iOS Xcode 4 properties access

Question

I switched to Xcode 4 recently and I don't really understand this new way to write accessors. For example, in the application delegate class that is auto-generated when creating a new project, the window object is not declared in the @interface but just this way:

@property (nonatomic, retain) IBOutlet UIWindow *window;

Then, in the implementation file, we have the @synthesize window=_window;. And in the functions, we have either self.window OR _window.

For example:

[self.window makeKeyAndVisible]; // in didFinishLaunchingWithOptions function
[_window release]; // in dealloc function

Can you explain me the difference, why there is nothing in the @interface, why we do @synthesize window=_window; instead of @synthesize window; and what is the difference between self.window and _window, I mean when do I have to call one more than the other?

I'm a bit lost, and feel like the new code I doing trying to do the same in not working properly...

Thanks!

Answer 1

1. "Why is there nothing in the @interface"

   The runtime is synthesizing the ivar for you.

2. "Why do we do @synthesize window=_window;

   This means that the window property will use an ivar named _window (by default the ivar name is the name of the property)

3. "What is the difference between self.window and _window?"

   The former is using the window "getter" method (ie, foo = [self window]), and the latter is accessing the ivar directly.

4. "Why do I have to call one more than the other?"

   It is generally considered unsafe to use accessor methods in your dealloc method, which means using the ivar is preferred.