Question

I’m an objective-c newbie and I just can’t understand why it is a bad idea to release object that doesn’t belong to me.

Let’s say I have this in the method called Europe

//initForStringTheory is a class init method;
Collider *LHC = [Collider initForStringTheory]; 

//Colliders is a NSMutableArray
[Colliders addObject: LHC]  

[LHC release]

I’m adviced not to release LHC in Europe as Europe does not own LHC, it only has a pointer to it. And therefore I should make good use of the autorelease pool and do

//newCollider is a pointer for the newly created instance in initForStringTheory
return [newCollider autorelease]; 

in initForStringTheory. But why?

Doesn’t the pointer in Europe points to the instance too? Why can’t I just release LHC in Europe instead of returning a autoreleasing newCollider in the init method?

Answer 1

It’s all a question of what initForStringTheory returns – if this is a convenience initialization method such as [NSArray arrayWithObjects...] or [NSString stringWithFormat...], it creates an autoreleased instance that you do not need to release after adding it to your collection. However, if this is an initialization that returns an initialized non-autoreleased instance, it is your responsibility to release the local instance definition in your method right after adding it to the collection.

The retain count of the object is incremented by 1 when it is added, so you don’t want it to be 2 because you haven’t released/autoreleased it after instantiation (would result in a memory leak because the object will never become deallocated, even when it’s out of the array).

Read this:
http://developer.apple.com/library/mac/#documentation/CoreFoundation/Conceptual/CFMemoryMgmt/Concepts/Ownership.html