13 use Scalar::Util 'weaken', 'isweak', 'reftype', 'blessed';
17 use Class::MOP::Mixin::AttributeCore;
18 use Class::MOP::Mixin::HasAttributes;
19 use Class::MOP::Mixin::HasMethods;
20 use Class::MOP::Class;
21 use Class::MOP::Attribute;
22 use Class::MOP::Method;
25 *IS_RUNNING_ON_5_10 = ($] < 5.009_005)
29 # this is either part of core or set up appropriately by MRO::Compat
30 *check_package_cache_flag = \&mro::get_pkg_gen;
35 $Class::MOP::{VERSION} ? ${ $Class::MOP::{VERSION} } : ()
39 # Metaclasses are singletons, so we cache them here.
40 # there is no need to worry about destruction though
41 # because they should die only when the program dies.
42 # After all, do package definitions even get reaped?
43 # Anonymous classes manage their own destruction.
46 sub get_all_metaclasses { %METAS }
47 sub get_all_metaclass_instances { values %METAS }
48 sub get_all_metaclass_names { keys %METAS }
49 sub get_metaclass_by_name { $METAS{$_[0]} }
50 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
51 sub weaken_metaclass { weaken($METAS{$_[0]}) }
52 sub metaclass_is_weak { isweak($METAS{$_[0]}) }
53 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
54 sub remove_metaclass_by_name { delete $METAS{$_[0]}; return }
56 # This handles instances as well as class names
58 return unless defined $_[0];
59 my $class = blessed($_[0]) || $_[0];
60 return $METAS{$class};
64 # We only cache metaclasses, meaning instances of
65 # Class::MOP::Class. We do not cache instance of
66 # Class::MOP::Package or Class::MOP::Module. Mostly
67 # because I don't yet see a good reason to do so.
71 Class::MOP::Deprecated::deprecated(
72 'The Class::MOP::load_class subroutine is deprecated.'
73 . ' Please use Class::Load instead.' );
74 goto &Class::Load::load_class;
77 sub load_first_existing_class {
78 Class::MOP::Deprecated::deprecated(
79 'The Class::MOP::load_first_existing_class subroutine is deprecated.'
80 . ' Please use Class::Load instead.' );
81 goto &Class::Load::load_first_existing_class;
85 Class::MOP::Deprecated::deprecated(
86 'The Class::MOP::is_class_loaded subroutine is deprecated.'
87 . ' Please use Class::Load instead.' );
88 goto &Class::Load::is_class_loaded;
91 sub _definition_context {
93 @context{qw(package file line)} = caller(1);
96 definition_context => \%context,
100 ## ----------------------------------------------------------------------------
101 ## Setting up our environment ...
102 ## ----------------------------------------------------------------------------
103 ## Class::MOP needs to have a few things in the global perl environment so
104 ## that it can operate effectively. Those things are done here.
105 ## ----------------------------------------------------------------------------
107 # ... nothing yet actually ;)
109 ## ----------------------------------------------------------------------------
111 ## ----------------------------------------------------------------------------
112 ## The code below here is to bootstrap our MOP with itself. This is also
113 ## sometimes called "tying the knot". By doing this, we make it much easier
114 ## to extend the MOP through subclassing and such since now you can use the
115 ## MOP itself to extend itself.
117 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
118 ## ----------------------------------------------------------------------------
120 # We need to add in the meta-attributes here so that
121 # any subclass of Class::MOP::* will be able to
122 # inherit them using _construct_instance
124 ## --------------------------------------------------------
125 ## Class::MOP::Mixin::HasMethods
127 Class::MOP::Mixin::HasMethods->meta->add_attribute(
128 Class::MOP::Attribute->new('_methods' => (
131 # we just alias the original method
132 # rather than re-produce it here
133 '_method_map' => \&Class::MOP::Mixin::HasMethods::_method_map
135 default => sub { {} },
136 _definition_context(),
140 Class::MOP::Mixin::HasMethods->meta->add_attribute(
141 Class::MOP::Attribute->new('method_metaclass' => (
144 # we just alias the original method
145 # rather than re-produce it here
146 'method_metaclass' => \&Class::MOP::Mixin::HasMethods::method_metaclass
148 default => 'Class::MOP::Method',
149 _definition_context(),
153 Class::MOP::Mixin::HasMethods->meta->add_attribute(
154 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
157 # we just alias the original method
158 # rather than re-produce it here
159 'wrapped_method_metaclass' => \&Class::MOP::Mixin::HasMethods::wrapped_method_metaclass
161 default => 'Class::MOP::Method::Wrapped',
162 _definition_context(),
166 ## --------------------------------------------------------
167 ## Class::MOP::Mixin::HasMethods
169 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
170 Class::MOP::Attribute->new('attributes' => (
172 # NOTE: we need to do this in order
173 # for the instance meta-object to
174 # not fall into meta-circular death
176 # we just alias the original method
177 # rather than re-produce it here
178 '_attribute_map' => \&Class::MOP::Mixin::HasAttributes::_attribute_map
180 default => sub { {} },
181 _definition_context(),
185 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
186 Class::MOP::Attribute->new('attribute_metaclass' => (
189 # we just alias the original method
190 # rather than re-produce it here
191 'attribute_metaclass' => \&Class::MOP::Mixin::HasAttributes::attribute_metaclass
193 default => 'Class::MOP::Attribute',
194 _definition_context(),
198 ## --------------------------------------------------------
199 ## Class::MOP::Package
201 Class::MOP::Package->meta->add_attribute(
202 Class::MOP::Attribute->new('package' => (
204 # NOTE: we need to do this in order
205 # for the instance meta-object to
206 # not fall into meta-circular death
208 # we just alias the original method
209 # rather than re-produce it here
210 'name' => \&Class::MOP::Package::name
212 _definition_context(),
216 Class::MOP::Package->meta->add_attribute(
217 Class::MOP::Attribute->new('namespace' => (
220 # we just alias the original method
221 # rather than re-produce it here
222 'namespace' => \&Class::MOP::Package::namespace
225 default => sub { \undef },
226 _definition_context(),
230 ## --------------------------------------------------------
231 ## Class::MOP::Module
234 # yeah this is kind of stretching things a bit,
235 # but truthfully the version should be an attribute
236 # of the Module, the weirdness comes from having to
237 # stick to Perl 5 convention and store it in the
238 # $VERSION package variable. Basically if you just
239 # squint at it, it will look how you want it to look.
240 # Either as a package variable, or as a attribute of
241 # the metaclass, isn't abstraction great :)
243 Class::MOP::Module->meta->add_attribute(
244 Class::MOP::Attribute->new('version' => (
247 # we just alias the original method
248 # rather than re-produce it here
249 'version' => \&Class::MOP::Module::version
252 default => sub { \undef },
253 _definition_context(),
258 # By following the same conventions as version here,
259 # we are opening up the possibility that people can
260 # use the $AUTHORITY in non-Class::MOP modules as
263 Class::MOP::Module->meta->add_attribute(
264 Class::MOP::Attribute->new('authority' => (
267 # we just alias the original method
268 # rather than re-produce it here
269 'authority' => \&Class::MOP::Module::authority
272 default => sub { \undef },
273 _definition_context(),
277 ## --------------------------------------------------------
280 Class::MOP::Class->meta->add_attribute(
281 Class::MOP::Attribute->new('superclasses' => (
284 # we just alias the original method
285 # rather than re-produce it here
286 'superclasses' => \&Class::MOP::Class::superclasses
289 default => sub { \undef },
290 _definition_context(),
294 Class::MOP::Class->meta->add_attribute(
295 Class::MOP::Attribute->new('instance_metaclass' => (
297 # NOTE: we need to do this in order
298 # for the instance meta-object to
299 # not fall into meta-circular death
301 # we just alias the original method
302 # rather than re-produce it here
303 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
305 default => 'Class::MOP::Instance',
306 _definition_context(),
310 Class::MOP::Class->meta->add_attribute(
311 Class::MOP::Attribute->new('immutable_trait' => (
313 'immutable_trait' => \&Class::MOP::Class::immutable_trait
315 default => "Class::MOP::Class::Immutable::Trait",
316 _definition_context(),
320 Class::MOP::Class->meta->add_attribute(
321 Class::MOP::Attribute->new('constructor_name' => (
323 'constructor_name' => \&Class::MOP::Class::constructor_name,
326 _definition_context(),
330 Class::MOP::Class->meta->add_attribute(
331 Class::MOP::Attribute->new('constructor_class' => (
333 'constructor_class' => \&Class::MOP::Class::constructor_class,
335 default => "Class::MOP::Method::Constructor",
336 _definition_context(),
341 Class::MOP::Class->meta->add_attribute(
342 Class::MOP::Attribute->new('destructor_class' => (
344 'destructor_class' => \&Class::MOP::Class::destructor_class,
346 _definition_context(),
351 # we don't actually need to tie the knot with
352 # Class::MOP::Class here, it is actually handled
353 # within Class::MOP::Class itself in the
354 # _construct_class_instance method.
356 ## --------------------------------------------------------
357 ## Class::MOP::Mixin::AttributeCore
358 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
359 Class::MOP::Attribute->new('name' => (
361 # NOTE: we need to do this in order
362 # for the instance meta-object to
363 # not fall into meta-circular death
365 # we just alias the original method
366 # rather than re-produce it here
367 'name' => \&Class::MOP::Mixin::AttributeCore::name
369 _definition_context(),
373 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
374 Class::MOP::Attribute->new('accessor' => (
375 reader => { 'accessor' => \&Class::MOP::Mixin::AttributeCore::accessor },
376 predicate => { 'has_accessor' => \&Class::MOP::Mixin::AttributeCore::has_accessor },
377 _definition_context(),
381 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
382 Class::MOP::Attribute->new('reader' => (
383 reader => { 'reader' => \&Class::MOP::Mixin::AttributeCore::reader },
384 predicate => { 'has_reader' => \&Class::MOP::Mixin::AttributeCore::has_reader },
385 _definition_context(),
389 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
390 Class::MOP::Attribute->new('initializer' => (
391 reader => { 'initializer' => \&Class::MOP::Mixin::AttributeCore::initializer },
392 predicate => { 'has_initializer' => \&Class::MOP::Mixin::AttributeCore::has_initializer },
393 _definition_context(),
397 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
398 Class::MOP::Attribute->new('definition_context' => (
399 reader => { 'definition_context' => \&Class::MOP::Mixin::AttributeCore::definition_context },
400 _definition_context(),
404 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
405 Class::MOP::Attribute->new('writer' => (
406 reader => { 'writer' => \&Class::MOP::Mixin::AttributeCore::writer },
407 predicate => { 'has_writer' => \&Class::MOP::Mixin::AttributeCore::has_writer },
408 _definition_context(),
412 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
413 Class::MOP::Attribute->new('predicate' => (
414 reader => { 'predicate' => \&Class::MOP::Mixin::AttributeCore::predicate },
415 predicate => { 'has_predicate' => \&Class::MOP::Mixin::AttributeCore::has_predicate },
416 _definition_context(),
420 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
421 Class::MOP::Attribute->new('clearer' => (
422 reader => { 'clearer' => \&Class::MOP::Mixin::AttributeCore::clearer },
423 predicate => { 'has_clearer' => \&Class::MOP::Mixin::AttributeCore::has_clearer },
424 _definition_context(),
428 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
429 Class::MOP::Attribute->new('builder' => (
430 reader => { 'builder' => \&Class::MOP::Mixin::AttributeCore::builder },
431 predicate => { 'has_builder' => \&Class::MOP::Mixin::AttributeCore::has_builder },
432 _definition_context(),
436 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
437 Class::MOP::Attribute->new('init_arg' => (
438 reader => { 'init_arg' => \&Class::MOP::Mixin::AttributeCore::init_arg },
439 predicate => { 'has_init_arg' => \&Class::MOP::Mixin::AttributeCore::has_init_arg },
440 _definition_context(),
444 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
445 Class::MOP::Attribute->new('default' => (
446 # default has a custom 'reader' method ...
447 predicate => { 'has_default' => \&Class::MOP::Mixin::AttributeCore::has_default },
448 _definition_context(),
452 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
453 Class::MOP::Attribute->new('insertion_order' => (
454 reader => { 'insertion_order' => \&Class::MOP::Mixin::AttributeCore::insertion_order },
455 writer => { '_set_insertion_order' => \&Class::MOP::Mixin::AttributeCore::_set_insertion_order },
456 predicate => { 'has_insertion_order' => \&Class::MOP::Mixin::AttributeCore::has_insertion_order },
457 _definition_context(),
461 ## --------------------------------------------------------
462 ## Class::MOP::Attribute
463 Class::MOP::Attribute->meta->add_attribute(
464 Class::MOP::Attribute->new('associated_class' => (
466 # NOTE: we need to do this in order
467 # for the instance meta-object to
468 # not fall into meta-circular death
470 # we just alias the original method
471 # rather than re-produce it here
472 'associated_class' => \&Class::MOP::Attribute::associated_class
474 _definition_context(),
478 Class::MOP::Attribute->meta->add_attribute(
479 Class::MOP::Attribute->new('associated_methods' => (
480 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
481 default => sub { [] },
482 _definition_context(),
486 Class::MOP::Attribute->meta->add_method('clone' => sub {
488 $self->meta->clone_object($self, @_);
491 ## --------------------------------------------------------
492 ## Class::MOP::Method
493 Class::MOP::Method->meta->add_attribute(
494 Class::MOP::Attribute->new('body' => (
495 reader => { 'body' => \&Class::MOP::Method::body },
496 _definition_context(),
500 Class::MOP::Method->meta->add_attribute(
501 Class::MOP::Attribute->new('associated_metaclass' => (
502 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
503 _definition_context(),
507 Class::MOP::Method->meta->add_attribute(
508 Class::MOP::Attribute->new('package_name' => (
509 reader => { 'package_name' => \&Class::MOP::Method::package_name },
510 _definition_context(),
514 Class::MOP::Method->meta->add_attribute(
515 Class::MOP::Attribute->new('name' => (
516 reader => { 'name' => \&Class::MOP::Method::name },
517 _definition_context(),
521 Class::MOP::Method->meta->add_attribute(
522 Class::MOP::Attribute->new('original_method' => (
523 reader => { 'original_method' => \&Class::MOP::Method::original_method },
524 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
525 _definition_context(),
529 ## --------------------------------------------------------
530 ## Class::MOP::Method::Wrapped
533 # the way this item is initialized, this
534 # really does not follow the standard
535 # practices of attributes, but we put
536 # it here for completeness
537 Class::MOP::Method::Wrapped->meta->add_attribute(
538 Class::MOP::Attribute->new('modifier_table' => (
539 _definition_context(),
543 ## --------------------------------------------------------
544 ## Class::MOP::Method::Generated
546 Class::MOP::Method::Generated->meta->add_attribute(
547 Class::MOP::Attribute->new('is_inline' => (
548 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
550 _definition_context(),
554 Class::MOP::Method::Generated->meta->add_attribute(
555 Class::MOP::Attribute->new('definition_context' => (
556 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
557 _definition_context(),
562 ## --------------------------------------------------------
563 ## Class::MOP::Method::Inlined
565 Class::MOP::Method::Inlined->meta->add_attribute(
566 Class::MOP::Attribute->new('_expected_method_class' => (
567 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
568 _definition_context(),
572 ## --------------------------------------------------------
573 ## Class::MOP::Method::Accessor
575 Class::MOP::Method::Accessor->meta->add_attribute(
576 Class::MOP::Attribute->new('attribute' => (
578 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
580 _definition_context(),
584 Class::MOP::Method::Accessor->meta->add_attribute(
585 Class::MOP::Attribute->new('accessor_type' => (
586 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
587 _definition_context(),
591 ## --------------------------------------------------------
592 ## Class::MOP::Method::Constructor
594 Class::MOP::Method::Constructor->meta->add_attribute(
595 Class::MOP::Attribute->new('options' => (
597 'options' => \&Class::MOP::Method::Constructor::options
599 default => sub { +{} },
600 _definition_context(),
604 Class::MOP::Method::Constructor->meta->add_attribute(
605 Class::MOP::Attribute->new('associated_metaclass' => (
606 init_arg => "metaclass", # FIXME alias and rename
608 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
610 _definition_context(),
614 ## --------------------------------------------------------
615 ## Class::MOP::Instance
618 # these don't yet do much of anything, but are just
619 # included for completeness
621 Class::MOP::Instance->meta->add_attribute(
622 Class::MOP::Attribute->new('associated_metaclass',
623 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
624 _definition_context(),
628 Class::MOP::Instance->meta->add_attribute(
629 Class::MOP::Attribute->new('_class_name',
631 reader => { _class_name => \&Class::MOP::Instance::_class_name },
632 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
633 #default => sub { $_[0]->associated_metaclass->name },
634 _definition_context(),
638 Class::MOP::Instance->meta->add_attribute(
639 Class::MOP::Attribute->new('attributes',
640 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
641 _definition_context(),
645 Class::MOP::Instance->meta->add_attribute(
646 Class::MOP::Attribute->new('slots',
647 reader => { slots => \&Class::MOP::Instance::slots },
648 _definition_context(),
652 Class::MOP::Instance->meta->add_attribute(
653 Class::MOP::Attribute->new('slot_hash',
654 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
655 _definition_context(),
659 ## --------------------------------------------------------
660 ## Class::MOP::Object
662 # need to replace the meta method there with a real meta method object
663 Class::MOP::Object->meta->_add_meta_method('meta');
665 ## --------------------------------------------------------
668 # need to replace the meta method there with a real meta method object
669 Class::MOP::Mixin->meta->_add_meta_method('meta');
671 require Class::MOP::Deprecated unless our $no_deprecated;
673 # we need the meta instance of the meta instance to be created now, in order
674 # for the constructor to be able to use it
675 Class::MOP::Instance->meta->get_meta_instance;
677 # pretend the add_method never happenned. it hasn't yet affected anything
678 undef Class::MOP::Instance->meta->{_package_cache_flag};
680 ## --------------------------------------------------------
681 ## Now close all the Class::MOP::* classes
683 # NOTE: we don't need to inline the the accessors this only lengthens
684 # the compile time of the MOP, and gives us no actual benefits.
686 $_->meta->make_immutable(
687 inline_constructor => 0,
688 constructor_name => "_new",
689 inline_accessors => 0,
695 Class::MOP::Attribute
701 Class::MOP::Method::Generated
702 Class::MOP::Method::Inlined
704 Class::MOP::Method::Accessor
705 Class::MOP::Method::Constructor
706 Class::MOP::Method::Wrapped
708 Class::MOP::Method::Meta
711 $_->meta->make_immutable(
712 inline_constructor => 0,
713 constructor_name => undef,
714 inline_accessors => 0,
717 Class::MOP::Mixin::AttributeCore
718 Class::MOP::Mixin::HasAttributes
719 Class::MOP::Mixin::HasMethods
724 # ABSTRACT: A Meta Object Protocol for Perl 5
732 This module is a fully functioning meta object protocol for the
733 Perl 5 object system. It makes no attempt to change the behavior or
734 characteristics of the Perl 5 object system, only to create a
735 protocol for its manipulation and introspection.
737 That said, it does attempt to create the tools for building a rich set
738 of extensions to the Perl 5 object system. Every attempt has been made
739 to abide by the spirit of the Perl 5 object system that we all know
742 This documentation is sparse on conceptual details. We suggest looking
743 at the items listed in the L<SEE ALSO> section for more
744 information. In particular the book "The Art of the Meta Object
745 Protocol" was very influential in the development of this system.
747 =head2 What is a Meta Object Protocol?
749 A meta object protocol is an API to an object system.
751 To be more specific, it abstracts the components of an object system
752 (classes, object, methods, object attributes, etc.). These
753 abstractions can then be used to inspect and manipulate the object
754 system which they describe.
756 It can be said that there are two MOPs for any object system; the
757 implicit MOP and the explicit MOP. The implicit MOP handles things
758 like method dispatch or inheritance, which happen automatically as
759 part of how the object system works. The explicit MOP typically
760 handles the introspection/reflection features of the object system.
762 All object systems have implicit MOPs. Without one, they would not
763 work. Explicit MOPs are much less common, and depending on the
764 language can vary from restrictive (Reflection in Java or C#) to wide
765 open (CLOS is a perfect example).
767 =head2 Yet Another Class Builder! Why?
769 This is B<not> a class builder so much as a I<class builder
770 B<builder>>. The intent is that an end user will not use this module
771 directly, but instead this module is used by module authors to build
772 extensions and features onto the Perl 5 object system.
774 This system is used by L<Moose>, which supplies a powerful class
775 builder system built entirely on top of C<Class::MOP>.
777 =head2 Who is this module for?
779 This module is for anyone who has ever created or wanted to create a
780 module for the Class:: namespace. The tools which this module provides
781 make doing complex Perl 5 wizardry simpler, by removing such barriers
782 as the need to hack symbol tables, or understand the fine details of
785 =head2 What changes do I have to make to use this module?
787 This module was designed to be as unintrusive as possible. Many of its
788 features are accessible without B<any> change to your existing
789 code. It is meant to be a compliment to your existing code and not an
790 intrusion on your code base. Unlike many other B<Class::> modules,
791 this module B<does not> require you subclass it, or even that you
792 C<use> it in within your module's package.
794 The only features which requires additions to your code are the
795 attribute handling and instance construction features, and these are
796 both completely optional features. The only reason for this is because
797 Perl 5's object system does not actually have these features built
798 in. More information about this feature can be found below.
800 =head2 About Performance
802 It is a common misconception that explicit MOPs are a performance hit.
803 This is not a universal truth, it is a side-effect of some specific
804 implementations. For instance, using Java reflection is slow because
805 the JVM cannot take advantage of any compiler optimizations, and the
806 JVM has to deal with much more runtime type information as well.
808 Reflection in C# is marginally better as it was designed into the
809 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
810 Object System) was built to support an explicit MOP, and so
811 performance is tuned for it.
813 This library in particular does its absolute best to avoid putting
814 B<any> drain at all upon your code's performance. In fact, by itself
815 it does nothing to affect your existing code. So you only pay for what
818 =head2 About Metaclass compatibility
820 This module makes sure that all metaclasses created are both upwards
821 and downwards compatible. The topic of metaclass compatibility is
822 highly esoteric and is something only encountered when doing deep and
823 involved metaclass hacking. There are two basic kinds of metaclass
824 incompatibility; upwards and downwards.
826 Upwards metaclass compatibility means that the metaclass of a
827 given class is either the same as (or a subclass of) all of the
830 Downward metaclass compatibility means that the metaclasses of a
831 given class's ancestors are all either the same as (or a subclass
834 Here is a diagram showing a set of two classes (C<A> and C<B>) and
835 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
836 metaclass compatibility both upwards and downwards.
838 +---------+ +---------+
839 | Meta::A |<----| Meta::B | <....... (instance of )
840 +---------+ +---------+ <------- (inherits from)
843 +---------+ +---------+
845 +---------+ +---------+
847 In actuality, I<all> of a class's metaclasses must be compatible,
848 not just the class metaclass. That includes the instance, attribute,
849 and method metaclasses, as well as the constructor and destructor
852 C<Class::MOP> will attempt to fix some simple types of
853 incompatibilities. If all the metaclasses for the parent class are
854 I<subclasses> of the child's metaclasses then we can simply replace
855 the child's metaclasses with the parent's. In addition, if the child
856 is missing a metaclass that the parent has, we can also just make the
857 child use the parent's metaclass.
859 As I said this is a highly esoteric topic and one you will only run
860 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
861 are interested in why this is an issue see the paper I<Uniform and
862 safe metaclass composition> linked to in the L<SEE ALSO> section of
865 =head2 Using custom metaclasses
867 Always use the L<metaclass> pragma when using a custom metaclass, this
868 will ensure the proper initialization order and not accidentally
869 create an incorrect type of metaclass for you. This is a very rare
870 problem, and one which can only occur if you are doing deep metaclass
871 programming. So in other words, don't worry about it.
873 Note that if you're using L<Moose> we encourage you to I<not> use
874 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
875 roles to a class's metaclasses. This topic is covered at length in
876 various L<Moose::Cookbook> recipes.
880 The meta-object protocol is divided into 4 main sub-protocols:
882 =head2 The Class protocol
884 This provides a means of manipulating and introspecting a Perl 5
885 class. It handles symbol table hacking for you, and provides a rich
886 set of methods that go beyond simple package introspection.
888 See L<Class::MOP::Class> for more details.
890 =head2 The Attribute protocol
892 This provides a consistent representation for an attribute of a Perl 5
893 class. Since there are so many ways to create and handle attributes in
894 Perl 5 OO, the Attribute protocol provide as much of a unified
895 approach as possible. Of course, you are always free to extend this
896 protocol by subclassing the appropriate classes.
898 See L<Class::MOP::Attribute> for more details.
900 =head2 The Method protocol
902 This provides a means of manipulating and introspecting methods in the
903 Perl 5 object system. As with attributes, there are many ways to
904 approach this topic, so we try to keep it pretty basic, while still
905 making it possible to extend the system in many ways.
907 See L<Class::MOP::Method> for more details.
909 =head2 The Instance protocol
911 This provides a layer of abstraction for creating object instances.
912 Since the other layers use this protocol, it is relatively easy to
913 change the type of your instances from the default hash reference to
914 some other type of reference. Several examples are provided in the
915 F<examples/> directory included in this distribution.
917 See L<Class::MOP::Instance> for more details.
921 Note that this module does not export any constants or functions.
923 =head2 Utility functions
925 Note that these are all called as B<functions, not methods>.
929 =item B<Class::MOP::get_code_info($code)>
931 This function returns two values, the name of the package the C<$code>
932 is from and the name of the C<$code> itself. This is used by several
933 elements of the MOP to determine where a given C<$code> reference is
936 =item B<Class::MOP::class_of($instance_or_class_name)>
938 This will return the metaclass of the given instance or class name. If the
939 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
944 =head2 Metaclass cache functions
946 Class::MOP holds a cache of metaclasses. The following are functions
947 (B<not methods>) which can be used to access that cache. It is not
948 recommended that you mess with these. Bad things could happen, but if
949 you are brave and willing to risk it: go for it!
953 =item B<Class::MOP::get_all_metaclasses>
955 This will return a hash of all the metaclass instances that have
956 been cached by L<Class::MOP::Class>, keyed by the package name.
958 =item B<Class::MOP::get_all_metaclass_instances>
960 This will return a list of all the metaclass instances that have
961 been cached by L<Class::MOP::Class>.
963 =item B<Class::MOP::get_all_metaclass_names>
965 This will return a list of all the metaclass names that have
966 been cached by L<Class::MOP::Class>.
968 =item B<Class::MOP::get_metaclass_by_name($name)>
970 This will return a cached L<Class::MOP::Class> instance, or nothing
971 if no metaclass exists with that C<$name>.
973 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
975 This will store a metaclass in the cache at the supplied C<$key>.
977 =item B<Class::MOP::weaken_metaclass($name)>
979 In rare cases (e.g. anonymous metaclasses) it is desirable to
980 store a weakened reference in the metaclass cache. This
981 function will weaken the reference to the metaclass stored
984 =item B<Class::MOP::metaclass_is_weak($name)>
986 Returns true if the metaclass for C<$name> has been weakened
987 (via C<weaken_metaclass>).
989 =item B<Class::MOP::does_metaclass_exist($name)>
991 This will return true of there exists a metaclass stored in the
992 C<$name> key, and return false otherwise.
994 =item B<Class::MOP::remove_metaclass_by_name($name)>
996 This will remove the metaclass stored in the C<$name> key.
1004 There are very few books out on Meta Object Protocols and Metaclasses
1005 because it is such an esoteric topic. The following books are really
1006 the only ones I have found. If you know of any more, B<I<please>>
1007 email me and let me know, I would love to hear about them.
1011 =item I<The Art of the Meta Object Protocol>
1013 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1015 =item I<Putting MetaClasses to Work>
1017 =item I<Smalltalk: The Language>
1025 =item "Uniform and safe metaclass composition"
1027 An excellent paper by the people who brought us the original Traits paper.
1028 This paper is on how Traits can be used to do safe metaclass composition,
1029 and offers an excellent introduction section which delves into the topic of
1030 metaclass compatibility.
1032 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1034 =item "Safe Metaclass Programming"
1036 This paper seems to precede the above paper, and propose a mix-in based
1037 approach as opposed to the Traits based approach. Both papers have similar
1038 information on the metaclass compatibility problem space.
1040 L<http://citeseer.ist.psu.edu/37617.html>
1048 =item The Perl 6 MetaModel work in the Pugs project
1052 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1054 =item L<http://github.com/perl6/p5-modules/tree/master/Perl6-ObjectSpace/>
1064 =item CPAN Module Review of Class::MOP
1066 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1070 =head1 SIMILAR MODULES
1072 As I have said above, this module is a class-builder-builder, so it is
1073 not the same thing as modules like L<Class::Accessor> and
1074 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1075 with similar goals to this module. The one I have found which is most
1076 like this module is L<Class::Meta>, although its philosophy and the MOP it
1077 creates are very different from this modules.
1081 All complex software has bugs lurking in it, and this module is no
1084 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1085 web interface at L<http://rt.cpan.org>.
1087 You can also discuss feature requests or possible bugs on the Moose
1088 mailing list (moose@perl.org) or on IRC at
1089 L<irc://irc.perl.org/#moose>.
1091 =head1 ACKNOWLEDGEMENTS
1097 Thanks to Rob for actually getting the development of this module kick-started.