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<title>Issue 1432: random_shuffle signatures are inconsistent</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++11">C++11</a> status.</em></p>
<h3 id="1432"><a href="lwg-defects.html#1432">1432</a>. <code>random_shuffle</code> signatures are inconsistent</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#C++11">C++11</a>
 <b>Submitter:</b> INCITS <b>Opened:</b> 2010-08-25 <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#C++11">C++11</a> status.</p>
<p><b>Duplicate of:</b> <a href="lwg-closed.html#1433" title="random_shuffle and shuffle should have consistent signatures (Status: Dup)">1433</a></p>
<p><b>Discussion:</b></p>

<p><b>Addresses US-121, GB-119</b></p>

<p>
<code>random_shuffle</code> and <code>shuffle</code> should be consistent in how
they accept their source of randomness: either both by rvalue reference or 
both by lvalue reference.
</p>

<p><i>[
Post-Rapperswil, Daniel provided wording
]</i></p>


<p>
The signatures of the <code>shuffle</code> and <code>random_shuffle</code> algorithms are different
in regard to the support of rvalues and lvalues of the provided generator:
</p>

<blockquote><pre>
template&lt;class RandomAccessIterator, class RandomNumberGenerator&gt;
void random_shuffle(RandomAccessIterator first,
RandomAccessIterator last,
RandomNumberGenerator<b>&amp;&amp;</b> rand);
</pre></blockquote>

<blockquote><pre>
template&lt;class RandomAccessIterator, class UniformRandomNumberGenerator&gt;
void shuffle(RandomAccessIterator first,
RandomAccessIterator last,
UniformRandomNumberGenerator<b>&amp;</b> g);
</pre></blockquote>

<p>
The first form uses the perfect forwarding signature and that change compared to
<code>C++03</code> was done intentionally as shown in the first rvalue proposal 
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1690.html#Improved%20random_shuffle">papers</a>.
</p>

<p>
While it is true, that random generators are excellent examples of stateful
functors, there still exist good reasons to support rvalues as arguments:
</p>

<ol>
<li>If one of the shuffle algorithms is called with the intention to shuffle items with a reproducible ordering 
	from a given generator class, it makes sense to create a generator exactly at the call point.
</li>
<li>Other algorithms with similar need for stateful functors (like <code>std::generate</code> and <code>std::generate_n</code>) 
	accept both rvalues and lvalues as well.
</li>
<li>Given the deduction rules for perfect forwarding it is hard for a user to produce code that does the wrong thing 
unintentionally. Any lvalue generator will deduce an lvalue-reference and behave as in <code>C++03</code>. In the specific 
cases, where rvalues are provided, the argument will be accepted instead of being rejected.
</li>
</ol>

<p>
Arguments have been raised that accepting rvalues is error-prone or even fundamentally wrong. The author of this 
proposal disagrees with that position for two additional reasons:
</p>

<ol>
<li>Enforcing lvalues as arguments won't prevent user code to enforce what they
want. So given
<blockquote><pre>
my_generator get_generator(int size);
</pre></blockquote>
instead of writing
<blockquote><pre>
std::vector&lt;int&gt; v = ...;
std::shuffle(v.begin(), v.end(), get_generator(v.size()));
</pre></blockquote>
they will just write
<blockquote><pre>
std::vector&lt;int&gt; v = ...;
auto gen = get_generator(v.size());
std::shuffle(v.begin(), v.end(), gen);
</pre></blockquote>
and feel annoyed about the need for it.
</li>
<li>Generators may be copyable and movable, and random number engines are <em>required</em> to be <code>CopyConstructible</code> 
and this is obviously a generally useful property for such objects. It is also useful and sometimes necessary to start a 
generator with exactly a specific seed again and again and thus to provide a new generator (or a copy) for each call. The 
<code>CopyConstructible</code> requirements allow providing rvalues of generators and thus this idiom must be useful as well. 
Therefore preventing <code>[random_]shuffle</code> to accept rvalues is an unnecessary restriction which doesn't prevent any 
user-error, if there would exist one.
</li>
</ol>

<p>
Thus this proposal recommends to make both <code>shuffle</code> functions consistent and perfectly forward-able.
</p>

<blockquote><p>
Moved to Tentatively Ready after 6 positive votes on c++std-lib.
</p></blockquote>

<p><i>[
Adopted at 2010-11 Batavia
]</i></p>




<p id="res-1432"><b>Proposed resolution:</b></p>

<ol>
<li>Change [algorithms.general], header <code>&lt;algorithm&gt;</code> synopsis as indicated:
<blockquote><pre>
template&lt;class RandomAccessIterator, class UniformRandomNumberGenerator&gt;
void shuffle(RandomAccessIterator first, RandomAccessIterator last,
UniformRandomNumberGenerator&amp;<ins>&amp;</ins> rand);
</pre></blockquote>
</li>
<li>Change the prototype description of [alg.random.shuffle] as indicated:
<blockquote><pre>
template&lt;class RandomAccessIterator, class UniformRandomNumberGenerator&gt;
void shuffle(RandomAccessIterator first, RandomAccessIterator last,
UniformRandomNumberGenerator&amp;<ins>&amp;</ins> rand);
</pre></blockquote>
</li>
</ol>






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