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<title>Issue 1093: Multiple definitions for random_shuffle algorithm</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#Resolved">Resolved</a> status.</em></p>
<h3 id="1093"><a href="lwg-defects.html#1093">1093</a>. Multiple definitions for <code>random_shuffle</code> algorithm</h3>
<p><b>Section:</b> 26.7.13 <a href="https://wg21.link/alg.random.shuffle">[alg.random.shuffle]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-03-22 <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.random.shuffle">issues</a> in [alg.random.shuffle].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>

<p>
There are a couple of issues with the declaration of the <code>random_shuffle</code>
algorithm accepting a random number engine.
</p>

<ol style="list-style-type:lower-roman">
<li>
The Iterators must be shuffle iterators, yet this requirement is missing.
</li>
<li>
The <code>RandomNumberEngine</code> concept is now provided by the random number
library
(<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2836.pdf">n2836</a>)
and the placeholder should be removed.
</li>
</ol>

<p><i>[
2009-05-02 Daniel adds:
]</i></p>


<blockquote>
<p>
this issue completes adding necessary requirement to the
third new <code>random_shuffle</code> overload. The current suggestion is:
</p>

<blockquote><pre>
template&lt;RandomAccessIterator Iter, UniformRandomNumberGenerator Rand&gt;
requires ShuffleIterator&lt;Iter&gt;
void random_shuffle(Iter first, Iter last, Rand&amp;&amp; g);
</pre></blockquote>

<p>
IMO this is still insufficient and I suggest to add the requirement
</p>
<blockquote><pre>
Convertible&lt;Rand::result_type, Iter::difference_type&gt;
</pre></blockquote>
<p>
to the list (as the two other overloads already have).
</p>

<p>
Rationale:
</p>

<blockquote>
<p>
Its true that this third overload is somewhat different from the remaining
two. Nevertheless we know from <code>UniformRandomNumberGenerator</code>, that
it's <code>result_type</code> is an integral type and that it satisfies
<code>UnsignedIntegralLike&lt;result_type&gt;</code>.
</p>
<p>
To realize it's designated task, the algorithm has to invoke the
<code>Callable</code> aspect of <code>g</code> and needs to perform some algebra involving
it's <code>min()/max()</code> limits to compute another index value that
at this point is converted into <code>Iter::difference_type</code>. This is so,
because 24.3.5.7 <a href="https://wg21.link/random.access.iterators">[random.access.iterators]</a> uses this type as argument
of it's algebraic operators. Alternatively consider the equivalent
iterator algorithms in 24.4.3 <a href="https://wg21.link/iterator.operations">[iterator.operations]</a> with the same result.
</p>
<p>
This argument leads us to the conclusion that we also need
<code>Convertible&lt;Rand::result_type, Iter::difference_type&gt;</code> here.
</p>
</blockquote>

</blockquote>

<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote>
<p>
Alisdair notes that point (ii) has already been addressed.
</p>
<p>
We agree with the proposed resolution to point (i)
with Daniel's added requirement.
</p>
<p>
Move to Review.
</p>
</blockquote>

<p><i>[
2009-06-05 Daniel updated proposed wording as recommended in Batavia.
]</i></p>


<p><i>[
2009-07-28 Alisdair adds:
]</i></p>


<blockquote><p>
Revert to Open, with a note there is consensus on direction but the
wording needs updating to reflect removal of concepts.
</p></blockquote>

<p><i>[
2009-10 post-Santa Cruz:
]</i></p>


<blockquote><p>
Leave Open, Walter to work on it.
</p></blockquote>

<p><i>[
2010 Pittsburgh:  Moved to <del>NAD Editorial</del><ins>Resolved</ins>, addressed by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3056.pdf">N3056</a>.
]</i></p>




<p><b>Rationale:</b></p>
<p>
Solved by
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3056.pdf">N3056</a>.
</p>


<p id="res-1093"><b>Proposed resolution:</b></p>
<p>
Change in  [algorithms.syn] and 26.7.13 <a href="https://wg21.link/alg.random.shuffle">[alg.random.shuffle]</a>:
</p>

<blockquote><pre>
<del>concept UniformRandomNumberGenerator&lt;typename Rand&gt; { }</del>
template&lt;RandomAccessIterator Iter, UniformRandomNumberGenerator Rand&gt;
  <ins>requires ShuffleIterator&lt;Iter&gt; &amp;&amp;
  Convertible&lt;Rand::result_type, Iter::difference_type&gt;</ins>
  void random_shuffle(Iter first, Iter last, Rand&amp;&amp; g);
</pre></blockquote>






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