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).

Answer 2

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.

Answer 3

I haven't seen this pattern before, but it seems fairly pointless to me. I guess the intent is to keep the returned value safe if the client code calls "release" on the parent object. It doesn't really hurt anything, but I doubt that situation comes up all that often in well-designed libraries.

---

Ah, ok. from the documentation smorgan linked to, it seems this is now one of the methods that Apple is currently recommending that people use. I think I still prefer the old-school version:

- (NSString *) value
{
    return myValue;
}

- (void) setValue: (NSString *) newValue
{
    if (newValue != myValue)
    {
       [myValue autorelease]; // actually, I nearly always use 'release' here
       myValue = [newValue retain];
    }
}

Answer 4

From here http://www.macosxguru.net/article.php?story=20030713184140267

- (id)getMyInstance
    {
        return myInstanceVar ;
    }

or

- (id)getMyInstance
{
    return [[myInstanceVar retain] autorelease] ;
}

What's the difference ? The second one allows the caller to get an instance variable of a container object, dispose of the container and continue to play with the instance variable until the next release of the current autoreleased pool, without being hurt by the release of the instance variable indirectly generated by the release of its container:

aLocalVar = [aContainer getAnInstanceVar] ;
[aContainer release];
doSomething(aLocalVar);

If the "get" is implemented in the first form, you should write:

aLocalVar = [[aContainer getAnInstanceVar] retain];
[aContainer release];
doSomething(aLocalVar);
[aLovalVar release];

The first form is a little bit more efficent in term of code execution speed. However, if you are writing frameworks to be used by others, maybe the second version should be recommanded: it makes life a little bit easier to people using your framework: they don't have to think too much about what they are doing…;) If you choose the first style version, state it clearly in your documentation… Whatever way you will be choosing, remember that changing from version 1 to version 2 is save for client code, when going back from version 2 to version 1 will break existing client code…