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<title>Issue 26: Bad sentry example</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#TC1">TC1</a> status.</em></p>
<h3 id="26"><a href="lwg-defects.html#26">26</a>. Bad sentry example</h3>
<p><b>Section:</b> 31.7.5.2.4 <a href="https://wg21.link/istream.sentry">[istream.sentry]</a> <b>Status:</b> <a href="lwg-active.html#TC1">TC1</a>
 <b>Submitter:</b> Nathan Myers <b>Opened:</b> 1998-08-06 <b>Last modified:</b> 2021-06-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#istream.sentry">issues</a> in [istream.sentry].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TC1">TC1</a> status.</p>
<p><b>Discussion:</b></p>
<p>In paragraph 6, the code in the example: </p>

<pre>  template &lt;class charT, class traits = char_traits&lt;charT&gt; &gt;
  basic_istream&lt;charT,traits&gt;::sentry(
           basic_istream&lt;charT,traits&gt;&amp; is, bool noskipws = false) {
      ...
      int_type c;
      typedef ctype&lt;charT&gt; ctype_type;
      const ctype_type&amp; ctype = use_facet&lt;ctype_type&gt;(is.getloc());
      while ((c = is.rdbuf()-&gt;snextc()) != traits::eof()) {
        if (ctype.is(ctype.space,c)==0) {
          is.rdbuf()-&gt;sputbackc (c);
          break;
        }
      }
      ...
   }</pre>

<p>fails to demonstrate correct use of the facilities described. In
particular, it fails to use traits operators, and specifies incorrect
semantics. (E.g. it specifies skipping over the first character in the
sequence without examining it.) </p>


<p id="res-26"><b>Proposed resolution:</b></p>
<p>Remove the example above from  [istream::sentry]
paragraph 6.</p>


<p><b>Rationale:</b></p>
<p>The originally proposed replacement code for the example was not
correct. The LWG tried in Kona and again in Tokyo to correct it
without success. In Tokyo, an implementor reported that actual working
code ran over one page in length and was quite complicated. The LWG
decided that it would be counter-productive to include such a lengthy
example, which might well still contain errors.</p>





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