objective c - What's the difference between adding pseudo-private ivars in a class extension or in the @implementation block?

Question

What's the difference between putting pseudo-private instance variables in a class extension inside the .m file, or putting them in the newly introduced @implementation brackets like shown below?

Are there consequences, pros, cons over one or the other way? Is internal2 treated differently than internal3 in a way a programmer must care of? (of course there is a difference McKay would say but the question is if you care in practice or not).

// MyClass.m

@interface MyClass () {
    id internal2;
}
@end


@implementation MyClass {
    id internal3;
}

- (void)internalMethod {
    NSLog(@"%@ %@", internal2, internal3);
}

@end

source: http://www.mcubedsw.com/blog/index.php/site/comments/new_objective-c_features/

Answer 1

The main difference between the two approaches is that you can include the class extension in a separate header, whereas the @implementation ivars obviously have to go with the @implementation block in the .m file (and there can only be one @implementation for a given class (extensions not included)). The practical result of this is that you can have multiple levels of "private" ivars:

• MyClass.h: public ivars
• MyClass+Private.h: semi-private ivars
• MyClass.m: really private ivars

As a hypothetical example, pretend that MyClass is UIView. In that case, UIView.h is the header that we can all access, UIView+Private.h is the "private" header than only Apple can access, and UIView.m has stuff that only the people specifically responsible for UIView need to know about.