Question

I’m just beginning to learn about proper releasing of objects for memory management in iOS. My biggest question is based on the below code.

- (void)viewDidLoad {  
    [super viewDidLoad];  

    self.title = @"My View Controller";  

    UIBarButtonItem *item = [[UIBarButtonItem alloc]   
                             initWithBarButtonSystemItem:UIBarButtonSystemItemAdd   
                             target:self   
                             action:@selector(doSomething)];  
    self.navigationItem.rightBarButtonItem = item;  

    [item release];  
}  

Ok, I see that UIBarButtonItem has been created (alloc). Now, it is set to the rightBarButtonItem property in the following line:

self.navigationItem.rightBarButtonItem = item; 

Directly after this line, the item gets released.

[item release]; 

I assume that the item is copied into the rightBarButtonItem but how can I know this happens. If it gets passed by reference, I would have a problem wouldn’t I? Can someone clarify what is going on here and why releasing the object is legal? Much thanks.

Answer 1

This is because the rightBarButtonItem was declared retained. (See the UINavigationItem Documentation)

When you declare a property as retained:

@property(nonatomic, retain) UIBarButtonItem *rightBarButtonItem

You are saying that when you’ll change its value aNavItem.rightBarButtonItem = aNewItem you will send a [aNewItem retain] message.

Therefore you can safely release it afterwards (as it is retained by the rightBarButtonItem)

For more information, I highly recommend you take a look (monthly just to get used to it until it becomes second nature) at the Apple Memory Management Documentation