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<title>Issue 2087: iostream_category() and 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++14">C++14</a> status.</em></p>
<h3 id="2087"><a href="lwg-defects.html#2087">2087</a>. <code>iostream_category()</code> and <code>noexcept</code></h3>
<p><b>Section:</b> 31.5 <a href="https://wg21.link/iostreams.base">[iostreams.base]</a> <b>Status:</b> <a href="lwg-active.html#C++14">C++14</a>
 <b>Submitter:</b> Nicolai Josuttis <b>Opened:</b> 2011-09-22 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#iostreams.base">issues</a> in [iostreams.base].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++14">C++14</a> status.</p>
<p><b>Discussion:</b></p>

<p>
In <code>&lt;system_error&gt;</code> we have:
</p>

<blockquote><pre>
const error_category&amp; generic_category() noexcept;
const error_category&amp; system_category() noexcept;
</pre></blockquote>

<p>
In <code>&lt;future&gt;</code> we have:
</p>

<blockquote><pre>
const error_category&amp; future_category() noexcept;
</pre></blockquote>

<p>
But in <code>&lt;ios&gt;</code> we have:
</p>

<blockquote><pre>
const error_category&amp; iostream_category();
</pre></blockquote>

<p>
Is there any reason that <code>iostream_category()</code> is not declared with 
<code>noexcept</code> or is this an oversight?
</p>

<p>
Daniel:
<p/>
This looks like an oversight to me. We made the above
mentioned changes as part of noexcept-ifying the thread
library but <code>iostream_category()</code> was skipped, so it seems
to be forgotten. There should be no reason, why it cannot
be <code>noexcept</code>. When doing so, we should also make these functions
<code>noexcept</code> (similar to corresponding overloads):
</p>
<blockquote><pre>
error_code make_error_code(io_errc e);
error_condition make_error_condition(io_errc e);
</pre></blockquote>

<p>
Suggested wording provided by Daniel.
</p>

<p><i>[2013-04-20, Bristol]</i></p>


<p>
Unanimous.
<p/>
Resolution: move to tentatively ready. 
</p>

<p><i>[2013-09-29, Chicago]</i></p>


<p>
Apply to Working Paper
</p>


<p id="res-2087"><b>Proposed resolution:</b></p>
<p>This wording is relative to the FDIS.</p>

<ol><li>
<p>Change  [iostreams.base.overview], header <code>&lt;ios&gt;</code> synopsis 
as indicated:
</p>
<blockquote><pre>
#include &lt;iosfwd&gt;
namespace std {
  [&hellip;]
  error_code make_error_code(io_errc e) <ins>noexcept</ins>;
  error_condition make_error_condition(io_errc e) <ins>noexcept</ins>;
  const error_category&amp; iostream_category() <ins>noexcept</ins>;
}
</pre></blockquote>

</li>
<li>
<p>Change the prototype declarations in 31.5.6 <a href="https://wg21.link/error.reporting">[error.reporting]</a> as indicated:
</p>
<blockquote><pre>
error_code make_error_code(io_errc e) <ins>noexcept</ins>;
</pre></blockquote><blockquote>
<p>
-1- <i>Returns</i>: <code>error_code(static_cast&lt;int&gt;(e), iostream_category())</code>.
</p>
</blockquote><blockquote><pre>
error_condition make_error_condition(io_errc e) <ins>noexcept</ins>;
</pre></blockquote><blockquote>
<p>
-2- <i>Returns</i>: <code>error_condition(static_cast&lt;int&gt;(e), iostream_category())</code>.
</p>
</blockquote><blockquote><pre>
const error_category&amp; iostream_category() <ins>noexcept</ins>;
</pre></blockquote><blockquote>
<p>
-3- <i>Returns</i>: A reference to an object of a type derived from class <code>error_category</code>.
<p/>
-4- The object’s <code>default_error_condition</code> and <code>equivalent</code> virtual functions shall behave as specified
for the class <code>error_category</code>. The object’s <code>name</code> virtual function shall return a pointer to the string
<code>"iostream"</code>.
</p>
</blockquote>
</li>
</ol>






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