10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 our $VERSION = '0.26';
16 ## ----------------------------------------------------------------------------
17 ## Setting up our environment ...
18 ## ----------------------------------------------------------------------------
19 ## Class::MOP needs to have a few things in the global perl environment so
20 ## that it can operate effectively. Those things are done here.
21 ## ----------------------------------------------------------------------------
23 # ... nothing yet actually ;)
25 ## ----------------------------------------------------------------------------
27 ## ----------------------------------------------------------------------------
28 ## The code below here is to bootstrap our MOP with itself. This is also
29 ## sometimes called "tying the knot". By doing this, we make it much easier
30 ## to extend the MOP through subclassing and such since now you can use the
31 ## MOP itself to extend itself.
33 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
34 ## ----------------------------------------------------------------------------
36 # We need to add in the meta-attributes here so that
37 # any subclass of Class::MOP::* will be able to
38 # inherit them using &construct_instance
42 Class::MOP::Class->meta->add_attribute(
43 Class::MOP::Attribute->new('$: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::Attribute
75 Class::MOP::Attribute->meta->add_attribute(
76 Class::MOP::Attribute->new('name' => (
81 Class::MOP::Attribute->meta->add_attribute(
82 Class::MOP::Attribute->new('associated_class' => (
83 reader => 'associated_class'
87 Class::MOP::Attribute->meta->add_attribute(
88 Class::MOP::Attribute->new('accessor' => (
90 predicate => 'has_accessor',
94 Class::MOP::Attribute->meta->add_attribute(
95 Class::MOP::Attribute->new('reader' => (
97 predicate => 'has_reader',
101 Class::MOP::Attribute->meta->add_attribute(
102 Class::MOP::Attribute->new('writer' => (
104 predicate => 'has_writer',
108 Class::MOP::Attribute->meta->add_attribute(
109 Class::MOP::Attribute->new('predicate' => (
110 reader => 'predicate',
111 predicate => 'has_predicate',
115 Class::MOP::Attribute->meta->add_attribute(
116 Class::MOP::Attribute->new('init_arg' => (
117 reader => 'init_arg',
118 predicate => 'has_init_arg',
122 Class::MOP::Attribute->meta->add_attribute(
123 Class::MOP::Attribute->new('default' => (
124 # default has a custom 'reader' method ...
125 predicate => 'has_default',
130 # NOTE: (meta-circularity)
131 # This should be one of the last things done
132 # it will "tie the knot" with Class::MOP::Attribute
133 # so that it uses the attributes meta-objects
134 # to construct itself.
135 Class::MOP::Attribute->meta->add_method('new' => sub {
140 (defined $name && $name)
141 || confess "You must provide a name for the attribute";
142 $options{init_arg} = $name
143 if not exists $options{init_arg};
145 # return the new object
146 $class->meta->new_object(name => $name, %options);
149 Class::MOP::Attribute->meta->add_method('clone' => sub {
151 $self->meta->clone_object($self, @_);
162 Class::MOP - A Meta Object Protocol for Perl 5
166 # ... This will come later, for now see
167 # the other SYNOPSIS for more information
171 This module is an attempt to create a meta object protocol for the
172 Perl 5 object system. It makes no attempt to change the behavior or
173 characteristics of the Perl 5 object system, only to create a
174 protocol for its manipulation and introspection.
176 That said, it does attempt to create the tools for building a rich
177 set of extensions to the Perl 5 object system. Every attempt has been
178 made for these tools to keep to the spirit of the Perl 5 object
179 system that we all know and love.
181 =head2 What is a Meta Object Protocol?
183 A meta object protocol is an API to an object system.
185 To be more specific, it is a set of abstractions of the components of
186 an object system (typically things like; classes, object, methods,
187 object attributes, etc.). These abstractions can then be used to both
188 inspect and manipulate the object system which they describe.
190 It can be said that there are two MOPs for any object system; the
191 implicit MOP, and the explicit MOP. The implicit MOP handles things
192 like method dispatch or inheritance, which happen automatically as
193 part of how the object system works. The explicit MOP typically
194 handles the introspection/reflection features of the object system.
195 All object systems have implicit MOPs, without one, they would not
196 work. Explict MOPs however as less common, and depending on the
197 language can vary from restrictive (Reflection in Java or C#) to
198 wide open (CLOS is a perfect example).
200 =head2 Yet Another Class Builder!! Why?
202 This is B<not> a class builder so much as it is a I<class builder
203 B<builder>>. My intent is that an end user does not use this module
204 directly, but instead this module is used by module authors to
205 build extensions and features onto the Perl 5 object system.
207 =head2 Who is this module for?
209 This module is specifically for anyone who has ever created or
210 wanted to create a module for the Class:: namespace. The tools which
211 this module will provide will hopefully make it easier to do more
212 complex things with Perl 5 classes by removing such barriers as
213 the need to hack the symbol tables, or understand the fine details
216 =head2 What changes do I have to make to use this module?
218 This module was designed to be as unintrusive as possible. Many of
219 its features are accessible without B<any> change to your existsing
220 code at all. It is meant to be a compliment to your existing code and
221 not an intrusion on your code base. Unlike many other B<Class::>
222 modules, this module B<does not> require you subclass it, or even that
223 you C<use> it in within your module's package.
225 The only features which requires additions to your code are the
226 attribute handling and instance construction features, and these are
227 both completely optional features. The only reason for this is because
228 Perl 5's object system does not actually have these features built
229 in. More information about this feature can be found below.
231 =head2 A Note about Performance?
233 It is a common misconception that explict MOPs are performance drains.
234 But this is not a universal truth at all, it is an side-effect of
235 specific implementations. For instance, using Java reflection is much
236 slower because the JVM cannot take advantage of any compiler
237 optimizations, and the JVM has to deal with much more runtime type
238 information as well. Reflection in C# is marginally better as it was
239 designed into the language and runtime (the CLR). In contrast, CLOS
240 (the Common Lisp Object System) was built to support an explicit MOP,
241 and so performance is tuned for it.
243 This library in particular does it's absolute best to avoid putting
244 B<any> drain at all upon your code's performance. In fact, by itself
245 it does nothing to affect your existing code. So you only pay for
246 what you actually use.
248 =head2 About Metaclass compatibility
250 This module makes sure that all metaclasses created are both upwards
251 and downwards compatible. The topic of metaclass compatibility is
252 highly esoteric and is something only encountered when doing deep and
253 involved metaclass hacking. There are two basic kinds of metaclass
254 incompatibility; upwards and downwards.
256 Upwards metaclass compatibility means that the metaclass of a
257 given class is either the same as (or a subclass of) all of the
260 Downward metaclass compatibility means that the metaclasses of a
261 given class's anscestors are all either the same as (or a subclass
264 Here is a diagram showing a set of two classes (C<A> and C<B>) and
265 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
266 metaclass compatibility both upwards and downwards.
268 +---------+ +---------+
269 | Meta::A |<----| Meta::B | <....... (instance of )
270 +---------+ +---------+ <------- (inherits from)
273 +---------+ +---------+
275 +---------+ +---------+
277 As I said this is a highly esoteric topic and one you will only run
278 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
279 are interested in why this is an issue see the paper
280 I<Uniform and safe metaclass composition> linked to in the
281 L<SEE ALSO> section of this document.
283 =head2 Using custom metaclasses
285 Always use the metaclass pragma when using a custom metaclass, this
286 will ensure the proper initialization order and not accidentely
287 create an incorrect type of metaclass for you. This is a very rare
288 problem, and one which can only occur if you are doing deep metaclass
289 programming. So in other words, don't worry about it.
293 The protocol is divided into 3 main sub-protocols:
297 =item The Class protocol
299 This provides a means of manipulating and introspecting a Perl 5
300 class. It handles all of symbol table hacking for you, and provides
301 a rich set of methods that go beyond simple package introspection.
303 See L<Class::MOP::Class> for more details.
305 =item The Attribute protocol
307 This provides a consistent represenation for an attribute of a
308 Perl 5 class. Since there are so many ways to create and handle
309 atttributes in Perl 5 OO, this attempts to provide as much of a
310 unified approach as possible, while giving the freedom and
311 flexibility to subclass for specialization.
313 See L<Class::MOP::Attribute> for more details.
315 =item The Method protocol
317 This provides a means of manipulating and introspecting methods in
318 the Perl 5 object system. As with attributes, there are many ways to
319 approach this topic, so we try to keep it pretty basic, while still
320 making it possible to extend the system in many ways.
322 See L<Class::MOP::Method> for more details.
330 There are very few books out on Meta Object Protocols and Metaclasses
331 because it is such an esoteric topic. The following books are really
332 the only ones I have found. If you know of any more, B<I<please>>
333 email me and let me know, I would love to hear about them.
337 =item "The Art of the Meta Object Protocol"
339 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
341 =item "Putting MetaClasses to Work"
343 =item "Smalltalk: The Language"
351 =item Uniform and safe metaclass composition
353 An excellent paper by the people who brought us the original Traits paper.
354 This paper is on how Traits can be used to do safe metaclass composition,
355 and offers an excellent introduction section which delves into the topic of
356 metaclass compatibility.
358 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
360 =item Safe Metaclass Programming
362 This paper seems to precede the above paper, and propose a mix-in based
363 approach as opposed to the Traits based approach. Both papers have similar
364 information on the metaclass compatibility problem space.
366 L<http://citeseer.ist.psu.edu/37617.html>
374 =item The Perl 6 MetaModel work in the Pugs project
378 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
380 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
386 =head1 SIMILAR MODULES
388 As I have said above, this module is a class-builder-builder, so it is
389 not the same thing as modules like L<Class::Accessor> and
390 L<Class::MethodMaker>. That being said there are very few modules on CPAN
391 with similar goals to this module. The one I have found which is most
392 like this module is L<Class::Meta>, although it's philosophy and the MOP it
393 creates are very different from this modules.
397 All complex software has bugs lurking in it, and this module is no
398 exception. If you find a bug please either email me, or add the bug
403 I use L<Devel::Cover> to test the code coverage of my tests, below is the
404 L<Devel::Cover> report on this module's test suite.
406 ---------------------------- ------ ------ ------ ------ ------ ------ ------
407 File stmt bran cond sub pod time total
408 ---------------------------- ------ ------ ------ ------ ------ ------ ------
409 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 9.6 100.0
410 Class/MOP/Attribute.pm 100.0 100.0 91.7 73.8 100.0 28.4 92.1
411 Class/MOP/Class.pm 100.0 93.5 82.3 98.2 100.0 56.6 95.7
412 Class/MOP/Method.pm 100.0 64.3 52.9 80.0 100.0 3.5 85.3
413 metaclass.pm 100.0 100.0 80.0 100.0 n/a 1.9 97.4
414 ---------------------------- ------ ------ ------ ------ ------ ------ ------
415 Total 100.0 90.8 79.7 86.2 100.0 100.0 93.6
416 ---------------------------- ------ ------ ------ ------ ------ ------ ------
418 =head1 ACKNOWLEDGEMENTS
422 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
424 Thanks to Rob for actually getting the development of this module kick-started.
430 Stevan Little E<lt>stevan@iinteractive.comE<gt>
432 =head1 COPYRIGHT AND LICENSE
434 Copyright 2006 by Infinity Interactive, Inc.
436 L<http://www.iinteractive.com>
438 This library is free software; you can redistribute it and/or modify
439 it under the same terms as Perl itself.