iOS 5 Programming Cookbook

1.18. Typecasting with Automatic Reference Counting

Problem

You want to know how to use new typecasting facilities under Automatic Reference Counting in order to avoid memory leaks when working with Core Foundation objects inside your Objective-C code.

Solution

Use the __bridge, __bridge_transfer, and __bridge_retained typecasting specifiers.

Discussion

Typecasting is the process of pointing one value of type A to another value of type B. For instance, if you have a Core Foundation string object of type CFStringRef and you would like to place it inside an Objective-C string of type NSString, you can easily create an error:

- (BOOL)            application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{

  CFStringRef coreFoundationString =
  CFStringCreateWithCString(CFAllocatorGetDefault(),
                            "C String",
                            kCFStringEncodingUTF8);

  /* Compile time error!!! */
  NSString *objCString = coreFoundationString;

  self.window = [[UIWindow alloc] initWithFrame:
                 [[UIScreen mainScreen] bounds]];

  self.window.backgroundColor = [UIColor whiteColor];
  [self.window makeKeyAndVisible];
  return YES;
}

Here we are assigning the value of the Core Foundation string coreFoundationString to the Objective-C string of type NSString named objCString, our compiler will get confused because it doesn’t know what we are intending to do with the memory assigned to each one of these objects. Additionally, we will end up with a memory leak because the compiler doesn’t know how to get rid of the Core Foundation ...