Why is window autoreleased in appliciation:didFinishLaunchingWithOptions: and released in dealloc?

Question

I have created an iphone application using the empty template without ARC in xcode 4.2. I'm not currently using ARC because I want to learn the basics of reference counting. In the application delegate I have the following method:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
    // Override point for customization after application launch.
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
    return YES;
 }

Why is window autoreleased? Is it because AppDelegate won't be using it in the future? But it is being assigned to a instance variable. There is also a dealloc method where window is released. Why is it released when it is already autoreleased?

- (void)dealloc
{
    [_window release];
    [super dealloc];
}

Answer 1

The property of the window in .h file is declared as @property (nonatomic, retain) UIWindow *window;. The window has a retain property. So the UIWindow is retained by the setter method of the window variable. In the line self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease]; the new window alloced has +1 in retainCount because of the alloc and another +1 because of the window setter method resulting in a +2 retainCount. The autorelease is to decrease the retainCount back to +1. In the dealloc the retainCount goes to 0 and the window is deallocated.