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| <TITLE>Catching Exceptions</TITLE> |
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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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