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<title>Issue 3227: Ambiguity issue for extract in ordered and unordered associative containers</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="3227"><a href="lwg-active.html#3227">3227</a>. Ambiguity issue for <code>extract</code> in ordered and unordered associative containers</h3>
<p><b>Section:</b> 23.2.7 <a href="https://wg21.link/associative.reqmts">[associative.reqmts]</a>, 23.2.8 <a href="https://wg21.link/unord.req">[unord.req]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Konstantin Boyarinov <b>Opened:</b> 2019-06-25 <b>Last modified:</b> 2022-04-24</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#associative.reqmts">active issues</a> in [associative.reqmts].</p>
<p><b>View all other</b> <a href="lwg-index.html#associative.reqmts">issues</a> in [associative.reqmts].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Ordered and unordered associative containers in C++14 contained an issue, which caused an ambiguity
while invoking <code>std::map::erase</code> when <code>key_type</code> of the map can be constructed from
the iterator. In this case both overloads <code>erase(const key_type&amp;)</code> and
<code>erase(const_iterator)</code> could be chosen.
<p/>
The issue LWG <a href="lwg-defects.html#2059" title="C++0x ambiguity problem with map::erase (Status: C++17)">2059</a><sup><a href="https://cplusplus.github.io/LWG/issue2059" title="Latest snapshot">(i)</a></sup> was reported and resolved in C++17 by adding an extra overload
for <code>erase</code> in ordered and unordered associative containers which accepts <code>iterator</code>
as an argument.
<p/>
C++17 also introduced new functionality for splicing ordered and unordered maps and sets.
One of the extensions allows to extract a node from the container by passing either
<code>key_type&amp;</code> or <code>const_iterator</code> to the <code>extract()</code> member function:
</p>
<blockquote><pre>
node_type extract(const key_type&amp; x);
node_type extract(const_iterator position);
</pre></blockquote>
<p>
Providing these two <code>extract</code> overloads causes the same problem as for <code>erase</code>.
Consider the following example:
</p>
<blockquote><pre>
#include &lt;map&gt;
#include &lt;string&gt;

struct Key
{
  template &lt;typename T&gt;
  Key(const T&amp;) {}
};

bool operator&lt;(const Key&amp;, const Key&amp;) { return false; }

int main()
{
  using map_type = std::map&lt;Key, std::string&gt;;

  map_type m({ {Key(1), "a"}, {Key(2), "b"} });
  map_type::iterator it = m.begin();
  auto nh = m.extract(it);
}
</pre></blockquote>
<p>
In this case, call to <code>extract()</code> is ambiguous, because the overloads which accept
<code>const_iterator</code> and <code>key_type</code> are equally good matches for the argument
<code>it</code>.
<p/>
Consequently, this issue can be resolved in the same way as for <code>std::map::erase</code>
by adding an overload for <code>extract</code> which accepts <code>iterator</code> as an argument.
</p>

<p><i>[2019-07 Issue Prioritization]</i></p>

<p>Priority to 3 after discussion on the reflector.</p>

<p>
<strong>Previous resolution [SUPERSEDED]:</strong>
</p>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/N4820" title=" Working Draft, Standard for Programming Language C++">N4820</a>.</p>

<ol>
<li><p>Modify  [tab:container.assoc.req], Table 69 &mdash;
"Associative container requirements", as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 69 &mdash; Associative container requirements (in addition to container)
 [tab:container.assoc.req]</caption>
<tr>
<th>Expression</th>
<th>Return type</th>
<th>Assertion&#47;note pre-&#47;post-condition</th>
<th>Complexity</th>
</tr>

<tr>
<td colspan="4" align="center">
<code>&hellip;</code>
</td>
</tr>

<tr>
<td>
<code>a.extract(q)</code>
</td>
<td>
<code>node_type</code>
</td>
<td>
<i>Effects:</i> Removes the element<br/>
pointed to by <code>q</code>.<br/>
<i>Returns:</i> A <code>node_type</code> owning<br/>
that element.
</td>
<td>
amortized constant
</td>
</tr>

<tr>
<td>
<ins><code>a.extract(r)</code></ins>
</td>
<td>
<ins><code>node_type</code></ins>
</td>
<td>
<ins><i>Effects:</i> Removes the element<br/>
pointed to by <code>r</code>.<br/>
<i>Returns:</i> A <code>node_type</code> owning<br/>
that element.</ins>
</td>
<td>
<ins>amortized constant</ins>
</td>
</tr>

<tr>
<td colspan="4" align="center">
<code>&hellip;</code>
</td>
</tr>

</table>
</blockquote>
</li>

<li><p>Modify  [tab:container.assoc.req], Table 70 &mdash;
"Unordered associative container requirements", as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 70 &mdash; Unordered associative container requirements (in addition to container)
 [tab:container.hash.req]</caption>
<tr>
<th>Expression</th>
<th>Return type</th>
<th>Assertion&#47;note pre-&#47;post-condition</th>
<th>Complexity</th>
</tr>

<tr>
<td colspan="4" align="center">
<code>&hellip;</code>
</td>
</tr>

<tr>
<td>
<code>a.extract(q)</code>
</td>
<td>
<code>node_type</code>
</td>
<td>
<i>Effects:</i> Removes the element<br/>
pointed to by <code>q</code>.<br/>
<i>Returns:</i> A <code>node_type</code> owning<br/>
that element.
</td>
<td>
Average case
<code>&#x1d4aa;(1)</code>, worst case
<code>&#x1d4aa;(a.size())</code>.
</td>
</tr>

<tr>
<td>
<ins><code>a.extract(r)</code></ins>
</td>
<td>
<ins><code>node_type</code></ins>
</td>
<td>
<ins><i>Effects:</i> Removes the element<br/>
pointed to by <code>r</code>.<br/>
<i>Returns:</i> A <code>node_type</code> owning<br/>
that element.</ins>
</td>
<td>
<ins>Average case
<code>&#x1d4aa;(1)</code>, worst case
<code>&#x1d4aa;(a.size())</code>.</ins>
</td>
</tr>

<tr>
<td colspan="4" align="center">
<code>&hellip;</code>
</td>
</tr>

</table>
</blockquote>
</li>

<li><p>Modify 23.4.3.1 <a href="https://wg21.link/map.overview">[map.overview]</a>, class template <code>map</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.4.4.1 <a href="https://wg21.link/multimap.overview">[multimap.overview]</a>, class template <code>multimap</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.4.6.1 <a href="https://wg21.link/set.overview">[set.overview]</a>, class template <code>set</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.4.7.1 <a href="https://wg21.link/multiset.overview">[multiset.overview]</a>, class template <code>multiset</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.5.3.1 <a href="https://wg21.link/unord.map.overview">[unord.map.overview]</a>, class template <code>unordered_map</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.5.4.1 <a href="https://wg21.link/unord.multimap.overview">[unord.multimap.overview]</a>, class template <code>unordered_multimap</code>
synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.5.6.1 <a href="https://wg21.link/unord.set.overview">[unord.set.overview]</a>, class template <code>unordered_set</code>
synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.5.7.1 <a href="https://wg21.link/unord.multiset.overview">[unord.multiset.overview]</a>, class template <code>unordered_multiset</code>
synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2022-04-24; Daniel rebases wording on <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>]</i></p>



<p id="res-3227"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 23.2.7.1 <a href="https://wg21.link/associative.reqmts.general">[associative.reqmts.general]</a> as indicated:</p>

<blockquote>
<pre>
a.extract(q)
</pre>
<blockquote>
<p>
-108- <i>Result:</i> <code>node_type</code>
<p/>
-109- <i>Effects:</i> Removes the element pointed to by <code>q</code>.
<p/>
-110- <i>Returns:</i> A <code>node_type</code> owning that element.
<p/>
-111- <i>Complexity:</i> Amortized constant.
</p>
</blockquote>
<pre>
<ins>a.extract(r)</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Result:</i> <code>node_type</code></ins>
<p/>
<ins>-?- <i>Effects:</i> Removes the element pointed to by <code>r</code>.</ins>
<p/>
<ins>-?- <i>Returns:</i> A <code>node_type</code> owning that element.</ins>
<p/>
<ins>-?- <i>Complexity:</i> Amortized constant.</ins>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.2.8.1 <a href="https://wg21.link/unord.req.general">[unord.req.general]</a> as indicated:</p>

<blockquote>
<pre>
a.extract(q)
</pre>
<blockquote>
<p>
-141- <i>Result:</i> <code>node_type</code>
<p/>
-142- <i>Effects:</i> Removes the element pointed to by <code>q</code>.
<p/>
-143- <i>Returns:</i> A <code>node_type</code> owning that element.
<p/>
-144- <i>Complexity:</i> Average case <code>&#x1d4aa;(1)</code>, worst case <code>&#x1d4aa;(a.size())</code>.
</p>
</blockquote>
<pre>
<ins>a.extract(r)</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Result:</i> <code>node_type</code></ins>
<p/>
<ins>-?- <i>Effects:</i> Removes the element pointed to by <code>r</code>.</ins>
<p/>
<ins>-?- <i>Returns:</i> A <code>node_type</code> owning that element.</ins>
<p/>
<ins>-?- <i>Complexity:</i> Average case <code>&#x1d4aa;(1)</code>, worst case <code>&#x1d4aa;(a.size())</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 23.4.3.1 <a href="https://wg21.link/map.overview">[map.overview]</a>, class template <code>map</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.4.4.1 <a href="https://wg21.link/multimap.overview">[multimap.overview]</a>, class template <code>multimap</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.4.6.1 <a href="https://wg21.link/set.overview">[set.overview]</a>, class template <code>set</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.4.7.1 <a href="https://wg21.link/multiset.overview">[multiset.overview]</a>, class template <code>multiset</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.5.3.1 <a href="https://wg21.link/unord.map.overview">[unord.map.overview]</a>, class template <code>unordered_map</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.5.4.1 <a href="https://wg21.link/unord.multimap.overview">[unord.multimap.overview]</a>, class template <code>unordered_multimap</code>
synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.5.6.1 <a href="https://wg21.link/unord.set.overview">[unord.set.overview]</a>, class template <code>unordered_set</code>
synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 23.5.7.1 <a href="https://wg21.link/unord.multiset.overview">[unord.multiset.overview]</a>, class template <code>unordered_multiset</code>
synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<ins>node_type extract(iterator position);</ins>
node_type extract(const_iterator position);
node_type extract(const key_type&amp; x);
[&hellip;]
</pre>
</blockquote>
</li>
</ol>




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