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<title>Issue 2074: Off by one error in std::reverse_copy</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="2074"><a href="lwg-defects.html#2074">2074</a>. Off by one error in <code>std::reverse_copy</code></h3>
<p><b>Section:</b> 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> Peter Miller <b>Opened:</b> 2011-08-17 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.reverse">issues</a> in [alg.reverse].</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>

<p>
The output of the program below should be:
</p>
<blockquote><pre>
"three two one null \n"
</pre></blockquote>
<p>
But when <code>std::reverse_copy</code> is implemented as described in N3291 26.7.10 <a href="https://wg21.link/alg.reverse">[alg.reverse]</a> 
it's:
</p>
<blockquote><pre>
"null three two one \n"
</pre></blockquote>
<p>
because there's an off by one error in 26.7.10 <a href="https://wg21.link/alg.reverse">[alg.reverse]</a>&#47;4; the definition should read:
</p>
<blockquote><pre>
*(result + (last - first) <span style="color:#C80000;font-weight:bold">- 1</span> - i) = *(first + i)
</pre></blockquote>
<p>
Test program:
</p>
<blockquote><pre>
#include &lt;algorithm&gt;
#include &lt;iostream&gt;

template &lt;typename BiIterator, typename OutIterator&gt;
auto
reverse_copy_as_described_in_N3291(
  BiIterator first, BiIterator last, OutIterator result )
-&gt; OutIterator
{
  // 25.3.10&#47;4 [alg.reverse]:
  // "...such that for any non-negative integer i &lt; (last - first)..."
  for ( unsigned i = 0; i &lt; ( last - first ); ++i )
    // "...the following assignment takes place:"
    *(result + (last - first) - i) = *(first + i);

  // 25.3.10&#47;6
  return result + (last - first);
}

int main()
{
  using std::begin;
  using std::end;
  using std::cout;

  static const char*const in[3]  { "one", "two", "three" };
  const char*             out[4] { "null", "null", "null", "null" };

  reverse_copy_as_described_in_N3291( begin( in ), end( in ), out );

  for ( auto s : out )
    cout &lt;&lt; s &lt;&lt; ' ';

  cout &lt;&lt; std::endl;

  return 0;
}
</pre></blockquote>

<p><i>[2012, Kona]</i></p>

<p>
Move to Ready.
</p>

<p><i>[2012, Portland: applied to WP]</i></p>




<p id="res-2074"><b>Proposed resolution:</b></p>
<p>This wording is relative to the FDIS.</p>

<p>
Change 26.7.10 <a href="https://wg21.link/alg.reverse">[alg.reverse]</a> p4 as follows:
</p> 
<blockquote><pre>
template&lt;class BidirectionalIterator, class OutputIterator&gt;
  OutputIterator
    reverse_copy(BidirectionalIterator first,
                 BidirectionalIterator last, OutputIterator result);
</pre><blockquote><p>
-4- <i>Effects</i>: Copies the range [<code>first,last</code>) to the range [<code>result,result+(last-first)</code>) 
such that for any non-negative integer <code>i &lt; (last - first)</code> the following assignment takes place: 
<code>*(result + (last - first) <ins>- 1</ins> - i) = *(first + i)</code>.
<p/>
-5- <i>Requires</i>: The ranges [<code>first,last</code>) and [<code>result,result+(last-first)</code>) shall not overlap.
<p/>
-6- <i>Returns</i>: <code>result + (last - first)</code>.
<p/>
-7- <i>Complexity</i>: Exactly <code>last - first</code> assignments.
</p></blockquote></blockquote>





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