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<title>Issue 974: duration&lt;double&gt; should not implicitly convert to duration&lt;int&gt;</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="974"><a href="lwg-defects.html#974">974</a>. <code>duration&lt;double&gt;</code> should not implicitly convert to <code>duration&lt;int&gt;</code></h3>
<p><b>Section:</b> 30.5.2 <a href="https://wg21.link/time.duration.cons">[time.duration.cons]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2009-01-21 <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#time.duration.cons">issues</a> in [time.duration.cons].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The following code should not compile because it involves implicit truncation
errors (against the design philosophy of the <code>duration</code> library).
</p>

<blockquote><pre>
duration&lt;double&gt; d(3.5);
duration&lt;int&gt; i = d;  <span style="color:#C80000">// implicit truncation, should not compile</span>
</pre></blockquote>

<p>
This intent was codified in the example implementation which drove this proposal
but I failed to accurately translate the code into the specification in this
regard.
</p>

<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote>
<p>
We agree with the proposed resolution.
</p>
<p>
Move to Tentatively Ready.
</p>
</blockquote>

<p><i>[
2009-07 Frankfurt
]</i></p>


<blockquote><p>
Moved from Tentatively Ready to Open only because the wording needs to be
improved for <code>enable_if</code> type constraining, possibly following Robert's
formula.
</p></blockquote>

<p><i>[
2009-08-01 Howard adds:
]</i></p>


<blockquote><p>
Addressed by <a href="lwg-defects.html#1177" title="Improve &quot;diagnostic required&quot; wording (Status: C++11)">1177</a><sup><a href="https://cplusplus.github.io/LWG/issue1177" title="Latest snapshot">(i)</a></sup>.
</p></blockquote>

<p><i>[
2009-10 Santa Cruz:
]</i></p>


<blockquote><p>
Not completely addressed by <a href="lwg-defects.html#1177" title="Improve &quot;diagnostic required&quot; wording (Status: C++11)">1177</a><sup><a href="https://cplusplus.github.io/LWG/issue1177" title="Latest snapshot">(i)</a></sup>.  Move to Ready.
</p></blockquote>



<p id="res-974"><b>Proposed resolution:</b></p>
<p>
Change 30.5.2 <a href="https://wg21.link/time.duration.cons">[time.duration.cons]</a>, p4:
</p>

<blockquote>
<pre>
template &lt;class Rep2, class Period2&gt; 
  duration(const duration&lt;Rep2, Period2&gt;&amp; d);
</pre>
<blockquote><p>
-4- <i>Requires:</i> <code>treat_as_floating_point&lt;rep&gt;::value</code>
shall be <code>true</code> or <ins>both</ins> <code>ratio_divide&lt;Period2,
period&gt;::type::den</code> shall be 1
<ins>and <code>treat_as_floating_point&lt;Rep2&gt;::value</code>
shall be <code>false</code></ins>.
Diagnostic required.
[<i>Note:</i> This requirement prevents implicit truncation error when
converting between integral-based <code>duration</code> types. Such a
construction could easily lead to confusion about the value of the
<code>duration</code>. &mdash; <i>end note</i>]
</p></blockquote>
</blockquote>





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