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<title>Issue 3960: How does chrono::parse handle duplicated data?</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#New">New</a> status.</em></p>
<h3 id="3960"><a href="lwg-active.html#3960">3960</a>. How does <code>chrono::parse</code> handle duplicated data?</h3>
<p><b>Section:</b> 30.13 <a href="https://wg21.link/time.parse">[time.parse]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2023-07-28 <b>Last modified:</b> 2023-11-03</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#time.parse">active issues</a> in [time.parse].</p>
<p><b>View all other</b> <a href="lwg-index.html#time.parse">issues</a> in [time.parse].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
A call to <code>chrono::parse</code> can encounter redundant or contradictory
data, e.g.
<pre><blockquote>stringstream("2023 2022") &gt;&gt; chrono::parse("%Y %C%y", yr);
</blockquote></pre>
or even simpler:
<pre><blockquote>stringstream("2023 2022") &gt;&gt; chrono::parse("%Y %Y", yr);
</blockquote></pre>
These cases can both be successfully parsed, as the input stream contains the
expected values in the expected format. But what value will <code>yr</code> have
after it returns?
</p>

<p><i>[2023-11-02; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
"Should allow setting <code>failbit</code> if an implementation does detect
inconsistent values."
"Yes, that's what MSVC does, so the wording needs something like:
<ins>it is unspecified whether <code>setstate(ios_base::failbit)</code>
is called on the <code>basic_istream</code> or whether an unspecified value
is assigned to the <code>p</code> argument.</ins>"
</p>



<p id="res-3960"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4950" title=" Working Draft, Standard for Programming Language C++">N4950</a>.
</p>

<ol>

<li><p>Modify 30.13 <a href="https://wg21.link/time.parse">[time.parse]</a> as indicated:</p>

<blockquote>
<p>
-17-
If the <code>from_stream</code> overload fails to parse everything specified by
the format string, or if insufficient information is parsed to specify
a complete duration, time point, or calendrical data structure,
<code>setstate(ios_base::failbit)</code> is called on the <code>basic_istream</code>.
</p>
<p><ins>
-?-
A format string can contain multiple flags that correspond to the same
information, (e.g., <code>"%Y %C%y"</code> or <code>"%R %H:%M"</code>). If such a
format string is successfully parsed but the extracted dates or times are
not consistent, the value assigned to the <code>tp</code> argument is unspecified.
</ins>
</p>
<p><ins>
[<i>Example</i>:
</ins>
<pre><blockquote><ins>chrono::year y;
stringstream("1905 1915") &gt;&gt; parse("%Y %C%y", y);
</ins></blockquote></pre>
<ins>
The value of <code>y</code> is unspecified; <code>y.ok()</code> can be <code>false</code>.
&mdash; <i>end example</i>]
</ins>
</p>

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





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