8 use Scalar::Util 'weaken';
10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 use Class::MOP::Class::Immutable;
16 our $VERSION = '0.35';
17 our $AUTHORITY = 'cpan:STEVAN';
20 # Metaclasses are singletons, so we cache them here.
21 # there is no need to worry about destruction though
22 # because they should die only when the program dies.
23 # After all, do package definitions even get reaped?
26 # means of accessing all the metaclasses that have
27 # been initialized thus far (for mugwumps obj browser)
28 sub get_all_metaclasses { %METAS }
29 sub get_all_metaclass_instances { values %METAS }
30 sub get_all_metaclass_names { keys %METAS }
31 sub get_metaclass_by_name { $METAS{$_[0]} }
32 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
33 sub weaken_metaclass { weaken($METAS{$_[0]}) }
34 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
35 sub remove_metaclass_by_name { $METAS{$_[0]} = undef }
38 # We only cache metaclasses, meaning instances of
39 # Class::MOP::Class. We do not cache instance of
40 # Class::MOP::Package or Class::MOP::Module. Mostly
41 # because I don't yet see a good reason to do so.
44 ## ----------------------------------------------------------------------------
45 ## Setting up our environment ...
46 ## ----------------------------------------------------------------------------
47 ## Class::MOP needs to have a few things in the global perl environment so
48 ## that it can operate effectively. Those things are done here.
49 ## ----------------------------------------------------------------------------
51 # ... nothing yet actually ;)
53 ## ----------------------------------------------------------------------------
55 ## ----------------------------------------------------------------------------
56 ## The code below here is to bootstrap our MOP with itself. This is also
57 ## sometimes called "tying the knot". By doing this, we make it much easier
58 ## to extend the MOP through subclassing and such since now you can use the
59 ## MOP itself to extend itself.
61 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
62 ## ----------------------------------------------------------------------------
64 # We need to add in the meta-attributes here so that
65 # any subclass of Class::MOP::* will be able to
66 # inherit them using &construct_instance
68 ## --------------------------------------------------------
69 ## Class::MOP::Package
71 Class::MOP::Package->meta->add_attribute(
72 Class::MOP::Attribute->new('$:package' => (
74 # NOTE: we need to do this in order
75 # for the instance meta-object to
76 # not fall into meta-circular death
78 # we just alias the original method
79 # rather than re-produce it here
80 'name' => \&Class::MOP::Package::name
82 init_arg => ':package',
86 Class::MOP::Package->meta->add_attribute(
87 Class::MOP::Attribute->new('%:namespace' => (
90 # we just alias the original method
91 # rather than re-produce it here
92 'namespace' => \&Class::MOP::Package::namespace
95 # protect this from silliness
96 init_arg => '!............( DO NOT DO THIS )............!',
97 default => sub { \undef }
102 # use the metaclass to construct the meta-package
103 # which is a superclass of the metaclass itself :P
104 Class::MOP::Package->meta->add_method('initialize' => sub {
106 my $package_name = shift;
107 $class->meta->new_object(':package' => $package_name, @_);
110 ## --------------------------------------------------------
111 ## Class::MOP::Module
114 # yeah this is kind of stretching things a bit,
115 # but truthfully the version should be an attribute
116 # of the Module, the weirdness comes from having to
117 # stick to Perl 5 convention and store it in the
118 # $VERSION package variable. Basically if you just
119 # squint at it, it will look how you want it to look.
120 # Either as a package variable, or as a attribute of
121 # the metaclass, isn't abstraction great :)
123 Class::MOP::Module->meta->add_attribute(
124 Class::MOP::Attribute->new('$:version' => (
127 # we just alias the original method
128 # rather than re-produce it here
129 'version' => \&Class::MOP::Module::version
132 # protect this from silliness
133 init_arg => '!............( DO NOT DO THIS )............!',
134 default => sub { \undef }
139 # By following the same conventions as version here,
140 # we are opening up the possibility that people can
141 # use the $AUTHORITY in non-Class::MOP modules as
144 Class::MOP::Module->meta->add_attribute(
145 Class::MOP::Attribute->new('$:authority' => (
148 # we just alias the original method
149 # rather than re-produce it here
150 'authority' => \&Class::MOP::Module::authority
153 # protect this from silliness
154 init_arg => '!............( DO NOT DO THIS )............!',
155 default => sub { \undef }
159 ## --------------------------------------------------------
162 Class::MOP::Class->meta->add_attribute(
163 Class::MOP::Attribute->new('%:attributes' => (
165 # NOTE: we need to do this in order
166 # for the instance meta-object to
167 # not fall into meta-circular death
169 # we just alias the original method
170 # rather than re-produce it here
171 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
173 init_arg => ':attributes',
174 default => sub { {} }
178 Class::MOP::Class->meta->add_attribute(
179 Class::MOP::Attribute->new('%:methods' => (
182 # we just alias the original method
183 # rather than re-produce it here
184 'get_method_map' => \&Class::MOP::Class::get_method_map
186 default => sub { {} }
190 Class::MOP::Class->meta->add_attribute(
191 Class::MOP::Attribute->new('$:attribute_metaclass' => (
194 # we just alias the original method
195 # rather than re-produce it here
196 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
198 init_arg => ':attribute_metaclass',
199 default => 'Class::MOP::Attribute',
203 Class::MOP::Class->meta->add_attribute(
204 Class::MOP::Attribute->new('$:method_metaclass' => (
207 # we just alias the original method
208 # rather than re-produce it here
209 'method_metaclass' => \&Class::MOP::Class::method_metaclass
211 init_arg => ':method_metaclass',
212 default => 'Class::MOP::Method',
216 Class::MOP::Class->meta->add_attribute(
217 Class::MOP::Attribute->new('$:instance_metaclass' => (
219 # NOTE: we need to do this in order
220 # for the instance meta-object to
221 # not fall into meta-circular death
223 # we just alias the original method
224 # rather than re-produce it here
225 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
227 init_arg => ':instance_metaclass',
228 default => 'Class::MOP::Instance',
233 # we don't actually need to tie the knot with
234 # Class::MOP::Class here, it is actually handled
235 # within Class::MOP::Class itself in the
236 # construct_class_instance method.
238 ## --------------------------------------------------------
239 ## Class::MOP::Attribute
241 Class::MOP::Attribute->meta->add_attribute(
242 Class::MOP::Attribute->new('name' => (
244 # NOTE: we need to do this in order
245 # for the instance meta-object to
246 # not fall into meta-circular death
248 # we just alias the original method
249 # rather than re-produce it here
250 'name' => \&Class::MOP::Attribute::name
255 Class::MOP::Attribute->meta->add_attribute(
256 Class::MOP::Attribute->new('associated_class' => (
258 # NOTE: we need to do this in order
259 # for the instance meta-object to
260 # not fall into meta-circular death
262 # we just alias the original method
263 # rather than re-produce it here
264 'associated_class' => \&Class::MOP::Attribute::associated_class
269 Class::MOP::Attribute->meta->add_attribute(
270 Class::MOP::Attribute->new('accessor' => (
271 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
272 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
276 Class::MOP::Attribute->meta->add_attribute(
277 Class::MOP::Attribute->new('reader' => (
278 reader => { 'reader' => \&Class::MOP::Attribute::reader },
279 predicate => { 'has_reader' => \&Class::MOP::Attribute::has_reader },
283 Class::MOP::Attribute->meta->add_attribute(
284 Class::MOP::Attribute->new('writer' => (
285 reader => { 'writer' => \&Class::MOP::Attribute::writer },
286 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
290 Class::MOP::Attribute->meta->add_attribute(
291 Class::MOP::Attribute->new('predicate' => (
292 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
293 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
297 Class::MOP::Attribute->meta->add_attribute(
298 Class::MOP::Attribute->new('clearer' => (
299 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
300 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
304 Class::MOP::Attribute->meta->add_attribute(
305 Class::MOP::Attribute->new('init_arg' => (
306 reader => { 'init_arg' => \&Class::MOP::Attribute::init_arg },
307 predicate => { 'has_init_arg' => \&Class::MOP::Attribute::has_init_arg },
311 Class::MOP::Attribute->meta->add_attribute(
312 Class::MOP::Attribute->new('default' => (
313 # default has a custom 'reader' method ...
314 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
318 Class::MOP::Attribute->meta->add_attribute(
319 Class::MOP::Attribute->new('associated_methods' => (
320 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
321 default => sub { [] }
325 # NOTE: (meta-circularity)
326 # This should be one of the last things done
327 # it will "tie the knot" with Class::MOP::Attribute
328 # so that it uses the attributes meta-objects
329 # to construct itself.
330 Class::MOP::Attribute->meta->add_method('new' => sub {
335 (defined $name && $name)
336 || confess "You must provide a name for the attribute";
337 $options{init_arg} = $name
338 if not exists $options{init_arg};
340 (Class::MOP::Attribute::is_default_a_coderef(\%options))
341 || confess("References are not allowed as default values, you must ".
342 "wrap then in a CODE reference (ex: sub { [] } and not [])")
343 if exists $options{default} && ref $options{default};
345 # return the new object
346 $class->meta->new_object(name => $name, %options);
349 Class::MOP::Attribute->meta->add_method('clone' => sub {
351 $self->meta->clone_object($self, @_);
354 ## --------------------------------------------------------
355 ## Class::MOP::Method
357 Class::MOP::Method->meta->add_attribute(
358 Class::MOP::Attribute->new('body' => (
359 reader => { 'body' => \&Class::MOP::Method::body },
363 ## --------------------------------------------------------
364 ## Class::MOP::Method::Wrapped
367 # the way this item is initialized, this
368 # really does not follow the standard
369 # practices of attributes, but we put
370 # it here for completeness
371 Class::MOP::Method::Wrapped->meta->add_attribute(
372 Class::MOP::Attribute->new('modifier_table')
375 ## --------------------------------------------------------
376 ## Class::MOP::Instance
379 # these don't yet do much of anything, but are just
380 # included for completeness
382 Class::MOP::Instance->meta->add_attribute(
383 Class::MOP::Attribute->new('meta')
386 Class::MOP::Instance->meta->add_attribute(
387 Class::MOP::Attribute->new('slots')
390 ## --------------------------------------------------------
391 ## Now close all the Class::MOP::* classes
394 # we don't need to inline the
395 # constructors or the accessors
396 # this only lengthens the compile
397 # time of the MOP, and gives us
398 # no actual benefits.
400 $_->meta->make_immutable(
401 inline_constructor => 0,
402 inline_accessors => 0,
408 Class::MOP::Attribute
414 Class::MOP::Method::Accessor
415 Class::MOP::Method::Wrapped
426 Class::MOP - A Meta Object Protocol for Perl 5
430 # ... This will come later, for now see
431 # the other SYNOPSIS for more information
435 This module is an attempt to create a meta object protocol for the
436 Perl 5 object system. It makes no attempt to change the behavior or
437 characteristics of the Perl 5 object system, only to create a
438 protocol for its manipulation and introspection.
440 That said, it does attempt to create the tools for building a rich
441 set of extensions to the Perl 5 object system. Every attempt has been
442 made for these tools to keep to the spirit of the Perl 5 object
443 system that we all know and love.
445 This documentation is admittedly sparse on details, as time permits
446 I will try to improve them. For now, I suggest looking at the items
447 listed in the L<SEE ALSO> section for more information. In particular
448 the book "The Art of the Meta Object Protocol" was very influential
449 in the development of this system.
451 =head2 What is a Meta Object Protocol?
453 A meta object protocol is an API to an object system.
455 To be more specific, it is a set of abstractions of the components of
456 an object system (typically things like; classes, object, methods,
457 object attributes, etc.). These abstractions can then be used to both
458 inspect and manipulate the object system which they describe.
460 It can be said that there are two MOPs for any object system; the
461 implicit MOP, and the explicit MOP. The implicit MOP handles things
462 like method dispatch or inheritance, which happen automatically as
463 part of how the object system works. The explicit MOP typically
464 handles the introspection/reflection features of the object system.
465 All object systems have implicit MOPs, without one, they would not
466 work. Explict MOPs however as less common, and depending on the
467 language can vary from restrictive (Reflection in Java or C#) to
468 wide open (CLOS is a perfect example).
470 =head2 Yet Another Class Builder!! Why?
472 This is B<not> a class builder so much as it is a I<class builder
473 B<builder>>. My intent is that an end user does not use this module
474 directly, but instead this module is used by module authors to
475 build extensions and features onto the Perl 5 object system.
477 =head2 Who is this module for?
479 This module is specifically for anyone who has ever created or
480 wanted to create a module for the Class:: namespace. The tools which
481 this module will provide will hopefully make it easier to do more
482 complex things with Perl 5 classes by removing such barriers as
483 the need to hack the symbol tables, or understand the fine details
486 =head2 What changes do I have to make to use this module?
488 This module was designed to be as unintrusive as possible. Many of
489 its features are accessible without B<any> change to your existsing
490 code at all. It is meant to be a compliment to your existing code and
491 not an intrusion on your code base. Unlike many other B<Class::>
492 modules, this module B<does not> require you subclass it, or even that
493 you C<use> it in within your module's package.
495 The only features which requires additions to your code are the
496 attribute handling and instance construction features, and these are
497 both completely optional features. The only reason for this is because
498 Perl 5's object system does not actually have these features built
499 in. More information about this feature can be found below.
501 =head2 A Note about Performance?
503 It is a common misconception that explict MOPs are performance drains.
504 But this is not a universal truth at all, it is an side-effect of
505 specific implementations. For instance, using Java reflection is much
506 slower because the JVM cannot take advantage of any compiler
507 optimizations, and the JVM has to deal with much more runtime type
508 information as well. Reflection in C# is marginally better as it was
509 designed into the language and runtime (the CLR). In contrast, CLOS
510 (the Common Lisp Object System) was built to support an explicit MOP,
511 and so performance is tuned for it.
513 This library in particular does it's absolute best to avoid putting
514 B<any> drain at all upon your code's performance. In fact, by itself
515 it does nothing to affect your existing code. So you only pay for
516 what you actually use.
518 =head2 About Metaclass compatibility
520 This module makes sure that all metaclasses created are both upwards
521 and downwards compatible. The topic of metaclass compatibility is
522 highly esoteric and is something only encountered when doing deep and
523 involved metaclass hacking. There are two basic kinds of metaclass
524 incompatibility; upwards and downwards.
526 Upwards metaclass compatibility means that the metaclass of a
527 given class is either the same as (or a subclass of) all of the
530 Downward metaclass compatibility means that the metaclasses of a
531 given class's anscestors are all either the same as (or a subclass
534 Here is a diagram showing a set of two classes (C<A> and C<B>) and
535 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
536 metaclass compatibility both upwards and downwards.
538 +---------+ +---------+
539 | Meta::A |<----| Meta::B | <....... (instance of )
540 +---------+ +---------+ <------- (inherits from)
543 +---------+ +---------+
545 +---------+ +---------+
547 As I said this is a highly esoteric topic and one you will only run
548 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
549 are interested in why this is an issue see the paper
550 I<Uniform and safe metaclass composition> linked to in the
551 L<SEE ALSO> section of this document.
553 =head2 Using custom metaclasses
555 Always use the metaclass pragma when using a custom metaclass, this
556 will ensure the proper initialization order and not accidentely
557 create an incorrect type of metaclass for you. This is a very rare
558 problem, and one which can only occur if you are doing deep metaclass
559 programming. So in other words, don't worry about it.
563 The protocol is divided into 3 main sub-protocols:
567 =item The Class protocol
569 This provides a means of manipulating and introspecting a Perl 5
570 class. It handles all of symbol table hacking for you, and provides
571 a rich set of methods that go beyond simple package introspection.
573 See L<Class::MOP::Class> for more details.
575 =item The Attribute protocol
577 This provides a consistent represenation for an attribute of a
578 Perl 5 class. Since there are so many ways to create and handle
579 atttributes in Perl 5 OO, this attempts to provide as much of a
580 unified approach as possible, while giving the freedom and
581 flexibility to subclass for specialization.
583 See L<Class::MOP::Attribute> for more details.
585 =item The Method protocol
587 This provides a means of manipulating and introspecting methods in
588 the Perl 5 object system. As with attributes, there are many ways to
589 approach this topic, so we try to keep it pretty basic, while still
590 making it possible to extend the system in many ways.
592 See L<Class::MOP::Method> for more details.
598 Class::MOP holds a cache of metaclasses, the following are functions
599 (B<not methods>) which can be used to access that cache. It is not
600 recommended that you mess with this, bad things could happen. But if
601 you are brave and willing to risk it, go for it.
605 =item B<get_all_metaclasses>
607 This will return an hash of all the metaclass instances that have
608 been cached by B<Class::MOP::Class> keyed by the package name.
610 =item B<get_all_metaclass_instances>
612 This will return an array of all the metaclass instances that have
613 been cached by B<Class::MOP::Class>.
615 =item B<get_all_metaclass_names>
617 This will return an array of all the metaclass names that have
618 been cached by B<Class::MOP::Class>.
620 =item B<get_metaclass_by_name ($name)>
622 =item B<store_metaclass_by_name ($name, $meta)>
624 =item B<weaken_metaclass ($name)>
626 =item B<does_metaclass_exist ($name)>
628 =item B<remove_metaclass_by_name ($name)>
636 There are very few books out on Meta Object Protocols and Metaclasses
637 because it is such an esoteric topic. The following books are really
638 the only ones I have found. If you know of any more, B<I<please>>
639 email me and let me know, I would love to hear about them.
643 =item "The Art of the Meta Object Protocol"
645 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
647 =item "Putting MetaClasses to Work"
649 =item "Smalltalk: The Language"
657 =item Uniform and safe metaclass composition
659 An excellent paper by the people who brought us the original Traits paper.
660 This paper is on how Traits can be used to do safe metaclass composition,
661 and offers an excellent introduction section which delves into the topic of
662 metaclass compatibility.
664 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
666 =item Safe Metaclass Programming
668 This paper seems to precede the above paper, and propose a mix-in based
669 approach as opposed to the Traits based approach. Both papers have similar
670 information on the metaclass compatibility problem space.
672 L<http://citeseer.ist.psu.edu/37617.html>
680 =item The Perl 6 MetaModel work in the Pugs project
684 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
686 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
696 =item CPAN Module Review of Class::MOP
698 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
702 =head1 SIMILAR MODULES
704 As I have said above, this module is a class-builder-builder, so it is
705 not the same thing as modules like L<Class::Accessor> and
706 L<Class::MethodMaker>. That being said there are very few modules on CPAN
707 with similar goals to this module. The one I have found which is most
708 like this module is L<Class::Meta>, although it's philosophy and the MOP it
709 creates are very different from this modules.
713 All complex software has bugs lurking in it, and this module is no
714 exception. If you find a bug please either email me, or add the bug
719 I use L<Devel::Cover> to test the code coverage of my tests, below is the
720 L<Devel::Cover> report on this module's test suite.
722 ---------------------------- ------ ------ ------ ------ ------ ------ ------
723 File stmt bran cond sub pod time total
724 ---------------------------- ------ ------ ------ ------ ------ ------ ------
725 Class/MOP.pm 97.7 100.0 88.9 94.7 100.0 3.2 96.6
726 Class/MOP/Attribute.pm 75.5 77.9 82.4 88.3 100.0 4.0 81.5
727 Class/MOP/Class.pm 96.9 88.8 72.1 98.2 100.0 35.8 91.4
728 Class/MOP/Class/Immutable.pm 88.2 60.0 n/a 95.5 100.0 0.5 84.6
729 Class/MOP/Instance.pm 86.4 75.0 33.3 86.2 100.0 1.2 87.5
730 Class/MOP/Method.pm 97.5 75.0 61.5 80.6 100.0 12.7 89.7
731 Class/MOP/Module.pm 100.0 n/a 55.6 100.0 100.0 0.1 90.7
732 Class/MOP/Object.pm 73.3 n/a 20.0 80.0 100.0 0.1 66.7
733 Class/MOP/Package.pm 94.6 71.7 33.3 100.0 100.0 42.2 87.0
734 metaclass.pm 100.0 100.0 83.3 100.0 n/a 0.2 97.7
735 ---------------------------- ------ ------ ------ ------ ------ ------ ------
736 Total 91.3 80.4 69.8 91.9 100.0 100.0 88.1
737 ---------------------------- ------ ------ ------ ------ ------ ------ ------
739 =head1 ACKNOWLEDGEMENTS
745 Thanks to Rob for actually getting the development of this module kick-started.
751 Stevan Little E<lt>stevan@iinteractive.comE<gt>
753 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
755 =head1 COPYRIGHT AND LICENSE
757 Copyright 2006 by Infinity Interactive, Inc.
759 L<http://www.iinteractive.com>
761 This library is free software; you can redistribute it and/or modify
762 it under the same terms as Perl itself.