Class::Virtual - Base class for virtual base classes.


  package My::Virtual::Idaho;
  use base qw(Class::Virtual);

  __PACKAGE__->virtual_methods(qw(new foo bar this that));

  package My::Private::Idaho;
  use base qw(My::Virtual::Idaho);

  # Check to make sure My::Private::Idaho implemented everything
  my @missing = __PACKAGE__->missing_methods;
  die __PACKAGE__ . ' forgot to implement ' . join ', ', @missing 
      if @missing;

  # If My::Private::Idaho forgot to implement new(), the program will
  # halt and yell about that.
  my $idaho = My::Private::Idaho->new;

  # See what methods we're obligated to implement.
  my @must_implement = __PACKAGE__->virtual_methods;


THIS MODULE IS DISCOURAGED! Avoid using it for new code. There's nothing wrong with it, but there are better ways to accomplish the same thing. Look into the Moose ecosystem.

This is a base class for implementing virtual base classes (what some people call an abstract class). Kinda kooky. It allows you to explicitly declare what methods are virtual and that must be implemented by subclasses. This might seem silly, since your program will halt and catch fire when an unimplemented virtual method is hit anyway, but there's some benefits.

The error message is more informative. Instead of the usual "Can't locate object method" error, you'll get one explaining that a virtual method was left unimplemented.

Subclass authors can explicitly check to make sure they've implemented all the necessary virtual methods. When used as part of a regression test, it will shield against the virtual method requirements changing out from under the subclass.

Finally, subclass authors can get an explicit list of everything they're expected to implement.

Doesn't hurt and it doesn't slow you down.


  my @must_implement = Sub::Class->virtual_methods;

This is an accessor to the list of virtual_methods. Virtual base classes will declare their list of virtual methods. Subclasses will look at them. Once the virtual methods are set they cannot be undone.

  my @missing_methods = Sub::Class->missing_methods;

Returns a list of methods Sub::Class has not yet implemented.


Autoloaded methods are currently not recognized. I have no idea how to solve this.


Michael G Schwern <>


Copyright 2000-2015 Michael G Schwern

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.