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<title>Issue 2952: iterator_traits should work for pointers to cv T</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++20">C++20</a> status.</em></p>
<h3 id="2952"><a href="lwg-defects.html#2952">2952</a>. <code>iterator_traits</code> should work for pointers to <i>cv</i> <code>T</code></h3>
<p><b>Section:</b> 24.3.2.3 <a href="https://wg21.link/iterator.traits">[iterator.traits]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Billy Robert O'Neal III <b>Opened:</b> 2017-03-27 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#iterator.traits">issues</a> in [iterator.traits].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>iterator_traits</code> accepts pointer to <code>volatile T*</code>, but then says that the <code>value_type</code> is 
<code>volatile T</code>, instead of <code>T</code>, which is inconsistent for what it does for pointer to <code>const T</code>. 
We should either reject <code>volatile</code> outright or give the right answer.
</p>

<p><i>[2017-03-30, David Krauss comments]</i></p>

<p>
<code>volatile</code> pointers may not be well-behaved random-access iterators. When simple access incurs side effects, 
the multiple-pass guarantee depends on underlying (hardware) semantics.
</p>

<p><i>[2017-07 Toronto Wed Issue Prioritization]</i></p>

<p>Priority 0; move to Ready</p>


<p id="res-2952"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4659">N4659</a>.
</p>

<ol>
<li><p>Change 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a> as indicated:</p>

<blockquote>
<pre>
<i>// 24.4 <a href="https://wg21.link/iterator.primitives">[iterator.primitives]</a>, primitives</i>
template&lt;class Iterator&gt; struct iterator_traits;
template&lt;class T&gt; struct iterator_traits&lt;T*&gt;;
<del>template&lt;class T&gt; struct iterator_traits&lt;const T*&gt;;</del>
</pre>
</blockquote>
</li>

<li><p>Change 24.3.2.3 <a href="https://wg21.link/iterator.traits">[iterator.traits]</a> as indicated:</p>

<blockquote>
<p>
-3- It is specialized for pointers as
</p>
<blockquote>
<pre>
namespace std {
  template&lt;class T&gt; struct iterator_traits&lt;T*&gt; {
    using difference_type = ptrdiff_t;
    using value_type = <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins>;
    using pointer = T*;
    using reference = T&amp;;
    using iterator_category = random_access_iterator_tag;
  };
}
</pre>
</blockquote>
<p>
<del>and for pointers to <code>const</code> as</del>
</p>
<blockquote>
<pre>
<del>namespace std {
  template&lt;class T&gt; struct iterator_traits&lt;const T*&gt; {
    using difference_type = ptrdiff_t;
    using value_type = T;
    using pointer = const T*;
    using reference = const T&amp;;
    using iterator_category = random_access_iterator_tag;
  };
}</del>
</pre>
</blockquote>
</blockquote>
</li>
</ol>





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