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<!DOCTYPE html>
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<html lang="en">
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<title>Issue 4242: ranges::distance does not work with volatile iterators</title>
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<meta property="og:title" content="Issue 4242: ranges::distance does not work with volatile iterators">
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<meta property="og:description" content="C++ library issue. Status: New">
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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#New">New</a> status.</em></p>
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<h3 id="4242"><a href="lwg-active.html#4242">4242</a>. <code class='backtick'>ranges::distance</code> does not work with volatile iterators</h3>
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<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#New">New</a>
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<b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-04-12 <b>Last modified:</b> 2025-04-13</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all other</b> <a href="lwg-index.html#range.iter.op.distance">issues</a> in [range.iter.op.distance].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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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>
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by returning <code>last - static_cast&lt;const decay_t&lt;I&gt;&amp;&gt;(first)</code> when the
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two are subtractable. However, this will cause a hard error if <code>first</code> is volatile-qualified
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(<a href="https://godbolt.org/z/5M3d9rMjE">demo</a>):
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</p>
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<blockquote><pre>
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#include &lt;iterator&gt;
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int main() {
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int arr[] = {1, 2, 3};
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int* volatile ptr = arr;
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// return std::distance(ptr, arr + 3); // this is ok
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return std::ranges::distance(ptr, arr + 3); // <span style="color:#C80000;font-weight:bold">hard error</span>
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}
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</pre></blockquote>
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<p>
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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".
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</p>
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<p id="res-4242"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<ol>
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<li><p>Modify 24.4.4.3 <a href="https://wg21.link/range.iter.op.distance">[range.iter.op.distance]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class I, sized_sentinel_for&lt;decay_t&lt;I&gt;&gt; S&gt;
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constexpr iter_difference_t&lt;decay_t&lt;I&gt;&gt; ranges::distance(I&amp;&amp; first, S last);
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</pre>
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<blockquote>
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<p>
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-3- <i>Effects</i>: Equivalent to:
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</p>
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<pre>
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<ins>if constexpr (!is_array_v&lt;remove_reference_t&lt;I&gt;&gt;)
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return last - first;
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else</ins>
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return last - static_cast&lt;<del>const</del> decay_t&lt;I&gt;<del>&amp;</del>&gt;(first);
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</pre>
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</blockquote>
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</body>
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</html>

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