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<title>Issue 515: Random number engine traits</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="515"><a href="lwg-closed.html#515">515</a>. Random number engine traits</h3>
<p><b>Section:</b> 29.5.2 <a href="https://wg21.link/rand.synopsis">[rand.synopsis]</a>, 99 [tr.rand.synopsis] <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Walter Brown <b>Opened:</b> 2005-07-03 <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#rand.synopsis">issues</a> in [rand.synopsis].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
To accompany the concept of a pseudo-random number engine as defined in Table 17,
we propose and recommend an adjunct template, engine_traits, to be declared in
[tr.rand.synopsis] as:
</p>
<blockquote><pre>
template&lt; class PSRE &gt;
class engine_traits;
</pre></blockquote>
<p>
This template's primary purpose would be as an aid to generic programming involving
pseudo-random number engines.  Given only the facilities described in tr1, it would
be very difficult to produce any algorithms involving the notion of a generic engine.
The intent of this proposal is to  provide, via engine_traits&lt;&gt;, sufficient
descriptive information to allow an algorithm to employ a pseudo-random number engine
without regard to its exact type, i.e., as a template parameter.
</p>
<p>
For example, today it is not possible to write an efficient generic function that
requires any specific number of random bits.  More specifically, consider a
cryptographic application that internally needs 256 bits of randomness per call:
</p>
<blockquote><pre>
template&lt; class Eng, class InIter, class OutIter &gt;
void crypto( Eng&amp; e, InIter in, OutIter out );
</pre></blockquote>
<p>
Without knowning the number of bits of randomness produced per call to a provided
engine, the algorithm has no means of determining how many times to call the engine.
</p>
<p>
In a new section [tr.rand.eng.traits], we proposed to define the engine_traits
template as: 
</p>
<blockquote><pre>
template&lt; class PSRE &gt;
class engine_traits
{
  static  std::size_t  bits_of_randomness = 0u;
  static  std::string  name()  { return "unknown_engine"; }
  // TODO: other traits here
};
</pre></blockquote>
<p>
Further, each engine described in [tr.rand.engine] would be accompanied by a
complete specialization of this new engine_traits template.
</p>



<p id="res-515"><b>Proposed resolution:</b></p>
<p><i>[
Berlin:  Walter: While useful for implementation per TR1, N1932 has no need for this
feature.  Recommend close as NAD.
]</i></p>



<p><b>Rationale:</b></p>
<p>
Recommend NAD,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1932.pdf">N1932</a>,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2111.pdf">N2111</a>
covers this.  Already in WP.
</p>





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