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<title>Issue 643: Impossible "as if" clauses</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#CD1">CD1</a> status.</em></p>
<h3 id="643"><a href="lwg-defects.html#643">643</a>. Impossible "as if" clauses</h3>
<p><b>Section:</b> 31.10.3 <a href="https://wg21.link/filebuf">[filebuf]</a>, 28.3.4.3.3.3 <a href="https://wg21.link/facet.num.put.virtuals">[facet.num.put.virtuals]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Daniel Kr&uuml;gler <b>Opened:</b> 2007-02-20 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The current standard 14882:2003(E) as well as N2134 have the
following
defects:
</p>

<p>
31.10.3 <a href="https://wg21.link/filebuf">[filebuf]</a>/5 says:
</p>

<blockquote>
<p>
In order to support file I/O and multibyte/wide character conversion, conversions are performed using members of a 
facet, referred to as <code><i>a_codecvt</i></code> in following sections, obtained "as if" by
</p>
<blockquote><pre>
codecvt&lt;charT,char,typename traits::state_type&gt; <i>a_codecvt</i> =
  use_facet&lt;codecvt&lt;charT,char,typename traits::state_type&gt; &gt;(getloc());
</pre></blockquote>
</blockquote>

<p>
<code>use_facet</code> returns a <code>const facet</code> reference and no facet is
copyconstructible, so the codecvt construction should fail to compile.
</p>

<p>
A similar issue arises in 28.3.4.3.3.3 <a href="https://wg21.link/facet.num.put.virtuals">[facet.num.put.virtuals]</a>/15 for <code>num_punct</code>.
</p>


<p id="res-643"><b>Proposed resolution:</b></p>
<p>
In 31.10.3 <a href="https://wg21.link/filebuf">[filebuf]</a>/5 change the "as if" code
</p>

<blockquote><pre>
<ins>const </ins>codecvt&lt;charT,char,typename traits::state_type&gt;<ins>&amp;</ins> <i>a_codecvt</i> =
  use_facet&lt;codecvt&lt;charT,char,typename traits::state_type&gt; &gt;(getloc());
</pre></blockquote>

<p>
In 28.3.4.3.3.3 <a href="https://wg21.link/facet.num.put.virtuals">[facet.num.put.virtuals]</a>/15 (This is para 5 in N2134) change
</p>

<blockquote>
<p>
A local variable <code><i>punct</i></code> is initialized via
</p>
<blockquote><pre>
<ins>const </ins>numpunct&lt;charT&gt;<ins>&amp;</ins> <i>punct</i> = use_facet&lt; numpunct&lt;charT&gt; &gt;(<i>str</i>.getloc() )<ins>;</ins>
</pre></blockquote>
</blockquote>

<p>
(Please note also the additional provided trailing semicolon)
</p>






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