trouble with NSFastEnumeration protocol and ARC

Question

I'm having trouble implementing the NSFastEnumeration protocol while migrating Objective-C code to ARC. Could someone tell me, how to get rid of the following warnig (see code snippet)? Thanks in advance.

// I changed it due to ARC, was before
// - (NSUInteger) countByEnumeratingWithState: (NSFastEnumerationState*) state objects: (id*) stackbuf count: (NSUInteger) len
- (NSUInteger) countByEnumeratingWithState: (NSFastEnumerationState*) state objects: (__unsafe_unretained id *) stackbuf count: (NSUInteger) len
{
    ... 
    *stackbuf = [[ZBarSymbol alloc] initWithSymbol: sym]; //Warning: Assigning retained object to unsafe_unretained variable; object will be released after assignment
    ... 
}

- (id) initWithSymbol: (const zbar_symbol_t*) sym
{
    if(self = [super init]) {
        ... 
    }
    return(self);
}

Answer 1

With ARC, something has to hold a strong or autoreleasing reference to each object, otherwise it will be released (just as the warning says). Because stackbuf is __unsafe_unretained, it's not going to hang onto the ZBarSymbol for you.

If you create a temporary autoreleasing variable and stash your object there, it will live until the current autorelease pool is popped. You can then point stackbuf to it without complaint.

ZBarSymbol * __autoreleasing tmp = [[ZBarSymbol alloc] initWithSymbol: sym];
*stackbuf = tmp;