Why does Instruments think this is leaking memory ?

Question

I have an About View which I push onto a NavigationController. The view has one UILabel which is connected to an IBOutlet. In viewDidLoad I populate the UILabel with the bundle version number (a string). Testing with instruments suggested that the line marked with a comment is leaking memory: -

viewDidLoad {
    [super viewDidLoad];

    self.title = @"About";
    // Line below is the suggested culprit ***
    NSString *versionLabel = [[NSString alloc] initWithFormat:@"Version %@", 
            [[[NSBundle mainBundle] infoDictionary] objectForKey:(NSString *)kCFBundleVersionKey]];

    self.applicationVersion.text = versionLabel;
    [versionLabel release];
    versionLabel = nil;
}

I'm assuming it is suggesting the NSString and not anything else on the line ...

My questions is Why ?

Answer 1

You don't even need to create an instance of NSString in that case, simply use the following method that acts on the NSString class (not an instance):

NSString *versionLabel = [NSString stringWithFormat:@"Version %@", 
                                             [[[NSBundle mainBundle] infoDictionary] 
                                            objectForKey:(NSString*)kCFBundleVersionKey]];

If you use NSString this way, you do not have to release versionLabel because memory was never allocated.

Answer 2

My suspicion is that you're leaking the applicationVersion UILabel. That will cause the string to leak as a by-product. The most common reason for this on iPhone is failure to follow the NIB memory management rules.

Answer 3

It may actually be the mainBundle or infoDictionary that is leaking - it is possible that the system is caching one or other of those and thus they are being created and then never released.

Try adding in to your applicationDidFinishLaunching the code:

[[NSBundle mainBundle] infoDictionary];

Without any other code and see if Leaks points to that line as the location of the leak. In that case, caching is the issue and you can ignore it.