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<title>Issue 3672: common_iterator::operator-&gt;() should return by value</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="3672"><a href="lwg-defects.html#3672">3672</a>. <code>common_iterator::operator-&gt;()</code> should return by value</h3>
<p><b>Section:</b> 24.5.5.4 <a href="https://wg21.link/common.iter.access">[common.iter.access]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2022-02-12 <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>
24.5.5.4 <a href="https://wg21.link/common.iter.access">[common.iter.access]</a> p5 (5.1) says that <code>common_iterator&lt;T*, S&gt;::operator-&gt;()</code> 
returns <code>std::get&lt;T*&gt;(v_)</code> which has type <code>T* const&amp;</code>. That means that 
<code>iterator_traits::pointer</code> is <code>T* const&amp;</code> as well (this was recently clarified by LWG 
<a href="lwg-defects.html#3660" title="iterator_traits&lt;common_iterator&gt;::pointer should conform to &sect;[iterator.traits] (Status: C++23)">3660</a><sup><a href="https://cplusplus.github.io/LWG/issue3660" title="Latest snapshot">(i)</a></sup>). We have an actual pointer here, why are we returning it by reference?
<p/>
For the three bullet points in 24.5.5.4 <a href="https://wg21.link/common.iter.access">[common.iter.access]</a> p5, the second and third return by value 
anyway, so <code>decltype(auto)</code> is equivalent to <code>auto</code>. For the first bullet, it would make a lot 
more sense for raw pointers to be returned by value. That leaves the case where the iterator has an 
<code>operator-&gt;()</code> member, which could potentially benefit from returning by reference. But it must 
return something that is iterator-like or pointer-like, which we usually just pass by value. Casey suggested 
we should just change <code>common_iterator&lt;I, S&gt;::operator-&gt;()</code> to return by value in all cases.
<p/>
Libstdc++ has always returned by value, as an unintended consequence of using a union instead of 
<code>std::variant&lt;I, S&gt;</code>, so that it doesn't use <code>std::get&lt;I&gt;</code> to return the member.
</p>

<p><i>[2022-03-04; Reflector poll]</i></p>

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

<p><i>[2022-07-15; LWG telecon: move to Ready]</i></p>


<p><i>[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3672"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>.
</p>

<ol>
<li><p>Modify 24.5.5.1 <a href="https://wg21.link/common.iterator">[common.iterator]</a>, class template <code>common_iterator</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
constexpr decltype(auto) operator*();
constexpr decltype(auto) operator*() const
  requires <i>dereferenceable</i>&lt;const I&gt;;
constexpr <del>decltype(</del>auto<del>)</del> operator-&gt;() const
  requires <i>see below</i>;
[&hellip;]
</pre>
</blockquote>
</li>

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

<blockquote>
<pre>
constexpr <del>decltype(</del>auto<del>)</del> operator-&gt;() const
  requires <i>see below</i>;
</pre>
<blockquote>
<p>
-3- The expression in the <i>requires-clause</i> is equivalent to: [&hellip;]
<p/>
-4- <i>Preconditions:</i> <code>holds_alternative&lt;I&gt;(v_)</code> is <code>true</code>.
<p/>
-5- <i>Effects:</i>
</p>
<ol style="list-style-type:none">
<li><p>(5.1) &mdash; If <code>I</code> is a pointer type or if the expression <code>get&lt;I&gt;(v_).operator-&gt;()</code> 
is well-formed, equivalent to: <code>return get&lt;I&gt;(v_);</code></p></li>
<li><p>(5.2) &mdash; Otherwise, if <code>iter_reference_t&lt;I&gt;</code> is a reference type, equivalent to:</p>
<blockquote><pre>
auto&amp;&amp; tmp = *get&lt;I&gt;  (v_);
return addressof(tmp);
</pre></blockquote>
</li>
<li><p>(5.3) &mdash; Otherwise, equivalent to: <code>return <i>proxy</i>(*get&lt;I&gt;(v_));</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_;
  constexpr <i>proxy</i>(iter_reference_t&lt;I&gt;&amp;&amp; x)
    : keep_(std::move(x)) {}
public:
  constexpr const iter_value_t&lt;I&gt;* operator-&gt;() const noexcept {
    return addressof(keep_);
  }
};
</pre></blockquote>
</li>
</ol>
</blockquote>
</blockquote>
</li>
</ol>





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