Question

I was just reading through the boilerplate code for a universal app and I saw this:

- (void)dealloc
{
    ...
    [_navigationController release];
    ...
}

and yet also:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    ...
        self.navigationController = [[[UINavigationController alloc] initWithRootViewController:masterViewController] autorelease];
    ...
}

I’m not sure I understand the whole underscore thing, but I gather that it’s to protect you from accessing the ivar directly, and instead accessing it through the property (why? I have no idea… but I accept that this is what its for).

However, what I don’t get is that if self.navigationController is autoreleased then surely this will cause issues when _navigationController is released at the end?

Even more confusing is the fact that there is no retain bit in the @property for navigationController.

So as far as I can tell _navigationController accessed through the property navigationController is autoreleased and then directly accessed and release in dealloc (where it could already be autoreleased surely?).

lol, I hope someone can explain what’s going on here! I’m sure the boilerplate code probably isn’t wrong!

Thank you.

Answer 1

Missing from the question are two important statements: @property and @synthesize.

My best guess is that these are:

@property (retain, nonatomic) UINavigationController * navigationController;

and

@synthesize navigationController = _navigationController;

Since the @property specifies retain when the setter is called, “self.navigationController =”, any existing value is released and the new value is retained. Thus an autorelease of the new value is counter acted by the retain in the setter.

The @synthesize statement says to use _navigationController as the ivar for the property navigationController.

This is not ARC code.