iOS SDK : Debugging (part 3) - NSZombieEnabled

Create a new View-based Application named Zombie.

Create a new Objective-C class called FooBar.

Create one method called helloThere (Listing 1). Don’t forget to put the method’s signature in FooBar’s interface (Listing 2).

Listing 1. FooBar.m

#import "FooBar.h"
@implementation FooBar
-(void) sayHello {
 NSLog(@"Hello there...");
}
-(void) dealloc {
[super dealloc];
}
@end

Listing 2. FooBar.h

@interface FooBar : NSObject {
}
-(void) sayHello;
@end

Modify application:didFinishLaunchingWithOptions in ZombieAppDelegate (Listing 3). Don’t forget to import FooBar.

Listing 3. ZombieAppDelegate.m

#import "ZombieAppDelegate.h"
#import "ZombieViewController.h"
#import "FooBar.h"
@implementation ZombieAppDelegate
@synthesize window;
@synthesize viewController;
- (BOOL)application:(UIApplication *)application
           didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    FooBar * myFooBar = [[FooBar alloc] init];
    NSMutableArray * myArray = [[NSMutableArray alloc]
           initWithObjects:myFooBar,nil];
    [myFooBar dealloc];
    [[myArray objectAtIndex:0] sayHello];

    [window addSubview:viewController.view];
    [window makeKeyAndVisible];
    return YES;
}

- (void)applicationWillTerminate:(UIApplication *)application {
    // Save data if appropriate.
}

-(void)dealloc {
  [viewController release];
  [window release];
  [super dealloc];
}
@end


					  

Click the Run button to build and debug the application. Notice the first time you run the application you may not get an error at all (even though we sent a message to a deallocated object). This is part of what makes finding and fixing errors like this so hard—if the memory didn’t happen to get used for anything else in the meantime, it might remain valid for an unpredictable amount of time. Even if you do get an error message, it is not all that descriptive (Listing 4). The debugger only knows the sayHello message was sent to an object already freed. The debugger doesn’t know the object’s identity. To change this, you must enable zombies.

Listing 4. Debugger Console error logging when zombies are not enabled

Attaching to program: '/Users/bward/Library/Application Support/iPhone
Simulator/User/Applications/05688B53-AF22-4F21
-95D3AFFE2682A6EA/Zombie.app/Zombie', process 1351.
objc[1351]: FREED(id): message sayHello sent to freed object=0x521bd0
Program received signal: "EXC_BAD_INSTRUCTION".

In the pull-down menu next to the Run button, select Edit Active Scheme. Select Launch in the left column and then click the Arguments tab. This should let you edit the arguments passed to the app on launch (Figure 11).

Figure 11. The Executable Info window

Click the + and add the NSZombieEnabled variable to the environment variable list. Assign it the value YES and click OK.

Run the app again. Now the error message is more descriptive (Listing 5).