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/
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:
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.