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<title>Issue 4318: Have hive::erase_if reevaluate end() to avoid UB</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#Ready">Tentatively Ready</a> status.</em></p>
<h3 id="4318"><a href="lwg-active.html#4318">4318</a>. Have <code class='backtick'>hive::erase_if</code> reevaluate <code class='backtick'>end()</code> to avoid UB</h3>
<p><b>Section:</b> 23.3.9.6 <a href="https://wg21.link/hive.erasure">[hive.erasure]</a> <b>Status:</b> <a href="lwg-active.html#Ready">Tentatively Ready</a>
 <b>Submitter:</b> Frank Birbacher <b>Opened:</b> 2025-08-16 <b>Last modified:</b> 2025-08-29</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#hive.erasure">issues</a> in [hive.erasure].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Tentatively Ready">Tentatively Ready</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Background:  <a href="https://github.com/cplusplus/draft/pull/8162">https://github.com/cplusplus/draft/pull/8162</a>
<p/>
For 23.3.9.6 <a href="https://wg21.link/hive.erasure">[hive.erasure]</a> p2, the defining code must not cache the end-iterator. In case the last 
element of the sequence is removed, the past-the-end iterator will be invalidated. This will trigger UB 
in the loop condition. Instead, re-evaluate <code class='backtick'>end()</code> each time.
</p>

<p><i>[2025-08-29; Reflector poll]</i></p>

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



<p id="res-4318"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5014">N5014</a>.
</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> There are other ways to fix this code while keeping the caching behaviour, but I don't 
see any particular reason to do so for the definition of the effects.]
</p>
</blockquote>

<ol>

<li><p>Modify 23.3.9.6 <a href="https://wg21.link/hive.erasure">[hive.erasure]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T, class Allocator, class Predicate&gt;
  typename hive&lt;T, Allocator&gt;::size_type
    erase_if(hive&lt;T, Allocator&gt;&amp; c, Predicate pred);
</pre>
<blockquote>
<p>
-2- <i>Effects</i>: Equivalent to:
</p>
<blockquote><pre>
auto original_size = c.size();
for (auto i = c.begin()<del>, last = c.end()</del>; i != <del>last</del><ins>c.end()</ins>; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();
</pre></blockquote>
</blockquote>
</blockquote>

</li>

</ol>





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