使用弱引用时是否可以实现NSFastEnumeration？

Question

I have a collection which maintains weak references to its objects. I'd like it to conform to NSFastEnumeration , but the buffer provided by countByEnumeratingWithState:objects:count: uses unsafe_unretained references. That creates a gap during which a returned reference could become invalid but not zeroed.

That's fine in the general case -- if the collection stuffs its (currently valid but weakly-referenced) object into the buffer and returns it, then the caller will presumably create its own strong reference if needed. But that leaves two problems:

(1) I don't see any guarantee that the for(){} iteration construct itself creates a temporary strong reference to the object, so if the contents of the {x} block changes something outside the collection in a way that causes the object to be released, then it'll have a dangling reference.

(2) There's still a small gap while returning from countByEnumeratingWithState: during which activity on another thread could invalidate the reference. My collection isn't meant to be thread-safe, but it would be nice if it could at least safely store references to objects which could be referenced on another thread, as there's really no way to prevent that in any multi-threaded application.

Answer 1

You can't return a strong reference directly to the caller. The caller won't release it, and the fast enumeration protocol does not guarantee that you will get a chance to release it yourself when the caller is done.

Instead you can retain+autorelease the objects before you store them into the buffer. That would guarantee the objects stay alive while the caller uses them. It may hurt the "fast" part of fast enumeration, but you would still get the "convenient syntax" part. If you add a nil check after you read the weak variable then you can avoid storing nil pointers into the buffer.