Use copy-on-write semantics for large values

gistfile1.txt

final class Ref<T> {
  var val: T
  init(_ v: T) {val = v}
}

struct Box<T> {
    var ref: Ref<T>
    init(_ x: T) { ref = Ref(x) }

    var value: T {
        get { return ref.val }
        set {
          if !isKnownUniquelyReferenced(&ref) {
            ref = Ref(newValue)
            return
          }
          ref.val = newValue
        }
    }
}