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<title>Issue 3802: flat_foo allocator-extended constructors lack move semantics</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="3802"><a href="lwg-active.html#3802">3802</a>. <code>flat_<i>foo</i></code> allocator-extended constructors lack move semantics</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> Arthur O'Dwyer <b>Opened:</b> 2022-10-25 <b>Last modified:</b> 2023-06-16</p>
<p><b>Priority: </b>2
</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>
Compare 23.6.4.2 <a href="https://wg21.link/priqueue.cons">[priqueue.cons]</a>'s overload set
</p>
<blockquote><pre>
priority_queue(const Compare&amp;, const Container&amp;);
priority_queue(const Compare&amp;, Container&amp;&amp;);
template&lt;class Alloc&gt; priority_queue(const Compare&amp;, const Container&amp;, const Alloc&amp;);
template&lt;class Alloc&gt; priority_queue(const Compare&amp;, Container&amp;&amp;, const Alloc&amp;);
</pre></blockquote>
<p>
against 23.6.8 <a href="https://wg21.link/flat.map">[flat.map]</a>'s overload set
</p>
<blockquote><pre>
flat_map(key_container_type, mapped_container_type);
template&lt;class Allocator&gt; flat_map(const key_container_type&amp;, const mapped_container_type&amp;, const Allocator&amp; a);
</pre></blockquote>
<p>
I see two issues here:
</p>
<ol style="list-style-type: none">
<li><p>(A) The allocator-extended ctor of <code>flat_map</code> always copies the <code>key_container</code> and <code>value_container</code>, 
when it should be move-enabled.</p></li>
<li><p>(B) Almost certainly the <code>Allocator</code> parameter should be named <code>Alloc</code> instead, and there should be a 
separate "Constructors with allocators" section with wording similar to 23.6.4.3 <a href="https://wg21.link/priqueue.cons.alloc">[priqueue.cons.alloc]</a> explaining that 
these ctors don't participate in overload resolution unless 
<code>uses_allocator_v&lt;KeyContainer, Alloc&gt; &amp;&amp; uses_allocator_v&lt;MappedContainer, Alloc&gt;</code>.</p></li>
</ol>
<p>
I suggest this overload set to replace the two overloads above:
</p>
<blockquote><pre>
flat_map(key_container_type, mapped_container_type);
template&lt;class Alloc&gt; flat_map(const key_container_type&amp;, const mapped_container_type&amp;, const Alloc&amp; a);
template&lt;class Alloc&gt; flat_map(const key_container_type&amp;, mapped_container_type&amp;&amp;, const Alloc&amp; a);
template&lt;class Alloc&gt; flat_map(key_container_type&amp;&amp;, const mapped_container_type&amp;, const Alloc&amp; a);
template&lt;class Alloc&gt; flat_map(key_container_type&amp;&amp;, mapped_container_type&amp;&amp;, const Alloc&amp; a);
</pre></blockquote>
<p>
This preserves the apparent assumption that <code>KeyContainer(std::move(kc))</code> is always efficient but 
<code>KeyContainer(std::move(kc), otheralloc)</code> might not be. Similar wording changes would have to be made to all the 
<code>flat_<i>foo</i></code> containers.
<p/>
Tony Table:
</p>
<blockquote><pre>
template&lt;class T, class Comp = std::less&lt;T&gt;, class Container = std::pmr::vector&lt;T&gt;&gt;
using pmr_flat_set = std::flat_set&lt;T, Comp, Container&gt;;

std::pmr::vector&lt;pmr_flat_set&lt;int&gt;&gt; vs;
std::pmr::vector&lt;int&gt; data = {1,2,3};

vs.reserve(1);
vs.emplace_back(std::move(data));
  // constructs-in-place with the argument list (std::move(data), get_allocator())
  // BEFORE: copies (causes heap traffic)
  // AFTER: moves (no heap traffic)
</pre></blockquote>

<p><i>[2022-11-04; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>

<p><i>[2023-06-14 Varna]</i></p>

<p>Mentioned in <a href="https://wg21.link/P2767R0" title=" flat_map/flat_set omnibus">P2767R0</a>, but not resolved by it.</p>



<p id="res-3802"><b>Proposed resolution:</b></p>





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