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<title>Issue 1241: unique_copy needs to require EquivalenceRelation</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#C++11">C++11</a> status.</em></p>
<h3 id="1241"><a href="lwg-defects.html#1241">1241</a>. <code>unique_copy</code> needs to require <code>EquivalenceRelation</code></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#C++11">C++11</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2009-10-17 <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#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p>
A lot of fixes were silently applied during concept-time and we should
not lose them again. The Requires clause of 26.7.9 <a href="https://wg21.link/alg.unique">[alg.unique]</a>/5
doesn't mention that <code>==</code> and the predicate need to satisfy an
<code>EquivalenceRelation</code>, as it is correctly said for <code>unique</code>. 
This was intentionally fixed during conceptification, were we had:
</p>

<blockquote><pre>
template&lt;InputIterator InIter, class OutIter&gt;
  requires OutputIterator&lt;OutIter, RvalueOf&lt;InIter::value_type&gt;::type&gt;
        &amp;&amp; EqualityComparable&lt;InIter::value_type&gt;
        &amp;&amp; HasAssign&lt;InIter::value_type, InIter::reference&gt;
        &amp;&amp; Constructible&lt;InIter::value_type, InIter::reference&gt;
  OutIter unique_copy(InIter first, InIter last, OutIter result);

template&lt;InputIterator InIter, class OutIter,
         EquivalenceRelation&lt;auto, InIter::value_type&gt; Pred&gt;
  requires OutputIterator&lt;OutIter, RvalueOf&lt;InIter::value_type&gt;::type&gt;
        &amp;&amp; HasAssign&lt;InIter::value_type, InIter::reference&gt;
        &amp;&amp; Constructible&lt;InIter::value_type, InIter::reference&gt;
        &amp;&amp; CopyConstructible&lt;Pred&gt;
  OutIter unique_copy(InIter first, InIter last, OutIter result, Pred pred);
</pre></blockquote>

<p>
Note that EqualityComparable implied an equivalence relation.
</p>

<p><i>[
N.B. <code>adjacent_find</code> was also specified to require
<code>EquivalenceRelation</code>, but that was considered as a defect in
concepts, see <a href="lwg-closed.html#1000" title="adjacent_find is over-constrained (Status: NAD Concepts)">1000</a><sup><a href="https://cplusplus.github.io/LWG/issue1000" title="Latest snapshot">(i)</a></sup>
]</i></p>


<p><i>[
2009-10-31 Howard adds:
]</i></p>


<blockquote><p>
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
</p></blockquote>



<p id="res-1241"><b>Proposed resolution:</b></p>
<p>
Change 26.7.9 <a href="https://wg21.link/alg.unique">[alg.unique]</a>/5 as indicated:
</p>

<blockquote><pre>
template&lt;class InputIterator, class OutputIterator&gt;
  OutputIterator
    unique_copy(InputIterator first, InputIterator last, OutputIterator result);

template&lt;class InputIterator, class OutputIterator, class BinaryPredicate&gt;
  OutputIterator
    unique_copy(InputIterator first, InputIterator last,
                OutputIterator result, BinaryPredicate pred);
</pre>
<blockquote><p>
<i>Requires:</i> <ins>The comparison function shall be an equivalence
relation.</ins> The ranges <code>[first,last)</code> and
<code>[result,result+(last-first))</code> shall not overlap. The expression
<code>*result = *first</code> shall be valid. If neither
<code>InputIterator</code> nor <code>OutputIterator</code> meets the
requirements of forward iterator then the value type of
<code>InputIterator</code> shall be <code>CopyConstructible</code> (34) and
<code>CopyAssignable</code> (table 36). Otherwise <code>CopyConstructible</code>
is not required.
</p></blockquote>
</blockquote>





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