Section 13.12. Processing Exceptions

13.12. Processing Exceptions

Catch exception objects in most-derived-first order.

The only drawback to using method calls to detect particular types of exceptions:

      if ( X::TooBig->caught(  ) ) {

is that you have to be careful about the order in which you try your alternatives. For example, if X::WaaaaayTooBig inherits from X:TooBig, the following code won't work correctly:

      
     # If the attempt fails...
     if ($EVAL_ERROR) {         
# If the candidate was considered too big, go with the maximum allowed...
         if ( X::TooBig->caught(  ) ) {             my @range = $EVAL_ERROR->get_range(  );             $value = $range[-1];         }         
# If the candidate was considered waaaaay too big, rethrow the exception...
         elsif ( X::WaaaaayTooBig->caught(  ) ) {             $EVAL_ERROR->rethrow(  );         }         
# etc.
     }

The problem is that if an X::WaaaaayTooBig exception is thrown, $EVAL_ERROR will refer to an X::WaaaaayTooBig object. But the X::WaaaaayTooBig class inherits from the X::TooBig class, so an X::WaaaaayTooBig object is an X::TooBig object. That means the first if test will succeed, and the specialized derived-class exception will be treated like a generic base-class exception instead.

The solution is simple: whenever you're determining the type of an exception you just caught, test for the most-derived classes first.