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<title>Issue 1101: unique requirements</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#NAD_Editorial">NAD Editorial</a> status.</em></p>
<h3 id="1101"><a href="lwg-closed.html#1101">1101</a>. <code>unique</code> requirements</h3>
<p><b>Section:</b> 26.7.9 <a href="https://wg21.link/alg.unique">[alg.unique]</a> <b>Status:</b> <a href="lwg-active.html#NAD_Editorial">NAD Editorial</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2009-04-25 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#alg.unique">active issues</a> in [alg.unique].</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.unique">issues</a> in [alg.unique].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD Editorial">NAD Editorial</a> status.</p>
<p><b>Discussion:</b></p>
<p>
From Message c++std-core-14160 Howard wrote:
</p>

<blockquote><p>
It was the intent of the rvalue reference proposal for unique to only require MoveAssignable:
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1860.html#25.2.9%20-%20Unique">N1860</a>.
</p></blockquote>

<p>
And Pete replied:
</p>

<blockquote><p>
That was overridden by the subsequent changes made for concepts in
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2573.pdf">N2573</a>,
which reimposed the C++03 requirements.
</p></blockquote>

<p>
My impression is that this overwrite was a simple (unintentional) mistake.
Wording below to correct it.
</p>

<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote>
<p>
Howard notes this issue resolves a discrepancy between the synopsis
and the description.
</p>
<p>
Move to NAD Editorial.
</p>
</blockquote>


<p id="res-1101"><b>Proposed resolution:</b></p>
<p>
Change 26.7.9 <a href="https://wg21.link/alg.unique">[alg.unique]</a>:
</p>

<blockquote><pre>
template&lt;ForwardIterator Iter&gt; 
  requires OutputIterator&lt;Iter, <ins>RvalueOf&lt;</ins>Iter::reference<ins>&gt;::type</ins>&gt; 
        &amp;&amp; EqualityComparable&lt;Iter::value_type&gt; 
  Iter unique(Iter first, Iter last); 

template&lt;ForwardIterator Iter, EquivalenceRelation&lt;auto, Iter::value_type&gt; Pred&gt; 
  requires OutputIterator&lt;Iter, RvalueOf&lt;Iter::reference&gt;::type&gt; 
        &amp;&amp; CopyConstructible&lt;Pred&gt; 
  Iter unique(Iter first, Iter last, Pred pred);
</pre></blockquote>

<p>
Note that the synopsis in  [algorithms.syn] is already correct.
</p>






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