Exceptions

• Upon failure, a method may abort its execution
• We do not wish the whole program to abort
• Mechanism:
  1. method throws an exception
  2. caller method catches it
  3. called method handles it if it can
  4. otherwise it re-throws the exception
• Exceptions are passed on the method calling hierarchy levels until one of the method can handle it
• If exceptions reaches main(), the program is aborted

An exception is a class. Exceptions can be thrown and caught by methods. If a method throws an exception, it must be declared: returnType methodName( arguments) throw (ExceptionName)

• The TimeStamp::update() method obtains the current time through the operating system, which is outside the program's control
• update() does not know how to deal with a failure directly, as it can only update the time; should failure occur, control is delegated to higher-level methods

class TimeStampException {
public:
TimeStampException();
$\tilde{\;}$ TimeStampException();
}

void TimeStamp::update(void) throw (TimeStampException) {
using namespace std;
struct timeval tv;
struct timezone tz;
try {
int retVal = gettimeofday(&tv, &tz);
if (retVal == -1) {
cerr « "TimeStamp::updateTimeStamp(): "
« "could not get system time" « endl;
throw TimeStampException();
}
} catch (...) {
cerr « "TimeStamp::updateTimeStamp(): "
« "could not get system time" « endl;
throw TimeStampException();
}
timestamp = tv.tv_sec;
}