSObject { |
|---|
| NSString *title; |
| } |
| + (void)classMethod; |
| @end |
you cannot refer to title within the implementation of classMethod.
self
Within the body of a class method, self refers to the class object itself. You might implement a factory method like this:
| + (id)myClass { |
|---|
| return [[[self alloc] init] autorelease]; |
| } |
In this method, self refers to the class to which the message was sent. If you created a subclass of MyClass:
| @interface MySubClass : MyClass { |
|---|
| } |
| @end |
and then sent a myClass message to the subclass:
id instance = [MySubClass myClass];
at runtime, within the body of the myClass method, self would refer to the MySubClass class (and so the method would return an instance of the subclass).