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<title>Issue 3741: std::chrono::abs(duration) is ill-formed with non-reduced periods</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="3741"><a href="lwg-closed.html#3741">3741</a>. <code>std::chrono::abs(duration)</code> is ill-formed with non-reduced periods</h3>
<p><b>Section:</b> 30.5.10 <a href="https://wg21.link/time.duration.alg">[time.duration.alg]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Charlie Barto <b>Opened:</b> 2022-07-16 <b>Last modified:</b> 2022-11-30</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently 30.5.10 <a href="https://wg21.link/time.duration.alg">[time.duration.alg]</a> specifies <code>abs(duration)</code> as:
</p>
<blockquote><p>
<i>Returns:</i> if <code>d &gt;= d.zero()</code>, return <code>d</code>, otherwise return <code>-d</code>.
</p></blockquote>
<p>
Because unary minus on durations is defined to return <code>common_type_t&lt;duration>(-rep_)</code>, 
and <code>common_type_t</code> for durations is specified to reduce the period, this is ill-formed 
with durations such as <code>duration&lt;int, ratio&lt;1000, 1000&gt;&gt;</code>, or any other 
type where the numerator and denominator of the period are not coprime.
</p>

<p><i>[2022-08-23; Reflector poll: NAD]</i></p>

<p>
Not ill-formed, implementation should do a conversion.
Changing it to return the reduced duration as an improvement would be for LEWG.
</p>

<p><i>[2022-11-30 LWG telecon. Status changed: Tentatively NAD &rarr; NAD.]</i></p>



<p id="res-3741"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 30.2 <a href="https://wg21.link/time.syn">[time.syn]</a>, header <code>&lt;chrono&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
<i>// 30.5.10 <a href="https://wg21.link/time.duration.alg">[time.duration.alg]</a>, specialized algorithms</i>
template&lt;class Rep, class Period&gt;
  constexpr <ins>common_type_t&lt;</ins>duration&lt;Rep, Period&gt;<ins>&gt;</ins> abs(duration&lt;Rep, Period&gt; d);
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 30.5.10 <a href="https://wg21.link/time.duration.alg">[time.duration.alg]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> This will cause <code>abs</code> to reduce the period before returning it, 
much like the other arithmetic operators. 
<p/>
This is not a breaking change, because code that was using <code>abs</code> with a non-reduced 
period before did not compile.
]
</p>
</blockquote>

<blockquote>
<pre>
template&lt;class Rep, class Period&gt;
  constexpr <ins>common_type_t&lt;</ins>duration&lt;Rep, Period&gt;<ins>&gt;</ins> abs(duration&lt;Rep, Period&gt; d);
</pre>
<blockquote>
<p>
-1- <i>Constraints:</i> <code>numeric_limits&lt;Rep&gt;::is_signed</code> is <code>true</code>.
<p/>
-2- <i>Returns:</i> If <code>d &gt;= d.zero()</code>, return <code><ins>+</ins>d</code>, otherwise return <code>-d</code>.
</p>
</blockquote>
</blockquote>
</li>

</ol>





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