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<title>Issue 61: Ambiguity in iostreams exception policy</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="61"><a href="lwg-defects.html#61">61</a>. Ambiguity in iostreams exception policy</h3>
<p><b>Section:</b> 31.7.5.4 <a href="https://wg21.link/istream.unformatted">[istream.unformatted]</a> <b>Status:</b> <a href="lwg-active.html#TC1">TC1</a>
 <b>Submitter:</b> Matt Austern <b>Opened:</b> 1998-08-06 <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#istream.unformatted">issues</a> in [istream.unformatted].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TC1">TC1</a> status.</p>
<p><b>Discussion:</b></p>
<p>The introduction to the section on unformatted input (27.6.1.3)
says that every unformatted input function catches all exceptions that
were thrown during input, sets badbit, and then conditionally rethrows
the exception. That seems clear enough. Several of the specific
functions, however, such as get() and read(), are documented in some
circumstances as setting eofbit and/or failbit. (The standard notes,
correctly, that setting eofbit or failbit can sometimes result in an
exception being thrown.) The question: if one of these functions
throws an exception triggered by setting failbit, is this an exception
&quot;thrown during input&quot; and hence covered by 27.6.1.3, or does
27.6.1.3 only refer to a limited class of exceptions? Just to make
this concrete, suppose you have the following snippet. </p>

<pre>  
  char buffer[N];
  istream is;
  ...
  is.exceptions(istream::failbit); // Throw on failbit but not on badbit.
  is.read(buffer, N);</pre>

<p>Now suppose we reach EOF before we've read N characters. What
iostate bits can we expect to be set, and what exception (if any) will
be thrown? </p>


<p id="res-61"><b>Proposed resolution:</b></p>
<p>
In 27.6.1.3, paragraph 1, after the sentence that begins
&quot;If an exception is thrown...&quot;, add the following
parenthetical comment: &quot;(Exceptions thrown from 
<code>basic_ios&lt;&gt;::clear()</code> are not caught or rethrown.)&quot;
</p>


<p><b>Rationale:</b></p>
<p>The LWG looked to two alternative wordings, and choose the proposed
resolution as better standardese.</p>





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