Memory leak even though every alloc is released

Question

The last two days I spent with hunting down memory leaks. I´ve read the documentation and searched the internet for good information (e.g. Owen Goss "Finding and Fixing Memory Leaks in iOS Apps"), but still I have too many mysteries to solve.

For example this piece of code lights up in Instruments again and again. I tried my best but can´t fix it.

- (void) updateUserDefaults
{
    // alloc temporary Array for object´s positions
    NSMutableArray *tArray = [[NSMutableArray alloc] init];

    // store locations of objects
    for (int i=0; i<[originalOrigins count]; ++i) {
        CGPoint foo = [self.view viewWithTag:100+i].center;
        NSString *moo = NSStringFromCGPoint(foo);
        [tArray addObject:moo];
        [moo release]; //?
    }

    // retrieve all stored positions for all objects
    NSMutableArray *zettelPannedOrigins = [[[[NSUserDefaults standardUserDefaults] objectForKey:@"zettelPannedOrigins"] mutableCopy] retain];

    // replace with objects from this level
    [zettelPannedOrigins replaceObjectAtIndex:zettelAtIndexInTonebank withObject:tArray];

    // save
    [[NSUserDefaults standardUserDefaults] setObject:zettelPannedOrigins forKey:@"zettelPannedOrigins"];
    [[NSUserDefaults standardUserDefaults] synchronize];

    // clean up memory
    [tArray release];
    [zettelPannedOrigins release]; //?

}

What I think might be interesting for others too is, that I release what I alloc. But still it is leaking. This I can´t answer with the documentation. Or can I?

Answer 1

NSMutableArray *zettelPannedOrigins = [[[[NSUserDefaults standardUserDefaults] objectForKey:@"zettelPannedOrigins"] mutableCopy] retain];

This will have a retain count of 2, as mutableCopy retains it once and you are calling retain on it again. Don't call retain here.

Remember, if you call a method with new, alloc, retain or copy in the name, you then own that object and the retain count goes up.

Answer 2

[NSObject mutableCopy]; will give you back an object with increased retainCount by 1 so you do not need another 'retain'.

NSMutableArray *zettelPannedOrigins = [[[[NSUserDefaults standardUserDefaults] objectForKey:@"zettelPannedOrigins"] mutableCopy] autorelease];

This should do the job :)