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<title>Issue 340: interpretation of has_facet&lt;Facet&gt;(loc)</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="340"><a href="lwg-defects.html#340">340</a>. interpretation of <code>has_facet&lt;Facet&gt;(loc)</code></h3>
<p><b>Section:</b> 28.3.3.1.2.1 <a href="https://wg21.link/locale.category">[locale.category]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Martin Sebor <b>Opened:</b> 2001-09-18 <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#locale.category">issues</a> in [locale.category].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
It's unclear whether 22.1.1.1.1, p3 says that
<code>has_facet&lt;Facet&gt;(loc)</code> returns true for any <code>Facet</code>
from Table 51 or whether it includes Table 52 as well:
</p>

<blockquote><p>
For any locale <code>loc</code> either constructed, or returned by
locale::classic(), and any facet <code>Facet</code> that is a member of a
standard category, <code>has_facet&lt;Facet&gt;(loc)</code> is true. Each
locale member function which takes a <code>locale::category</code>
argument operates on the corresponding set of facets.
</p></blockquote>

<p>
It seems that it comes down to which facets are considered to be members of a
standard category. Intuitively, I would classify all the facets in Table 52 as
members of their respective standard categories, but there are an unbounded set
of them...
</p>

<p>
The paragraph implies that, for instance, <code>has_facet&lt;num_put&lt;C,
OutputIterator&gt; &gt;(loc)</code> must always return true. I don't think that's
possible. If it were, then <code>use_facet&lt;num_put&lt;C, OutputIterator&gt;
&gt;(loc)</code> would have to return a reference to a distinct object for each
valid specialization of <code>num_put&lt;C, OutputIteratory&gt;</code>, which is
clearly impossible.
</p>

<p>
On the other hand, if none of the facets in Table 52 is a member of a standard
category then none of the locale member functions that operate on entire
categories of facets will work properly.
</p>

<p>
It seems that what p3 should mention that it's required (permitted?)
to hold only for specializations of <code>Facet</code> from Table 52 on
<code>C</code> from the set { <code>char</code>, <code>wchar_t</code> }, and
<code>InputIterator</code> and <code>OutputIterator</code> from the set of
{
{i,o}<code>streambuf_iterator</code>&lt;{<code>char</code>,<code>wchar_t</code>}<code>&gt;</code>
}.
</p>


<p id="res-340"><b>Proposed resolution:</b></p>
<p>In 28.3.3.1.2.1 <a href="https://wg21.link/locale.category">[locale.category]</a>, paragraph 3, change
"that is a member of a standard category" to "shown in Table 51".</p>


<p><b>Rationale:</b></p>
<p>The facets in Table 52 are an unbounded set.  Locales should not be
required to contain an infinite number of facets.</p> 

<p>It's not necessary to talk about which values of InputIterator and
OutputIterator must be supported.  Table 51 already contains a
complete list of the ones we need.</p>






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