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<title>Issue 2689: Parallel versions of std::copy and std::move shouldn't be in order</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++17">C++17</a> status.</em></p>
<h3 id="2689"><a href="lwg-defects.html#2689">2689</a>. Parallel versions of <code>std::copy</code> and <code>std::move</code> shouldn't be in order</h3>
<p><b>Section:</b> 26.7.1 <a href="https://wg21.link/alg.copy">[alg.copy]</a>, 26.7.2 <a href="https://wg21.link/alg.move">[alg.move]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2016-03-23 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</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++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
26.3.5 <a href="https://wg21.link/algorithms.parallel.overloads">[algorithms.parallel.overloads]</a>/2 says that "Unless otherwise specified, the semantics of 
<code>ExecutionPolicy</code> algorithm overloads are identical to their overloads without."
<p/>
There's no "otherwise specified" for the <code>ExecutionPolicy</code> overloads for <code>std::copy</code> and <code>std::move</code>, 
so the requirement that they "start[] from first and proceed[] to last" in the original algorithm's description would 
seem to apply, which defeats the whole point of adding a parallel overload.
</p>

<p><i>[2016-05 Issues Telecon]</i></p>

<p>
Marshall noted that all three versions of copy have subtly different wording, and suggested that they should not.
</p>


<p id="res-2689"><b>Proposed resolution:</b></p>
<p>
This wording is relative to N4582.
</p>
<ol>
<li><p>Insert the following paragraphs after 26.7.1 <a href="https://wg21.link/alg.copy">[alg.copy]</a>/4:</p>

<blockquote><pre>
<ins>template&lt;class ExecutionPolicy, class InputIterator, class OutputIterator&gt;
  OutputIterator copy(ExecutionPolicy&amp;&amp; policy, InputIterator first, InputIterator last,
                      OutputIterator result);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires</i>: The ranges <code>[first, last)</code> and <code>[result, result + (last - first))</code> shall not 
overlap.</ins>
<p/>
<ins>-?- <i>Effects</i>: Copies elements in the range <code>[first, last)</code> into the range <code>[result, result + (last -
first))</code>. For each non-negative integer <code>n &lt; (last - first)</code>, performs <code>*(result + n) = *(first + n)</code>.</ins>
<p/>
<ins>-?- <i>Returns</i>: <code>result + (last - first)</code>.</ins>
<p/>
<ins>-?- <i>Complexity</i>: Exactly <code>last - first</code> assignments.</ins>
</p>
</blockquote>
</blockquote>
</li>

<li><p>Insert the following paragraphs after 26.7.2 <a href="https://wg21.link/alg.move">[alg.move]</a>/4:</p>

<blockquote><pre>
<ins>template&lt;class ExecutionPolicy, class InputIterator, class OutputIterator&gt;
  OutputIterator move(ExecutionPolicy&amp;&amp; policy, InputIterator first, InputIterator last,
                      OutputIterator result);</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires</i>: The ranges <code>[first, last)</code> and <code>[result, result + (last - first))</code> shall not 
overlap.</ins>
<p/>
<ins>-?- <i>Effects</i>: Moves elements in the range <code>[first, last)</code> into the range <code>[result, result + (last -
first))</code>. For each non-negative integer <code>n &lt; (last - first)</code>, performs 
<code>*(result + n) = std::move(*(first + n))</code>.</ins>
<p/>
<ins>-?- <i>Returns</i>: <code>result + (last - first)</code>.</ins>
<p/>
<ins>-?- <i>Complexity</i>: Exactly <code>last - first</code> assignments.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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