[swift-dev] COW for non-mutating methods

Patrick Pijnappel patrickpijnappel at gmail.com
Thu Dec 1 05:29:09 CST 2016

I'm implementing a COW big int type but am running into problems with
non-mutating functions (e.g. the + operator). Simplified example code below
shows AFAIK the default way to implement COW, but the non-mutating method
doesn't identify the reference as unique (even with -O), resulting in a
needless copy.

I've tried everything I could think of, but only inout parameters seem to
work. How does the standard library do this, for e.g. String + String?

struct Foo {

  var storage = Storage()

  class Storage { var x = 0 }

  init(_ x: Int) { storage.x = x }

  mutating func negate() {

    if !isKnownUniquelyReferenced(&storage) {



    storage.x = -storage.x


  func negated() -> Foo {

    var result = self // This counts as a second reference


    return result



func test() {

  var a = Foo(5)



  let b = Foo(5)

  let c = b.negated()







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