diotalevi has asked for the wisdom of the Perl Monks concerning the following question:

The following code shows that Class::Delegation does two things wrong. It masks the real exception and it misreports the exception as a non-existant table method. So what's a perl hacker to do? Write all my delegation code manually and toss C::D?

$w = wrapper->instance;
$w->table;
# Dies with the error "Could not delegate wrapper::table"

package wrapper;
sub instance {
    return bless { schema => database->new() }, shift();
}
sub schema { $_[0]->{schema} }
use Class::Delegation
    send => '-ALL',
    to => 'schema';

package database;
sub new { bless {}, shift() }
sub table { $_[0]->column }
sub column { die "Some exception" }

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Re: Class::Delegation is incompatible with exceptions?
by Jenda (Abbot) on Jun 13, 2003 at 15:24 UTC

    I would say he's to fix the module and send a patch to the author ;-) In this case it seems all you have to do is to change the exception handling code in Class::Delegation so that it only changes some exceptions and rethrows the others without change. (Note I have no idea what is C::D supposed to do.)

    Jenda
    Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
       -- Rick Osborne

    Edit by castaway: Closed small tag in signature

[reply]

      I altered my local copy to rethrow exceptions but I don't understand the problem domain well enough to know whether that is a valid thing to do or whether I have to be more careful.

      For anyone that is interested, my alterations:

      # diff -u Delegation.pm~ Delegation.pm
--- Delegation.pm~      Mon Apr 22 23:20:23 2002
+++ Delegation.pm       Fri Jun 13 11:22:45 2003
@@ -53,6 +53,7 @@
                                next DELEGATOR if exists $delegated->{
+$to[0]};
                                foreach my $as (@as) {
                                        push @results, delegate($deleg
+ated,$wantarray,$invocant,$to[0],$as,\@args);
+                                       die $@ if $@;
                                }
                        }
                        elsif (@as==1) {
@@ -60,6 +61,7 @@
                                foreach my $to (@to) {
                                        next if exists $delegated->{$t
+o};
                                        push @results, delegate($deleg
+ated,$wantarray,$invocant,$to,$as[0],\@args);
+                                       die $@ if $@;
                                }
                        }
                        else {
@@ -69,6 +71,7 @@
                                        my $as = shift @as;
                                        next if exists $delegated->{$t
+o};
                                        push @results, delegate($deleg
+ated,$wantarray,$invocant,$to,$as,\@args);
+                                       die $@ if $@;
                                }
                        }
                }

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[reply]
[d/l]

        I think that the best thing to do would be to change the lines 96-97 from 

                    ? eval { [$target->$as(@$args)] }
            : eval {  $target->$as(@$args)  };

        [download]
        to 
                    ? [$target->$as(@$args)]
            : $target->$as(@$args);

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        I think the eval simply doesn't belong there. We know that the method does exist already thanks to the 
        return unless eval { $target->can($as) };

        [download]
        above and if the method feels like throwing an exception I don't think the Class::Delegate should interfere. I think we should ask TheDamian why is the eval{} there.

        Jenda
        Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
           -- Rick Osborne

        Edit by castaway: Closed small tag in signature

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[d/l]
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Re: Class::Delegation is incompatible with exceptions?
by adrianh (Chancellor) on Jun 13, 2003 at 15:29 UTC
[reply]
Solved! Re: Class::Delegation *is* compatible with exceptions?
by diotalevi (Canon) on Jun 15, 2003 at 22:54 UTC

    To "fix" this problem for Class::Delegation version 1.06 (the highest currently available on CPAN.org right now) requires installing an autoload handler into the appropriate slot. In this case because the wrapper:: class delegates I'd install AUTOLOAD into that package. My current fix is to just rethrow $@ which appears to be the fix.

    This is a problem because this behaviour isn't documented - I had to read the source code to understand it. It turns out that C::D looks for a pre-existing AUTOLOAD function and if it isn't present then it installs a function which croaks with the message "Could not delegate %s". This just overrides C::D's default autohandler.

    sub wrapper::AUTOLOAD {

    # See http://www.perlmonks.org/index.pl?node_id=265697
    # "Class::Delegation is incompatible with exceptions?" for
    # discussion of this function and why it is needed to get C::D
    # to support exceptions properly.
    if ($@ and
        not $@ =~ m{ Class\WDelegation\.pm\s+line\s+\d+.$ }xm) {
        die $@;
    }

    Carp::confess( "Could not delegate " . $AUTOLOAD );
}

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