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<title>Issue 3879: erase_if for flat_{,multi}set is incorrectly specified</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="3879"><a href="lwg-defects.html#3879">3879</a>. <code>erase_if</code> for <code>flat_{,multi}set</code> is incorrectly specified</h3>
<p><b>Section:</b> 23.6.11.6 <a href="https://wg21.link/flat.set.erasure">[flat.set.erasure]</a>, 23.6.12.6 <a href="https://wg21.link/flat.multiset.erasure">[flat.multiset.erasure]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2023-02-09 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</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>
The current specification of <code>erase_if</code> for <code>flat_{,multi}set</code>
calls <code>ranges::remove_if</code> on the set, which is obviously incorrect &mdash;
the set only present constant views of its elements.
</p>

<p><i>[Issaquah 2023-02-09; LWG]</i></p>

<p>Move to Immediate for C++23</p>

<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3879"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>
<ol>
<li><p>Modify 23.6.11.6 <a href="https://wg21.link/flat.set.erasure">[flat.set.erasure]</a> as indicated:</p>
<blockquote>
<pre>
template&lt;class Key, class Compare, class KeyContainer, class Predicate&gt;
  typename flat_set&lt;Key, Compare, KeyContainer&gt;::size_type
    erase_if(flat_set&lt;Key, Compare, KeyContainer&gt;&amp; c, Predicate pred);
</pre>
<blockquote>
<p>
<del>-1- <i>Effects:</i> Equivalent to:</del>
</p>
<blockquote>
<pre>
<del>auto [erase_first, erase_last] = ranges::remove_if(c, pred);
auto n = erase_last - erase_first;
c.erase(erase_first, erase_last);
return n;</del>
</pre>
</blockquote>
<p>
<ins>
-1- <i>Preconditions:</i> <code>Key</code> meets the <i>Cpp17MoveAssignable</i> requirements.
</ins>
<p/>
<ins>
-2- <i>Effects:</i> Let <i>E</i> be <code>bool(pred(as_const(e)))</code>. 
Erases all elements <code>e</code> in <code>c</code> for which <i>E</i> holds.
</ins>
<p/>
<ins>-3- <i>Returns:</i> The number of elements erased.</ins>
<p/>
<ins>-4- <i>Complexity:</i> Exactly <code>c.size()</code> applications of the predicate.</ins>
<p/>
<ins>-5- <i>Remarks:</i> Stable (16.4.6.8 <a href="https://wg21.link/algorithm.stable">[algorithm.stable]</a>). 
If an invocation of <code>erase_if</code> exits via an exception, <code>c</code> is in a valid but
unspecified state (3.67 <a href="https://wg21.link/defns.valid">[defns.valid]</a>). </ins>
<p/>
<ins>
[<i>Note 1</i>: <code>c</code> still meets its invariants, but can be empty. &mdash; <i>end note</i>]
</ins>
</p>
</blockquote>
</blockquote>
</li>
<li><p>Modify 23.6.12.6 <a href="https://wg21.link/flat.multiset.erasure">[flat.multiset.erasure]</a> as indicated:</p>
<blockquote>
<pre>
template&lt;class Key, class Compare, class KeyContainer, class Predicate&gt;
  typename flat_multiset&lt;Key, Compare, KeyContainer&gt;::size_type
    erase_if(flat_multiset&lt;Key, Compare, KeyContainer&gt;&amp; c, Predicate pred);
</pre>
<blockquote>
<p>
<del>-1- <i>Effects:</i> Equivalent to:</del>
</p>
<blockquote>
<pre>
<del>auto [erase_first, erase_last] = ranges::remove_if(c, pred);
auto n = erase_last - erase_first;
c.erase(erase_first, erase_last);
return n;</del>
</pre>
</blockquote>
<p>
<ins>
-1- <i>Preconditions:</i> <code>Key</code> meets the <i>Cpp17MoveAssignable</i> requirements.
</ins>
<p/>
<ins>
-2- <i>Effects:</i> Let <i>E</i> be <code>bool(pred(as_const(e)))</code>. 
Erases all elements <code>e</code> in <code>c</code> for which <i>E</i> holds.
</ins>
<p/>
<ins>-3- <i>Returns:</i> The number of elements erased.</ins>
<p/>
<ins>-4- <i>Complexity:</i> Exactly <code>c.size()</code> applications of the predicate.</ins>
<p/>
<ins>-5- <i>Remarks:</i> Stable (16.4.6.8 <a href="https://wg21.link/algorithm.stable">[algorithm.stable]</a>). 
If an invocation of <code>erase_if</code> exits via an exception, <code>c</code> is in a valid but
unspecified state (3.67 <a href="https://wg21.link/defns.valid">[defns.valid]</a>). </ins>
<p/>
<ins>
[<i>Note 1</i>: <code>c</code> still meets its invariants, but can be empty. &mdash; <i>end note</i>]
</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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