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Moose::Manual::Concepts - Moose OO concepts


In the past, you may not have thought too much about the difference between packages and classes, attributes and methods, constructors and methods, etc. With Moose, these are all conceptually separate things, even though under the hood they're implemented with plain old Perl.

Our meta-object protocol (aka MOP) provides well-defined introspection features for each of those concepts, and Moose in turn provides distinct sugar for each of them. Moose also introduces additional concepts such as roles, method modifiers, and declarative delegation.

Knowing what these concepts mean in Moose-speak, and how they used to be done in old school Perl 5 OO is a good way to start learning to use Moose.


When you say "use Moose" in a package, you are making your package a class. At its simplest, a class will consist simply of attributes and/or methods. It can also include roles, method modifiers, and more.

A class has zero or more attributes.

A class has zero or more methods.

A class has zero or more superclasses (aka parent classes). A class inherits from its superclass(es).

A class has zero or more method modifiers. These modifiers can apply to its own methods or methods that are inherited from its ancestors.

A class does (and consumes) zero or more roles.

A class has a constructor and a destructor. These are provided for you "for free" by Moose.

The constructor accepts named parameters corresponding to the class's attributes and uses them to initialize an object instance.

A class has a metaclass, which in turn has meta-attributes, meta-methods, and meta-roles. This metaclass describes the class.

A class is usually analogous to a category of nouns, like "People" or "Users".

  package Person;

  use Moose;
  # now it's a Moose class!


An attribute is a property of the class that defines it. It always has a name, and it may have a number of other properties.

These properties can include a read/write flag, a type, accessor method names, delegations, a default value, and more.

Attributes are not methods, but defining them causes various accessor methods to be created. At a minimum, a normal attribute will always have a reader accessor method. Many attributes also have other methods, such as a writer method, clearer method, and predicate method ("has it been set?").

An attribute may also define delegations, which will create additional methods based on the delegation mapping.

By default, Moose stores attributes in the object instance, which is a hashref, but this is invisible to the author of a Moose-based class! It is best to think of Moose attributes as "properties" of the opaque object instance. These properties are accessed through well-defined accessor methods.

An attribute is something that the class's members have. For example, People have first and last names. Users have passwords and last login datetimes.

  has 'first_name' => (
      is  => 'rw',
      isa => 'Str',


A method is very straightforward. Any subroutine you define in your class is a method.

Methods correspond to verbs, and are what your objects can do. For example, a User can login.

  sub login { ... }


A role is something that a class does. We also say that classes consume roles. For example, a Machine class might do the Breakable role, and so could a Bone class. A role is used to define some concept that cuts across multiple unrelated classes, like "breakability", or "has a color".

A role has zero or more attributes.

A role has zero or more methods.

A role has zero or more method modifiers.

A role has zero or more required methods.

A required method is not implemented by the role. Required methods say "to use this Role you must implement this method".

A role has zero or more excluded roles.

An excluded role is a role that the role doing the excluding says it cannot be combined with.

Roles are composed into classes (or other roles). When a role is composed into a class, its attributes and methods are "flattened" into the class. Roles do not show up in the inheritance hierarchy. When a role is composed, its attributes and methods appear as if they were defined in the consuming class.

Role are somewhat like mixins or interfaces in other OO languages.

  package Breakable;

  use Moose::Role;

  requires 'break';

  has 'is_broken' => (
      is  => 'rw',
      isa => 'Bool',

  after 'break' => sub {
      my $self = shift;


Method modifiers

A method modifier is a hook that is called when a named method is called. For example, you could say "before calling login(), call this modifier first". Modifiers come in different flavors like "before", "after", "around", and "augment", and you can apply more than one modifier to a single method.

Method modifiers are often used as an alternative to overriding a method in a parent class. They are also used in roles as a way of modifying methods in the consuming class.

Under the hood, a method modifier is just a plain old Perl subroutine that gets called before or after (or around, etc.) some named method.

  before 'login' => sub {
      my $self = shift;
      my $pw   = shift;

      warn "Called login() with $pw\n";


Moose also comes with a (miniature) type system. This allows you to define types for attributes. Moose has a set of built-in types based on what Perl provides, such as Str, Num, Bool, HashRef, etc.

In addition, every class name in your application can also be used as a type name.

Finally, you can define your own types, either as subtypes or entirely new types, with their own constraints. For example, you could define a type PosInt, a subtype of Int which only allows positive numbers.


Moose attributes provide declarative syntax for defining delegations. A delegation is a method which calls some method on an attribute to do its real work.


A constructor creates an object instance for the class. In old school Perl, this was usually done by defining a method called new() which in turn called bless on a reference.

With Moose, this new() method is created for you, and it simply does the right thing. You should never need to define your own constructor!

Sometimes you want to do something whenever an object is created. In those cases, you can provide a BUILD() method in your class. Moose will call this for you after creating a new object.


This is a special method called when an object instance goes out of scope. You can specialize what your class does in this method if you need to, but you usually don't.

With old school Perl 5, this is the DESTROY() method, but with Moose it is the DEMOLISH() method.

Object instance

An object instance is a specific noun in the class's "category". For example, one specific Person or User. An instance is created by the class's constructor.

An instance has values for its attributes. For example, a specific person has a first and last name.

In old school Perl 5, this is often a blessed hash reference. With Moose, you should never need to know what your object instance actually is. (Okay, it's usually a blessed hashref with Moose, too.)

Moose vs old school summary


A metaclass is a class that describes classes. With Moose, every class you define gets a meta() method. It returns a Moose::Meta::Class object, which has an introspection API that can tell you about the class it represents.

  my $meta = User->meta();

  for my $attribute ( $meta->get_all_attributes ) {
      print $attribute->name(), "\n";

      if ( $attribute->has_type_constraint ) {
          print "  type: ", $attribute->type_constraint->name, "\n";

  for my $method ( $meta->get_all_methods ) {
      print $method->name, "\n";

Almost every concept we defined earlier has a meta class, so we have Moose::Meta::Class, Moose::Meta::Attribute, Moose::Meta::Method, Moose::Meta::Role, Moose::Meta::TypeConstraint, Moose::Meta::Instance, and so on.


One of the great things about Moose is that if you dig down and find that it does something the "wrong way", you can change it by extending a metaclass. For example, you can have arrayref based objects, you can make your constructors strict (no unknown parameters allowed!), you can define a naming scheme for attribute accessors, you can make a class a Singleton, and much, much more.

Many of these extensions require surprisingly small amounts of code, and once you've done it once, you'll never have to hand-code "your way of doing things" again. Instead you'll just load your favorite extensions.

  package MyWay::User;

  use Moose;
  use MooseX::StrictConstructor
  use MooseX::MyWay;

  has ...;


So you're sold on Moose. Time to learn how to really use it.

If you want to see how Moose would translate directly into old school Perl 5 OO code, check out Moose::Manual::Unsweetened. This might be helpful for quickly wrapping your brain around some aspects of "the Moose way".

Or you can skip that and jump straight to Moose::Manual::Classes and the rest of the Moose::Manual.

After that we recommend that you start with the Moose::Cookbook. If you work your way through all the recipes under the basics section, you should have a pretty good sense of how Moose works, and all of its basic OO features.

After that, check out the Role recipes. If you're really curious, go on and read the Meta and Extending recipes, but those are mostly there for people who want to be Moose wizards and change how Moose works.


Dave Rolsky <>


Copyright 2008-2009 by Infinity Interactive, Inc.

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

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