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<title>Issue 4350: Should flat adaptors use insert_range when available?</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="4350"><a href="lwg-active.html#4350">4350</a>. Should flat adaptors use <code class='backtick'>insert_range</code> when available?</h3>
<p><b>Section:</b> 23.6.8 <a href="https://wg21.link/flat.map">[flat.map]</a>, 23.6.9 <a href="https://wg21.link/flat.multimap">[flat.multimap]</a>, 23.6.11 <a href="https://wg21.link/flat.set">[flat.set]</a>, 23.6.12 <a href="https://wg21.link/flat.multiset">[flat.multiset]</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 other</b> <a href="lwg-index-open.html#flat.map">active issues</a> in [flat.map].</p>
<p><b>View all other</b> <a href="lwg-index.html#flat.map">issues</a> in [flat.map].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
As specified in 23.6.3.4 <a href="https://wg21.link/queue.mod">[queue.mod]</a>,
<code class='backtick'>std::queue::push_range</code> is guaranteed to use <code class='backtick'>c.append_range</code> if that exists.
For <code class='backtick'>flat_map</code> and its flat friends, we only ever use <code class='backtick'>c.insert</code> and never try
to use <code class='backtick'>c.insert_range</code>, <code class='backtick'>c.append_range</code>, etc.
LWG thinks the "as if by" wording allows implementations to use <code class='backtick'>insert_range</code>
when the container type being adapted is <code class='backtick'>std::vector</code> but not for user-defined
containers.
Should the flat adaptors follow <code class='backtick'>std::queue</code> and guarantee that they will use
the <code class='backtick'>xxx_range</code> member if they exist?
That would mean that user containers need to ensure that any member functions
with those names must behave as expected, but we already require that for
<code class='backtick'>std::queue</code>.
</p>


<p id="res-4350"><b>Proposed resolution:</b></p>
<p>
</p>





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