package NEXT; use Carp; use strict; sub ancestors { my @inlist = @_; my @outlist = (); while (@inlist) { push @outlist, shift @inlist; no strict 'refs'; unshift @inlist, @{"$outlist[-1]::ISA"}; } return @outlist; } sub AUTOLOAD { my ($self) = @_; my $caller = (caller(1))[3]; my $wanted = $NEXT::AUTOLOAD || 'NEXT::AUTOLOAD'; undef $NEXT::AUTOLOAD; my ($caller_class, $caller_method) = $caller =~ m{(.*)::(.*)}g; my ($wanted_class, $wanted_method) = $wanted =~ m{(.*)::(.*)}g; croak "Can't call $wanted from $caller" unless $caller_method eq $wanted_method; local $NEXT::NEXT{$self,$wanted_method} = $NEXT::NEXT{$self,$wanted_method}; unless (@{$NEXT::NEXT{$self,$wanted_method}||[]}) { my @forebears = ancestors ref $self; while (@forebears) { last if shift @forebears eq $caller_class } no strict 'refs'; @{$NEXT::NEXT{$self,$wanted_method}} = map { *{"${_}::$caller_method"}{CODE}||() } @forebears unless $wanted_method eq 'AUTOLOAD'; @{$NEXT::NEXT{$self,$wanted_method}} = map { (*{"${_}::AUTOLOAD"}{CODE}) ? "${_}::AUTOLOAD" : () } @forebears unless @{$NEXT::NEXT{$self,$wanted_method}||[]}; } my $call_method = shift @{$NEXT::NEXT{$self,$wanted_method}}; return unless defined $call_method; if (ref $call_method eq 'CODE') { return shift()->$call_method(@_) } else { # AN AUTOLOAD no strict 'refs'; ${$call_method} = $caller_method eq 'AUTOLOAD' && ${"${caller_class}::AUTOLOAD"} || $wanted; return $call_method->(@_); } } 1; __END__ =head1 NAME NEXT.pm - Provide a pseudo-class NEXT that allows method redispatch =head1 SYNOPSIS use NEXT; package A; sub A::method { print "$_[0]: A method\n"; $_[0]->NEXT::method() } sub A::DESTROY { print "$_[0]: A dtor\n"; $_[0]->NEXT::DESTROY() } package B; use base qw( A ); sub B::AUTOLOAD { print "$_[0]: B AUTOLOAD\n"; $_[0]->NEXT::AUTOLOAD() } sub B::DESTROY { print "$_[0]: B dtor\n"; $_[0]->NEXT::DESTROY() } package C; sub C::method { print "$_[0]: C method\n"; $_[0]->NEXT::method() } sub C::AUTOLOAD { print "$_[0]: C AUTOLOAD\n"; $_[0]->NEXT::AUTOLOAD() } sub C::DESTROY { print "$_[0]: C dtor\n"; $_[0]->NEXT::DESTROY() } package D; use base qw( B C ); sub D::method { print "$_[0]: D method\n"; $_[0]->NEXT::method() } sub D::AUTOLOAD { print "$_[0]: D AUTOLOAD\n"; $_[0]->NEXT::AUTOLOAD() } sub D::DESTROY { print "$_[0]: D dtor\n"; $_[0]->NEXT::DESTROY() } package main; my $obj = bless {}, "D"; $obj->method(); # Calls D::method, A::method, C::method $obj->missing_method(); # Calls D::AUTOLOAD, B::AUTOLOAD, C::AUTOLOAD # Clean-up calls D::DESTROY, B::DESTROY, A::DESTROY, C::DESTROY =head1 DESCRIPTION NEXT.pm adds a pseudoclass named C to any program that uses it. If a method C calls C<$self->NEXT::m()>, the call to C is redispatched as if the calling method had not originally been found. In other words, a call to C<$self->NEXT::m()> resumes the depth-first, left-to-right search of C<$self>'s class hierarchy that resulted in the original call to C. Note that this is not the same thing as C<$self->SUPER::m()>, which begins a new dispatch that is restricted to searching the ancestors of the current class. C<$self->NEXT::m()> can backtrack past the current class -- to look for a suitable method in other ancestors of C<$self> -- whereas C<$self->SUPER::m()> cannot. A typical use would be in the destructors of a class hierarchy, as illustrated in the synopsis above. Each class in the hierarchy has a DESTROY method that performs some class-specific action and then redispatches the call up the hierarchy. As a result, when an object of class D is destroyed, the destructors of I its parent classes are called (in depth-first, left-to-right order). Another typical use of redispatch would be in C'ed methods. If such a method determined that it was not able to handle a particular call, it might choose to redispatch that call, in the hope that some other C (above it, or to its left) might do better. Note that it is a fatal error for any method (including C) to attempt to redispatch any method except itself. For example: sub D::oops { print "oops!\n"; $_[0]->NEXT::other_method() } =head1 AUTHOR Damian Conway (damian@conway.org) =head1 BUGS AND IRRITATIONS Because it's a module, not an integral part of the interpreter, NEXT.pm has to guess where the surrounding call was found in the method look-up sequence. In the presence of diamond inheritance patterns it occasionally guesses wrong. It's also too slow (despite caching). Comment, suggestions, and patches welcome. =head1 COPYRIGHT Copyright (c) 2000-2001, Damian Conway. All Rights Reserved. This module is free software. It may be used, redistributed and/or modified under the same terms as Perl itself.