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.
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