# man > Class::Virtual

## NAME
    [Class::Virtual](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AVirtual/markdown) - Base class for virtual base classes.

## SYNOPSIS
      package [My::Virtual::Idaho](https://www.chedong.com/phpMan.php/perldoc/My%3A%3AVirtual%3A%3AIdaho/markdown);
      use base qw([Class::Virtual](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AVirtual/markdown));

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


      package [My::Private::Idaho](https://www.chedong.com/phpMan.php/perldoc/My%3A%3APrivate%3A%3AIdaho/markdown);
      use base qw([My::Virtual::Idaho](https://www.chedong.com/phpMan.php/perldoc/My%3A%3AVirtual%3A%3AIdaho/markdown));

      # Check to make sure [My::Private::Idaho](https://www.chedong.com/phpMan.php/perldoc/My%3A%3APrivate%3A%3AIdaho/markdown) implemented everything
      my @missing = __PACKAGE__->missing_methods;
      die __PACKAGE__ . ' forgot to implement ' . join ', ', @missing
          if @missing;

      # If [My::Private::Idaho](https://www.chedong.com/phpMan.php/perldoc/My%3A%3APrivate%3A%3AIdaho/markdown) forgot to implement new(), the program will
      # halt and yell about that.
      my $idaho = [My::Private::Idaho](https://www.chedong.com/phpMan.php/perldoc/My%3A%3APrivate%3A%3AIdaho/markdown)->new;

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

## DESCRIPTION
    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.

### Methods
    virtual_methods
          [Virtual::Class](https://www.chedong.com/phpMan.php/perldoc/Virtual%3A%3AClass/markdown)->virtual_methods(@virtual_methods);
          my @must_implement = [Sub::Class](https://www.chedong.com/phpMan.php/perldoc/Sub%3A%3AClass/markdown)->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.

    missing_methods
          my @missing_methods = [Sub::Class](https://www.chedong.com/phpMan.php/perldoc/Sub%3A%3AClass/markdown)->missing_methods;

        Returns a list of methods [Sub::Class](https://www.chedong.com/phpMan.php/perldoc/Sub%3A%3AClass/markdown) has not yet implemented.

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

## AUTHOR
    Michael G Schwern <<schwern@pobox.com>>

## LEGAL
    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.

    See <<http://dev.perl.org/licenses/>>

## SEE ALSO
    [Class::Virtually::Abstract](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AVirtually%3A%3AAbstract/markdown)