Class::ActsLike - Perl extension for identifying class behavior similarities


  package HappyFunBuilding;

  use Class::ActsLike qw( Bakery Arcade );


  $building->bake( 'cookies' ) if $building->acts_like( 'Bakery' );
  $building->play( 'pinball' ) if $building->acts_like( 'Arcade' );


Note: This is a deprecated module. Use Class::Roles for new development. This module uses Class::Roles internally and exists for the purpose of backwards compatibility. The philosophy of the documentation still applies, though, so enjoy reading it!

Polymorphism is a fundamental building block of object orientation. Any two objects that can receive the same messages with identical semantics can substitute for each other, regardless of their internal implementations.

Much of the introductory literature explains this concept in terms of inheritance. While inheritance is one way for two different classes to provide different behavior for the same methods, it is not the only way. Perl modules such as the DBDs or Test::MockObject prove that classes do not have to inherit from a common ancestor to be polymorphically equivalent.

Class::ActsLike provides an alternative to isa().

The example class defined above marks HappyFunBuilding as acting like both the Bakery and Arcade classes. It is not necessary to create an ancestor class of Building, or to have HappyFunBuilding inherit from both Bakery and Arcade. As well, one could say:

  package FauxArcade;

  use Arcade;
  use Class::ActsLike 'Arcade';

  sub new
        my $class = shift;
        bless { _arcade => Arcade->new() }, $class;

Provided that the FauxArcade now delegates all methods an Arcade object can receive to the contained Arcade object, this expresses the has-a relationship more accurately. Code which requires an Arcade object should, when handed an object, check to see if the object acts like an Arcade object. The FauxArcade is suitable for any sort of Hollywood production where the real Arcade is unnecessary. This is why actors always seem so good at pinball.

This technique fulfills two goals:

  • To allow you to check that the class or object you receive can handle the types of messages you're going to send it.

  • To avoid dictating how the class or object you receive handles the messages: inheritance, delegation, composition, or re-implementation.

By default, a new class automatically acts like itself, whether you use Class::ActsLike in its package. It also automatically acts like all of its parent classes, again without having had Class::ActsLike used in its namespace.


Class::ActsLike installs a new method named acts_like() in the UNIVERSAL package, so it is available to all classes and objects. Call it directly on an object or a class name:

  $class_or_object->acts_like( $potentially_emulated_class );

It returns true or false, depending on whether the class or class of the object acts like the target class.


chromatic chromatic at wgz dot org


Allison Randal, for debating the theory of this idea. Dan Sugalski and Dave Rolsky for understanding the idea.

Larry Wall and the rest of the Perl 6 design team for adding roles to Perl 6.


Copyright (c) 2003 and 2005, chromatic.

You may use, modify, and distribute this software under the same terms as Perl 5.8.x. There is no guarantee or warranty. If something breaks, migrate to Class::Roles, then file a bug, write a test, and send a patch.


perl(1), Class::Roles, Perl 6 Apocalypse 12 and Synopsis 12.