How can I determine if a value in a Dictionary is uniquely referenced?

Using Swift
1

I have an object pool (that requires getting the objects out of a Dictionary for various reasons) and I need to determine if they're uniquely referenced when I get them out, or else they may still be in use. I've played around with lots of things like:

        if cache[id] == nil {
            let object = MyObject()
            cache[id] = object
            return object
        }
        unowned(unsafe) var object = cache[id]!
        if isKnownUniquelyReferenced(&object) {
            return object
        }

However, this always returns false for isKnownUniquelyReferenced, even when there don't seem to be any references besides that of the map's. Any ideas what to do?

2

Yes, that's documented:

isKnownUniquelyReferenced(_:) checks only for strong references to the given object—if object has additional weak or unowned references, the result may still be true . Because weak and unowned references cannot be the only reference to an object, passing a weak or unowned reference as object always results in false .

You can get the address of elements of a dictionary directly, so isKnownUniquelyReferenced(&cache[id]) should work.

EDIT: Works for me (in a compiled app; playgrounds are not reliable for reference counting):

class MyObject {}
var cache = [Int: MyObject]()
do {
  var obj = MyObject()
  if isKnownUniquelyReferenced(&obj) {
    print("1 - Unique")
  } else {
    print("1 - Not unique")
  }
  cache[3] = obj
}
if isKnownUniquelyReferenced(&cache[3]) {
 print("2 - Unique")
} else {
 print("2 - Not unique")
}
do {
  let obj = cache[3]
  DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(10), execute: { _ = obj })
  if isKnownUniquelyReferenced(&cache[3]) {
   print("3 - Unique")
  } else {
   print("3 - Not unique")
  }
}

prints:

1 - Unique
2 - Unique
3 - Not unique
3

It looks like I can store a Wrapper around the value to the dictionary, and then checking if the value is uniquely referenced will work.