#!/usr/bin/perl

{
    package Obj;
    my $next_id = 0;
    our %linklist;
    sub new {
        my($class, $label, $prev, $next) = @_;
        my $id = $next_id++;
        my $prev_id = $prev->{id};
        my $next_id = $next->{id};
        my $self = bless {
                          id => $id,
                          label => $label,
                          prev => $prev_id,
                          next => $next_id,
                         }, $class;
        $prev->{next} = $id;
        $next->{prev} = $id;
        $linklist{$id} = $self;
        $self;
    }
    sub next {
        my $self = shift;
        if (defined $self->{next}) {
            $linklist{$self->{next}};
        }
    }
    sub prev {
        my $self = shift;
        if (defined $self->{prev}) {
            $linklist{$self->{prev}};
        }
    }
    sub traverse_forward {
        my($self) = @_;
        print $self->{label};
        while(defined $self->{next}) {
            $self = $linklist{$self->{next}};
            print " " . $self->{label};
        }
        print "\n";
    }
    sub traverse_backward {
        my($self) = @_;
        print $self->{label};
        while(defined $self->{prev}) {
            $self = $linklist{$self->{prev}};
            print " " . $self->{label};
        }
        print "\n";
    }
    sub DESTROY {
        warn "DESTROY @_";
    }
}

{
    package Obj2;
    use Scalar::Util qw(weaken);
    sub new {
        my($class, $label, $prev, $next) = @_;
        my $self = bless {
                          label => $label,
                          prev => $prev,
                          next => $next,
                         }, $class;
        $prev->{next} = $self;
        $next->{prev} = $self;
#       weaken $self->{prev} if $self->{prev}; # Hier!
#       weaken $next->{prev} if $next->{prev}; # Hier!
        $self;
    }
    sub next {
        my $self = shift;
        $self->{next};
    }
    sub prev {
        my $self = shift;
        $self->{prev};
    }
    sub traverse_forward {
        my($self) = @_;
        print $self->{label};
        while(defined $self->{next}) {
            $self = $self->{next};
            print " " . $self->{label};
        }
        print "\n";
    }
    sub traverse_backward {
        my($self) = @_;
        print $self->{label};
        while(defined $self->{prev}) {
            $self = $self->{prev};
            print " " . $self->{label};
        }
        print "\n";
    }
    sub DESTROY {
        warn "DESTROY @_";
    }
}

{
    my $x1 = Obj->new("Null", undef, undef);
    my $x2 = Obj->new("Eins", $x1, undef);
    my $x3 = Obj->new("Zwei", $x2, undef);

    $x1->traverse_forward;
    $x3->traverse_backward;

    %Obj::linklist = ();
}

{
    my $x1 = Obj2->new("Null", undef, undef);
    my $x2 = Obj2->new("Eins", $x1, undef);
    my $x3 = Obj2->new("Zwei", $x2, undef);

    $x1->traverse_forward;
    $x3->traverse_backward;

}