[swift-evolution] [Pitch] Reference storage for enum associated values

[Joe Groff]

Another way to address this might be by allowing property behaviors, a feature proposed in https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md to allow for factoring out property implementations, could also apply to `case` declarations. "Weak" and "unowned" could be considered to be property behaviors once we have them, so you'd write something like:

class Foo {
  var parent: Foo?
    behavior weak
}

to define a property with "weak" behavior. We could potentially allow you to apply behaviors to 'case' associated values too:

enum ParentChild {
  case Parent(Parent) behavior unowned
  case Child(Parent) behavior unowned
}

-Joe

> On May 3, 2016, at 8:07 AM, Marc Prud'hommeaux via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> The following code currently has a retain cycle and memory leak:
> 
> enum ParentChild {
>     case SonOf(Parent)
>     case DaughterOf(Parent)
> }
> 
> class Parent {
>     lazy var son: Child = Child(parentChild: .SonOf(self))
>     lazy var daughter: Child = Child(parentChild: .DaughterOf(self))
>     deinit { print("deinit Parent") }
> }
> 
> class Child {
>     var parentChild: ParentChild
>     init(parentChild: ParentChild) {
>         self.parentChild = parentChild
>     }
>     deinit { print("deinit Child") }
> }
> 
> 
> do {
>     let parent = Parent()
>     parent.son
>     parent.daughter
> }
> 
> 
> Child.parentChild cannot be declared unowned because ParentChild is a value type. I propose adding the ability to declare the reference storage class for an enum's associated value, like so:
> 
> enum ParentChild {
>     case SonOf(unowned Parent)
>     case DaughterOf(unowned Parent)
> }
> 
> The only current alternative is to have some intermediate reference type that itself holds the reference, akin to the old "Box" type that we used to use to work around enum limitations. E.g., this is our current cumbersome work-around:
> 
> /// An unowned reference to a value, which is useful for maintaining parent-child relations through value types like enums
> public struct UnownedRef<T: AnyObject> {
>     public unowned let value: T
>     public init(_ value: T) { self.value = value }
> }
> 
> enum ParentChild {
>     case SonOf(UnownedRef<Parent>)
>     case DaughterOf(UnownedRef<Parent>)
> }
> 
> 
> class Parent {
>     lazy var son: Child = Child(parentChild: .SonOf(UnownedRef(self)))
>     lazy var daughter: Child = Child(parentChild: .DaughterOf(UnownedRef(self)))
>     deinit { print("deinit Foo") }
> }
> 
> class Child {
>     var parentChild: ParentChild
>     init(parentChild: ParentChild) {
>         self.parentChild = parentChild
>     }
> 
>     deinit { print("deinit Child") }
> }
> 
> The storage type of an enum would, of course, be limited to reference types, and when the storage class is weak, it would require that the stored type be Optional, just like when declaring a weak variable.
> 
> What do people think?
> 
> 	-Marc
> 
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