Class method

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:

+ (void)classMethod;

To send a message to a class, you put the name of the class as the receiver in the message expression:

[MyClass classMethod];

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).

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