[swift-users] Problem with mutable views and COW

[Zhao Xin]

>Why is the second check false, even if the property is marked as unowned for
the view?

Please search the mailing list, this is not the first time it comes as a
new question. Shortly speaking, it is `false` only because you used
`unowned`. If you you can grantee it always exists. Just use it directly,
this is what `unowned` for. If you can't grantee that. You should use
`weak` and check it with `if let` or `if foo == nil`

Zhaoxin


On Fri, Nov 18, 2016 at 8:05 PM, Adrian Zubarev via swift-users <
swift-users at swift.org> wrote:

> Hi there,
>
> I just can’t get my head around mutable views and COW.
>
> Here is a small example:
>
> final class Storage {
>
>     var keys: [String] = []
>     var values: [Int] = []
> }
>
> public struct Document {
>
>     var _storageReference: Storage
>
>     public init() {
>
>         self._storageReference = Storage()
>     }
>
>     public init(_ values: DocumentValues) {
>
>         self._storageReference = values._storageReference
>     }
>
>     public var values: DocumentValues {
>
>         get { return DocumentValues(self) }
>
>         set { self = Document(newValue) }
>     }
> }
>
> public struct DocumentValues : MutableCollection {
>
>     unowned var _storageReference: Storage
>
>     init(_ document: Document) {
>
>         self._storageReference = document._storageReference
>     }
>
>     public var startIndex: Int {
>
>         return self._storageReference.keys.startIndex
>     }
>
>     public var endIndex: Int {
>
>         return self._storageReference.keys.endIndex
>     }
>
>     public func index(after i: Int) -> Int {
>
>         return self._storageReference.keys.index(after: i)
>     }
>
>     public subscript(position: Int) -> Int {
>
>         get { return _storageReference.values[position] }
>
>         set { self._storageReference.values[position] = newValue } // That will break COW
>     }
> }
>
> First of all the _storageReference property is unowned because I wanted
> to check the following:
>
> var document = Document()
>
> print(CFGetRetainCount(document._storageReference)) //=> 2
> print(isKnownUniquelyReferenced(&document._storageReference)) // true
>
> var values = document.values
>
> print(CFGetRetainCount(values._storageReference)) //=> 2
> print(isKnownUniquelyReferenced(&values._storageReference)) // false
>
> Why is the second check false, even if the property is marked as unowned
> for the view?
>
> Next up, I don’t have an idea how to correctly COW optimize this view.
> Assume the following scenario:
>
> Scenario A:
>
> var document = Document()
>
> // just assume we already added some values and can mutate safely on a given index
> // mutation in place
> document.values[0] = 10
>
> VS:
>
> Scenario B:
>
> var document = Document()
>
> let copy = document
>
> // just assume we already added some values and can mutate safely on a given index
> // mutation in place
> document.values[0] = 10 // <--- this should only mutate `document` but not `copy`
>
> We could change the subscript setter on the mutable view like this:
>
> set {
>
>     if !isKnownUniquelyReferenced(&self._storageReference) {
>
>         self._storageReference = ... // clone
>     }
>     self._storageReference.values[position] = newValue
> }
>
> There is only one problem here. We’d end up cloning the storage every
> time, because as shown in the very first example, even with unowned the
> function isKnownUniquelyReferenced will return false for scenario A.
>
> Any suggestions?
>
> PS: In general I also wouldn’t want to use unowned because the view
> should be able to outlive it’s parent.
>
>
>
> --
> Adrian Zubarev
> Sent with Airmail
>
>
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>
>
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