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.send { color:#77bb77; }
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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>This doesn’t make any sense.</p>
<p>Somewhere from the Swift book:</p>
<blockquote>
<p>Weak and unowned references enable one instance in a reference cycle to refer to the other instance <strong>without keeping a strong hold on it</strong>. The instances can then refer to each other without creating a strong reference cycle.</p>
</blockquote>
<p>From the sdlib of the function <code>isKnownUniquelyReferenced</code>:</p>
<blockquote>
<p>Returns a Boolean value indicating whether the given object is a class instance known to have <strong>a single strong reference</strong>.</p>
</blockquote>
<p><code>unowned</code> doesn’t increase the reference count, so the view in the examples I showed should have no strong reference to that class instance. The only strong reference that exists is from <code>Document</code>. So why exactly is the result of the second <code>isKnownUniquelyReferenced</code> call <code>false</code> again?</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_1479477213018533888" 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 14:50:57, Zhao Xin (<a href="mailto:owenzx@gmail.com">owenzx@gmail.com</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div><div></div><div>
<title></title>
<div dir="ltr">
<div class="gmail_default" style="font-family:georgia,serif">
><span style="font-family:arial,sans-serif;font-size:12.800000190734863px">Why
is the second check false, even if the property is marked
as </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);border:1px solid rgb(234,234,234);margin:0px 2px;padding:0px 5px;word-break:normal;word-wrap:normal">unowned</code><span style="font-family:arial,sans-serif;font-size:12.800000190734863px"> for
the view?</span></div>
<div class="gmail_default" style="font-family:georgia,serif">
<br></div>
<div class="gmail_default"><span style="font-size:12.800000190734863px">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`</span></div>
<div class="gmail_default"><span style="font-size:12.800000190734863px"><br></span></div>
<div class="gmail_default"><span style="font-size:12.800000190734863px">Zhaoxin</span></div>
<div class="gmail_default"><span style="font-size:12.800000190734863px"><br></span></div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Fri, Nov 18, 2016 at 8:05 PM, Adrian
Zubarev via swift-users <span dir="ltr"><<a href="mailto:swift-users@swift.org" target="_blank">swift-users@swift.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="word-wrap:break-word">
<div class="m_5359467609274298590bloop_markdown">
<p>Hi there,</p>
<p>I just can’t get my head around mutable views and COW.</p>
<p>Here is a small example:</p>
<pre><code class="m_5359467609274298590swift">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.<wbr>startIndex
}
public var endIndex: Int {
return self._storageReference.keys.<wbr>endIndex
}
public func index(after i: Int) -> Int {
return self._storageReference.keys.<wbr>index(after: i)
}
public subscript(position: Int) -> Int {
get { return _storageReference.values[<wbr>position] }
set { self._storageReference.values[<wbr>position] = newValue } // That will break COW
}
}
</code></pre>
<p>First of all the <code>_storageReference</code> property is
<code>unowned</code> because I wanted to check the following:</p>
<pre><code class="m_5359467609274298590swift">var document = Document()
print(CFGetRetainCount(<wbr>document._storageReference)) //=> 2
print(<wbr>isKnownUniquelyReferenced(&<wbr>document._storageReference)) // true
var values = document.values
print(CFGetRetainCount(values.<wbr>_storageReference)) //=> 2
print(<wbr>isKnownUniquelyReferenced(&<wbr>values._storageReference)) // false
</code></pre>
<p>Why is the second check false, even if the property is marked as
<code>unowned</code> for the view?</p>
<p>Next up, I don’t have an idea how to correctly COW optimize this
view. Assume the following scenario:</p>
<p>Scenario A:</p>
<pre><code class="m_5359467609274298590swift">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>VS:</p>
<p>Scenario B:</p>
<pre><code class="m_5359467609274298590swift">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>We could change the subscript setter on the mutable view like
this:</p>
<pre><code class="m_5359467609274298590swift">set {
if !isKnownUniquelyReferenced(&<wbr>self._storageReference) {
self._storageReference = ... // clone
}
self._storageReference.values[<wbr>position] = newValue
}
</code></pre>
<p>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
<code>unowned</code> the function
<code>isKnownUniquelyReferenced</code> will return false for
scenario A.</p>
<p>Any suggestions?</p>
<p>PS: In general I also wouldn’t want to use <code>unowned</code>
because the view should be able to outlive it’s parent.</p>
</div>
<div class="m_5359467609274298590bloop_original_html">
<div id="m_5359467609274298590bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px;color:rgba(0,0,0,1.0);margin:0px;line-height:auto">
<span class="HOEnZb"><font color="#888888"><br></font></span></div>
<span class="HOEnZb"><font color="#888888"><br></font></span>
<div id="m_5359467609274298590bloop_sign_1479469826202990080" class="m_5359467609274298590bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">
<span class="HOEnZb"><font color="#888888">-- <br>
Adrian Zubarev<br>
Sent with Airmail</font></span></div>
</div>
</div>
<div class="m_5359467609274298590bloop_markdown"></div>
</div>
<br>
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