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<title>Issue 3546: common_iterator's postfix-proxy is not quite right</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++23">C++23</a> status.</em></p>
<h3 id="3546"><a href="lwg-defects.html#3546">3546</a>. <code>common_iterator</code>'s <code><i>postfix-proxy</i></code> is not quite right</h3>
<p><b>Section:</b> 24.5.5.5 <a href="https://wg21.link/common.iter.nav">[common.iter.nav]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2021-04-23 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#common.iter.nav">issues</a> in [common.iter.nav].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<a href="https://wg21.link/p2259r1">P2259R1</a> modeled <code>common_iterator::operator++(int)</code>'s 
<code><i>postfix-proxy</i></code> class on the existing proxy class used by <code>common_iterator::operator-&gt;</code>, 
but in doing so it overlooked two differences:
</p>
<ul>
<li><p><code>operator-&gt;</code>'s proxy is only used when <code>iter_reference_t&lt;I&gt;</code> is not a
reference type; this is not the case for <code><i>postfix-proxy</i></code>;</p></li>
<li><p><code>operator-&gt;</code> returns a prvalue proxy, while <code>operator++</code>'s <code><i>postfix-proxy</i></code>
is returned by (elidable) move, so the latter needs to require <code>iter_value_t&lt;I&gt;</code> to be 
<code>move_constructible</code>.</p></li>
</ul>
<p>
The proposed wording has been implemented and tested.
</p>

<p><i>[2021-05-10; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>


<p><i>[2021-05-17; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>

<p><i>[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3546"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4885">N4885</a>.
</p>

<ol>
<li><p>Modify 24.5.5.5 <a href="https://wg21.link/common.iter.nav">[common.iter.nav]</a> as indicated:</p>

<blockquote>
<pre>
decltype(auto) operator++(int);
</pre>
<blockquote>
<p>
-4- <i>Preconditions:</i> <code>holds_alternative&lt;I&gt;(v_)</code> is <code>true</code>.
<p/>
-5- <i>Effects:</i> If <code>I</code> models <code>forward_iterator</code>, equivalent to:
</p>
<blockquote><pre>
common_iterator tmp = *this;
++*this;
return tmp;
</pre></blockquote>
<p>
Otherwise, if <code>requires (I&amp; i) { { *i++ } -&gt; <i>can-reference</i>; }</code> is <code>true</code> 
or <code>constructible_from&lt;iter_value_t&lt;I&gt;, iter_reference_t&lt;I&gt;&gt; <ins>&amp;&amp;
move_constructible&lt;iter_value_t&lt;I&gt;&gt;</ins></code> is <code>false</code>, equivalent to:
</p>
<blockquote><pre>
return get&lt;I&gt;(v_)++;
</pre></blockquote>
<p>
Otherwise, equivalent to:
</p>
<blockquote><pre>
<i>postfix-proxy</i> p(**this);
++*this;
return p;
</pre></blockquote>
<p>
where <code><i>postfix-proxy</i></code> is the exposition-only class:
</p>
<blockquote><pre>
class <i>postfix-proxy</i> {
  iter_value_t&lt;I&gt; keep_;
  <i>postfix-proxy</i>(iter_reference_t&lt;I&gt;&amp;&amp; x)
    : keep_(std::<del>move</del><ins>forward&lt;iter_reference_t&lt;I&gt;&gt;</ins>(x)) {}
public:
  const iter_value_t&lt;I&gt;&amp; operator*() const {
    return keep_;
  }
};
</pre></blockquote>
</blockquote>
</blockquote>
</li>
</ol>





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