Catching Exceptions

Previous file Top of Document Contents Index page Next file

Apache C++ Standard Library User's Guide

29.3 Catching Exceptions

By default a stream does not throw any exceptions. You must explicitly activate an exception because a stream contains an exception mask. Each flag in this mask corresponds to one of the error flags. For example, once the badbit flag is set in the exception mask, an exception is thrown each time the badbit flag is set in the stream state.

NOTE -- An example of using an exception mask is provided by the streams layer, which catches bad_alloc exceptions thrown during allocation of its internal resources, iword and pword. It then setsbadbit or failbit. An exception would be thrown only if the corresponding bit in the exception mask is set. The exception thrown isios_base::failure.

The following code demonstrates how to activate an exception on an input stream object in:

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;
}

//1 In calling the exceptions() 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 setstate() and exception() raise an exception in case the exception mask requires it.]

//2 Objects thrown by the stream's operations are of types derived from std::ios_base::failure. Hence this catch clause catches all stream exceptions in principle. We qualify this generalization because a stream might fail to catch certain exceptions like std::bad_alloc, for example, so that exceptions other than std::ios_base::failure might be raised. That's how exception handling in C++ works: you never know what exceptions will be raised.

Generally, it is a good idea to activate the badbit exception and suppress the eofbit and failbit exceptions, because the latter do not represent exceptional states. A badbit situation, however, is likely to be a serious error condition similar to the memory shortage indicated by a bad_alloc exception. Unless you want to suppress exceptions thrown by iostreams altogether, we would recommend that you switch on the badbit exception and turn off eofbit and failbit.

Previous file Top of Document Contents Index page Next file