[swift-users] Checking whether an object is uniquely referenced, using a weak reference to it

[Andrew Trick]

> On Jun 28, 2016, at 9:32 AM, Jordan Rose via swift-users <swift-users at swift.org> wrote:
> 
> 
>> On Jun 27, 2016, at 18:52, Tim Vermeulen <tvermeulen at me.com> wrote:
>> 
>> Thanks for your reply. It didn’t clear up everything, though. The official documentation says "Weak references do not affect the result of this function.”, which suggests that weak (and unowned) references intentionally aren’t counted. The docs only mention the implementation of copy-on-write behaviour as a use case (which also happens to be what I’m using it for).
> 
> I would expect that weak references are important to count for COW, since you can observe changes through them. Dave?
> 
> 
>> 
>> Couldn’t there just a be a function that returns the reference count of a given object as an Int? It would make everything a lot easier (i.e. it wouldn’t need inout because it can just create a reference to that object, find the reference count, then subtract 1).
> 
> As we’ve said for a long time in Objective-C, asking for the reference count of an object is meaningless. isUniquelyReferenced only works because it’s conservative: because it only checks for “exactly 1”, it’s safe from threading issues and autorelease pools. We do not plan to add a -retainCount equivalent to Swift.
> 
> Jordan

Specifically, isUniquelyReferenced guarantees “exactly 1” if it returns ‘true’. It does not guarantee ‘> 1’ by returning ‘false’. In other words, the implementation reserves the right to return ‘false’ whenever it feels like it. The user can’t directly control the number of local references that exist. So, although it’s critical for optimization, you shouldn’t build any visible program behavior around isUniquelyReferenced.

I don’t have a good answer on the weak reference semantics, but I *suspect* that considering weak references in the count is neither feasible for objc-style weak references, nor useful since we don’t expose weak references to CoW storage.

-Andy