Question

I’m a newer in Objective C. I found can access the private variable outside. I just get a warning Like follows:

@interface foo : NSObject
{
   @private 
        int b;
}
-(id) init;
@end
//omit the implement
int main()
{
    foo *a = [[foo alloc] init];
    printf("%d", a->b);
}

So dose the private keyword only work on its subclass? If Yes, why need protected keyword

Answer 1

The code you provided accesses ab, not b. Furthermore, you’re using the class name foo, but you appear to be trying to access an instance variable named ab. If you want to access an instance variable, you’ll first need to create an instance of the class:

foo f = [foo new];

The @private keyword prevents you from accessing b from anywhere other than the methods defined by class foo.

Update:

I’m assure the codes is correct now. So why I can access the variable outside.

If you’re compiling with an Objective-C compiler, you normally can’t:

If you’re able to get the code to compile and access b, there’s something odd going on. What compiler are you using, and what settings?