how to create an object using self?

I thought I understood the use of self while referring to anything in the current class. After encountering this warning and subsequent run failure, I have googled many variants of "define self" or "usage of self" and gotten nowhere. This problem is how to create an object without the warning, and understand why.

#import <Cocoa/Cocoa.h>

@interface Foo : NSObject {  
   Foo *obj;  
}  
-(void)beta;  
@end  

#import "Foo.h"  
@implementation Foo  
   -(void)beta{  
   obj = [self new];      // 'Foo' may not respond to '-new'  
}  
@end    

Note, if I substitute Foo for self, there's no problem. I thought the class name and self were equivalent, but obviously the compiler doesn't think so.

1. Perhaps an explanation of what's wrong here will not only solve my problem but also enlighten my understanding of the usage of self.

开发者_JAVA百科

2. Are there any tutorials about proper usage of self? I couldn't find anything beyond something like "self is the receiver of the message," which didn't help me at all.

self references the receiver of a message, so in this specific case, self references the object on which -beta was invoked. (Within the scope of a class method, self references the class' Class object)

Now, since self in this case references the object that the method was invoked on, the compiler gives you a warning because new is not an instance method of Foo. new is a class method inherited from NSObject. So, the correct way to do this would be to retrieve the Class object from self, then invoke new on that:

- (void) beta {  
  obj = [[self class] new];
}