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<title>Issue 1206: Incorrect requires for move_backward and copy_backward</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="1206"><a href="lwg-defects.html#1206">1206</a>. Incorrect requires for <code>move_backward</code> and <code>copy_backward</code></h3>
<p><b>Section:</b> 26.7.2 <a href="https://wg21.link/alg.move">[alg.move]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2009-09-13 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</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>
26.7.2 <a href="https://wg21.link/alg.move">[alg.move]</a>, p6 says:
</p>

<blockquote>
<pre>
template&lt;class BidirectionalIterator1, class BidirectionalIterator2&gt;
  BidirectionalIterator2
    move_backward(BidirectionalIterator1 first,
                  BidirectionalIterator1 last,
                  BidirectionalIterator2 result);
</pre>
<blockquote>
<p>...</p>
<p>
<i>Requires:</i> <code>result</code> shall not be in the range
<code>[first,last)</code>.
</p>
</blockquote>
</blockquote>

<p>
This is essentially an "off-by-one" error.
</p>

<p>
When <code>result == last</code>, which
<em>is</em> allowed by this specification, then the range <code>[first, last)</code>
is being move assigned into the range <code>[first, last)</code>.  The <code>move</code>
(forward) algorithm doesn't allow self move assignment, and neither should
<code>move_backward</code>.  So <code>last</code> should be included in the range which
<code>result</code> can not be in.
</p>

<p>
Conversely, when <code>result == first</code>, which <em>is not</em> allowed by this
specification, then the range <code>[first, last)</code>
is being move assigned into the range <code>[first - (last-first), first)</code>.
I.e. into a <em>non-overlapping</em> range.  Therefore <code>first</code> should
not be included in the range which <code>result</code> can not be in.
</p>

<p>
The same argument applies to <code>copy_backward</code> though copy assigning elements
to themselves (<code>result == last</code>) should be harmless (though is disallowed
by <code>copy</code>).
</p>

<p><i>[
2010 Pittsburgh:  Moved to Ready.
]</i></p>




<p id="res-1206"><b>Proposed resolution:</b></p>
<p>
Change 26.7.2 <a href="https://wg21.link/alg.move">[alg.move]</a>, p6:
</p>

<blockquote>
<pre>
template&lt;class BidirectionalIterator1, class BidirectionalIterator2&gt;
  BidirectionalIterator2
    move_backward(BidirectionalIterator1 first,
                  BidirectionalIterator1 last,
                  BidirectionalIterator2 result);
</pre>
<blockquote>
<p>...</p>
<p>
<i>Requires:</i> <code>result</code> shall not be in the range
<code><del>[</del><ins>(</ins>first,last<ins>]</ins><del>)</del></code>.
</p>
</blockquote>
</blockquote>

<p>
Change 26.7.1 <a href="https://wg21.link/alg.copy">[alg.copy]</a>, p13:
</p>

<blockquote>
<pre>
template&lt;class BidirectionalIterator1, class BidirectionalIterator2&gt;
  BidirectionalIterator2
    copy_backward(BidirectionalIterator1 first,
                  BidirectionalIterator1 last,
                  BidirectionalIterator2 result);
</pre>
<blockquote>
<p>...</p>
<p>
<i>Requires:</i> <code>result</code> shall not be in the range
<code><del>[</del><ins>(</ins>first,last<ins>]</ins><del>)</del></code>.
</p>
</blockquote>
</blockquote>





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