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 <H2>29.3 Catching Exceptions</H2>
 <A NAME="idx736"><!></A>
 <A NAME="idx737"><!></A>
 <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>
 <BLOCKQUOTE><HR><B>
 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>
 </B><HR></BLOCKQUOTE>
 <A NAME="idx738"><!></A>
 <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;
 }
 </PRE></UL>
 <TABLE CELLPADDING="3">

 <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.
 </TABLE>
 <A NAME="idx739"><!></A>
 <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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	<H2>29.3 Catching Exceptions</H2>
	<A NAME="idx736"><!></A>
	<A NAME="idx737"><!></A>
	<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>
	<BLOCKQUOTE><HR><B>
	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>
	</B><HR></BLOCKQUOTE>
	<A NAME="idx738"><!></A>
	<P>The following code demonstrates how to activate an exception on an input stream object <SAMP>in</SAMP>:</P>

	<UL><PRE>
	extern std::istream& in;
	try {
	in.exceptions(std::ios_base::badbit \|
	std::ios_base::failbit); //1
	in >> x;
	// do lots of other stream i/o
	}
	catch(std::ios_base::failure& exc) { //2
	std::cerr << exc.what() << std::endl;
	throw;
	}
	</PRE></UL>
	<TABLE CELLPADDING="3">

	<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.
	</TABLE>
	<A NAME="idx739"><!></A>
	<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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