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<title>Issue 3867: Should std::basic_osyncstream's move assignment operator be noexcept?</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="3867"><a href="lwg-defects.html#3867">3867</a>. Should <code>std::basic_osyncstream</code>'s move assignment operator be <code>noexcept</code>?</h3>
<p><b>Section:</b> 31.11.3.1 <a href="https://wg21.link/syncstream.osyncstream.overview">[syncstream.osyncstream.overview]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2023-01-29 <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#syncstream.osyncstream.overview">issues</a> in [syncstream.osyncstream.overview].</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>
The synopsis of <code>std::basic_osyncstream</code> (31.11.3.1 <a href="https://wg21.link/syncstream.osyncstream.overview">[syncstream.osyncstream.overview]</a>) indicates that it's 
member functions behave as if it hold a <code>std::basic_syncbuf</code> as its subobject, and according to 
16.3.3.5 <a href="https://wg21.link/functions.within.classes">[functions.within.classes]</a>, <code>std::basic_osyncstream</code>'s move assignment operator should call 
<code>std::basic_syncbuf</code>'s move assignment operator.
<p/>
However, currently <code>std::basic_osyncstream</code>'s move assignment operator is <code>noexcept</code>, while 
<code>std::basic_syncbuf</code>'s is not. So when an exception is thrown from move assignment between <code>std::basic_syncbuf</code> 
objects, <code>std::terminate</code> should be called.
<p/>
It's clarified in LWG <a href="lwg-defects.html#3498" title="Inconsistent noexcept-specifiers for basic_syncbuf (Status: C++23)">3498</a><sup><a href="https://cplusplus.github.io/LWG/issue3498" title="Latest snapshot">(i)</a></sup> that an exception can escape from <code>std::basic_syncbuf</code>'s move 
assignment operator. Is there any reason that an exception shouldn't escape from <code>std::basic_osyncstream</code>'s 
move assignment operator? 
</p>

<p><i>[2023-02-06; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after seven 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-3867"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>

<ol>
<li><p>Modify 31.11.3.1 <a href="https://wg21.link/syncstream.osyncstream.overview">[syncstream.osyncstream.overview]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class charT, class traits = char_traits&lt;charT&gt;, class Allocator = allocator&lt;charT&gt;&gt;
  class basic_osyncstream : public basic_ostream&lt;charT, traits&gt; {
  public:
    [&hellip;]
    using syncbuf_type = basic_syncbuf&lt;charT, traits, Allocator&gt;;
    [&hellip;]
    
    <i>// assignment</i>
    basic_osyncstream&amp; operator=(basic_osyncstream&amp;&amp;) <del>noexcept</del>;

    [&hellip;]
  
  private:
    syncbuf_type sb; <i>// exposition only</i>
  };
}
</pre>
</blockquote>
</li>

</ol>




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