1Moose::Manual::MOP(3) User Contributed Perl DocumentationMoose::Manual::MOP(3)
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6 Moose::Manual::MOP - The Moose (and Class::MOP) meta API
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9 version 2.2203
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12 Moose provides a powerful introspection API built on top of
13 "Class::MOP". "MOP" stands for Meta-Object Protocol. In plainer
14 English, a MOP is an API for performing introspection on classes,
15 attributes, methods, and so on.
16 In fact, it is "Class::MOP" that provides many of Moose's core
18 features, including attributes, before/after/around method modifiers,
19 and immutability. In most cases, Moose takes an existing "Class::MOP"
20 class and subclasses it to add additional features. Moose also adds
21 some entirely new features of its own, such as roles, the augment
22 modifier, and types.
23 If you're interested in the MOP, it's important to know about
25 "Class::MOP" so you know what docs to read. Often, the introspection
26 method that you're looking for is defined in a "Class::MOP" class,
27 rather than Moose itself.
28 The MOP provides more than just read-only introspection. It also lets
30 you add attributes and methods, apply roles, and much more. In fact,
31 all of the declarative Moose sugar is simply a thin layer on top of the
32 MOP API.
33 If you want to write Moose extensions, you'll need to learn some of the
35 MOP API. The introspection methods are also handy if you want to
36 generate docs or inheritance graphs, or do some other runtime
37 reflection.
38 This document is not a complete reference for the meta API. We're just
40 going to cover some of the highlights, and give you a sense of how it
41 all works. To really understand it, you'll have to read a lot of other
42 docs, and possibly even dig into the Moose guts a bit.
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45 The usual entry point to the meta API is through a class's metaclass
46 object, which is a Moose::Meta::Class. This is available by calling the
47 "meta" method on a class or object:
48 package User;
50 use Moose;
52 my $meta = __PACKAGE__->meta;
54 The "meta" method is added to a class when it uses Moose.
56 You can also use "Class::MOP::Class->initialize($name)" to get a
58 metaclass object for any class. This is safer than calling
59 "$class->meta" when you're not sure that the class has a meta method.
60 The "Class::MOP::Class->initialize" constructor will return an existing
62 metaclass if one has already been created (via Moose or some other
63 means). If it hasn't, it will return a new "Class::MOP::Class" object.
64 This will work for classes that use Moose, meta API classes, and
65 classes which don't use Moose at all.
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68 The metaclass object can tell you about a class's attributes, methods,
69 roles, parents, and more. For example, to look at all of the class's
70 attributes:
71 for my $attr ( $meta->get_all_attributes ) {
73 print $attr->name, "\n";
74 }
75 The "get_all_attributes" method is documented in "Class::MOP::Class".
77 For Moose-using classes, it returns a list of Moose::Meta::Attribute
78 objects for attributes defined in the class and its parents.
79 You can also get a list of methods:
81 for my $method ( $meta->get_all_methods ) {
83 print $method->fully_qualified_name, "\n";
84 }
85 Now we're looping over a list of Moose::Meta::Method objects. Note that
87 some of these objects may actually be a subclass of
88 Moose::Meta::Method, as Moose uses different classes to represent
89 wrapped methods, delegation methods, constructors, etc.
90 We can look at a class's parent classes and subclasses:
92 for my $class ( $meta->linearized_isa ) {
94 print "$class\n";
95 }
96 for my $subclass ( $meta->subclasses ) {
98 print "$subclass\n";
99 }
100 Note that both these methods return class names, not metaclass objects.
102
104 The metaclass object can change the class directly, by adding
105 attributes, methods, etc.
106 As an example, we can add a method to a class:
108 $meta->add_method( 'say' => sub { print @_, "\n" } );
110 Or an attribute:
112 $meta->add_attribute( 'size' => ( is => 'rw', isa => 'Int' ) );
114 Obviously, this is much more cumbersome than using Perl syntax or Moose
116 sugar for defining methods and attributes, but this API allows for very
117 powerful extensions.
118 You might remember that we've talked about making classes immutable
120 elsewhere in the manual. This is a good practice. However, once a class
121 is immutable, calling any of these update methods will throw an
122 exception.
123 You can make a class mutable again simply by calling
125 "$meta->make_mutable". Once you're done changing it, you can restore
126 immutability by calling "$meta->make_immutable".
127 However, the most common use for this part of the meta API is as part
129 of Moose extensions. These extensions should assume that they are being
130 run before you make a class immutable.
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133 If you're interested in extending Moose, we recommend reading all of
134 the "Meta" and "Extending" recipes in the Moose::Cookbook. Those
135 recipes show various practical applications of the MOP.
136 If you'd like to write your own extensions, one of the best ways to
138 learn more about this is to look at other similar extensions to see how
139 they work. You'll probably also need to read various API docs,
140 including the docs for the various "Moose::Meta::*" and "Class::MOP::*"
141 classes.
142 Finally, we welcome questions on the Moose mailing list and IRC.
144 Information on the mailing list, IRC, and more references can be found
145 in the Moose.pm docs.
146
148 • Stevan Little <stevan@cpan.org>
149 • Dave Rolsky <autarch@urth.org>
151 • Jesse Luehrs <doy@cpan.org>
153 • Shawn M Moore <sartak@cpan.org>
155 • יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
157 • Karen Etheridge <ether@cpan.org>
159 • Florian Ragwitz <rafl@debian.org>
161 • Hans Dieter Pearcey <hdp@cpan.org>
163 • Chris Prather <chris@prather.org>
165 • Matt S Trout <mstrout@cpan.org>
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169 This software is copyright (c) 2006 by Infinity Interactive, Inc.
170 This is free software; you can redistribute it and/or modify it under
172 the same terms as the Perl 5 programming language system itself.
173perl v5.36.0 2023-02-06 Moose::Manual::MOP(3)