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<title>Issue 2041: Stage 2 accumulate incompatibilty</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++11">C++11</a> status.</em></p>
<h3 id="2041"><a href="lwg-defects.html#2041">2041</a>. Stage 2 accumulate incompatibilty</h3>
<p><b>Section:</b> 28.3.4.3.2.3 <a href="https://wg21.link/facet.num.get.virtuals">[facet.num.get.virtuals]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Howard Hinnant <b>Opened:</b> 2011-03-09 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#facet.num.get.virtuals">active issues</a> in [facet.num.get.virtuals].</p>
<p><b>View all other</b> <a href="lwg-index.html#facet.num.get.virtuals">issues</a> in [facet.num.get.virtuals].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p><code>num_get</code> Stage 2 accumulation changed between C++03 and the current C++0x working draft. The sentences:</p>

<blockquote><p>
If it is not discarded, then a check is made to determine if <code>c</code> is allowed as the
next character of an input field of the conversion specifier returned by stage 1.
If so it is accumulated.
</p></blockquote>

<p>have been dropped from 28.3.4.3.2.3 <a href="https://wg21.link/facet.num.get.virtuals">[facet.num.get.virtuals]</a>, Stage 2, paragraph 3 that begins:</p>

<blockquote><p>
If <code>discard</code> is true, [&hellip;]
</p></blockquote>

<p>Consider this code:</p>

<blockquote><pre>
#include &lt;sstream&gt;
#include &lt;iostream&gt;

int main(void)
{
   std::istringstream s("8cz");
   long i = 0;
   char c;
   s &gt;&gt; i;
   if (!s.fail())
       std::cout &lt;&lt; "i = " &lt;&lt; i &lt;&lt; '\n';
   else
   {
       std::cout &lt;&lt; "s &gt;&gt; i failed\n";
       s.clear();
   }
   s &gt;&gt; c;
   if (!s.fail())
       std::cout &lt;&lt; "c = " &lt;&lt; c &lt;&lt; '\n';
   else
       std::cout &lt;&lt; "s &gt;&gt; c failed\n";
}
</pre></blockquote>

<p>C++0x currently prints out:</p>

<blockquote><pre>
s &gt;&gt; i failed
c = z
</pre></blockquote>

<p>However C++03 conforming implementations will output:</p>

<blockquote><pre>
i = 8
c = c
</pre></blockquote>

<p>I believe we need to restore C++03 compatibility.</p>



<p id="res-2041"><b>Proposed resolution:</b></p>

<p>Add to 28.3.4.3.2.3 <a href="https://wg21.link/facet.num.get.virtuals">[facet.num.get.virtuals]</a>, Stage 2:</p>

<blockquote><p>
If <code>discard</code> is true, then if <code>'.'</code> has not yet been accumulated, then the position of the character
is remembered, but the character is otherwise ignored. Otherwise, if <code>'.'</code> has already been
accumulated, the character is discarded and Stage 2 terminates. <ins>If it is not discarded, then a 
check is made to determine if <code>c</code> is allowed as the next character of an input field of the 
conversion specifier returned by stage 1. If so it is accumulated.</ins>
<p/>
If the character is either discarded or accumulated then in is advanced by <code>++in</code> and processing
returns to the beginning of stage 2.
</p></blockquote>





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