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<title>Issue 3317: Incorrect operator&lt;&lt; for floating-point durations</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++20">C++20</a> status.</em></p>
<h3 id="3317"><a href="lwg-defects.html#3317">3317</a>. Incorrect <code>operator&lt;&lt;</code> for floating-point durations</h3>
<p><b>Section:</b> 30.5.11 <a href="https://wg21.link/time.duration.io">[time.duration.io]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> United States <b>Opened:</b> 2019-11-05 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#time.duration.io">issues</a> in [time.duration.io].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses <a href="https://github.com/cplusplus/nbballot/issues/330">US 334</a></b></p>

<p>
<code>operator&lt;&lt;</code> for floating-point durations always produces output with six digits 
after the decimal point, and doesn't use the stream's locale either.
</p>
<p>
Proposed change:
<p/>
Rewrite the specification to not rely on <code>to_string()</code> for floating-point formatting.
</p>

<p><i>[2019-11 Status to Ready during Wednesday morning issue processing in Belfast.]</i></p>



<p id="res-3317"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4835">N4835</a>.</p>

<ol>
<li><p>Modify 30.5.11 <a href="https://wg21.link/time.duration.io">[time.duration.io]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class charT, class traits, class Rep, class Period>
  basic_ostream&lt;charT, traits&gt;&amp;
    operator&lt;&lt;(basic_ostream&lt;charT, traits&gt;&amp; os, const duration&lt;Rep, Period&gt;&amp; d);
</pre>
<blockquote>
<p>
<del>-1- <i>Requires:</i> <code>Rep</code> is an integral type whose integer conversion rank 
(6.9.6 <a href="https://wg21.link/conv.rank">[conv.rank]</a>) is greater than or equal to that of <code>short</code>, or a floating-point type. 
<code>charT</code> is <code>char</code> or <code>wchar_t</code>.</del>
<p/>
-2- <i>Effects:</i> <del>Forms a <code>basic_string&lt;charT, traits&gt;</code> from <code>d.count()</code> using 
<code>to_string</code> if <code>charT</code> is <code>char</code>, or <code>to_wstring</code> if <code>charT</code> is 
<code>wchar_t</code>. Appends the units suffix described below to the <code>basic_string</code>.
Inserts the resulting <code>basic_string</code> into <code>os</code>. [<i>Note:</i> This specification ensures 
that the result of this streaming operation will obey the width and alignment properties of the stream. 
&mdash; <i>end note</i>]</del><ins>Inserts the duration <code>d</code> onto the stream <code>os</code> as if it 
were implemented as follows:</ins>
<blockquote><pre>
<ins>basic_ostringstream&lt;charT, traits&gt; s;
s.flags(os.flags());
s.imbue(os.getloc());
s.precision(os.precision());
s &lt;&lt; d.count() &lt;&lt; <i>units_suffix</i>;
return os &lt;&lt; s.str();</ins>
</pre></blockquote>
<p/>
-3- <del>The units suffix</del><ins><code><i>units_suffix</i></code></ins> depends on the type 
<code>Period::type</code> as follows:
<ol style="list-style-type: none">
<li><p>(3.1) &mdash; If <code>Period::type</code> is <code>atto</code>, <del>the suffix</del><ins><code><i>units_suffix</i></code></ins> 
is <code>"as"</code>.</p></li>
<li><p>(3.2) &mdash; Otherwise, if <code>Period::type</code> is <code>femto</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"fs"</code>.</p></li>
<li><p>(3.3) &mdash; Otherwise, if <code>Period::type</code> is <code>pico</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"ps"</code>.</p></li>
<li><p>(3.4) &mdash; Otherwise, if <code>Period::type</code> is <code>nano</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"ns"</code>.</p></li>
<li><p>(3.5) &mdash; Otherwise, if <code>Period::type</code> is <code>micro</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"&micro;s"</code> 
(<code>"\u00b5\u0073"</code>).</p></li>
<li><p>(3.6) &mdash; Otherwise, if <code>Period::type</code> is <code>milli</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"ms"</code>.</p></li>
<li><p>(3.7) &mdash; Otherwise, if <code>Period::type</code> is <code>centi</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"cs"</code>.</p></li>
<li><p>(3.8) &mdash; Otherwise, if <code>Period::type</code> is <code>deci</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"ds"</code>.</p></li>
<li><p>(3.9) &mdash; Otherwise, if <code>Period::type</code> is <code>ratio&lt;1&gt;</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"s"</code>.</p></li>
<li><p>(3.10) &mdash; Otherwise, if <code>Period::type</code> is <code>deca</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"das"</code>.</p></li>
<li><p>(3.11) &mdash; Otherwise, if <code>Period::type</code> is <code>hecto</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"hs"</code>.</p></li>
<li><p>(3.12) &mdash; Otherwise, if <code>Period::type</code> is <code>kilo</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"ks"</code>.</p></li>
<li><p>(3.13) &mdash; Otherwise, if <code>Period::type</code> is <code>mega</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"Ms"</code>.</p></li>
<li><p>(3.14) &mdash; Otherwise, if <code>Period::type</code> is <code>giga</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"Gs"</code>.</p></li>
<li><p>(3.15) &mdash; Otherwise, if <code>Period::type</code> is <code>tera</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"Ts"</code>.</p></li>
<li><p>(3.16) &mdash; Otherwise, if <code>Period::type</code> is <code>peta</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"Ps"</code>.</p></li>
<li><p>(3.17) &mdash; Otherwise, if <code>Period::type</code> is <code>exa</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"Es"</code>.</p></li>
<li><p>(3.18) &mdash; Otherwise, if <code>Period::type</code> is <code>ratio&lt;60&gt;</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"min"</code>.</p></li>
<li><p>(3.19) &mdash; Otherwise, if <code>Period::type</code> is <code>ratio&lt;3600&gt;</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"h"</code>.</p></li>
<li><p>(3.20) &mdash; Otherwise, if <code>Period::type</code> is <code>ratio&lt;86400&gt;</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"d"</code>.</p></li>
<li><p>(3.21) &mdash; Otherwise, if <code>Period::type::den == 1</code>, 
<del>the suffix</del><ins><code><i>units_suffix</i></code></ins> is <code>"<i>[num</i>]s"</code>.</p></li>
<li><p>(3.22) &mdash; Otherwise, <del>the suffix</del><ins><code><i>units_suffix</i></code></ins> 
is <code>"[<i>num</i>/<i>den</i>]s"</code>.</p></li>
</ol>
<p/>
In the list above the use of <code><i>num</i></code> and <code><i>den</i></code> refer to the static data members 
of <code>Period::type</code>, which are converted to arrays of <code>charT</code> using a decimal conversion with 
no leading zeroes.
<p/>
-4- If <code>Period::type</code> is <code>micro</code>, but the character U+00B5 cannot be represented in the 
encoding used for <code>charT</code>, <del>the unit suffix</del><ins><code><i>units_suffix</i></code></ins> 
<code>"us"</code> is used instead of <code>"&micro;s"</code>.
<p/>
-5- <i>Returns:</i> <code>os</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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