About "self"

Class method can't refer derectly to instance variables. Within the body of  a class method, self refers to the class object itself. For example:

@interface Myclass : NSObject 
+ (id)classMethod;
@end

 

Implementation of the classMethod like this, let's call it method_A:

 

+ (id)classMethod
{
      return [[[self alloc] init] autorelease];
}

 

the "self" in the body  of the method  named "classMethod"  refers to the class object rather than the instance variable.  

Also you can do like this, let's call it method_B:

 

+ (id)classMethod
{
      return [[[MyClass alloc] init] autorelease];
}

 

So what's the difference between the two methods above:

Imagine that you create a subclass of Myclass, maybe like this:

 

@interface MySubClass: MyClass 
@end

 

And now you want to  create a instance of MySubClass by class method like "[MySubClass classMethod]" ,but wait, if you use  method_A, that's OK,  you create a instance of MySubClass successfully,if you use method_B, eh...and you can see that method_B return a instance of MyClass, that's not what you wanted.