Why should a self-implemented getter retain and autorelease the returned object?

Question

Example:

- (NSString*) title {
    return [[title retain] autorelease];
}

The setter actually retained it already, right? and actually nobo

Answer 1

Compare this code

  return [[title retain] release]; // releases immediately

with this

  return [[title retain] autorelease]; // releases at end of current run loop (or if autorelease pool is drained earlier)

The second one guarantees that a client will have a non-dealloced object to work with.

This can be useful in a situation like this (client code):

 NSString *thing = [obj title];
 [obj setTitle:nil]; // here you could hit retainCount 0!
 NSLog(@"Length %d", [thing length]); // here thing might be dealloced already!

The retain (and use of autorelease instead of release) in your title method prevents this code from blowing up. The autoreleased object will not have its release method called until AFTER the current call stack is done executing (end of the current run loop). This gives all client code in the call stack a chance to use this object without worrying about it getting dealloc'ed.

The Important Thing To Remember: This ain't Java, Ruby or PHP. Just because you have a reference to an object in yer [sic] variable does NOT ensure that you won't get it dealloc'ed from under you. You have to retain it, but then you'd have to remember to release it. Autorelease lets you avoid this. You should always use autorelease unless you're dealing with properties or loops with many iterations (and probably not even then unless a problem occurs).