Weak object in an NSDictionary?

问题:

I would like to store a zeroing weak reference to an object in a NSDictionary. This is for a reference to a parent NSDictionary, so I can crawl back up a large structure without searching.

I can not use __weak here; even if my local reference is weak, the NSDictionary will store a strong reference to the object that was weakly referenced. And, of course, NSDictionary can't have nil objects.

I'm on iOS, not Mac, so NSHashTable isn't available. And I only want one object to be weak; the rest should still be strong.

(I'm going to post my answer, so I have something to mark as accepted if there's no better answer. But I'm hoping someone has a better answer.)

回答1:

In iOS 6+ you can use NSMapTable and choose if you want objects and/or keys to be weak or strong.

回答2:

I've settled on defining a "container" class with a single member, like this:

@interface Parent : NSObject
@property (nonatomic, weak) id parent;
@end

@implementation Parent

@synthesize parent = _parent;

- (id)initWithParent: (id)parent;
{
    if (( self = [super init] )) {
        _parent = parent;
    }
    return self;
}
@end

And using it:

id parentRef = [[Parent alloc] initWithParent: parent];
[newElement setObject: parentRef forKey: ParentKey];

I think this will work for me, but it seems crazy that there's no better way built in to Foundation.

回答3:

There is a built-in way; just use:

[array addObject:[NSValue valueWithNonretainedObject:object]];

Then to access the object later, use:

id object = [[array objectAtIndex:i] nonretainedObjectValue];

If the value has been released since you added it then "object" will be an invalid pointer under iOS 4.x, and (I'm assuming) will be nil under 5.x with ARC enabled.

EDIT: my assumption was wrong - it's not weak under ARC (see discussion below).