Question

I have these two buttons hooked up to these two methods (they’re nearly identical)

-(void)moveOneImageNewer{
    int num = [numberOfImage intValue];
    num--;

    numberOfImage = [[NSString stringWithFormat:@"%i",num] retain];

    //Load the image
    [self loadImage];
}

-(void)moveOneImageOlder{
    int num = [numberOfImage intValue];
    num++;
    numberOfImage = [NSString stringWithFormat:@"%i",num];

    //Load the image
    [self loadImage];
}

If I hit either of them twice (or once each, basically if they get called a total of two times) I get an EXC_BAD_ACCESS. If I throw a retain on: numberOfImage = [[NSString stringWithFormat:@"%i",num]retain] it’s fine though. Can someone explain why this is? I did an NSZombie on the instruments and traced it back to this stringWithFormat call. Thanks in advance!

Answer 1

+stringWithFormat: doesn’t contain ‘new’, ‘alloc’, ‘copy’, or ‘retain’, so it should be expected that you have to retain the return value of it if you want the new NSString it creates to stick around.

Edited to include this handy link duskwuff kindly dug up: http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html#//apple_ref/doc/uid/20000994-BAJHFBGH