private final class COWBoxImpl<T> : NonObjectiveCBase {
	var value: T
  init(_ value: T) {
    self.value = value
#if LOGGING
    print("new box: \(value)")
#endif
  }
}

public struct COWBox<T> {
	private var boxed: COWBoxImpl<T>

  init(_ value: T) {
    self.boxed = COWBoxImpl(value)
  }

	public var value: T {
		get { return boxed.value }
		set {
			if isUniquelyReferenced(&boxed) {
				boxed.value = newValue
			} else {
				boxed = COWBoxImpl(newValue)
			}
		}
    // FIXME: materializeForSet
	}
}

struct COWPoint {
  private struct Impl {
    var x: Double
    var y: Double
  }

  private var impl: COWBox<Impl>

  init(x: Double, y: Double) {
    self.impl = COWBox(Impl(x: x, y: y))
  }
  
  var x: Double {
    get { return impl.value.x }
    set { impl.value.x = newValue }
  }

  var y: Double {
    get { return impl.value.y }
    set { impl.value.y = newValue }
  }
}

// Testing
var pt = COWPoint(x: 0, y: 0)
pt.x = 10
var pt2 = pt
pt.x = 20
print(pt.x, pt2.x)