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<title>Issue 3595: Exposition-only classes proxy and postfix-proxy for common_iterator 
should be fully constexpr</title>
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should be fully constexpr">
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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="3595"><a href="lwg-defects.html#3595">3595</a>. Exposition-only classes <code><i>proxy</i></code> and <code><i>postfix-proxy</i></code> for <code>common_iterator</code> 
should be fully <code>constexpr</code></h3>
<p><b>Section:</b> 24.5.5.4 <a href="https://wg21.link/common.iter.access">[common.iter.access]</a>, 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> Hewill Kang <b>Opened:</b> 2021-09-18 <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.access">issues</a> in [common.iter.access].</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>
LWG <a href="lwg-defects.html#3574" title="common_iterator should be completely constexpr-able (Status: C++23)">3574</a><sup><a href="https://cplusplus.github.io/LWG/issue3574" title="Latest snapshot">(i)</a></sup> added <code>constexpr</code> to all member functions and friends of <code>common_iterator</code> to make it 
fully <code>constexpr</code>, but accidentally omitted the exposition-only classes <code><i>proxy</i></code> and 
<code><i>postfix-proxy</i></code> defined for some member functions. We should make these two classes fully <code>constexpr</code>, too.
</p>

<p><i>[2021-09-24; Reflector poll]</i></p>

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

<p><i>[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3595"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 24.5.5.4 <a href="https://wg21.link/common.iter.access">[common.iter.access]</a> as indicated:</p>

<blockquote>
<pre>
decltype(auto) operator-&gt;() const
  requires <i>see below</i>;
</pre>
<blockquote>
<p>
-3- [&hellip;]
<p/>
-4- [&hellip;]
<p/>
-5- <i>Effects:</i>
</p>
<ol style="list-style-type: none">
<li><p>(5.1) &mdash; [&hellip;]</p></li>
<li><p>(5.2) &mdash; [&hellip;]</p></li>
<li><p>(5.3) &mdash; Otherwise, equivalent to: <code>return <i>proxy</i>(*get&lt;I&gt;(<i>v_</i>));</code> 
where <code><i>proxy</i></code> is the exposition-only class:</p>
<blockquote><pre>
class <i>proxy</i> {
  iter_value_t&lt;I&gt; keep_;
  <ins>constexpr</ins> <i>proxy</i>(iter_reference_t&lt;I&gt;&amp;&amp; x)
    : keep_(std::move(x)) {}
public:
  <ins>constexpr</ins> const iter_value_t&lt;I&gt;* operator-&gt;() const <ins>noexcept</ins> {
    return addressof(keep_);
  }
};
</pre></blockquote>
</li>
</ol>
</blockquote>
</blockquote>
</li>

<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- [&hellip;]
<p/>
-5- <i>Effects:</i> If I models forward_iterator, equivalent to:
<p/>
[&hellip;]
<p/>
Otherwise, if [&hellip;]
<p/>
[&hellip;]
<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_;
  <ins>constexpr</ins> <i>postfix-proxy</i>(iter_reference_t&lt;I&gt;&amp;&amp; x)
    : keep_(std::forward&lt;iter_reference_t&lt;I&gt;&gt;(x)) {}
public:
  <ins>constexpr</ins> const iter_value_t&lt;I&gt;&amp; operator*() const <ins>noexcept</ins> {
    return keep_;
  }
};
</pre></blockquote>
</blockquote>
</blockquote>
</li>

</ol>





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