Question

I have some doubts about unsafe_unretained and weak keywords: as a read they are quite the same and the only difference is that the weak one is set to null if the pointed object is released.

Now I the code below and it crash at point [#2] during the [instanceOfTheView setDelegate:self]

but if in I4vMainView declaration [#1] I substitute

@property (nonatomic, weak) id <I4vDraggingFileProt> delegate;

with

@property (nonatomic, unsafe_unretained) id <I4vDraggingFileProt> delegate;

it works perfectly. What is the reason for this behavior? Thanks

Details: Target 10.7 compiling with ARC. Xcode 4.5.2 . Apple LLVM 4.1

In the class I4vMainView I have:

//----------- I4vMainView.h --------
@protocol I4vDraggingFileProt <NSObject>
   -(void) anURLWasDeopped: (NSURL *) droppedUrl;
@end    

@interface I4vMainView : NSView <NSDraggingDestination>{
    NSImageCell *imageCell;
    NSImage * image;
}

@property (nonatomic, weak) id <I4vDraggingFileProt> delegate;  // [#1]

While in the caller

//----------- I4vViewController.h --------
@class I4vMainView;

@protocol I4vDraggingFileProt <NSObject>
    -(void) anURLWasDeopped: (NSURL *) droppedUrl;
@end

@interface I4vViewController : NSViewController <I4vDraggingFileProt>{
    I4vMainView * mv;
}

-(void) anURLWasDeopped: (NSURL *) droppedUrl;

@end


//----------- I4vViewController.m --------
@implementation I4vViewController
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Initialization code here.
    }
    return self;
}

- (void)loadView{
    mv = [[I4vMainView alloc] init];
    [mv setDelegate:self];    // <-- [#2]
    [self setView:mv];       
}

-(void) anURLWasDeopped: (NSURL *) droppedUrl{
    // ...
}

@end

Add:

With delegate declared as

@property (nonatomic, weak) id <I4vDraggingFileProt> delegate;

i have this Error

<I4vMainView: 0x10060bf40> objc[4773]: cannot form weak reference to instance (0x10061bf10) of class I4vViewController

and back-trace goes thoroughly _objc_trap <- objc_stroreWeak <- -[I4vMainView setDelegate:] <- [I4vViewController view]

Answer 1

I found the answer: as said in weak property for delegate cannot be formed

You cannot currently create zeroing-weak references to instances of the following classes (emphasis mine):

NSATSTypesetter, NSColorSpace, NSFont, NSFontManager, NSFontPanel, NSImage, NSMenuView, NSParagraphStyle, NSSimpleHorizontalTypesetter, NSTableCellView, NSTextView, NSViewController, NSWindow, and NSWindowController. In addition, in OS X no classes in the AV Foundation framework support weak references.

For declared properties, you should use assign instead of weak; for variables you should use __unsafe_unretained instead of __weak.

In addition, you cannot create weak references from instances of NSHashTable, NSMapTable, or NSPointerArray under ARC.