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.error { color:#AA0000; }</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="bloop_markdown"><p>What exactly to you mean by unsafe/invalide? If you refer to <code>unowned</code> than yes this is incorrect. That was only part of the first question about <code>isKnownUniquelyReferenced</code>, where I myself missed the part from the docs that says that weak references don’t affect the result.</p>
<p>In general <code>DocumentValues</code> has a strong reference to the storage and should be able to outlive <code>Document</code>.</p>
<p>I borrowed the main idea from:</p>
<ul>
<li>proposal: <a href="https://github.com/natecook1000/swift-evolution/blob/nc-dictionary-collections/proposals/0000-dictionary-key-and-value-collections.md">https://github.com/natecook1000/swift-evolution/blob/nc-dictionary-collections/proposals/0000-dictionary-key-and-value-collections.md</a></li>
<li>implementation: <a href="https://github.com/apple/swift/compare/master...natecook1000:nc-dictionary">https://github.com/apple/swift/compare/master…natecook1000:nc-dictionary</a></li>
</ul>
<p>But it uses some buildin stuff that I don’t have. :/</p>
<p></p></div><div class="bloop_original_html"><style>body{font-family:Helvetica,Arial;font-size:13px}</style><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div> <br> <div id="bloop_sign_1479483851703318016" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br><p class="airmail_on">Am 18. November 2016 um 16:43:40, Karl (<a href="mailto:razielim@gmail.com">razielim@gmail.com</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div></div><div>
<title></title>
<br class="">
<div>
<blockquote type="cite" class="">
<div class="">On 18 Nov 2016, at 16:40, Karl <<a href="mailto:raziel.im+swift-users@gmail.com" class="">raziel.im+swift-users@gmail.com</a>> wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class="">
<div class="">
<blockquote type="cite" class="">
<div class="">On 18 Nov 2016, at 13:05, Adrian Zubarev via
swift-users <<a href="mailto:swift-users@swift.org" class="">swift-users@swift.org</a>> wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div class="bloop_markdown" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(254, 254, 254);">
<p style="margin: 15px 0px; -webkit-margin-before: 0px;" class="">
Hi there,</p>
<p style="margin: 15px 0px;" class="">I just can’t get my head
around mutable views and COW.</p>
<p style="margin: 15px 0px;" class="">Here is a small example:</p>
<pre style="margin: 15px 0px; font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(204, 204, 204); overflow: auto; padding: 4px 8px; word-break: normal; word-wrap: normal;" class=""><code class="swift" style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 0px; margin: 0px; padding: 0px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;">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
}
}
</code></pre>
<p style="margin: 15px 0px;" class="">First of all the<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">_storageReference</code><span class="Apple-converted-space"> </span>property is<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal;" class="">unowned</code><span class="Apple-converted-space"> </span>because I wanted to check the
following:</p>
<pre style="margin: 15px 0px; font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(204, 204, 204); overflow: auto; padding: 4px 8px; word-break: normal; word-wrap: normal;" class=""><code class="swift" style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 0px; margin: 0px; padding: 0px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;">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
</code></pre>
<p style="margin: 15px 0px;" class="">Why is the second check
false, even if the property is marked as<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">unowned</code><span class="Apple-converted-space"> </span>for the view?</p>
<p style="margin: 15px 0px;" class="">Next up, I don’t have an idea
how to correctly COW optimize this view. Assume the following
scenario:</p>
<p style="margin: 15px 0px;" class="">Scenario A:</p>
<pre style="margin: 15px 0px; font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(204, 204, 204); overflow: auto; padding: 4px 8px; word-break: normal; word-wrap: normal;" class=""><code class="swift" style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 0px; margin: 0px; padding: 0px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;">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
</code></pre>
<p style="margin: 15px 0px;" class="">VS:</p>
<p style="margin: 15px 0px;" class="">Scenario B:</p>
<pre style="margin: 15px 0px; font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(204, 204, 204); overflow: auto; padding: 4px 8px; word-break: normal; word-wrap: normal;" class=""><code class="swift" style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 0px; margin: 0px; padding: 0px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;">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`
</code></pre>
<p style="margin: 15px 0px;" class="">We could change the subscript
setter on the mutable view like this:</p>
<pre style="margin: 15px 0px; font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(204, 204, 204); overflow: auto; padding: 4px 8px; word-break: normal; word-wrap: normal;" class=""><code class="swift" style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 0px; margin: 0px; padding: 0px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;">set {
if !isKnownUniquelyReferenced(&self._storageReference) {
self._storageReference = ... // clone
}
self._storageReference.values[position] = newValue
}
</code></pre>
<p style="margin: 15px 0px;" class="">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<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">unowned</code><span class="Apple-converted-space"> </span>the function<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal;" class="">isKnownUniquelyReferenced</code><span class="Apple-converted-space"> </span>will return false for scenario
A.</p>
<p style="margin: 15px 0px;" class="">Any suggestions?<span class="Apple-converted-space"> </span></p>
<p style="margin: 15px 0px;" class="">PS: In general I also
wouldn’t want to use<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">unowned</code><span class="Apple-converted-space"> </span>because the view should be
able to outlive it’s parent.</p>
<div style="margin: 15px 0px;" class=""><br class="webkit-block-placeholder"></div>
</div>
<div class="bloop_original_html" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(254, 254, 254);">
<div id="bloop_customfont" style="font-family: Helvetica, Arial; font-size: 13px; margin: 0px;" class=""><br class=""></div>
<br class="">
<div id="bloop_sign_1479469826202990080" class="bloop_sign">
<div style="font-family: helvetica, arial; font-size: 13px;" class="">-- <br class="">
Adrian Zubarev<br class="">
Sent with Airmail</div>
</div>
</div>
<div class="bloop_markdown" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(254, 254, 254);">
<div style="margin: 15px 0px; -webkit-margin-before: 0px;" class=""><br class="webkit-block-placeholder"></div>
</div>
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</blockquote>
</div>
<br class="">
<div class=""><br class=""></div>
<div class="">This is kind of an invalid/unsafe design IMO;
DocumentValues may escape the scope of the Document and the
underlying storage may be deallocated.</div>
<div class=""><br class=""></div>
<div class="">Instead, I’d recommend a function:</div>
<div class=""><br class=""></div>
<blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class="">
<div class=""><font face="Courier" class="">func
withDocumentValues<T>(_ invoke: (inout DocumentValues)->T)
-> T {</font></div>
<div class=""><font face="Courier" class="">var view =
DocumentValues(self)</font></div>
<div class=""><font face="Courier" class="">
defer { _fixLifetime(view) }</font></div>
<div class=""></div>
</blockquote>
</div>
</div>
</blockquote>
<div><br class=""></div>
Oops.. actually, I think this should be:</div>
<div>
<div class="" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">
<blockquote class="" style="margin: 0px 0px 0px 40px; border: none; padding: 0px;">
<div class=""><font face="Courier" class="">
defer { _fixLifetime(self) }</font></div>
<div class=""><font face="Courier" class=""><br class=""></font></div>
</blockquote>
</div>
<blockquote type="cite" class="">
<div class="">
<div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">
<blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class="">
<div class=""><font face="Courier" class="">
return invoke(&view)</font></div>
<div class=""><font face="Courier" class="">}</font></div>
<div class=""><font face="Courier" class=""><br class=""></font></div>
</blockquote>
(unfortunately, this isn’t completely safe because somebody could
still copy the <font face="Courier" class="">DocumentValues</font>
from their closure, the same way you can copy the pointer from
String’s <font face="Courier" class="">withCString</font>, but
that’s a limitation of Swift right now)
<div class=""><br class=""></div>
<div class="">CC: John McCall, because I read his suggestion in the
thread about contiguous memory/borrowing that we could have a
generalised <font face="Courier" class="">@noescape</font>. In this
example, you would want the <font face="Courier" class="">DocumentValues</font> parameter in the closure to be <font face="Courier" class="">@noescape</font>.</div>
<div class="">
<div class="">
<div class="">
<div class=""><br class=""></div>
<div class="">- Karl</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<br class="">
</div></div></span></blockquote></div><div class="bloop_markdown"><p></p></div></body></html>