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<title>Issue 2967: std::equal on empty ranges</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="2967"><a href="lwg-closed.html#2967">2967</a>. <code>std::equal</code> on empty ranges</h3>
<p><b>Section:</b> 26.6.13 <a href="https://wg21.link/alg.equal">[alg.equal]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Gennaro Prota <b>Opened:</b> 2017-05-26 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#alg.equal">issues</a> in [alg.equal].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The description of the <code>std::equal()</code> algorithm in the standard doesn't
make clear what the result of it is on empty ranges:
</p>
<blockquote><pre>
std::equal(first, first, second) ; // what does this return?
</pre></blockquote>
<p>
It should IMHO return true (two empty ranges are always equal).
</p>

<p><i>[2017-07 Toronto Monday issue prioritization]</i></p>

<p>Closing as NAD; the existing wording covers empty sequences</p>


<p id="res-2967"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4659">N4659</a>.</p>

<ol>
<li>
<p>Edit 26.6.13 <a href="https://wg21.link/alg.equal">[alg.equal]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> The current wording presented below uses two times the unusual phrase "[&hellip;] return [&hellip;]" 
instead of "[&hellip;] return<b>s</b> [&hellip;]". The project editor is kindly asked to consider to replace these unusual 
wording forms by the usual one. &mdash; <i>end drafting note</i>]
</p>
</blockquote>

<blockquote>
<pre>
template&lt;class InputIterator1, class InputIterator2&gt;
  bool equal(InputIterator1 first1, InputIterator1 last1,
             InputIterator2 first2);
[&hellip;]
</pre>
<blockquote>
<p>
-1- <i>Remarks:</i> If <code>last2</code> was not given in the argument list, it denotes <code>first2 + (last1 - first1)</code> below.
<p/>
-2- <i>Returns:</i> <ins>If <code>[first1, last1)</code> and <code>[first2, last2)</code> are both empty, returns <code>true</code>.</ins> If 
<code>last1 - first1 != last2 - first2</code>, return <code>false</code>. Otherwise return <code>true</code> if for every iterator 
<code>i</code> in the range <code>[first1, last1)</code> the following corresponding conditions hold: 
<code>*i == *(first2 + (i - first1)), pred(*i, *(first2 + (i - first1))) != false</code>. Otherwise, returns <code>false</code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
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