Iphone: Confused about memory managment in HelloWorld

Question

In the HelloWorld example you can find the following code:
MyViewController *aViewController = [[MyViewController alloc] initWithNibName:@"ControllerView" bundle:[NSBundle mainBundle]];
self.myViewController = aViewController;
[aViewController release];
The myViewController is released in the dealloc method.
I dont see why I need to declare and assign the temporary "aViewController" just to assign it to the instance variable. Wouldn't it be enough to write:
self.myViewController = [[MyViewController alloc] initWithNibName:@"ControllerView" bundle:[NSBundle mainBundle]];
As I see it, myViewConroller will keep a reference to the object, and it will be released once the dealloc method is invoked.

Answer 1

The property "myViewController" most probably has been declared as such:
@property (nonatomic, retain) MyViewController* myViewController;
This means that every time something is assigned to it, the ref-count for that object is incremented (as 'retain' is called).
So, if you were to do this:
self.myViewController = [[MyViewController alloc] initWithNibName:@"ControllerView" bundle:[NSBundle mainBundle]];
You would be incrementing the ref-count for the newly allocated object and the object will then have a refcount of 2 (as by default 'alloc' sets ref-count to 1). Now, even if myViewController is released in dealloc, it will only decrement the count to '1' from '2'; this results in a memory leak.
So the golden rule is, *every time* you do an 'alloc' or 'copy' it is your responsibility to 'release' that object. And for this reason you see it being assigned and then being released again. This is what happens:
MyViewController *aViewController = [[MyViewController alloc] initWithNibName:@"ControllerView" bundle:[NSBundle mainBundle]];
Ref Count for aViewController is 1
self.myViewController = aViewController;
Ref Count for aViewController is 2 (as self.myViewController does a retain on it)
[aViewController release];
Ref count for aViewController is now 1
Later when myViewController is released, ref-count of the object pointed by 'aViewController' or 'myViewController' is then set to 0 and at that point it's released from memory.