1Class::MOP(3)         User Contributed Perl Documentation        Class::MOP(3)
2
3
4

NAME

6       Class::MOP - A Meta Object Protocol for Perl 5
7

VERSION

9       version 2.2206
10

DESCRIPTION

12       This module is a fully functioning meta object protocol for the Perl 5
13       object system. It makes no attempt to change the behavior or
14       characteristics of the Perl 5 object system, only to create a protocol
15       for its manipulation and introspection.
16
17       That said, it does attempt to create the tools for building a rich set
18       of extensions to the Perl 5 object system. Every attempt has been made
19       to abide by the spirit of the Perl 5 object system that we all know and
20       love.
21
22       This documentation is sparse on conceptual details. We suggest looking
23       at the items listed in the "SEE ALSO" section for more information. In
24       particular the book "The Art of the Meta Object Protocol" was very
25       influential in the development of this system.
26
27   What is a Meta Object Protocol?
28       A meta object protocol is an API to an object system.
29
30       To be more specific, it abstracts the components of an object system
31       (classes, object, methods, object attributes, etc.). These abstractions
32       can then be used to inspect and manipulate the object system which they
33       describe.
34
35       It can be said that there are two MOPs for any object system; the
36       implicit MOP and the explicit MOP. The implicit MOP handles things like
37       method dispatch or inheritance, which happen automatically as part of
38       how the object system works. The explicit MOP typically handles the
39       introspection/reflection features of the object system.
40
41       All object systems have implicit MOPs. Without one, they would not
42       work. Explicit MOPs are much less common, and depending on the language
43       can vary from restrictive (Reflection in Java or C#) to wide open (CLOS
44       is a perfect example).
45
46   Yet Another Class Builder! Why?
47       This is not a class builder so much as a class builder builder. The
48       intent is that an end user will not use this module directly, but
49       instead this module is used by module authors to build extensions and
50       features onto the Perl 5 object system.
51
52       This system is used by Moose, which supplies a powerful class builder
53       system built entirely on top of "Class::MOP".
54
55   Who is this module for?
56       This module is for anyone who has ever created or wanted to create a
57       module for the Class:: namespace. The tools which this module provides
58       make doing complex Perl 5 wizardry simpler, by removing such barriers
59       as the need to hack symbol tables, or understand the fine details of
60       method dispatch.
61
62   What changes do I have to make to use this module?
63       This module was designed to be as unobtrusive as possible. Many of its
64       features are accessible without any change to your existing code. It is
65       meant to be a complement to your existing code and not an intrusion on
66       your code base. Unlike many other Class:: modules, this module does not
67       require you subclass it, or even that you "use" it in within your
68       module's package.
69
70       The only features which require additions to your code are the
71       attribute handling and instance construction features, and these are
72       both completely optional features. The only reason for this is because
73       Perl 5's object system does not actually have these features built in.
74       More information about this feature can be found below.
75
76   About Performance
77       It is a common misconception that explicit MOPs are a performance hit.
78       This is not a universal truth, it is a side-effect of some specific
79       implementations. For instance, using Java reflection is slow because
80       the JVM cannot take advantage of any compiler optimizations, and the
81       JVM has to deal with much more runtime type information as well.
82
83       Reflection in C# is marginally better as it was designed into the
84       language and runtime (the CLR). In contrast, CLOS (the Common Lisp
85       Object System) was built to support an explicit MOP, and so performance
86       is tuned for it.
87
88       This library in particular does its absolute best to avoid putting any
89       drain at all upon your code's performance. In fact, by itself it does
90       nothing to affect your existing code. So you only pay for what you
91       actually use.
92
93   About Metaclass compatibility
94       This module makes sure that all metaclasses created are both upwards
95       and downwards compatible. The topic of metaclass compatibility is
96       highly esoteric and is something only encountered when doing deep and
97       involved metaclass hacking. There are two basic kinds of metaclass
98       incompatibility; upwards and downwards.
99
100       Upwards metaclass compatibility means that the metaclass of a given
101       class is either the same as (or a subclass of) all of the metaclasses
102       of the class's ancestors.
103
104       Downward metaclass compatibility means that the metaclasses of a given
105       class's ancestors are all the same as (or a subclass of) that class's
106       metaclass.
107
108       Here is a diagram showing a set of two classes ("A" and "B") and two
109       metaclasses ("Meta::A" and "Meta::B") which have correct metaclass
110       compatibility both upwards and downwards.
111
112           +---------+     +---------+
113           | Meta::A |<----| Meta::B |      <....... (instance of  )
114           +---------+     +---------+      <------- (inherits from)
115                ^               ^
116                :               :
117           +---------+     +---------+
118           |    A    |<----|    B    |
119           +---------+     +---------+
120
121       In actuality, all of a class's metaclasses must be compatible, not just
122       the class metaclass. That includes the instance, attribute, and method
123       metaclasses, as well as the constructor and destructor classes.
124
125       "Class::MOP" will attempt to fix some simple types of
126       incompatibilities. If all the metaclasses for the parent class are
127       subclasses of the child's metaclasses then we can simply replace the
128       child's metaclasses with the parent's. In addition, if the child is
129       missing a metaclass that the parent has, we can also just make the
130       child use the parent's metaclass.
131
132       As I said this is a highly esoteric topic and one you will only run
133       into if you do a lot of subclassing of Class::MOP::Class. If you are
134       interested in why this is an issue see the paper Uniform and safe
135       metaclass composition linked to in the "SEE ALSO" section of this
136       document.
137
138   Using custom metaclasses
139       Always use the metaclass pragma when using a custom metaclass, this
140       will ensure the proper initialization order and not accidentally create
141       an incorrect type of metaclass for you. This is a very rare problem,
142       and one which can only occur if you are doing deep metaclass
143       programming. So in other words, don't worry about it.
144
145       Note that if you're using Moose we encourage you to not use the
146       metaclass pragma, and instead use Moose::Util::MetaRole to apply roles
147       to a class's metaclasses. This topic is covered at length in various
148       Moose::Cookbook recipes.
149

PROTOCOLS

151       The meta-object protocol is divided into 4 main sub-protocols:
152
153   The Class protocol
154       This provides a means of manipulating and introspecting a Perl 5 class.
155       It handles symbol table hacking for you, and provides a rich set of
156       methods that go beyond simple package introspection.
157
158       See Class::MOP::Class for more details.
159
160   The Attribute protocol
161       This provides a consistent representation for an attribute of a Perl 5
162       class. Since there are so many ways to create and handle attributes in
163       Perl 5 OO, the Attribute protocol provide as much of a unified approach
164       as possible. Of course, you are always free to extend this protocol by
165       subclassing the appropriate classes.
166
167       See Class::MOP::Attribute for more details.
168
169   The Method protocol
170       This provides a means of manipulating and introspecting methods in the
171       Perl 5 object system. As with attributes, there are many ways to
172       approach this topic, so we try to keep it pretty basic, while still
173       making it possible to extend the system in many ways.
174
175       See Class::MOP::Method for more details.
176
177   The Instance protocol
178       This provides a layer of abstraction for creating object instances.
179       Since the other layers use this protocol, it is relatively easy to
180       change the type of your instances from the default hash reference to
181       some other type of reference. Several examples are provided in the
182       examples/ directory included in this distribution.
183
184       See Class::MOP::Instance for more details.
185

FUNCTIONS

187       Note that this module does not export any constants or functions.
188
189   Utility functions
190       Note that these are all called as functions, not methods.
191
192       Class::MOP::get_code_info($code)
193
194       This function returns two values, the name of the package the $code is
195       from and the name of the $code itself. This is used by several elements
196       of the MOP to determine where a given $code reference is from.
197
198       Class::MOP::class_of($instance_or_class_name)
199
200       This will return the metaclass of the given instance or class name.  If
201       the class lacks a metaclass, no metaclass will be initialized, and
202       "undef" will be returned.
203
204       You should almost certainly be using "Moose::Util::find_meta" instead.
205
206   Metaclass cache functions
207       "Class::MOP" holds a cache of metaclasses. The following are functions
208       (not methods) which can be used to access that cache. It is not
209       recommended that you mess with these. Bad things could happen, but if
210       you are brave and willing to risk it: go for it!
211
212       Class::MOP::get_all_metaclasses
213
214       This will return a hash of all the metaclass instances that have been
215       cached by Class::MOP::Class, keyed by the package name.
216
217       Class::MOP::get_all_metaclass_instances
218
219       This will return a list of all the metaclass instances that have been
220       cached by Class::MOP::Class.
221
222       Class::MOP::get_all_metaclass_names
223
224       This will return a list of all the metaclass names that have been
225       cached by Class::MOP::Class.
226
227       Class::MOP::get_metaclass_by_name($name)
228
229       This will return a cached Class::MOP::Class instance, or nothing if no
230       metaclass exists with that $name.
231
232       Class::MOP::store_metaclass_by_name($name, $meta)
233
234       This will store a metaclass in the cache at the supplied $key.
235
236       Class::MOP::weaken_metaclass($name)
237
238       In rare cases (e.g. anonymous metaclasses) it is desirable to store a
239       weakened reference in the metaclass cache. This function will weaken
240       the reference to the metaclass stored in $name.
241
242       Class::MOP::metaclass_is_weak($name)
243
244       Returns true if the metaclass for $name has been weakened (via
245       "weaken_metaclass").
246
247       Class::MOP::does_metaclass_exist($name)
248
249       This will return true of there exists a metaclass stored in the $name
250       key, and return false otherwise.
251
252       Class::MOP::remove_metaclass_by_name($name)
253
254       This will remove the metaclass stored in the $name key.
255
256       Some utility functions (such as "Class::MOP::load_class") that were
257       previously defined in "Class::MOP" regarding loading of classes have
258       been extracted to Class::Load. Please see Class::Load for
259       documentation.
260

SEE ALSO

262   Books
263       There are very few books out on Meta Object Protocols and Metaclasses
264       because it is such an esoteric topic. The following books are really
265       the only ones I have found. If you know of any more, please email me
266       and let me know, I would love to hear about them.
267
268       The Art of the Meta Object Protocol
269       Advances in Object-Oriented Metalevel Architecture and Reflection
270       Putting MetaClasses to Work
271       Smalltalk: The Language
272
273   Papers
274       "Uniform and safe metaclass composition"
275           An excellent paper by the people who brought us the original Traits
276           paper.  This paper is on how Traits can be used to do safe
277           metaclass composition, and offers an excellent introduction section
278           which delves into the topic of metaclass compatibility.
279
280           <http://scg.unibe.ch/archive/papers/Duca05ySafeMetaclassTrait.pdf>
281
282       "Safe Metaclass Programming"
283           This paper seems to precede the above paper, and propose a mix-in
284           based approach as opposed to the Traits based approach. Both papers
285           have similar information on the metaclass compatibility problem
286           space.
287
288           <http://citeseer.ist.psu.edu/37617.html>
289
290   Prior Art
291       The Perl 6 MetaModel work in the Pugs project
292           <http://github.com/perl6/p5-modules/tree/master/Perl6-ObjectSpace/>
293
294   Articles
295       CPAN Module Review of Class::MOP
296           <http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
297

SIMILAR MODULES

299       As I have said above, this module is a class-builder-builder, so it is
300       not the same thing as modules like Class::Accessor and
301       Class::MethodMaker. That being said there are very few modules on CPAN
302       with similar goals to this module. The one I have found which is most
303       like this module is Class::Meta, although its philosophy and the MOP it
304       creates are very different from this modules.
305

BUGS

307       All complex software has bugs lurking in it, and this module is no
308       exception.
309
310       Please report any bugs to "bug-class-mop@rt.cpan.org", or through the
311       web interface at <http://rt.cpan.org>.
312
313       You can also discuss feature requests or possible bugs on the Moose
314       mailing list (moose@perl.org) or on IRC at <irc://irc.perl.org/#moose>.
315

ACKNOWLEDGEMENTS

317       Rob Kinyon
318           Thanks to Rob for actually getting the development of this module
319           kick-started.
320

AUTHORS

322       •   Stevan Little <stevan@cpan.org>
323
324       •   Dave Rolsky <autarch@urth.org>
325
326       •   Jesse Luehrs <doy@cpan.org>
327
328       •   Shawn M Moore <sartak@cpan.org>
329
330       •   יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
331
332       •   Karen Etheridge <ether@cpan.org>
333
334       •   Florian Ragwitz <rafl@debian.org>
335
336       •   Hans Dieter Pearcey <hdp@cpan.org>
337
338       •   Chris Prather <chris@prather.org>
339
340       •   Matt S Trout <mstrout@cpan.org>
341
343       This software is copyright (c) 2006 by Infinity Interactive, Inc.
344
345       This is free software; you can redistribute it and/or modify it under
346       the same terms as the Perl 5 programming language system itself.
347
348
349
350perl v5.38.0                      2023-07-23                     Class::MOP(3)
Impressum