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<title>Issue 612: numeric_limits::is_modulo insufficiently defined</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#CD1">CD1</a> status.</em></p>
<h3 id="612"><a href="lwg-defects.html#612">612</a>. numeric_limits::is_modulo insufficiently defined</h3>
<p><b>Section:</b> 17.3.5.2 <a href="https://wg21.link/numeric.limits.members">[numeric.limits.members]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Chris Jefferson <b>Opened:</b> 2006-11-10 <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#numeric.limits.members">issues</a> in [numeric.limits.members].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
18.2.1.2 55 states that "A type is modulo if it is possible to add two
positive numbers together and have a result that wraps around to a
third number that is less".
This seems insufficient for the following reasons:
</p>

<ol>
<li>Doesn't define what that value received is.</li>
<li>Doesn't state the result is repeatable</li>
<li> Doesn't require that doing addition, subtraction and other
operations on all values is defined behaviour.</li>
</ol>

<p><i>[
Batavia: Related to
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2144.pdf">N2144</a>.
Pete: is there an ISO definition of modulo?  Underflow on signed behavior is undefined.
]</i></p>


<p><i>[
Bellevue:  accept resolution, move to ready status.
Does this mandate that is_modulo be true on platforms for which int
happens to b modulo? A: the standard already seems to require that.
]</i></p>



<p id="res-612"><b>Proposed resolution:</b></p>
<p>
Suggest 17.3.5.2 <a href="https://wg21.link/numeric.limits.members">[numeric.limits.members]</a>, paragraph 57 is amended to:
</p>

<blockquote><p>
A type is modulo if<ins>,</ins> <del>it is possible to add two positive numbers
and have a result that wraps around to a third number that is less.</del>
<ins>given any operation involving +,- or * on values of that type whose value
would fall outside the range <code>[min(), max()]</code>, then the value returned
differs from the true value by an integer multiple of <code>(max() - min() +
1)</code>.</ins>
</p></blockquote>






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