func isUniquelyReferenced<T : NonObjectiveCBase>(inout: T)

Returns true iff object is a non-@objc class instance with a single strong reference.

  • Does not modify object; the use of inout is an implementation artifact.
  • Weak references do not affect the result of this function.

Useful for implementing the copy-on-write optimization for the deep storage of value types:

mutating func modifyMe(arg: X) {
  if isUniquelyReferenced(&myStorage) {
  else {
    myStorage = myStorage.createModified(arg)

This function is safe to use for mutating functions in multithreaded code because a false positive would imply that there is already a user-level data race on the value being mutated.


func isUniquelyReferenced<T : NonObjectiveCBase>(inout object: T) -> Bool