Perl Best Practices

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.