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<title>Issue 4242: ranges::distance does not work with volatile iterators</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#WP">WP</a> status.</em></p>
<h3 id="4242"><a href="lwg-defects.html#4242">4242</a>. <code class='backtick'>ranges::distance</code> does not work with volatile iterators</h3>
<p><b>Section:</b> 24.4.4.3 <a href="https://wg21.link/range.iter.op.distance">[range.iter.op.distance]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-04-12 <b>Last modified:</b> 2025-06-23</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.iter.op.distance">issues</a> in [range.iter.op.distance].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
After LWG <a href="lwg-defects.html#3664" title="LWG 3392 broke std::ranges::distance(a, a+3) (Status: C++23)">3664</a><sup><a href="https://cplusplus.github.io/LWG/issue3664" title="Latest snapshot">(i)</a></sup>, <code class='backtick'>ranges::distance</code> computes the distance between <code class='backtick'>last</code> and <code class='backtick'>first</code>
by returning <code>last - static_cast&lt;const decay_t&lt;I&gt;&amp;&gt;(first)</code> when the 
two are subtractable. However, this will cause a hard error if <code>first</code> is volatile-qualified
(<a href="https://godbolt.org/z/Pz1cGxKEW">demo</a>):
</p>
<blockquote><pre>
#include &lt;iterator&gt;

int main() {
  int arr[] = {1, 2, 3};
  int* volatile ptr = arr;
  // return std::distance(ptr, arr + 3);      // this is ok
  return std::ranges::distance(ptr, arr + 3); // <span  style="color:#C80000;font-weight:bold">hard error</span>
}
</pre></blockquote>
<p>
The resolution changes the <i>Effects</i> of LWG <a href="lwg-defects.html#3664" title="LWG 3392 broke std::ranges::distance(a, a+3) (Status: C++23)">3664</a><sup><a href="https://cplusplus.github.io/LWG/issue3664" title="Latest snapshot">(i)</a></sup> from "cute" to "noncute".
</p>

<p><i>[2025-06-12; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after eight votes in favour during reflector poll.
</p>
<p>"Would prefer <code class='backtick'>auto(first)</code> but the current wording is correct."</p>

<p><i>[Sofia 2025-06-21; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4242"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<ol>

<li><p>Modify 24.4.4.3 <a href="https://wg21.link/range.iter.op.distance">[range.iter.op.distance]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class I, sized_sentinel_for&lt;decay_t&lt;I&gt;&gt; S&gt;
  constexpr iter_difference_t&lt;decay_t&lt;I&gt;&gt; ranges::distance(I&amp;&amp; first, S last);
</pre>
<blockquote>
<p>
-3- <i>Effects</i>: Equivalent to:
</p>
<pre>
<ins>if constexpr (!is_array_v&lt;remove_reference_t&lt;I&gt;&gt;)
  return last - first;
else</ins>
  return last - static_cast&lt;<del>const</del> decay_t&lt;I&gt;<del>&amp;</del>&gt;(first);
</pre>
</blockquote>
</blockquote>

</li>

</ol>





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