Difference between using ObjectIdentifier() and '===' Operator

Question

Let\'s say I\'m implementing a root class in Swift, which I declare adopts the Equatable protocol (I want to be able to tell if an array of my type contains a g

Answer 1

There is no difference for class instance, see the following comments in ObjectIdentifier.swift:

  /// Creates an instance that uniquely identifies the given class instance.
  ///
  /// The following example creates an example class `A` and compares instances
  /// of the class using their object identifiers and the identical-to
  /// operator (`===`):
  ///
  ///     class IntegerRef {
  ///         let value: Int
  ///         init(_ value: Int) {
  ///             self.value = value
  ///         }
  ///     }
  ///
  ///     let x = IntegerRef(10)
  ///     let y = x
  ///
  ///     print(ObjectIdentifier(x) == ObjectIdentifier(y))
  ///     // Prints "true"
  ///     print(x === y)
  ///     // Prints "true"
  ///
  ///     let z = IntegerRef(10)
  ///     print(ObjectIdentifier(x) == ObjectIdentifier(z))
  ///     // Prints "false"
  ///     print(x === z)
  ///     // Prints "false"
  ///

It also becomes apparent from the implementation of == for ObjectIdentifier, which just compares the pointers to the object storage:

  public static func == (x: ObjectIdentifier, y: ObjectIdentifier) -> Bool {
    return Bool(Builtin.cmp_eq_RawPointer(x._value, y._value))
  }

which is what the === operator does as well:

public func === (lhs: AnyObject?, rhs: AnyObject?) -> Bool {
  switch (lhs, rhs) {
  case let (l?, r?):
    return Bool(Builtin.cmp_eq_RawPointer(
        Builtin.bridgeToRawPointer(Builtin.castToUnknownObject(l)),
        Builtin.bridgeToRawPointer(Builtin.castToUnknownObject(r))
      ))
  case (nil, nil):
    return true
  default:
    return false
  }
}

ObjectIdentifier conforms to Hashable, so it is useful if you want to implement that protocol for your class:

extension MyClass: Hashable {
    var hashValue: Int {
        return ObjectIdentifier(self).hashValue
    }
}

An object identifier can also be created for meta types (e.g. ObjectIdentifier(Float.self)) for which === is not defined.