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<TITLE>Catching Exceptions</TITLE>
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<H2>29.3 Catching Exceptions</H2>
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<P>By default a stream does not throw any exceptions. You must explicitly activate an exception because a stream contains an<I> exception mask</I>. Each flag in this mask corresponds to one of the error flags. For example, once the <SAMP>badbit</SAMP> flag is set in the exception mask, an exception is thrown each time the <SAMP>badbit</SAMP> flag is set in the stream state.</P>
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NOTE -- An example of using an exception mask is provided by the streams layer, which catches <SAMP>bad_alloc</SAMP> exceptions thrown during allocation of its internal resources, <SAMP>iword</SAMP> and <SAMP>pword</SAMP><B>. </B>It then sets<B> </B><SAMP>badbit</SAMP> or <SAMP>failbit</SAMP><B>. </B>An exception would be thrown only if the corresponding bit in the exception mask is set. The exception thrown is<B> </B><SAMP>ios_base::failure</SAMP><B>.</B>
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<P>The following code demonstrates how to activate an exception on an input stream object <SAMP>in</SAMP>:</P>

<UL><PRE>
extern std::istream&amp; in;
try {
  in.exceptions(std::ios_base::badbit   |
                std::ios_base::failbit);                       //1
  in &gt;&gt; x;
  // do lots of other stream i/o
}
catch(std::ios_base::failure&amp; exc) {                            //2
  std::cerr &lt;&lt; exc.what() &lt;&lt; std::endl; 
  throw;
}
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<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>In calling the <SAMP>exceptions()</SAMP> function, you indicate what flags in the stream's state shall cause an exception to be thrown. [Each change of either the stream state or the exception mask can result in an exception being thrown. This is because the functions <SAMP>setstate()</SAMP> and <SAMP>exception()</SAMP> raise an exception in case the exception mask requires it.]
<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>Objects thrown by the stream's operations are of types derived from <SAMP>std::ios_base::failure</SAMP>. Hence this catch clause catches all stream exceptions in principle. We qualify this generalization because a stream might fail to catch certain exceptions like <SAMP>std::bad_alloc</SAMP>, for example, so that exceptions other than <SAMP>std::ios_base::failure</SAMP> might be raised. That's how exception handling in C++ works: you never know what exceptions will be raised.
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<P>Generally, it is a good idea to activate the <SAMP>badbit</SAMP> exception and suppress the <SAMP>eofbit</SAMP> and <SAMP>failbit</SAMP> exceptions, because the latter do not represent exceptional states. A <SAMP>badbit</SAMP> situation, however, is likely to be a serious error condition similar to the memory shortage indicated by a <SAMP>bad_alloc</SAMP> exception. Unless you want to suppress exceptions thrown by iostreams altogether, we would recommend that you switch on the <SAMP>badbit</SAMP> exception and turn off <SAMP>eofbit</SAMP> and <SAMP>failbit</SAMP>.</P>

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