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

It's not just for cases where someone releases the container, since in that case it's more obvious that they should retain the object themselves. Consider this code:

NSString* newValue = @"new";
NSString* oldValue = [foo someStringValue];
[foo setSomeStringValue:newValue];
// Go on to do something with oldValue

This looks reasonable, but if neither the setter nor the getter uses autorelease the "Go on to do something" part will likely crash, because oldValue has now been deallocated (assuming nobody else had retained it). You usually want to use Technique 1 or Technique 2 from Apple's accessor method examples so code like the above will work as most people will expect.