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<title>Issue 4352: "operations on flat_multimap are equivalent to those of flat_map"</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#New">New</a> status.</em></p>
<h3 id="4352"><a href="lwg-active.html#4352">4352</a>. "operations on flat_multimap are equivalent to those of flat_map"</h3>
<p><b>Section:</b> 23.6.9.1 <a href="https://wg21.link/flat.multimap.overview">[flat.multimap.overview]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2025-09-05 <b>Last modified:</b> 2025-09-05</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
23.6.9.1 <a href="https://wg21.link/flat.multimap.overview">[flat.multimap.overview]</a> p4 says:
</p>
<blockquote>
Except as otherwise noted, operations on <code class='backtick'>flat_multimap</code> are equivalent to
those of <code class='backtick'>flat_map</code>, except that <code class='backtick'>flat_multimap</code> operations do not remove
or replace elements with equal keys.
<p>
[<i>Example 1</i>: <code class='backtick'>flat_multimap</code> constructors and emplace do not erase
non-unique elements after sorting them.
&mdash; <i>end example</i>]
</p>
</blockquote>

<p>
This doesn't really work, because there is no <code class='backtick'>flat_map</code> operation that is
equivalent to the constructors and <code class='backtick'>insert</code> and <code class='backtick'>insert_range</code> functions that
take a <code class='backtick'>sorted_equivalent_t</code> tag argument. The <code class='backtick'>flat_map</code> operations that the
wording intends to refer to take a <code class='backtick'>sorted_unique_t</code> tag argument.
It's a bit of a stretch to say they're equivalent when they don't even have
the same parameters, so we need some weasel words here.
</p>



<p id="res-4352"><b>Proposed resolution:</b></p>
<p>
</p>





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