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<title>Issue 3842: Unclear wording for precision in chrono-format-spec</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++23">C++23</a> status.</em></p>
<h3 id="3842"><a href="lwg-defects.html#3842">3842</a>. Unclear wording for <i>precision</i> in <i>chrono-format-spec</i></h3>
<p><b>Section:</b> 30.12 <a href="https://wg21.link/time.format">[time.format]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2022-12-14 <b>Last modified:</b> 2024-01-29</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#time.format">active issues</a> in [time.format].</p>
<p><b>View all other</b> <a href="lwg-index.html#time.format">issues</a> in [time.format].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
30.12 <a href="https://wg21.link/time.format">[time.format]</a> says:
</p>
<blockquote><p>
[&hellip;] Giving a <i>precision</i> specification in the <i>chrono-format-spec</i> is valid only for 
<code>std::chrono::duration</code> types where the representation type
<code>Rep</code> is a floating-point type. 
For all other <code>Rep</code> types, an exception of type <code>format_error</code> is thrown if the 
<i>chrono-format-spec</i> contains a <i>precision</i> specification. [&hellip;]
</p></blockquote>
<p>
It's unclear whether the restriction in the first sentence applies to all types, or only duration types. 
The second sentence seems to restrict the exceptional case to only types with a non-floating-point <code>Rep</code>, 
but what about types with no <code>Rep</code> type at all?
<p/>
Can you use a precision with <code>sys_time&lt;duration&lt;float&gt;&gt;</code>? That is not a <code>duration</code> type at all, 
so does the restriction apply? What about <code>hh_mm_ss&lt;duration&lt;int&gt;&gt;</code>? That's not a 
<code>duration</code> type, but it uses one, and its <code>Rep</code> is not a floating-point type. 
What about <code>sys_info</code>? That's not a <code>duration</code> and doesn't have any associated <code>duration</code>, 
or <code>Rep</code> type.
<p/>
What is the intention here?
<p/>
Less importantly, I don't like the use of <code>Rep</code> here. That's the template parameter of the 
<code>duration</code> class template, but that name isn't in scope here. Why aren't we talking about the 
<code>duration</code> type's <code>rep</code> type, which is the public name for it? Or about a concrete 
specialization <code>duration&lt;Rep, Period&gt;</code>, instead of the class template?
<p/>
The suggested change below would preserve the intended meaning, but with more &hellip; <i>precision</i>.
</p>

<p><i>[2023-01-06; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after eight votes in favour during reflector poll.
</p>

<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3842"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 30.12 <a href="https://wg21.link/time.format">[time.format]</a> as indicated:</p>

<blockquote>
<p>
-1- [&hellip;]
<p/>
The productions <i>fill-and-align</i>, <i>width</i>, and <i>precision</i> are described in 28.5.2 <a href="https://wg21.link/format.string">[format.string]</a>. 
Giving a <i>precision</i> specification in the <i>chrono-format-spec</i> is valid only for 
<ins>types that are specializations of</ins> <code>std::chrono::duration</code> <del>types where the representation type
<code>Rep</code> is</del><ins>for which the nested <i>typedef-name</i> <code>rep</code> denotes</ins> a floating-point type. 
For all other <del><code>Rep</code></del> types, an exception of type <code>format_error</code> is thrown if the 
<i>chrono-format-spec</i> contains a <i>precision</i> specification. [&hellip;]

</p>
</blockquote>
</li>
</ol>






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