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<title>Issue 3571: flush_emit should set badbit if the emit call fails</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="3571"><a href="lwg-defects.html#3571">3571</a>. <code>flush_emit</code> should set <code>badbit</code> if the <code>emit</code> call fails</h3>
<p><b>Section:</b> 31.7.6.5 <a href="https://wg21.link/ostream.manip">[ostream.manip]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2021-06-19 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#ostream.manip">issues</a> in [ostream.manip].</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>
<code>basic_osyncstream::emit</code> is specified to set <code>badbit</code> if it fails
(31.11.3.3 <a href="https://wg21.link/syncstream.osyncstream.members">[syncstream.osyncstream.members]</a> p1), but the <code>emit</code> part of the
<code>flush_emit</code> manipulator isn't, even though the <code>flush</code> part does set
<code>badbit</code> if it fails.
<p/>
More generally, given an <code>osyncstream s</code>, <code>s &lt;&lt; flush_emit;</code> should
probably have the same behavior as <code>s.flush(); s.emit()</code>.
<p/>
The reference implementation linked in <a href="https://wg21.link/P0753R2" title=" Manipulators for C++ Synchronized Buffered Ostream">P0753R2</a> does set <code>badbit</code> on
failure, so at least this part appears to be an oversight. As
discussed in LWG <a href="lwg-defects.html#3570" title="basic_osyncstream::emit should be an unformatted output function (Status: C++23)">3570</a><sup><a href="https://cplusplus.github.io/LWG/issue3570" title="Latest snapshot">(i)</a></sup>, <code>basic_osyncstream::emit</code> should probably be an
unformatted output function, so the <code>emit</code> part of <code>flush_emit</code> should do so too.
</p>

<p><i>[2021-06-23; Reflector poll]</i></p>

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

<p><i>[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3571"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4885">N4885</a>.
</p>

<ol>
<li><p>Modify 31.7.6.5 <a href="https://wg21.link/ostream.manip">[ostream.manip]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class charT, class traits&gt;
  basic_ostream&lt;charT, traits&gt;&amp; flush_emit(basic_ostream&lt;charT, traits&gt;&amp; os);
</pre>
<blockquote>
<p>
-12- <i>Effects:</i> Calls <code>os.flush()</code>. Then, if <code>os.rdbuf()</code> is a 
<code>basic_syncbuf&lt;charT, traits, Allocator&gt;*</code>, called <code>buf</code> for the purpose of exposition, 
<ins>behaves as an unformatted output function (31.7.6.4 <a href="https://wg21.link/ostream.unformatted">[ostream.unformatted]</a>) of <code>os</code>. After 
constructing a <code>sentry</code> object,</ins> calls <code>buf-&gt;emit()</code>. <ins>If that call returns 
<code>false</code>, calls <code>os.setstate(ios_base::badbit)</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>

</ol>




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