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<title>Issue 2405: rotate()'s return value is incorrect when middle == first</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#NAD">NAD</a> status.</em></p>
<h3 id="2405"><a href="lwg-closed.html#2405">2405</a>. <code>rotate()</code>'s return value is incorrect when <code>middle == first</code></h3>
<p><b>Section:</b> 26.7.11 <a href="https://wg21.link/alg.rotate">[alg.rotate]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2014-06-14 <b>Last modified:</b> 2014-06-17</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.rotate">issues</a> in [alg.rotate].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
When LWG <a href="lwg-defects.html#488" title="rotate throws away useful information (Status: CD1)">488</a><sup><a href="https://cplusplus.github.io/LWG/issue488" title="Latest snapshot">(i)</a></sup> was resolved, the intention was to return "where the subrange <code>[first, middle)</code> starts after 
the rotate is performed". However, this wasn't achieved in one case.
<p/>
When <code>middle == last</code>, <code>rotate()</code> does nothing and returns <code>first</code>. This is good.
<p/>
When <code>middle == first</code>, <code>rotate()</code> does nothing and returns <code>last</code>. This is bad. In addition to being 
inconsistent with the other do-nothing case, it prevents <code>rotate()</code> from providing the useful guarantee that LWG 
<a href="lwg-defects.html#488" title="rotate throws away useful information (Status: CD1)">488</a><sup><a href="https://cplusplus.github.io/LWG/issue488" title="Latest snapshot">(i)</a></sup> wanted: when <code>[first, last)</code> is non-empty, <code>rotate()</code>'s return value should always be 
dereferenceable to get the originally-first element.
<p/>
Howard Hinnant:
<p/>
As the author of LWG <a href="lwg-defects.html#488" title="rotate throws away useful information (Status: CD1)">488</a><sup><a href="https://cplusplus.github.io/LWG/issue488" title="Latest snapshot">(i)</a></sup> I can assure everyone that the edge cases the proposed resolution specifies were 
not specified by accident. <code>rotate(i, i, j)</code> should return <code>j</code> and <code>rotate(i, j, j)</code> should return <code>i</code>.  
Doing otherwise will break working code. These return values were motivated by the uses of <code>rotate</code> in the implementation 
of algorithms such as <code>stable_partition</code> and <code>inplace_merge</code>. The results of these edge cases were not chosen lightly.
<p/>
Also a good read:
<p/>
<a href="http://www.cs.rpi.edu/~musser/gsd/notes-on-programming-2006-10-13.pdf">notes-on-programming-2006-10-13</a>
<p/>
Summary: NAD.
</p>

<p><i>[2014-06-16 Rapperswill]</i></p>

<p>
Closed as NAD.
</p>




<p id="res-2405"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3936.</p>

<ol>
<li><p>Change 26.7.11 <a href="https://wg21.link/alg.rotate">[alg.rotate]</a> p2 as indicated:</p>

<blockquote>
<p>
-2- <i>Returns</i>: <ins>If <code>middle == first</code>, returns <code>first</code>. Otherwise, returns</ins> <code>first + (last - middle)</code>.
</p>
</blockquote>
</li>

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