10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 use Class::MOP::Class::Immutable;
16 our $VERSION = '0.31';
18 ## ----------------------------------------------------------------------------
19 ## Setting up our environment ...
20 ## ----------------------------------------------------------------------------
21 ## Class::MOP needs to have a few things in the global perl environment so
22 ## that it can operate effectively. Those things are done here.
23 ## ----------------------------------------------------------------------------
25 # ... nothing yet actually ;)
27 ## ----------------------------------------------------------------------------
29 ## ----------------------------------------------------------------------------
30 ## The code below here is to bootstrap our MOP with itself. This is also
31 ## sometimes called "tying the knot". By doing this, we make it much easier
32 ## to extend the MOP through subclassing and such since now you can use the
33 ## MOP itself to extend itself.
35 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
36 ## ----------------------------------------------------------------------------
38 # We need to add in the meta-attributes here so that
39 # any subclass of Class::MOP::* will be able to
40 # inherit them using &construct_instance
42 ## Class::MOP::Package
44 Class::MOP::Package->meta->add_attribute(
45 Class::MOP::Attribute->new('$:package' => (
47 # NOTE: we need to do this in order
48 # for the instance meta-object to
49 # not fall into meta-circular death
50 'name' => sub { (shift)->{'$:package'} }
52 init_arg => ':package',
58 Class::MOP::Class->meta->add_attribute(
59 Class::MOP::Attribute->new('%:attributes' => (
61 # NOTE: we need to do this in order
62 # for the instance meta-object to
63 # not fall into meta-circular death
64 'get_attribute_map' => sub { (shift)->{'%:attributes'} }
66 init_arg => ':attributes',
71 Class::MOP::Class->meta->add_attribute(
72 Class::MOP::Attribute->new('$:attribute_metaclass' => (
73 reader => 'attribute_metaclass',
74 init_arg => ':attribute_metaclass',
75 default => 'Class::MOP::Attribute',
79 Class::MOP::Class->meta->add_attribute(
80 Class::MOP::Attribute->new('$:method_metaclass' => (
81 reader => 'method_metaclass',
82 init_arg => ':method_metaclass',
83 default => 'Class::MOP::Method',
87 Class::MOP::Class->meta->add_attribute(
88 Class::MOP::Attribute->new('$:instance_metaclass' => (
90 # NOTE: we need to do this in order
91 # for the instance meta-object to
92 # not fall into meta-circular death
93 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
95 init_arg => ':instance_metaclass',
96 default => 'Class::MOP::Instance',
100 ## Class::MOP::Attribute
102 Class::MOP::Attribute->meta->add_attribute(
103 Class::MOP::Attribute->new('name' => (
105 # NOTE: we need to do this in order
106 # for the instance meta-object to
107 # not fall into meta-circular death
108 'name' => sub { (shift)->{name} }
113 Class::MOP::Attribute->meta->add_attribute(
114 Class::MOP::Attribute->new('associated_class' => (
116 # NOTE: we need to do this in order
117 # for the instance meta-object to
118 # not fall into meta-circular death
119 'associated_class' => sub { (shift)->{associated_class} }
124 Class::MOP::Attribute->meta->add_attribute(
125 Class::MOP::Attribute->new('accessor' => (
126 reader => 'accessor',
127 predicate => 'has_accessor',
131 Class::MOP::Attribute->meta->add_attribute(
132 Class::MOP::Attribute->new('reader' => (
134 predicate => 'has_reader',
138 Class::MOP::Attribute->meta->add_attribute(
139 Class::MOP::Attribute->new('writer' => (
141 predicate => 'has_writer',
145 Class::MOP::Attribute->meta->add_attribute(
146 Class::MOP::Attribute->new('predicate' => (
147 reader => 'predicate',
148 predicate => 'has_predicate',
152 Class::MOP::Attribute->meta->add_attribute(
153 Class::MOP::Attribute->new('init_arg' => (
154 reader => 'init_arg',
155 predicate => 'has_init_arg',
159 Class::MOP::Attribute->meta->add_attribute(
160 Class::MOP::Attribute->new('default' => (
161 # default has a custom 'reader' method ...
162 predicate => 'has_default',
167 # NOTE: (meta-circularity)
168 # This should be one of the last things done
169 # it will "tie the knot" with Class::MOP::Attribute
170 # so that it uses the attributes meta-objects
171 # to construct itself.
172 Class::MOP::Attribute->meta->add_method('new' => sub {
177 (defined $name && $name)
178 || confess "You must provide a name for the attribute";
179 $options{init_arg} = $name
180 if not exists $options{init_arg};
182 # return the new object
183 $class->meta->new_object(name => $name, %options);
186 Class::MOP::Attribute->meta->add_method('clone' => sub {
188 $self->meta->clone_object($self, @_);
191 ## Try and close Class::MOP::*
193 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
194 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
195 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
196 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
197 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
198 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
209 Class::MOP - A Meta Object Protocol for Perl 5
213 # ... This will come later, for now see
214 # the other SYNOPSIS for more information
218 This module is an attempt to create a meta object protocol for the
219 Perl 5 object system. It makes no attempt to change the behavior or
220 characteristics of the Perl 5 object system, only to create a
221 protocol for its manipulation and introspection.
223 That said, it does attempt to create the tools for building a rich
224 set of extensions to the Perl 5 object system. Every attempt has been
225 made for these tools to keep to the spirit of the Perl 5 object
226 system that we all know and love.
228 This documentation is admittedly sparse on details, as time permits
229 I will try to improve them. For now, I suggest looking at the items
230 listed in the L<SEE ALSO> section for more information. In particular
231 the book "The Art of the Meta Object Protocol" was very influential
232 in the development of this system.
234 =head2 What is a Meta Object Protocol?
236 A meta object protocol is an API to an object system.
238 To be more specific, it is a set of abstractions of the components of
239 an object system (typically things like; classes, object, methods,
240 object attributes, etc.). These abstractions can then be used to both
241 inspect and manipulate the object system which they describe.
243 It can be said that there are two MOPs for any object system; the
244 implicit MOP, and the explicit MOP. The implicit MOP handles things
245 like method dispatch or inheritance, which happen automatically as
246 part of how the object system works. The explicit MOP typically
247 handles the introspection/reflection features of the object system.
248 All object systems have implicit MOPs, without one, they would not
249 work. Explict MOPs however as less common, and depending on the
250 language can vary from restrictive (Reflection in Java or C#) to
251 wide open (CLOS is a perfect example).
253 =head2 Yet Another Class Builder!! Why?
255 This is B<not> a class builder so much as it is a I<class builder
256 B<builder>>. My intent is that an end user does not use this module
257 directly, but instead this module is used by module authors to
258 build extensions and features onto the Perl 5 object system.
260 =head2 Who is this module for?
262 This module is specifically for anyone who has ever created or
263 wanted to create a module for the Class:: namespace. The tools which
264 this module will provide will hopefully make it easier to do more
265 complex things with Perl 5 classes by removing such barriers as
266 the need to hack the symbol tables, or understand the fine details
269 =head2 What changes do I have to make to use this module?
271 This module was designed to be as unintrusive as possible. Many of
272 its features are accessible without B<any> change to your existsing
273 code at all. It is meant to be a compliment to your existing code and
274 not an intrusion on your code base. Unlike many other B<Class::>
275 modules, this module B<does not> require you subclass it, or even that
276 you C<use> it in within your module's package.
278 The only features which requires additions to your code are the
279 attribute handling and instance construction features, and these are
280 both completely optional features. The only reason for this is because
281 Perl 5's object system does not actually have these features built
282 in. More information about this feature can be found below.
284 =head2 A Note about Performance?
286 It is a common misconception that explict MOPs are performance drains.
287 But this is not a universal truth at all, it is an side-effect of
288 specific implementations. For instance, using Java reflection is much
289 slower because the JVM cannot take advantage of any compiler
290 optimizations, and the JVM has to deal with much more runtime type
291 information as well. Reflection in C# is marginally better as it was
292 designed into the language and runtime (the CLR). In contrast, CLOS
293 (the Common Lisp Object System) was built to support an explicit MOP,
294 and so performance is tuned for it.
296 This library in particular does it's absolute best to avoid putting
297 B<any> drain at all upon your code's performance. In fact, by itself
298 it does nothing to affect your existing code. So you only pay for
299 what you actually use.
301 =head2 About Metaclass compatibility
303 This module makes sure that all metaclasses created are both upwards
304 and downwards compatible. The topic of metaclass compatibility is
305 highly esoteric and is something only encountered when doing deep and
306 involved metaclass hacking. There are two basic kinds of metaclass
307 incompatibility; upwards and downwards.
309 Upwards metaclass compatibility means that the metaclass of a
310 given class is either the same as (or a subclass of) all of the
313 Downward metaclass compatibility means that the metaclasses of a
314 given class's anscestors are all either the same as (or a subclass
317 Here is a diagram showing a set of two classes (C<A> and C<B>) and
318 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
319 metaclass compatibility both upwards and downwards.
321 +---------+ +---------+
322 | Meta::A |<----| Meta::B | <....... (instance of )
323 +---------+ +---------+ <------- (inherits from)
326 +---------+ +---------+
328 +---------+ +---------+
330 As I said this is a highly esoteric topic and one you will only run
331 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
332 are interested in why this is an issue see the paper
333 I<Uniform and safe metaclass composition> linked to in the
334 L<SEE ALSO> section of this document.
336 =head2 Using custom metaclasses
338 Always use the metaclass pragma when using a custom metaclass, this
339 will ensure the proper initialization order and not accidentely
340 create an incorrect type of metaclass for you. This is a very rare
341 problem, and one which can only occur if you are doing deep metaclass
342 programming. So in other words, don't worry about it.
346 The protocol is divided into 3 main sub-protocols:
350 =item The Class protocol
352 This provides a means of manipulating and introspecting a Perl 5
353 class. It handles all of symbol table hacking for you, and provides
354 a rich set of methods that go beyond simple package introspection.
356 See L<Class::MOP::Class> for more details.
358 =item The Attribute protocol
360 This provides a consistent represenation for an attribute of a
361 Perl 5 class. Since there are so many ways to create and handle
362 atttributes in Perl 5 OO, this attempts to provide as much of a
363 unified approach as possible, while giving the freedom and
364 flexibility to subclass for specialization.
366 See L<Class::MOP::Attribute> for more details.
368 =item The Method protocol
370 This provides a means of manipulating and introspecting methods in
371 the Perl 5 object system. As with attributes, there are many ways to
372 approach this topic, so we try to keep it pretty basic, while still
373 making it possible to extend the system in many ways.
375 See L<Class::MOP::Method> for more details.
383 There are very few books out on Meta Object Protocols and Metaclasses
384 because it is such an esoteric topic. The following books are really
385 the only ones I have found. If you know of any more, B<I<please>>
386 email me and let me know, I would love to hear about them.
390 =item "The Art of the Meta Object Protocol"
392 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
394 =item "Putting MetaClasses to Work"
396 =item "Smalltalk: The Language"
404 =item Uniform and safe metaclass composition
406 An excellent paper by the people who brought us the original Traits paper.
407 This paper is on how Traits can be used to do safe metaclass composition,
408 and offers an excellent introduction section which delves into the topic of
409 metaclass compatibility.
411 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
413 =item Safe Metaclass Programming
415 This paper seems to precede the above paper, and propose a mix-in based
416 approach as opposed to the Traits based approach. Both papers have similar
417 information on the metaclass compatibility problem space.
419 L<http://citeseer.ist.psu.edu/37617.html>
427 =item The Perl 6 MetaModel work in the Pugs project
431 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
433 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
439 =head1 SIMILAR MODULES
441 As I have said above, this module is a class-builder-builder, so it is
442 not the same thing as modules like L<Class::Accessor> and
443 L<Class::MethodMaker>. That being said there are very few modules on CPAN
444 with similar goals to this module. The one I have found which is most
445 like this module is L<Class::Meta>, although it's philosophy and the MOP it
446 creates are very different from this modules.
450 All complex software has bugs lurking in it, and this module is no
451 exception. If you find a bug please either email me, or add the bug
456 I use L<Devel::Cover> to test the code coverage of my tests, below is the
457 L<Devel::Cover> report on this module's test suite.
459 ---------------------------- ------ ------ ------ ------ ------ ------ ------
460 File stmt bran cond sub pod time total
461 ---------------------------- ------ ------ ------ ------ ------ ------ ------
462 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 19.8 100.0
463 Class/MOP/Attribute.pm 100.0 100.0 91.7 61.2 100.0 14.3 87.9
464 Class/MOP/Class.pm 97.6 91.3 77.3 98.4 100.0 56.4 93.2
465 Class/MOP/Instance.pm 91.1 75.0 33.3 91.7 100.0 6.8 90.7
466 Class/MOP/Method.pm 97.6 60.0 52.9 76.9 100.0 1.6 82.6
467 metaclass.pm 100.0 100.0 83.3 100.0 n/a 1.0 97.7
468 ---------------------------- ------ ------ ------ ------ ------ ------ ------
469 Total 97.5 88.5 75.5 82.8 100.0 100.0 91.2
470 ---------------------------- ------ ------ ------ ------ ------ ------ ------
472 =head1 ACKNOWLEDGEMENTS
476 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
478 Thanks to Rob for actually getting the development of this module kick-started.
484 Stevan Little E<lt>stevan@iinteractive.comE<gt>
486 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
488 =head1 COPYRIGHT AND LICENSE
490 Copyright 2006 by Infinity Interactive, Inc.
492 L<http://www.iinteractive.com>
494 This library is free software; you can redistribute it and/or modify
495 it under the same terms as Perl itself.