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<title>Issue 2039: Issues with std::reverse and std::copy_if</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++14">C++14</a> status.</em></p>
<h3 id="2039"><a href="lwg-defects.html#2039">2039</a>. Issues with <code>std::reverse</code> and <code>std::copy_if</code></h3>
<p><b>Section:</b> 26.7.1 <a href="https://wg21.link/alg.copy">[alg.copy]</a>, 26.7.10 <a href="https://wg21.link/alg.reverse">[alg.reverse]</a> <b>Status:</b> <a href="lwg-active.html#C++14">C++14</a>
 <b>Submitter:</b> Nikolay Ivchenkov <b>Opened:</b> 2011-03-02 <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.copy">active issues</a> in [alg.copy].</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.copy">issues</a> in [alg.copy].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++14">C++14</a> status.</p>
<p><b>Discussion:</b></p>

<ol>
<li><p>In the description of <code>std::reverse</code></p>
<blockquote><p>
<i>Effects</i>: For each non-negative integer <code>i &lt;= (last - first)/2</code>, applies <code>iter_swap</code> 
to all pairs of iterators <code>first + i</code>, <code>(last - i) - 1</code>.
</p></blockquote>
<p>
should be changed to
</p><blockquote><p>
<i>Effects</i>: For each non-negative integer <code>i <strong>&lt;</strong> (last - first)/2</code>, applies <code>iter_swap</code> 
to all pairs of iterators <code>first + i</code>, <code>(last - i) - 1</code>.
</p></blockquote><p>
Here <code>i</code> shall be strictly less than <code>(last - first)/2</code>.
</p>
</li>
<li><p>In the description of <code>std::copy_if</code> <i>Returns</i> paragraph is missing.</p></li>
</ol>

<p><i>[2011-03-02: Daniel drafts wording]</i></p>



<p id="res-2039"><b>Proposed resolution:</b></p>
<ol>
<li><p>Modify 26.7.10 <a href="https://wg21.link/alg.reverse">[alg.reverse]</a> p. 1 as indicated:</p>

<blockquote><p>
1 <i>Effects</i>: For each non-negative integer <code>i &lt;<del>=</del> (last - first)/2</code>, applies <code>iter_swap</code> 
to all pairs of iterators <code>first + i</code>, <code>(last - i) - 1</code>.
</p></blockquote>
</li>

<li><p>Add the following <i>Returns</i> element after 26.7.1 <a href="https://wg21.link/alg.copy">[alg.copy]</a> p. 9:</p>

<blockquote><pre>
template&lt;class InputIterator, class OutputIterator, class Predicate&gt;
OutputIterator copy_if(InputIterator first, InputIterator last,
   OutputIterator result, Predicate pred);
</pre><blockquote><p>
8 <i>Requires</i>: The ranges <code>[first,last)</code> and <code>[result,result + (last - first))</code> shall not overlap.
<p/>
9 <i>Effects</i>: Copies all of the elements referred to by the iterator <code>i</code> in the range <code>[first,last)</code> 
for which <code>pred(*i)</code> is true.
<p/>
<ins>?? <i>Returns</i>: The end of the resulting range.</ins>
<p/>
10 <i>Complexity</i>: Exactly <code>last - first</code> applications of the corresponding predicate.
<p/>
11 <i>Remarks</i>: Stable.
</p></blockquote></blockquote>
</li>
</ol>





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