Why does 'self' protect memory space?

Question

In a navigation based app, I initialize an array in my app delegate without using self. When accessed in the RootViewController's cellForRowAtIndexPath:, all array objects are there and I can see it is an NSCFArray. Once the app loads, I click a table cell and in didSelectRowAtIndexPath:, that same array has a type of NSArray, no objects and I get a EXC_ BAD_ACCESS error. If I precede the array with self in the app delegate, all is fine. Why is that?

Below is the app delegate .h file:

@interface MyAppDelegate : NSObject <UIApplicationDelegate> {

  UIWindow *window;
  UINavigationController *navigationController;
  NSMutableDictionary *aDict;
  NSArray *aArray;
}

@property (nonatomic, retain) IBOutlet UIWindow *window;
@property (nonatomic, retain) IBOutlet UINavigationController *navigationController;
@property (nonatomic, retain) NSArray *aArray;
@property (nonatomic, retain) NSArray *aDict
@end

The array and dictionary are synthesized in the .m file. I initialize the array in the app delegate .m file like this:

aArray = [self.aDict allKeys];

It is accessed like this in both root controller methods:

theDelegate = [[UIApplication sharedApplication] delegate]; 
[theDelegate.aArray objectAtIndex:2];

Only when I reach didSelectRowAtIndexPath: does it fail. Doing this in the app delegate makes everything work:

self.aArray = [self.aDict allKeys];

I didn't do anything to myArray between cellForRowAtIndexPath: and didSelectRowAtIndexPath:. Why does it fail in the first scenario?

Answer 1

That is because self.aArray makes use of the synthesized property. You defined the property as (nonatomic, retain), this means that whatever it is set to, that value will be retained. What was happening when you didn't use this was that the allKeys was being auto-released, and by the time you actually used it, it was gone. Using the property retained that value, making it live past its scope.

Hope that made sense, sorry if I misunderstood you.

Further reading:

• Apple Docs - Memory Management Rules
• Apple Docs - Declared Properties
• Apple Docs - NSAutoreleasePool
• Question Regarding How NSAutoreleasePool Works

I'm sure there are more reader friendly articles out there, but now you know what to look up: Objective-C memory management, and Objective-C properties. It's pretty important so I recommend you do this.