Question

I want to ask a stupid question about the iPhone application. I am the green of the iPhone app. I read the following code in the Apple website.

    MyViewController *aViewController = [[MyViewController alloc] initWithNibName:@"MyViewController" bundle:[NSBundle mainBundle]];
    [self setMyViewController:aViewController];
    [aViewController release];

And I have a question, how it means of the ‘release’ in the line 3?

Does it presents the memory clear? or the program take control of that object? or other meanings. Thank you very much.

Answer 1

When you alloc something, the object you get will have a retain count of 1 – this means that this object is currently being used by someone, so it should not be removed from memory. If you call retain on an object it will increase the retain count, meaning the object is being used by 2 things. If the retain count reaches 0, it implies that the object is no longer being used by anything and it can be removed from memory. You can decrease an object’s retain count by calling release on the object.

In your example, aViewController is alloc‘d and after line 1 has a retain count of +1.

It is then set as the view controller in line 2. This method is therefor taking ownership of the object, so should retain it for its own use.

Line 3, we don’t want anything more to do with the view controller, so we release our hold of it. The retain count decreases by one – and it is now up to the new owner to release it when it is finished with it.

You might find it helpful to read through the memory management section of this tutrial