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<title>Issue 2716: Specification of shuffle and sample disallows lvalue URNGs</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="2716"><a href="lwg-defects.html#2716">2716</a>. Specification of <code>shuffle</code> and <code>sample</code> disallows lvalue <code>URNG</code>s</h3>
<p><b>Section:</b> 26.7.12 <a href="https://wg21.link/alg.random.sample">[alg.random.sample]</a>, 26.7.13 <a href="https://wg21.link/alg.random.shuffle">[alg.random.shuffle]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2016-05-24 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</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>
<code>std::shuffle</code> and <code>std::sample</code> each accepts a <code>UniformRandomNumberGenerator&amp;&amp;</code> argument 
and says that "<code>UniformRandomNumberGenerator</code> shall meet the requirements of a uniform random number generator 
(29.5.3.3 <a href="https://wg21.link/rand.req.urng">[rand.req.urng]</a>) type".
<p/>
29.5.3.3 <a href="https://wg21.link/rand.req.urng">[rand.req.urng]</a>/2 in turn starts with "A class <code>G</code> satisfies the requirements of a uniform random 
number generator if [&hellip;]". 
<p/>
If an lvalue is passed, <code>UniformRandomNumberGenerator</code> is a reference type, not a class, and in fact expressions 
like <code>G::min()</code> will not compile if <code>G</code> is a reference type.</p>

<p><i>[2016-06 Oulu]</i></p>

<p>Moved to P0/Ready during issues prioritization.</p>
<p>Friday: status to Immediate</p>


<p id="res-2716"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4582.</p>
<ol>
<li><p>Edit 26.7.12 <a href="https://wg21.link/alg.random.sample">[alg.random.sample]</a>/1 as indicated:</p>

<blockquote>
<pre>
template&lt;class PopulationIterator, class SampleIterator,
         class Distance, class UniformRandomNumberGenerator&gt;
  SampleIterator sample(PopulationIterator first, PopulationIterator last,
                        SampleIterator out, Distance n,
                        UniformRandomNumberGenerator&amp;&amp; g);
</pre>
<blockquote>
<p>
-1- <i>Requires::</i>
</p>
<ol style="list-style-type: none">
<li>[&hellip;]</li>
<li>(1.6) &mdash; <code><ins>remove_reference_t&lt;</ins>UniformRandomNumberGenerator<ins>&gt;</ins></code> 
shall meet the requirements of a uniform random number generator
type (26.6.1.3) whose return type is convertible to <code>Distance</code>.</li>
<li>[&hellip;]</li>
</ol>
</blockquote>
</blockquote>
</li>

<li><p>Edit 26.7.13 <a href="https://wg21.link/alg.random.shuffle">[alg.random.shuffle]</a>/2 as indicated:</p>

<blockquote>
<pre>
template&lt;class RandomAccessIterator, class UniformRandomNumberGenerator&gt;
  void shuffle(RandomAccessIterator first,
               RandomAccessIterator last,
               UniformRandomNumberGenerator&amp;&amp; g);
</pre>
<blockquote>
<p>
-1- [&hellip;]
<p/>
-2- <i>Requires:</i> <code>RandomAccessIterator</code> shall satisfy the requirements of <code>ValueSwappable</code> (17.6.3.2). 
The type <code><ins>remove_reference_t&lt;</ins>UniformRandomNumberGenerator<ins>&gt;</ins></code> shall meet 
the requirements of a uniform random number generator (26.6.1.3) type whose return type is convertible to 
<code>iterator_traits&lt;RandomAccessIterator&gt;::difference_type</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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