Question

I am really puzzled by this. I believe I am managing memory the correct way but executing the code suggests that I am double releasing the object. Here is the code and then I’ll explain what is happening.

@protocol SomeDelegate <NSObject>
@required
- (id)initWithCols:(NSUInteger)Cols Rows:(NSUInteger)Rows;
@end


@interface SomeObject : NSObject <SomeDelegate> {
}
- (id)initWithCols:(NSUInteger)Cols Rows:(NSUInteger)Rows;
@end


@interface Layout : UIView {
  id<SomeDelegate> someDelegate;
}
@property(retain) id<SomeDelegate> someDelegate;
- (id)initWithFrame:(CGRect)aRect Cols:(NSUInteger)Cols Rows:(NSUInteger)Rows;
@end


@implementation Layout
@synthesize someDelegate;
- (id)initWithFrame:(CGRect)aRect Cols:(NSUInteger)Cols Rows:(NSUInteger)Rows {

  if(self = [super initWithFrame:aRect]) {
    cols = Cols; 
    rows = Rows;
    id<SomeDelegate> delegate = [[SomeObject alloc] initWithCols:cols Rows:rows];
    [self setSomeDelegate:delegate];
    //[delegate release];
  }
  return self;
}

-(void)dealloc {
    [someDelegate release];
    [super dealloc];
}

@end

Now when I uncomment out the “//[delegate release];” line in the constructor of the Layout class, then I get a “EXC_BAD_ACCESS” error and the application crashes when it attempts to dealloc. I have traced the crash to the release of the someDelegate object in the dealloc method of Layout class. If I leave it commented then the application works fine.

Can someone please explain why this is happening as it appears to be going against everything I have read about memory management in Objective-C.

Just a note that the code example actually works, however my code doesn’t which follows the example. Could there be something inside of my actual SomeObject that is causing an autorelease?

Thanks in advance.

Answer 1

First, go back and reread the memory management rules just to make sure you are not missing anything obvious in your use of delegate elsewhere.

Next, turn on NSZombieEnabled (in your executable settings, Arguments panel, add an environment variable NSZombieEnabled set to YES).

Then add a dealloc method to your delagate if it does not have one already (make sure you call [super dealloc]!) and put a break point on there – that will tell you when your delagate is deallocated which will tell you when it is being released.

Alternatively, add trivial release/autorelease methods to your delegate class which do nothing but call through, and then breakpoint them and that will tell you exactly when it is being released.

Three final comments: in the standard naming convention for Objective C/Cocoa, you should have lowercase parameter fields, ie it should be:

- (id)initWithFrame:(CGRect)aRect cols:(NSUInteger)Cols rows:(NSUInteger)Rows;

When your ivar and property are named identically, it is very easy to accidently use the wrong one, so I recommend using a different ivar name and property name to avoid confusion, either use an _ prefix like Apple, or some other prefix to avoid confusion with Apple as well:

  id<SomeDelegate> _someDelegate;

@synthesize someDelegate = _someDelegate;

And Apple recomends against using setters/getters in init/dealloc, so your init code should be:

_someDelegate = [[SomeObject alloc] initWithCols:cols Rows:rows];