Moose::Manual::MOP - phpMan

NAME
    Moose::Manual::MOP - The Moose (and Class::MOP) meta API

VERSION
    version 2.2200

INTRODUCTION
    Moose provides a powerful introspection API built on top of
    "Class::MOP". "MOP" stands for Meta-Object Protocol. In plainer English,
    a MOP is an API for performing introspection on classes, attributes,
    methods, and so on.

    In fact, it is "Class::MOP" that provides many of Moose's core features,
    including attributes, before/after/around method modifiers, and
    immutability. In most cases, Moose takes an existing "Class::MOP" class
    and subclasses it to add additional features. Moose also adds some
    entirely new features of its own, such as roles, the augment modifier,
    and types.

    If you're interested in the MOP, it's important to know about
    "Class::MOP" so you know what docs to read. Often, the introspection
    method that you're looking for is defined in a "Class::MOP" class,
    rather than Moose itself.

    The MOP provides more than just *read-only* introspection. It also lets
    you add attributes and methods, apply roles, and much more. In fact, all
    of the declarative Moose sugar is simply a thin layer on top of the MOP
    API.

    If you want to write Moose extensions, you'll need to learn some of the
    MOP API. The introspection methods are also handy if you want to
    generate docs or inheritance graphs, or do some other runtime
    reflection.

    This document is not a complete reference for the meta API. We're just
    going to cover some of the highlights, and give you a sense of how it
    all works. To really understand it, you'll have to read a lot of other
    docs, and possibly even dig into the Moose guts a bit.

GETTING STARTED
    The usual entry point to the meta API is through a class's metaclass
    object, which is a Moose::Meta::Class. This is available by calling the
    "meta" method on a class or object:

      package User;

      use Moose;

      my $meta = __PACKAGE__->meta;

    The "meta" method is added to a class when it uses Moose.

    You can also use "Class::MOP::Class->initialize($name)" to get a
    metaclass object for any class. This is safer than calling
    "$class->meta" when you're not sure that the class has a meta method.

    The "Class::MOP::Class->initialize" constructor will return an existing
    metaclass if one has already been created (via Moose or some other
    means). If it hasn't, it will return a new "Class::MOP::Class" object.
    This will work for classes that use Moose, meta API classes, and classes
    which don't use Moose at all.

USING THE METACLASS OBJECT
    The metaclass object can tell you about a class's attributes, methods,
    roles, parents, and more. For example, to look at all of the class's
    attributes:

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

    The "get_all_attributes" method is documented in "Class::MOP::Class".
    For Moose-using classes, it returns a list of Moose::Meta::Attribute
    objects for attributes defined in the class and its parents.

    You can also get a list of methods:

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

    Now we're looping over a list of Moose::Meta::Method objects. Note that
    some of these objects may actually be a subclass of Moose::Meta::Method,
    as Moose uses different classes to represent wrapped methods, delegation
    methods, constructors, etc.

    We can look at a class's parent classes and subclasses:

      for my $class ( $meta->linearized_isa ) {
          print "$class\n";
      }

      for my $subclass ( $meta->subclasses ) {
          print "$subclass\n";
      }

    Note that both these methods return class *names*, not metaclass
    objects.

ALTERING CLASSES WITH THE MOP
    The metaclass object can change the class directly, by adding
    attributes, methods, etc.

    As an example, we can add a method to a class:

      $meta->add_method( 'say' => sub { print @_, "\n" } );

    Or an attribute:

      $meta->add_attribute( 'size' => ( is => 'rw', isa  => 'Int' ) );

    Obviously, this is much more cumbersome than using Perl syntax or Moose
    sugar for defining methods and attributes, but this API allows for very
    powerful extensions.

    You might remember that we've talked about making classes immutable
    elsewhere in the manual. This is a good practice. However, once a class
    is immutable, calling any of these update methods will throw an
    exception.

    You can make a class mutable again simply by calling
    "$meta->make_mutable". Once you're done changing it, you can restore
    immutability by calling "$meta->make_immutable".

    However, the most common use for this part of the meta API is as part of
    Moose extensions. These extensions should assume that they are being run
    before you make a class immutable.

GOING FURTHER
    If you're interested in extending Moose, we recommend reading all of the
    "Meta" and "Extending" recipes in the Moose::Cookbook. Those recipes
    show various practical applications of the MOP.

    If you'd like to write your own extensions, one of the best ways to
    learn more about this is to look at other similar extensions to see how
    they work. You'll probably also need to read various API docs, including
    the docs for the various "Moose::Meta::*" and "Class::MOP::*" classes.

    Finally, we welcome questions on the Moose mailing list and IRC.
    Information on the mailing list, IRC, and more references can be found
    in the Moose.pm docs.

AUTHORS
    *   Stevan Little <stevan AT cpan.org>

    *   Dave Rolsky <autarch AT urth.org>

    *   Jesse Luehrs <doy AT cpan.org>

    *   Shawn M Moore <sartak AT cpan.org>

    *   יובל קוג'מן (Yuval Kogman) <nothingmuch AT woobling.org>

    *   Karen Etheridge <ether AT cpan.org>

    *   Florian Ragwitz <rafl AT debian.org>

    *   Hans Dieter Pearcey <hdp AT cpan.org>

    *   Chris Prather <chris AT prather.org>

    *   Matt S Trout <mstrout AT cpan.org>

COPYRIGHT AND LICENSE
    This software is copyright (c) 2006 by Infinity Interactive, Inc.

    This is free software; you can redistribute it and/or modify it under
    the same terms as the Perl 5 programming language system itself.