Question

I had a library designed for ARC and iOS 5, but a customer wanted to build for iOS 4.x. I converted properties with the weak qualifer to unsafe_unretained, however now it seems I am running into memory corruption-esq crashes.
In the case of the crash, I have a delegate property like this

@property (unsafe_unretained) id<MYDelegateProtocol> delegate;

and I @synthesize the ivar.

whenever I want to call a method on the delegate, I check if it is nil, and if it is not, then I call the method, since I have no optional methods in that protocol, I don’t check respondsToSelector.

However, since changing the weak to unsafe_unretained, I have found that apparently the reference goes away, and I am left with a dangling pointer. If I put a breakpoint on ObjC Exceptions, (for unrecognized message), and then po the address of the object, I see that it is an object that is not even implementing the protocol of the delegate, so of course it crashes.

When using unsafe_unretained how can I know that the reference has “gone away”? My code relied on the pointer being zeroed out before.

Answer 1

When using unsafe_unretained how can I know that the reference has
“gone away”? My code relied on the pointer being zeroed out before.

This has to be determined at compile time by you. Often, the object that is the delegate should set the delegate property of the object delegating to it to nil in the dealloc method. For example:

- (void)dealloc
{
    if (_httpRequest.delegate == self)
        _httpRequest.delegate = nil;
}

Hope this helps!