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<title>Issue 4000: flat_map::insert_range's Effects is not quite right</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="4000"><a href="lwg-active.html#4000">4000</a>. <code>flat_map::insert_range</code>'s <i>Effects</i> is not quite right</h3>
<p><b>Section:</b> 23.6.8.7 <a href="https://wg21.link/flat.map.modifiers">[flat.map.modifiers]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-10-23 <b>Last modified:</b> 2025-09-05</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#flat.map.modifiers">active issues</a> in [flat.map.modifiers].</p>
<p><b>View all other</b> <a href="lwg-index.html#flat.map.modifiers">issues</a> in [flat.map.modifiers].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>flat_map::insert_range</code> adds elements to the <code>containers</code> member via:
</p>
<blockquote><pre>
for (const auto&amp; e : rg) {
  c.keys.insert(c.keys.end(), e.first);
  c.values.insert(c.values.end(), e.second);
}
</pre></blockquote>
<p>
which is incorrect because <code>rg</code>'s value type may not be a <code>pair</code> (<code>tuple</code>, for instance), 
which means that <code>.first</code> and <code>.second</code> are not valid in such cases.
</p>

<p><i>[2024-02-22; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll in October 2023.
</p>

<p>
"This is <a href="https://wg21.link/P2767" title=" flat_map/flat_set omnibus">P2767</a> section 6 which LWG looked at in Varna."
</p>

<p><i>[2025-09-05; LWG telecon]</i></p>

<p>This will be resolved by <a href="https://wg21.link/P3567" title=" `flat_meow` Fixes">P3567</a>.</p>



<p id="res-4000"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4964" title=" Working Draft, Programming Languages — C++">N4964</a>.
</p>

<ol>

<li><p>Modify 23.6.8.7 <a href="https://wg21.link/flat.map.modifiers">[flat.map.modifiers]</a> as indicated:</p>

<blockquote><pre>
template&lt;<i>container-compatible-range</i>&lt;value_type&gt; R&gt;
  void insert_range(R&amp;&amp; rg);
</pre>
<blockquote>
<p>
-12- <i>Effects</i>: Adds elements to <code>c</code> as if by:
</p>
<blockquote><pre>
for (<ins>value_type</ins><del>const auto&amp;</del> e : rg) {
  c.keys.insert(c.keys.end(), <ins>std::move(</ins>e.first<ins>)</ins>);
  c.values.insert(c.values.end(), <ins>std::move(</ins>e.second<ins>)</ins>);
}
</pre></blockquote>
<p>
[&hellip;]
</p>
</blockquote>
</blockquote>

</li>

</ol>





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