anandabits

// This example shows how closed protocols can be emulated in Swift today.
// The technique leverages Swift's support for public (but not open) classes.

// First, it's worth observing that there is an almost trivial technique that can be used when
// it is possible to specify a (possibly abstract) superclass for all conforiming types.
// Simply declare a public (not open) base class and a protocol with a Self inheritance constraint.
// Swift does not support open subclasses of a public superclass so no classes outside the module
// will be able to meet the self inheritance constraint.
public class FooBase {}
public protocol Foo where Self: FooBase {}

dabrahams

extension ObjectIdentifier {
  /// Returns true if the object identified by `self` is uniquely referenced.
  ///
  /// - Requires: the object identified by `self` exists.
  /// - Note: will only work when called from a mutating method
  internal func _liveObjectIsUniquelyReferenced() -> Bool {
    var me = self
    return withUnsafeMutablePointer(to: &me) {
      $0.withMemoryRebound(to: AnyObject.self, capacity: 1) {
        _isUnique(&$0.pointee)