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

Hi again ,

I'm having loads'o trouble calling functions using code ref's from with an AUTOLOAD method - I have a package that should have a configurable way of handling exceptions - configuration being via the selection of a function from the standard functions e.g. die() or croak(), such that AUTOLOAD should cause a die() or croak() (or whatsoever) as appropriate. Assuming (in its simplest form), a package as follows ... 

package fred;

use Carp;

our $DEFAULT_HANDLER = 'die';
.
. or 
.
our $DEFAULT_HANDLER = \¨

our $AUTOLOAD;

sub AUTOLOAD {
print STDERR "AUTOLOAD(@_) - $AUTOLOAD\n";
  my $self = shift();
  $DEFAULT_HANDLER-<('Died via indirection'); # a la Schwarz & Phoe
+nix
  &$DEFAULT_HANDLER('Died via default');
}

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and a call such as ... 
fred->raise_exception("Some text");

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If either or both the definition of $DEFAULT_HANDLER and the call to the referenced function are changed, the problem continues to manifest itself as follows ... 
AUTOLOAD(fred Some text) - fred::raise_exception
AUTOLOAD(Some text) - fred::die

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The reference gets converted to the configured function call and then the search begins again for a method belonging to the package i.e. at no time does the configured function get called !! I'm using Strawberry perl on Win2K. Am I being spectacularly dense and missing something so obvious it's untrue, or is/may there be a problem ? Anybody any ideas ?? TIA ,
At last, a user level that best describes my experience :-))

Replies are listed 'Best First'.
Re: Problem using code ref's in AUTOLOAD
by bruceb3 (Pilgrim) on Aug 27, 2007 at 21:59 UTC
    The POD perlsub says that you can't a reference to a built in function using the syntax \&CORE::die. 
    ...
Even though it looks like a regular function call, it
isn't: you can't take a reference to it, 
such as the incorrect "\&CORE::open" might appear to produce.
...

    To get around this you could use an anonymous subroutine as the value to $DEFAULT_HANDLER. 

    our $DEFAULT_HANDLER = sub { die @_ };

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    Making this change seems to work. 

    bruce:1:~/tmp $ cat p.pl
#!/usr/bin/perl
# vim: sw=4

use strict;
use warnings;

package fred;

our $DEFAULT_HANDLER = sub { die @_ };
our $AUTOLOAD;

sub AUTOLOAD {
    print STDERR "AUTOLOAD(@_) - $AUTOLOAD\n";
    my $self = shift;
    $DEFAULT_HANDLER->('died via indirection');
}

package main;

my $fred = fred->raise_exception("some test");
bruce:1:~/tmp $ ./p.pl
AUTOLOAD(fred some test) - fred::raise_exception
died via indirection at ./p.pl line 9.
bruce:1:~/tmp $

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[d/l]
[select]
      Mmm, OK, looks like I'm being way dense ... you're right WRT code ref's, however, it doesn't explain why a call of the form 
      package fred;

our $DEFAULT_HANDLER = 'die';

sub AUTOLOAD {
  $&ref("Some text");
}

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      doesn't work ... or my ignorance exceeds even my normal standards, and it does ?
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[d/l]

        You are being very harsh on yourself. It looks like when a sub is referenced that doesn't exist autoload will only ever look in the current package. So when you are making subroutine call (as opposed to a method call) AUTOLOAD will only ever search for fred::die, which doesn't exist, hence the repeated calls to AUTOLOAD. Interesting.

        If the call to the missing subroutine is made as a method call, AUTOLOAD will search the current package and then the packages listed in the @ISA array. To get this to work the handler has to be called as a method, e.g. 

        bruce:1:~/tmp $ cat p.pl
#!/usr/bin/perl
# vim: sw=4

#use strict;
#use warnings;

package bert;

sub die {
    die @_;
}

package fred;

our @ISA = qw( bert );

our $DEFAULT_HANDLER = "die";
our $AUTOLOAD;

sub AUTOLOAD {
    print STDERR "AUTOLOAD(@_) - $AUTOLOAD\n";
    my $self = shift;
    $self->$DEFAULT_HANDLER('died via indirection');
}

package main;

my $fred = fred->raise_exception("some test");
bruce:1:~/tmp $ ./p.pl
AUTOLOAD(fred some test) - fred::raise_exception
fred died via indirection at ./p.pl line 10.
bruce:1:~/tmp $

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        So how to make a package inherit from "CORE" ?

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[d/l]

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