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<title>Issue 3905: Type of std::fexcept_t</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#WP">WP</a> status.</em></p>
<h3 id="3905"><a href="lwg-defects.html#3905">3905</a>. Type of <code>std::fexcept_t</code></h3>
<p><b>Section:</b> 29.3.1 <a href="https://wg21.link/cfenv.syn">[cfenv.syn]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Sam Elliott <b>Opened:</b> 2023-03-13 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>&lt;cfenv&gt;</code>, as specified in 29.3.1 <a href="https://wg21.link/cfenv.syn">[cfenv.syn]</a>, requires <code>fexcept_t</code> to be
an integer type:
</p>
<blockquote><pre>
using fexcept_t = <i>integer type</i>;
</pre></blockquote>
<p>
<code>&lt;cfenv&gt;</code> was initially added to the (first) Technical Report on
C++ Library Extensions via <a href="https://wg21.link/N1568" title=" Proposed additions to TR-1 to improve compatibility with C99">N1568</a> and then integrated into the 
C++ Working Draft N2009 in Berlin (April, 2006).
<p/>
However, C99 does not actually require that <code>fexcept_t</code> is
an integer type, it only requires:
</p>
<blockquote>
<p>
The type <code>fexcept_t</code> represents the floating-point status flags
collectively, including any status the implementation associates
with the flags.
</p>
</blockquote>
<p>
Relaxing this requirement should not cause conforming C++ implementations to no longer be conforming. 
In fact, this should enable conforming C implementations to become conforming C++ implementations without 
an ABI break. The only incompatibility I foresee is where a user's program is initializing a <code>std::fexcept_t</code> 
with an integer value, which would become invalid on some C++ implementations (but not those that were 
previously conforming).
</p>

<p><i>[2023-05-24; Reflector poll]</i></p>

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

<p><i>[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3905"><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 29.3.1 <a href="https://wg21.link/cfenv.syn">[cfenv.syn]</a>, header <code>&lt;cfenv&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
namespace std {
  // <i>types</i>
  using fenv_t = <i>object type</i>;
  using fexcept_t = <i><del>integer</del><ins>object</ins> type</i>;
  [&hellip;]
}
</pre>
</blockquote>

</li>

</ol>





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