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<title>Issue 1149: Reformulating NonemptyRange axiom</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_Concepts">NAD Concepts</a> status.</em></p>
<h3 id="1149"><a href="lwg-closed.html#1149">1149</a>. Reformulating NonemptyRange axiom</h3>
<p><b>Section:</b> 99 [rand.concept.urng] <b>Status:</b> <a href="lwg-active.html#NAD_Concepts">NAD Concepts</a>
 <b>Submitter:</b> Walter Brown <b>Opened:</b> 2009-06-25 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD Concepts">NAD Concepts</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In 99 [rand.concept.urng], we have the following:
</p>
<blockquote><pre>
concept UniformRandomNumberGenerator&lt;typename G&gt; : Callable&lt;G&gt; {
  ...
  axiom NonemptyRange(G&amp; g) {
    G::min() &lt; G::max();
  }
  ...
}
</pre></blockquote>

<p>
Since the parameter <code>G</code> is in scope throughout the concept, there is no
need for the axiom to be further parameterized, and so the axiom can be
slightly simplified as:
</p>

<blockquote><pre>
axiom NonemptyRange()  {
  G::min() &lt; G::max();
}
</pre></blockquote>

<p>
We can further reformulate so as to avoid any axiom machinery as:
</p>

<blockquote><pre>
requires True&lt; G::min() &lt; G::max() &gt;;
</pre></blockquote>

<p>
This is not only a simpler statement of the same requirement, but also
forces the requirement to be checked.
</p>

<p><i>[
Post-Rapperswil:
]</i></p>


<blockquote><p>
Moved to Tentatively Ready after 5 positive votes on c++std-lib.
</p></blockquote>


<p id="res-1149"><b>Proposed resolution:</b></p>
<p>
In 99 [rand.concept.urng], replace the <code>NonemptyRange</code> axiom by:
</p>

<blockquote><pre>
<del>axiom NonemptyRange(G&amp; g) { 
   G::min() &lt; G::max(); 
}</del>
<ins>requires True&lt; G::min() &lt; G::max() &gt;;</ins>
</pre></blockquote>






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