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<title>Issue 2325: minmax_element()'s behavior differing from max_element()'s should be noted</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="2325"><a href="lwg-defects.html#2325">2325</a>. <code>minmax_element()</code>'s behavior differing from <code>max_element()</code>'s should be noted</h3>
<p><b>Section:</b> 26.8.9 <a href="https://wg21.link/alg.min.max">[alg.min.max]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2013-09-21 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#alg.min.max">active issues</a> in [alg.min.max].</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.min.max">issues</a> in [alg.min.max].</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>
26.8.9 <a href="https://wg21.link/alg.min.max">[alg.min.max]</a>/23 says that <code>max_element()</code> finds the <em>first</em> biggest element, while /25 says that 
<code>minmax_element()</code> finds the <em>last</em> biggest element. This significant difference is unusual &mdash; it means that 
<code>minmax_element(args)</code> is not equivalent to <code>make_pair(min_element(args), max_element(args))</code>, whereas the other 
major "two for one" algorithm <code>equal_range(args)</code> is equivalent to <code>make_pair(lower_bound(args), upper_bound(args))</code>.  
<code>minmax_element()</code>'s behavior is intentional &mdash; it is a fundamental consequence of the <code>3N/2</code> algorithm &mdash; 
but the Standardese does not draw attention to this in any way. This wording came from LWG <a href="lwg-defects.html#715" title="minmax_element complexity is too lax (Status: CD1)">715</a><sup><a href="https://cplusplus.github.io/LWG/issue715" title="Latest snapshot">(i)</a></sup>'s resolution (which 
changed the semantics but didn't mention it), citing CLRS for the algorithm &mdash; but CLRS doesn't mention the behavior for 
equivalent elements! The wording here deeply confused me (as an STL maintainer fixing an incorrect implementation) until I walked 
through the algorithm by hand and figured out the fundamental reason. It would be really nice for the Standard to provide a hint 
that something magical is happening here.
</p>

<p><i>[2014-06-06 Library reflector vote]</i></p>

<p>
The issue has been identified as Tentatively Ready based on six votes in favour.
</p>



<p id="res-2325"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3691.</p>

<ol>
<li><p>Add a footnote to 26.8.9 <a href="https://wg21.link/alg.min.max">[alg.min.max]</a>/25 as indicated:</p>

<blockquote><pre>
template&lt;class ForwardIterator&gt;
  pair&lt;ForwardIterator, ForwardIterator&gt;
    minmax_element(ForwardIterator first, ForwardIterator last);
template&lt;class ForwardIterator, class Compare&gt;
  pair&lt;ForwardIterator, ForwardIterator&gt;
    minmax_element(ForwardIterator first, ForwardIterator last, Compare comp);
</pre><blockquote>
<p>
-25- <i>Returns:</i> <code>make_pair(first, first)</code> if <code>[first,last)</code> is empty, otherwise <code>make_pair(m, M)</code>, where
<code>m</code> is the first iterator in <code>[first,last)</code> such that no iterator in the range refers to a smaller element,
and where <code>M</code> is the last iterator <ins>[Footnote: This behavior intentionally differs from <code>max_element()</code>.]</ins> 
in <code>[first,last)</code> such that no iterator in the range refers to a larger element.
</p>
</blockquote></blockquote>
</li>

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