10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 our $VERSION = '0.30';
16 ## ----------------------------------------------------------------------------
17 ## Setting up our environment ...
18 ## ----------------------------------------------------------------------------
19 ## Class::MOP needs to have a few things in the global perl environment so
20 ## that it can operate effectively. Those things are done here.
21 ## ----------------------------------------------------------------------------
23 # ... nothing yet actually ;)
25 ## ----------------------------------------------------------------------------
27 ## ----------------------------------------------------------------------------
28 ## The code below here is to bootstrap our MOP with itself. This is also
29 ## sometimes called "tying the knot". By doing this, we make it much easier
30 ## to extend the MOP through subclassing and such since now you can use the
31 ## MOP itself to extend itself.
33 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
34 ## ----------------------------------------------------------------------------
36 # We need to add in the meta-attributes here so that
37 # any subclass of Class::MOP::* will be able to
38 # inherit them using &construct_instance
42 Class::MOP::Class->meta->add_attribute(
43 Class::MOP::Attribute->new('$:package' => (
45 init_arg => ':package',
49 Class::MOP::Class->meta->add_attribute(
50 Class::MOP::Attribute->new('%:attributes' => (
51 reader => 'get_attribute_map',
52 init_arg => ':attributes',
57 Class::MOP::Class->meta->add_attribute(
58 Class::MOP::Attribute->new('$:attribute_metaclass' => (
59 reader => 'attribute_metaclass',
60 init_arg => ':attribute_metaclass',
61 default => 'Class::MOP::Attribute',
65 Class::MOP::Class->meta->add_attribute(
66 Class::MOP::Attribute->new('$:method_metaclass' => (
67 reader => 'method_metaclass',
68 init_arg => ':method_metaclass',
69 default => 'Class::MOP::Method',
73 Class::MOP::Class->meta->add_attribute(
74 Class::MOP::Attribute->new('$:instance_metaclass' => (
75 reader => 'instance_metaclass',
76 init_arg => ':instance_metaclass',
77 default => 'Class::MOP::Instance',
81 ## Class::MOP::Attribute
83 Class::MOP::Attribute->meta->add_attribute(
84 Class::MOP::Attribute->new('name' => (
89 Class::MOP::Attribute->meta->add_attribute(
90 Class::MOP::Attribute->new('associated_class' => (
91 reader => 'associated_class'
95 Class::MOP::Attribute->meta->add_attribute(
96 Class::MOP::Attribute->new('accessor' => (
98 predicate => 'has_accessor',
102 Class::MOP::Attribute->meta->add_attribute(
103 Class::MOP::Attribute->new('reader' => (
105 predicate => 'has_reader',
109 Class::MOP::Attribute->meta->add_attribute(
110 Class::MOP::Attribute->new('writer' => (
112 predicate => 'has_writer',
116 Class::MOP::Attribute->meta->add_attribute(
117 Class::MOP::Attribute->new('predicate' => (
118 reader => 'predicate',
119 predicate => 'has_predicate',
123 Class::MOP::Attribute->meta->add_attribute(
124 Class::MOP::Attribute->new('init_arg' => (
125 reader => 'init_arg',
126 predicate => 'has_init_arg',
130 Class::MOP::Attribute->meta->add_attribute(
131 Class::MOP::Attribute->new('default' => (
132 # default has a custom 'reader' method ...
133 predicate => 'has_default',
138 # NOTE: (meta-circularity)
139 # This should be one of the last things done
140 # it will "tie the knot" with Class::MOP::Attribute
141 # so that it uses the attributes meta-objects
142 # to construct itself.
143 Class::MOP::Attribute->meta->add_method('new' => sub {
148 (defined $name && $name)
149 || confess "You must provide a name for the attribute";
150 $options{init_arg} = $name
151 if not exists $options{init_arg};
153 # return the new object
154 $class->meta->new_object(name => $name, %options);
157 Class::MOP::Attribute->meta->add_method('clone' => sub {
159 $self->meta->clone_object($self, @_);
170 Class::MOP - A Meta Object Protocol for Perl 5
174 # ... This will come later, for now see
175 # the other SYNOPSIS for more information
179 This module is an attempt to create a meta object protocol for the
180 Perl 5 object system. It makes no attempt to change the behavior or
181 characteristics of the Perl 5 object system, only to create a
182 protocol for its manipulation and introspection.
184 That said, it does attempt to create the tools for building a rich
185 set of extensions to the Perl 5 object system. Every attempt has been
186 made for these tools to keep to the spirit of the Perl 5 object
187 system that we all know and love.
189 =head2 What is a Meta Object Protocol?
191 A meta object protocol is an API to an object system.
193 To be more specific, it is a set of abstractions of the components of
194 an object system (typically things like; classes, object, methods,
195 object attributes, etc.). These abstractions can then be used to both
196 inspect and manipulate the object system which they describe.
198 It can be said that there are two MOPs for any object system; the
199 implicit MOP, and the explicit MOP. The implicit MOP handles things
200 like method dispatch or inheritance, which happen automatically as
201 part of how the object system works. The explicit MOP typically
202 handles the introspection/reflection features of the object system.
203 All object systems have implicit MOPs, without one, they would not
204 work. Explict MOPs however as less common, and depending on the
205 language can vary from restrictive (Reflection in Java or C#) to
206 wide open (CLOS is a perfect example).
208 =head2 Yet Another Class Builder!! Why?
210 This is B<not> a class builder so much as it is a I<class builder
211 B<builder>>. My intent is that an end user does not use this module
212 directly, but instead this module is used by module authors to
213 build extensions and features onto the Perl 5 object system.
215 =head2 Who is this module for?
217 This module is specifically for anyone who has ever created or
218 wanted to create a module for the Class:: namespace. The tools which
219 this module will provide will hopefully make it easier to do more
220 complex things with Perl 5 classes by removing such barriers as
221 the need to hack the symbol tables, or understand the fine details
224 =head2 What changes do I have to make to use this module?
226 This module was designed to be as unintrusive as possible. Many of
227 its features are accessible without B<any> change to your existsing
228 code at all. It is meant to be a compliment to your existing code and
229 not an intrusion on your code base. Unlike many other B<Class::>
230 modules, this module B<does not> require you subclass it, or even that
231 you C<use> it in within your module's package.
233 The only features which requires additions to your code are the
234 attribute handling and instance construction features, and these are
235 both completely optional features. The only reason for this is because
236 Perl 5's object system does not actually have these features built
237 in. More information about this feature can be found below.
239 =head2 A Note about Performance?
241 It is a common misconception that explict MOPs are performance drains.
242 But this is not a universal truth at all, it is an side-effect of
243 specific implementations. For instance, using Java reflection is much
244 slower because the JVM cannot take advantage of any compiler
245 optimizations, and the JVM has to deal with much more runtime type
246 information as well. Reflection in C# is marginally better as it was
247 designed into the language and runtime (the CLR). In contrast, CLOS
248 (the Common Lisp Object System) was built to support an explicit MOP,
249 and so performance is tuned for it.
251 This library in particular does it's absolute best to avoid putting
252 B<any> drain at all upon your code's performance. In fact, by itself
253 it does nothing to affect your existing code. So you only pay for
254 what you actually use.
256 =head2 About Metaclass compatibility
258 This module makes sure that all metaclasses created are both upwards
259 and downwards compatible. The topic of metaclass compatibility is
260 highly esoteric and is something only encountered when doing deep and
261 involved metaclass hacking. There are two basic kinds of metaclass
262 incompatibility; upwards and downwards.
264 Upwards metaclass compatibility means that the metaclass of a
265 given class is either the same as (or a subclass of) all of the
268 Downward metaclass compatibility means that the metaclasses of a
269 given class's anscestors are all either the same as (or a subclass
272 Here is a diagram showing a set of two classes (C<A> and C<B>) and
273 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
274 metaclass compatibility both upwards and downwards.
276 +---------+ +---------+
277 | Meta::A |<----| Meta::B | <....... (instance of )
278 +---------+ +---------+ <------- (inherits from)
281 +---------+ +---------+
283 +---------+ +---------+
285 As I said this is a highly esoteric topic and one you will only run
286 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
287 are interested in why this is an issue see the paper
288 I<Uniform and safe metaclass composition> linked to in the
289 L<SEE ALSO> section of this document.
291 =head2 Using custom metaclasses
293 Always use the metaclass pragma when using a custom metaclass, this
294 will ensure the proper initialization order and not accidentely
295 create an incorrect type of metaclass for you. This is a very rare
296 problem, and one which can only occur if you are doing deep metaclass
297 programming. So in other words, don't worry about it.
301 The protocol is divided into 3 main sub-protocols:
305 =item The Class protocol
307 This provides a means of manipulating and introspecting a Perl 5
308 class. It handles all of symbol table hacking for you, and provides
309 a rich set of methods that go beyond simple package introspection.
311 See L<Class::MOP::Class> for more details.
313 =item The Attribute protocol
315 This provides a consistent represenation for an attribute of a
316 Perl 5 class. Since there are so many ways to create and handle
317 atttributes in Perl 5 OO, this attempts to provide as much of a
318 unified approach as possible, while giving the freedom and
319 flexibility to subclass for specialization.
321 See L<Class::MOP::Attribute> for more details.
323 =item The Method protocol
325 This provides a means of manipulating and introspecting methods in
326 the Perl 5 object system. As with attributes, there are many ways to
327 approach this topic, so we try to keep it pretty basic, while still
328 making it possible to extend the system in many ways.
330 See L<Class::MOP::Method> for more details.
338 There are very few books out on Meta Object Protocols and Metaclasses
339 because it is such an esoteric topic. The following books are really
340 the only ones I have found. If you know of any more, B<I<please>>
341 email me and let me know, I would love to hear about them.
345 =item "The Art of the Meta Object Protocol"
347 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
349 =item "Putting MetaClasses to Work"
351 =item "Smalltalk: The Language"
359 =item Uniform and safe metaclass composition
361 An excellent paper by the people who brought us the original Traits paper.
362 This paper is on how Traits can be used to do safe metaclass composition,
363 and offers an excellent introduction section which delves into the topic of
364 metaclass compatibility.
366 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
368 =item Safe Metaclass Programming
370 This paper seems to precede the above paper, and propose a mix-in based
371 approach as opposed to the Traits based approach. Both papers have similar
372 information on the metaclass compatibility problem space.
374 L<http://citeseer.ist.psu.edu/37617.html>
382 =item The Perl 6 MetaModel work in the Pugs project
386 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
388 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
394 =head1 SIMILAR MODULES
396 As I have said above, this module is a class-builder-builder, so it is
397 not the same thing as modules like L<Class::Accessor> and
398 L<Class::MethodMaker>. That being said there are very few modules on CPAN
399 with similar goals to this module. The one I have found which is most
400 like this module is L<Class::Meta>, although it's philosophy and the MOP it
401 creates are very different from this modules.
405 All complex software has bugs lurking in it, and this module is no
406 exception. If you find a bug please either email me, or add the bug
411 I use L<Devel::Cover> to test the code coverage of my tests, below is the
412 L<Devel::Cover> report on this module's test suite.
414 ---------------------------- ------ ------ ------ ------ ------ ------ ------
415 File stmt bran cond sub pod time total
416 ---------------------------- ------ ------ ------ ------ ------ ------ ------
417 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 9.6 100.0
418 Class/MOP/Attribute.pm 100.0 100.0 91.7 73.8 100.0 28.4 92.1
419 Class/MOP/Class.pm 100.0 93.5 82.3 98.2 100.0 56.6 95.7
420 Class/MOP/Method.pm 100.0 64.3 52.9 80.0 100.0 3.5 85.3
421 metaclass.pm 100.0 100.0 80.0 100.0 n/a 1.9 97.4
422 ---------------------------- ------ ------ ------ ------ ------ ------ ------
423 Total 100.0 90.8 79.7 86.2 100.0 100.0 93.6
424 ---------------------------- ------ ------ ------ ------ ------ ------ ------
426 =head1 ACKNOWLEDGEMENTS
430 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
432 Thanks to Rob for actually getting the development of this module kick-started.
438 Stevan Little E<lt>stevan@iinteractive.comE<gt>
440 =head1 COPYRIGHT AND LICENSE
442 Copyright 2006 by Infinity Interactive, Inc.
444 L<http://www.iinteractive.com>
446 This library is free software; you can redistribute it and/or modify
447 it under the same terms as Perl itself.