I want to subclass a class that I don't really control, as its objects are created by an other class, and I am looking for the best way to do this.

Here is the problem: in XML::Twig you create an XML::Twig object, and use it to parse XML data. During the parsing the XML::Twig object creates objects, in the XML::Twig::Elt class, for each element in the XML. It is easy to subclass XML::Twig, and add methods to it. It is however more difficult to subclass XML::Twig::Elt, as the objects are created within XML::Twig. It would be really convenient though, as you can define handlers on XML elements that are passed the element (in $_).

So I plan to add an elt_class option, that will let you write something like this:

my $t= XML::Twig->new( elt_class => 'my_elt_class',
                       p => sub { $_->cut         # an XML::Twig::Elt 
+method
                                    ->my_process; # a my_elt_class met
+hod
                                }
                     );

package my_elt_class;
BEGIN { @my_elt_class::ISA=('XML::Twig::Elt'); } # to inherit from XML
+::Twig::Elt

sub my_process
  { my $elt= shift;
    # stuff here
  }

[download]

So I have 2 questions: does this make sense? I know I like it, I was getting quite uncomfortable adding methods directly in the XML::Twig::Elt package all the time. And the second part: is the implementation below the best possible?

  # in XML::Twig, every time it needs to create an element
  $self->{elt_class} || 'XML::Twig::Elt';
  my $element=  $elt_class->new;

[download]

Here is a complete example, which creates a default element, then element from classes that redefine the new and print methods, just the print method, and then nothing, inheriting both from XML::Twig::Elt.

#!/usr/bin/perl -w
use strict;

foreach my $elt_class ( '', 'my_elt1', 'my_elt2', 'my_elt3')
  { warn "testing $elt_class";
    my $t= XML::Twig->new( elt_class => $elt_class);
    my $e= $t->add_elt;
    $e->print;
  }

package XML::Twig;

sub new 
  { my $class= shift;
    return bless { @_ }; 
  }

sub add_elt
  { my $self= shift;
    $self->{elt} ||= [];

    # this is where it all happens
    my $elt_class=  $self->{elt_class} || 'XML::Twig::Elt';
    my $e=  $elt_class->new;

    push @{$self->{elt}}, $e;
    return $e;
  }
  
package XML::Twig::Elt;
sub new
  { my $class= shift;
    warn "  new elt $class in XML::Twig::Elt\n";
    return bless { @_ }, $class; 
  }

sub print
  { print "  printing from XML::Twig::Elt\n"; }


# redefines new and print
package my_elt1;

# the BEGIN is needed or @my_elt1::ISA would not actually be initializ
+ed
BEGIN { @my_elt1::ISA= qw(XML::Twig::Elt); }

sub new
  {  my $class= shift;
     warn "  new my_elt1 in my_elt1\n";
     return bless { @_ }, $class; 
  }


sub print 
  { print "  printing from my_elt1\n"; }


# redefines print (new is inherited)
package my_elt2;

BEGIN { @my_elt2::ISA= qw(XML::Twig::Elt); }

sub print 
  { print "  printing from my_elt2\n"; }


# does not redefine anything, both new and print are inherited
package my_elt3;

BEGIN { @my_elt3::ISA= qw(XML::Twig::Elt); }

[download]

In reply to OO Perl: subclassing a class through an other one by mirod

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