> On 18 Nov 2016, at 16:40, Karl <raziel.im+swift-us...@gmail.com> wrote:
>
>
>> On 18 Nov 2016, at 13:05, Adrian Zubarev via swift-users
>> <swift-users@swift.org <mailto:swift-users@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
>>
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>> swift-users@swift.org <mailto:swift-users@swift.org>
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>> <https://lists.swift.org/mailman/listinfo/swift-users>
>
>
> This is kind of an invalid/unsafe design IMO; DocumentValues may escape the
> scope of the Document and the underlying storage may be deallocated.
>
> Instead, I’d recommend a function:
>
> func withDocumentValues<T>(_ invoke: (inout DocumentValues)->T) -> T {
> var view = DocumentValues(self)
> defer { _fixLifetime(view) }
Oops.. actually, I think this should be:
defer { _fixLifetime(self) }
> return invoke(&view)
> }
>
> (unfortunately, this isn’t completely safe because somebody could still copy
> the DocumentValues from their closure, the same way you can copy the pointer
> from String’s withCString, but that’s a limitation of Swift right now)
>
> CC: John McCall, because I read his suggestion in the thread about contiguous
> memory/borrowing that we could have a generalised @noescape. In this example,
> you would want the DocumentValues parameter in the closure to be @noescape.
>
> - Karl
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