<div dir="ltr"><div class="gmail_default" style="font-family:georgia,serif"><p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">protocol</span><span style="font-variant-ligatures:no-common-ligatures"> Copyable {</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">func</span><span style="font-variant-ligatures:no-common-ligatures"> copy() -> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(112,61,170)">Self</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures">}</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"></span><br></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">final</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">class</span><span style="font-variant-ligatures:no-common-ligatures"> Storage:</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">Copyable</span><span style="font-variant-ligatures:no-common-ligatures"> {</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">var</span><span style="font-variant-ligatures:no-common-ligatures"> keys: [</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(112,61,170)">String</span><span style="font-variant-ligatures:no-common-ligatures">] = []</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">var</span><span style="font-variant-ligatures:no-common-ligatures"> values: [</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(112,61,170)">Int</span><span style="font-variant-ligatures:no-common-ligatures">] = []</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">func</span><span style="font-variant-ligatures:no-common-ligatures"> copy() -> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">Storage</span><span style="font-variant-ligatures:no-common-ligatures"> {</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">let</span><span style="font-variant-ligatures:no-common-ligatures"> s = </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">Storage</span><span style="font-variant-ligatures:no-common-ligatures">()</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> s.</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">keys</span><span style="font-variant-ligatures:no-common-ligatures"> = </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">keys</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> s.</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">values</span><span style="font-variant-ligatures:no-common-ligatures"> = </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">values</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">return</span><span style="font-variant-ligatures:no-common-ligatures"> s</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> }</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures">}</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"></span><br></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">public</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">struct</span><span style="font-variant-ligatures:no-common-ligatures"> Document:</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">Copyable</span><span style="font-variant-ligatures:no-common-ligatures"> {</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">var</span><span style="font-variant-ligatures:no-common-ligatures"> _storageReference: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">Storage</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(186,45,162)"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)"> </span><span style="font-variant-ligatures:no-common-ligatures">public</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)"> </span><span style="font-variant-ligatures:no-common-ligatures">init</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)">() {</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(79,129,135)"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">self</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)">.</span><span style="font-variant-ligatures:no-common-ligatures">_storageReference</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)"> = </span><span style="font-variant-ligatures:no-common-ligatures">Storage</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)">()</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> }</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">public</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">init</span><span style="font-variant-ligatures:no-common-ligatures">(</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">_</span><span style="font-variant-ligatures:no-common-ligatures"> values: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">DocumentValues</span><span style="font-variant-ligatures:no-common-ligatures">) {</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(79,129,135)"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">self</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)">.</span><span style="font-variant-ligatures:no-common-ligatures">_storageReference</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)"> = values.</span><span style="font-variant-ligatures:no-common-ligatures">_storageReference</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> }</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">public</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">var</span><span style="font-variant-ligatures:no-common-ligatures"> values: </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">DocumentValues</span><span style="font-variant-ligatures:no-common-ligatures"> {</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">get</span><span style="font-variant-ligatures:no-common-ligatures"> { </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">return</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">DocumentValues</span><span style="font-variant-ligatures:no-common-ligatures">(</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">self</span><span style="font-variant-ligatures:no-common-ligatures">) }</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">set</span><span style="font-variant-ligatures:no-common-ligatures"> { </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">self</span><span style="font-variant-ligatures:no-common-ligatures"> = </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">Document</span><span style="font-variant-ligatures:no-common-ligatures">(newValue) }</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> }</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">public</span><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">func</span><span style="font-variant-ligatures:no-common-ligatures"> copy() -> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">Document</span><span style="font-variant-ligatures:no-common-ligatures"> {</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">var</span><span style="font-variant-ligatures:no-common-ligatures"> d = </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">Document</span><span style="font-variant-ligatures:no-common-ligatures">()</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(79,129,135)"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)"> d.</span><span style="font-variant-ligatures:no-common-ligatures">_storageReference</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)"> = </span><span style="font-variant-ligatures:no-common-ligatures">_storageReference</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)">.</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(49,89,93)">copy</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)">()</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"> </span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">return</span><span style="font-variant-ligatures:no-common-ligatures"> d</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"> }</span></p>
<p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures">}</span></p><p style="margin:0px;font-size:11px;line-height:normal;font-family:menlo;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures"><br></span></p></div><div class="gmail_default"><p style="font-family:menlo;margin:0px;font-size:11px;line-height:normal;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">var</span><span style="font-variant-ligatures:no-common-ligatures"> document = </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">Document</span><span style="font-variant-ligatures:no-common-ligatures">()</span></p>
<p style="font-family:menlo;margin:0px;font-size:11px;line-height:normal;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"></span><br></p>
<p style="font-family:menlo;margin:0px;font-size:11px;line-height:normal;color:rgb(0,0,0)"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(186,45,162)">let</span><span style="font-variant-ligatures:no-common-ligatures"> copy = document.</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(49,89,93)">copy</span><span style="font-variant-ligatures:no-common-ligatures">()</span></p>
<p style="font-family:menlo;margin:0px;font-size:11px;line-height:normal;color:rgb(0,0,0);min-height:13px"><span style="font-variant-ligatures:no-common-ligatures"></span><br></p>
<p style="font-family:menlo;margin:0px;font-size:11px;line-height:normal;color:rgb(0,132,0)"><span style="font-variant-ligatures:no-common-ligatures">// just assume we already added some values and can mutate safely on a given index</span></p>
<p style="font-family:menlo;margin:0px;font-size:11px;line-height:normal;color:rgb(0,132,0)"><span style="font-variant-ligatures:no-common-ligatures">// mutation in place</span></p>
<p style="font-family:menlo;margin:0px;font-size:11px;line-height:normal;color:rgb(0,132,0)"><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)">document.</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(79,129,135)">values</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(49,89,93)">[</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(39,42,216)">0</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(49,89,93)">]</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)"> = </span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(39,42,216)">10</span><span style="font-variant-ligatures:no-common-ligatures;color:rgb(0,0,0)"> </span><span style="font-variant-ligatures:no-common-ligatures">// <--- this will only mutate `document` but not `copy`</span></p><p style="margin:0px;line-height:normal"><span style="font-variant-ligatures:no-common-ligatures"><font face="georgia, serif" color="#000000"><br></font></span></p><p style="margin:0px;line-height:normal"><font color="#000000" face="georgia, serif">You use `class` property inside `struct`, so the `struct` is no longer copyable. You have to do it yourself.</font></p><p style="margin:0px;line-height:normal"><span style="font-variant-ligatures:no-common-ligatures"><font face="georgia, serif" color="#000000"><br></font></span></p><p style="margin:0px;line-height:normal"><span style="font-variant-ligatures:no-common-ligatures"><font face="georgia, serif" color="#000000">Zhaoxin</font></span></p></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Nov 18, 2016 at 10:09 PM, Adrian Zubarev <span dir="ltr"><<a href="mailto:adrian.zubarev@devandartist.com" target="_blank">adrian.zubarev@devandartist.com</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_8960081155502691664bloop_markdown"><p>Ups sorry for CCing the post to the evolution list. That happens from time to time. :/ </p>
<p></p></div><div class="m_8960081155502691664bloop_original_html"><span class=""><div id="m_8960081155502691664bloop_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="m_8960081155502691664bloop_sign_1479478093471261952" class="m_8960081155502691664bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br></span><div><div class="h5"><p class="m_8960081155502691664airmail_on">Am 18. November 2016 um 15:07:43, Adrian Zubarev (<a href="mailto:adrian.zubarev@devandartist.com" target="_blank">adrian.zubarev@devandartist.<wbr>com</a>) schrieb:</p> <blockquote type="cite" class="m_8960081155502691664clean_bq"><span><div style="word-wrap:break-word"><div></div><div>
<div class="m_8960081155502691664bloop_markdown">
<p>I apologize about the <code>weak/unowned</code> issue I
mentioned. I kinda missed that portion from the docs <code>Weak
references do not affect the result of this function.</code>.</p>
<p>Okay it’s clear to me now why the result is evaluated as
<code>false</code>.</p>
<p>But how do I solve the COW problem for mutable views?</p>
</div>
<div class="m_8960081155502691664bloop_original_html">
<div id="m_8960081155502691664bloop_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="m_8960081155502691664bloop_sign_1479477967829885952" class="m_8960081155502691664bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">
-- <br>
Adrian Zubarev<br>
Sent with Airmail</div>
</div>
<br>
<p class="m_8960081155502691664airmail_on">Am 18. November 2016 um 15:04:55, Adrian
Zubarev (<a href="mailto:adrian.zubarev@devandartist.com" target="_blank">adrian.zubarev@devandartist.<wbr>com</a>)
schrieb:</p>
<blockquote type="cite" class="m_8960081155502691664clean_bq">
<div style="word-wrap:break-word">
<div>
<div class="m_8960081155502691664bloop_markdown">
<p><span>This doesn’t make any sense.</span></p>
<p><span>Somewhere from the Swift book:</span></p>
<blockquote>
<p><span>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.</span></p>
</blockquote>
<p><span>From the sdlib of the function
<code>isKnownUniquelyReferenced</code>:</span></p>
<blockquote>
<p><span>Returns a Boolean value indicating whether the given
object is a class instance known to have <strong>a single strong
reference</strong>.</span></p>
</blockquote>
<p><span><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?</span></p>
</div>
<div class="m_8960081155502691664bloop_original_html">
<div id="m_8960081155502691664bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px;color:rgba(0,0,0,1.0);margin:0px;line-height:auto">
<span><br></span></div>
<span><br></span>
<div id="m_8960081155502691664bloop_sign_1479477213018533888" class="m_8960081155502691664bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">
<span>-- <br>
Adrian Zubarev<br>
Sent with Airmail</span></div>
</div>
<span><br></span>
<p class="m_8960081155502691664airmail_on"><span>Am 18. November 2016 um 14:50:57, Zhao
Xin (<a href="mailto:owenzx@gmail.com" target="_blank">owenzx@gmail.com</a>)
schrieb:</span></p>
<blockquote type="cite" class="m_8960081155502691664clean_bq">
<div>
<div>
<div dir="ltr">
<div class="gmail_default" style="font-family:georgia,serif">
<span><span>><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></span></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_8960081155502691664m_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_8960081155502691664m_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._<a href="http://storageReference.keys.st">storageReference.keys.st</a><wbr>artIndex
}
public var endIndex: Int {
return self._storageReference.keys.en<wbr>dIndex
}
public func index(after i: Int) -> Int {
return self._<a href="http://storageReference.keys.in">storageReference.keys.in</a><wbr>dex(after: i)
}
public subscript(position: Int) -> Int {
get { return _storageReference.values[posit<wbr>ion] }
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_8960081155502691664m_5359467609274298590swift">var document = Document()
print(CFGetRetainCount(documen<wbr>t._storageReference)) //=> 2
print(isKnownUniquelyReference<wbr>d(&document._storageReference)<wbr>) // true
var values = document.values
print(CFGetRetainCount(values.<wbr>_storageReference)) //=> 2
print(isKnownUniquelyReference<wbr>d(&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_8960081155502691664m_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_8960081155502691664m_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_8960081155502691664m_5359467609274298590swift">set {
if !isKnownUniquelyReferenced(&se<wbr>lf._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_8960081155502691664m_5359467609274298590bloop_original_html">
<div id="m_8960081155502691664m_5359467609274298590bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px;color:rgba(0,0,0,1.0);margin:0px;line-height:auto">
<span class="m_8960081155502691664HOEnZb"><font color="#888888"><br></font></span></div>
<span class="m_8960081155502691664HOEnZb"><font color="#888888"><br></font></span>
<div id="m_8960081155502691664m_5359467609274298590bloop_sign_1479469826202990080" class="m_8960081155502691664m_5359467609274298590bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">
<span class="m_8960081155502691664HOEnZb"><font color="#888888">-- <br>
Adrian Zubarev<br>
Sent with Airmail</font></span></div>
</div>
</div>
<div class="m_8960081155502691664m_5359467609274298590bloop_markdown"></div>
</div>
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</div>
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</blockquote>
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<div class="m_8960081155502691664bloop_markdown"></div>
</div></div></span></blockquote></div></div></div><div class="m_8960081155502691664bloop_markdown"><p></p></div></div></blockquote></div><br></div>