Question

I haven’t done any iOS development since iOS 3, so my memory is a little hazy (though memory management was never anything I struggled with and my mind is quite clear on that).

I’m starting a new project and don’t understand why the skeleton code is structured the way it is:

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

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[[UIWindow alloc]
                    initWithFrame:[[UIScreen mainScreen] bounds]]
                   autorelease];
    // ... snip ...
}

1. Why would the window object be autoreleased? I’m pretty sure it never used to be this way in older iOS versions.
2. Where does _window come from? Is this just another way to access [self window]?

I’d have written this as:

- (void)dealloc
{
    [self.window release];
    [super dealloc];
}

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc]
                   initWithFrame:[[UIScreen mainScreen] bounds]];
    // ... snip ...
}

It was always drummed into me never to release an autoreleased object, and in fact doing so usually results in a segmentation fault.

Answer 1

In you second example you are leaking the window object since, the alloc will give the object a retain count of 1, the you assign it _window via the property which will also retain the object assigned to it.

It’s true you should not release an autorelease object, but in the dealloc you are releasing the iVar for the window property. You should always release any property that is declared as either retain or strong. (although not when using ARC).
The _window is now automatically generated as the iVar for the property window.

There some believes that you should not use self. properties in init or dealloc.
Thus the way I do it is:

 [_window release], _window = nil;

This wil set the _window to nil after releasing it, making sure that if any other thread might want to use this it will be calling on nil. Which could prevent a crash but could alo create some weird behavior. This is totally up to you.

---

You should move to ARC, which is a complier option to add release/autolease at compiletime fro you. There is no need to added these your self if you set the property correctly when using ARC.