12 use Class::Load 0.07 ();
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 goto &Class::Load::load_class;
74 sub load_first_existing_class {
75 goto &Class::Load::load_first_existing_class;
79 goto &Class::Load::is_class_loaded;
82 sub _definition_context {
84 @context{qw(package file line)} = caller(1);
87 definition_context => \%context,
91 ## ----------------------------------------------------------------------------
92 ## Setting up our environment ...
93 ## ----------------------------------------------------------------------------
94 ## Class::MOP needs to have a few things in the global perl environment so
95 ## that it can operate effectively. Those things are done here.
96 ## ----------------------------------------------------------------------------
98 # ... nothing yet actually ;)
100 ## ----------------------------------------------------------------------------
102 ## ----------------------------------------------------------------------------
103 ## The code below here is to bootstrap our MOP with itself. This is also
104 ## sometimes called "tying the knot". By doing this, we make it much easier
105 ## to extend the MOP through subclassing and such since now you can use the
106 ## MOP itself to extend itself.
108 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
109 ## ----------------------------------------------------------------------------
111 # We need to add in the meta-attributes here so that
112 # any subclass of Class::MOP::* will be able to
113 # inherit them using _construct_instance
115 ## --------------------------------------------------------
116 ## Class::MOP::Mixin::HasMethods
118 Class::MOP::Mixin::HasMethods->meta->add_attribute(
119 Class::MOP::Attribute->new('_methods' => (
122 # we just alias the original method
123 # rather than re-produce it here
124 '_method_map' => \&Class::MOP::Mixin::HasMethods::_method_map
126 default => sub { {} },
127 _definition_context(),
131 Class::MOP::Mixin::HasMethods->meta->add_attribute(
132 Class::MOP::Attribute->new('method_metaclass' => (
135 # we just alias the original method
136 # rather than re-produce it here
137 'method_metaclass' => \&Class::MOP::Mixin::HasMethods::method_metaclass
139 default => 'Class::MOP::Method',
140 _definition_context(),
144 Class::MOP::Mixin::HasMethods->meta->add_attribute(
145 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
148 # we just alias the original method
149 # rather than re-produce it here
150 'wrapped_method_metaclass' => \&Class::MOP::Mixin::HasMethods::wrapped_method_metaclass
152 default => 'Class::MOP::Method::Wrapped',
153 _definition_context(),
157 ## --------------------------------------------------------
158 ## Class::MOP::Mixin::HasMethods
160 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
161 Class::MOP::Attribute->new('attributes' => (
163 # NOTE: we need to do this in order
164 # for the instance meta-object to
165 # not fall into meta-circular death
167 # we just alias the original method
168 # rather than re-produce it here
169 '_attribute_map' => \&Class::MOP::Mixin::HasAttributes::_attribute_map
171 default => sub { {} },
172 _definition_context(),
176 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
177 Class::MOP::Attribute->new('attribute_metaclass' => (
180 # we just alias the original method
181 # rather than re-produce it here
182 'attribute_metaclass' => \&Class::MOP::Mixin::HasAttributes::attribute_metaclass
184 default => 'Class::MOP::Attribute',
185 _definition_context(),
189 ## --------------------------------------------------------
190 ## Class::MOP::Package
192 Class::MOP::Package->meta->add_attribute(
193 Class::MOP::Attribute->new('package' => (
195 # NOTE: we need to do this in order
196 # for the instance meta-object to
197 # not fall into meta-circular death
199 # we just alias the original method
200 # rather than re-produce it here
201 'name' => \&Class::MOP::Package::name
203 _definition_context(),
207 Class::MOP::Package->meta->add_attribute(
208 Class::MOP::Attribute->new('namespace' => (
211 # we just alias the original method
212 # rather than re-produce it here
213 'namespace' => \&Class::MOP::Package::namespace
216 default => sub { \undef },
217 _definition_context(),
221 ## --------------------------------------------------------
222 ## Class::MOP::Module
225 # yeah this is kind of stretching things a bit,
226 # but truthfully the version should be an attribute
227 # of the Module, the weirdness comes from having to
228 # stick to Perl 5 convention and store it in the
229 # $VERSION package variable. Basically if you just
230 # squint at it, it will look how you want it to look.
231 # Either as a package variable, or as a attribute of
232 # the metaclass, isn't abstraction great :)
234 Class::MOP::Module->meta->add_attribute(
235 Class::MOP::Attribute->new('version' => (
238 # we just alias the original method
239 # rather than re-produce it here
240 'version' => \&Class::MOP::Module::version
243 default => sub { \undef },
244 _definition_context(),
249 # By following the same conventions as version here,
250 # we are opening up the possibility that people can
251 # use the $AUTHORITY in non-Class::MOP modules as
254 Class::MOP::Module->meta->add_attribute(
255 Class::MOP::Attribute->new('authority' => (
258 # we just alias the original method
259 # rather than re-produce it here
260 'authority' => \&Class::MOP::Module::authority
263 default => sub { \undef },
264 _definition_context(),
268 ## --------------------------------------------------------
271 Class::MOP::Class->meta->add_attribute(
272 Class::MOP::Attribute->new('superclasses' => (
275 # we just alias the original method
276 # rather than re-produce it here
277 'superclasses' => \&Class::MOP::Class::superclasses
280 default => sub { \undef },
281 _definition_context(),
285 Class::MOP::Class->meta->add_attribute(
286 Class::MOP::Attribute->new('instance_metaclass' => (
288 # NOTE: we need to do this in order
289 # for the instance meta-object to
290 # not fall into meta-circular death
292 # we just alias the original method
293 # rather than re-produce it here
294 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
296 default => 'Class::MOP::Instance',
297 _definition_context(),
301 Class::MOP::Class->meta->add_attribute(
302 Class::MOP::Attribute->new('immutable_trait' => (
304 'immutable_trait' => \&Class::MOP::Class::immutable_trait
306 default => "Class::MOP::Class::Immutable::Trait",
307 _definition_context(),
311 Class::MOP::Class->meta->add_attribute(
312 Class::MOP::Attribute->new('constructor_name' => (
314 'constructor_name' => \&Class::MOP::Class::constructor_name,
317 _definition_context(),
321 Class::MOP::Class->meta->add_attribute(
322 Class::MOP::Attribute->new('constructor_class' => (
324 'constructor_class' => \&Class::MOP::Class::constructor_class,
326 default => "Class::MOP::Method::Constructor",
327 _definition_context(),
332 Class::MOP::Class->meta->add_attribute(
333 Class::MOP::Attribute->new('destructor_class' => (
335 'destructor_class' => \&Class::MOP::Class::destructor_class,
337 _definition_context(),
342 # we don't actually need to tie the knot with
343 # Class::MOP::Class here, it is actually handled
344 # within Class::MOP::Class itself in the
345 # _construct_class_instance method.
347 ## --------------------------------------------------------
348 ## Class::MOP::Mixin::AttributeCore
349 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
350 Class::MOP::Attribute->new('name' => (
352 # NOTE: we need to do this in order
353 # for the instance meta-object to
354 # not fall into meta-circular death
356 # we just alias the original method
357 # rather than re-produce it here
358 'name' => \&Class::MOP::Mixin::AttributeCore::name
360 _definition_context(),
364 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
365 Class::MOP::Attribute->new('accessor' => (
366 reader => { 'accessor' => \&Class::MOP::Mixin::AttributeCore::accessor },
367 predicate => { 'has_accessor' => \&Class::MOP::Mixin::AttributeCore::has_accessor },
368 _definition_context(),
372 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
373 Class::MOP::Attribute->new('reader' => (
374 reader => { 'reader' => \&Class::MOP::Mixin::AttributeCore::reader },
375 predicate => { 'has_reader' => \&Class::MOP::Mixin::AttributeCore::has_reader },
376 _definition_context(),
380 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
381 Class::MOP::Attribute->new('initializer' => (
382 reader => { 'initializer' => \&Class::MOP::Mixin::AttributeCore::initializer },
383 predicate => { 'has_initializer' => \&Class::MOP::Mixin::AttributeCore::has_initializer },
384 _definition_context(),
388 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
389 Class::MOP::Attribute->new('definition_context' => (
390 reader => { 'definition_context' => \&Class::MOP::Mixin::AttributeCore::definition_context },
391 _definition_context(),
395 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
396 Class::MOP::Attribute->new('writer' => (
397 reader => { 'writer' => \&Class::MOP::Mixin::AttributeCore::writer },
398 predicate => { 'has_writer' => \&Class::MOP::Mixin::AttributeCore::has_writer },
399 _definition_context(),
403 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
404 Class::MOP::Attribute->new('predicate' => (
405 reader => { 'predicate' => \&Class::MOP::Mixin::AttributeCore::predicate },
406 predicate => { 'has_predicate' => \&Class::MOP::Mixin::AttributeCore::has_predicate },
407 _definition_context(),
411 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
412 Class::MOP::Attribute->new('clearer' => (
413 reader => { 'clearer' => \&Class::MOP::Mixin::AttributeCore::clearer },
414 predicate => { 'has_clearer' => \&Class::MOP::Mixin::AttributeCore::has_clearer },
415 _definition_context(),
419 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
420 Class::MOP::Attribute->new('builder' => (
421 reader => { 'builder' => \&Class::MOP::Mixin::AttributeCore::builder },
422 predicate => { 'has_builder' => \&Class::MOP::Mixin::AttributeCore::has_builder },
423 _definition_context(),
427 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
428 Class::MOP::Attribute->new('init_arg' => (
429 reader => { 'init_arg' => \&Class::MOP::Mixin::AttributeCore::init_arg },
430 predicate => { 'has_init_arg' => \&Class::MOP::Mixin::AttributeCore::has_init_arg },
431 _definition_context(),
435 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
436 Class::MOP::Attribute->new('default' => (
437 # default has a custom 'reader' method ...
438 predicate => { 'has_default' => \&Class::MOP::Mixin::AttributeCore::has_default },
439 _definition_context(),
443 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
444 Class::MOP::Attribute->new('insertion_order' => (
445 reader => { 'insertion_order' => \&Class::MOP::Mixin::AttributeCore::insertion_order },
446 writer => { '_set_insertion_order' => \&Class::MOP::Mixin::AttributeCore::_set_insertion_order },
447 predicate => { 'has_insertion_order' => \&Class::MOP::Mixin::AttributeCore::has_insertion_order },
448 _definition_context(),
452 ## --------------------------------------------------------
453 ## Class::MOP::Attribute
454 Class::MOP::Attribute->meta->add_attribute(
455 Class::MOP::Attribute->new('associated_class' => (
457 # NOTE: we need to do this in order
458 # for the instance meta-object to
459 # not fall into meta-circular death
461 # we just alias the original method
462 # rather than re-produce it here
463 'associated_class' => \&Class::MOP::Attribute::associated_class
465 _definition_context(),
469 Class::MOP::Attribute->meta->add_attribute(
470 Class::MOP::Attribute->new('associated_methods' => (
471 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
472 default => sub { [] },
473 _definition_context(),
477 Class::MOP::Attribute->meta->add_method('clone' => sub {
479 $self->meta->clone_object($self, @_);
482 ## --------------------------------------------------------
483 ## Class::MOP::Method
484 Class::MOP::Method->meta->add_attribute(
485 Class::MOP::Attribute->new('body' => (
486 reader => { 'body' => \&Class::MOP::Method::body },
487 _definition_context(),
491 Class::MOP::Method->meta->add_attribute(
492 Class::MOP::Attribute->new('associated_metaclass' => (
493 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
494 _definition_context(),
498 Class::MOP::Method->meta->add_attribute(
499 Class::MOP::Attribute->new('package_name' => (
500 reader => { 'package_name' => \&Class::MOP::Method::package_name },
501 _definition_context(),
505 Class::MOP::Method->meta->add_attribute(
506 Class::MOP::Attribute->new('name' => (
507 reader => { 'name' => \&Class::MOP::Method::name },
508 _definition_context(),
512 Class::MOP::Method->meta->add_attribute(
513 Class::MOP::Attribute->new('original_method' => (
514 reader => { 'original_method' => \&Class::MOP::Method::original_method },
515 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
516 _definition_context(),
520 ## --------------------------------------------------------
521 ## Class::MOP::Method::Wrapped
524 # the way this item is initialized, this
525 # really does not follow the standard
526 # practices of attributes, but we put
527 # it here for completeness
528 Class::MOP::Method::Wrapped->meta->add_attribute(
529 Class::MOP::Attribute->new('modifier_table' => (
530 _definition_context(),
534 ## --------------------------------------------------------
535 ## Class::MOP::Method::Generated
537 Class::MOP::Method::Generated->meta->add_attribute(
538 Class::MOP::Attribute->new('is_inline' => (
539 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
541 _definition_context(),
545 Class::MOP::Method::Generated->meta->add_attribute(
546 Class::MOP::Attribute->new('definition_context' => (
547 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
548 _definition_context(),
553 ## --------------------------------------------------------
554 ## Class::MOP::Method::Inlined
556 Class::MOP::Method::Inlined->meta->add_attribute(
557 Class::MOP::Attribute->new('_expected_method_class' => (
558 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
559 _definition_context(),
563 ## --------------------------------------------------------
564 ## Class::MOP::Method::Accessor
566 Class::MOP::Method::Accessor->meta->add_attribute(
567 Class::MOP::Attribute->new('attribute' => (
569 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
571 _definition_context(),
575 Class::MOP::Method::Accessor->meta->add_attribute(
576 Class::MOP::Attribute->new('accessor_type' => (
577 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
578 _definition_context(),
582 ## --------------------------------------------------------
583 ## Class::MOP::Method::Constructor
585 Class::MOP::Method::Constructor->meta->add_attribute(
586 Class::MOP::Attribute->new('options' => (
588 'options' => \&Class::MOP::Method::Constructor::options
590 default => sub { +{} },
591 _definition_context(),
595 Class::MOP::Method::Constructor->meta->add_attribute(
596 Class::MOP::Attribute->new('associated_metaclass' => (
597 init_arg => "metaclass", # FIXME alias and rename
599 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
601 _definition_context(),
605 ## --------------------------------------------------------
606 ## Class::MOP::Instance
609 # these don't yet do much of anything, but are just
610 # included for completeness
612 Class::MOP::Instance->meta->add_attribute(
613 Class::MOP::Attribute->new('associated_metaclass',
614 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
615 _definition_context(),
619 Class::MOP::Instance->meta->add_attribute(
620 Class::MOP::Attribute->new('_class_name',
622 reader => { _class_name => \&Class::MOP::Instance::_class_name },
623 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
624 #default => sub { $_[0]->associated_metaclass->name },
625 _definition_context(),
629 Class::MOP::Instance->meta->add_attribute(
630 Class::MOP::Attribute->new('attributes',
631 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
632 _definition_context(),
636 Class::MOP::Instance->meta->add_attribute(
637 Class::MOP::Attribute->new('slots',
638 reader => { slots => \&Class::MOP::Instance::slots },
639 _definition_context(),
643 Class::MOP::Instance->meta->add_attribute(
644 Class::MOP::Attribute->new('slot_hash',
645 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
646 _definition_context(),
650 ## --------------------------------------------------------
651 ## Class::MOP::Object
653 # need to replace the meta method there with a real meta method object
654 Class::MOP::Object->meta->_add_meta_method('meta');
656 ## --------------------------------------------------------
659 # need to replace the meta method there with a real meta method object
660 Class::MOP::Mixin->meta->_add_meta_method('meta');
662 require Class::MOP::Deprecated unless our $no_deprecated;
664 # we need the meta instance of the meta instance to be created now, in order
665 # for the constructor to be able to use it
666 Class::MOP::Instance->meta->get_meta_instance;
668 # pretend the add_method never happenned. it hasn't yet affected anything
669 undef Class::MOP::Instance->meta->{_package_cache_flag};
671 ## --------------------------------------------------------
672 ## Now close all the Class::MOP::* classes
674 # NOTE: we don't need to inline the the accessors this only lengthens
675 # the compile time of the MOP, and gives us no actual benefits.
677 $_->meta->make_immutable(
678 inline_constructor => 0,
679 constructor_name => "_new",
680 inline_accessors => 0,
686 Class::MOP::Attribute
692 Class::MOP::Method::Generated
693 Class::MOP::Method::Inlined
695 Class::MOP::Method::Accessor
696 Class::MOP::Method::Constructor
697 Class::MOP::Method::Wrapped
699 Class::MOP::Method::Meta
700 Class::MOP::Method::Overload
703 $_->meta->make_immutable(
704 inline_constructor => 0,
705 constructor_name => undef,
706 inline_accessors => 0,
709 Class::MOP::Mixin::AttributeCore
710 Class::MOP::Mixin::HasAttributes
711 Class::MOP::Mixin::HasMethods
716 # ABSTRACT: A Meta Object Protocol for Perl 5
724 This module is a fully functioning meta object protocol for the
725 Perl 5 object system. It makes no attempt to change the behavior or
726 characteristics of the Perl 5 object system, only to create a
727 protocol for its manipulation and introspection.
729 That said, it does attempt to create the tools for building a rich set
730 of extensions to the Perl 5 object system. Every attempt has been made
731 to abide by the spirit of the Perl 5 object system that we all know
734 This documentation is sparse on conceptual details. We suggest looking
735 at the items listed in the L<SEE ALSO> section for more
736 information. In particular the book "The Art of the Meta Object
737 Protocol" was very influential in the development of this system.
739 =head2 What is a Meta Object Protocol?
741 A meta object protocol is an API to an object system.
743 To be more specific, it abstracts the components of an object system
744 (classes, object, methods, object attributes, etc.). These
745 abstractions can then be used to inspect and manipulate the object
746 system which they describe.
748 It can be said that there are two MOPs for any object system; the
749 implicit MOP and the explicit MOP. The implicit MOP handles things
750 like method dispatch or inheritance, which happen automatically as
751 part of how the object system works. The explicit MOP typically
752 handles the introspection/reflection features of the object system.
754 All object systems have implicit MOPs. Without one, they would not
755 work. Explicit MOPs are much less common, and depending on the
756 language can vary from restrictive (Reflection in Java or C#) to wide
757 open (CLOS is a perfect example).
759 =head2 Yet Another Class Builder! Why?
761 This is B<not> a class builder so much as a I<class builder
762 B<builder>>. The intent is that an end user will not use this module
763 directly, but instead this module is used by module authors to build
764 extensions and features onto the Perl 5 object system.
766 This system is used by L<Moose>, which supplies a powerful class
767 builder system built entirely on top of C<Class::MOP>.
769 =head2 Who is this module for?
771 This module is for anyone who has ever created or wanted to create a
772 module for the Class:: namespace. The tools which this module provides
773 make doing complex Perl 5 wizardry simpler, by removing such barriers
774 as the need to hack symbol tables, or understand the fine details of
777 =head2 What changes do I have to make to use this module?
779 This module was designed to be as unintrusive as possible. Many of its
780 features are accessible without B<any> change to your existing
781 code. It is meant to be a complement to your existing code and not an
782 intrusion on your code base. Unlike many other B<Class::> modules,
783 this module B<does not> require you subclass it, or even that you
784 C<use> it in within your module's package.
786 The only features which require additions to your code are the
787 attribute handling and instance construction features, and these are
788 both completely optional features. The only reason for this is because
789 Perl 5's object system does not actually have these features built
790 in. More information about this feature can be found below.
792 =head2 About Performance
794 It is a common misconception that explicit MOPs are a performance hit.
795 This is not a universal truth, it is a side-effect of some specific
796 implementations. For instance, using Java reflection is slow because
797 the JVM cannot take advantage of any compiler optimizations, and the
798 JVM has to deal with much more runtime type information as well.
800 Reflection in C# is marginally better as it was designed into the
801 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
802 Object System) was built to support an explicit MOP, and so
803 performance is tuned for it.
805 This library in particular does its absolute best to avoid putting
806 B<any> drain at all upon your code's performance. In fact, by itself
807 it does nothing to affect your existing code. So you only pay for what
810 =head2 About Metaclass compatibility
812 This module makes sure that all metaclasses created are both upwards
813 and downwards compatible. The topic of metaclass compatibility is
814 highly esoteric and is something only encountered when doing deep and
815 involved metaclass hacking. There are two basic kinds of metaclass
816 incompatibility; upwards and downwards.
818 Upwards metaclass compatibility means that the metaclass of a
819 given class is either the same as (or a subclass of) all of the
822 Downward metaclass compatibility means that the metaclasses of a
823 given class's ancestors are all the same as (or a subclass of) that
826 Here is a diagram showing a set of two classes (C<A> and C<B>) and
827 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
828 metaclass compatibility both upwards and downwards.
830 +---------+ +---------+
831 | Meta::A |<----| Meta::B | <....... (instance of )
832 +---------+ +---------+ <------- (inherits from)
835 +---------+ +---------+
837 +---------+ +---------+
839 In actuality, I<all> of a class's metaclasses must be compatible,
840 not just the class metaclass. That includes the instance, attribute,
841 and method metaclasses, as well as the constructor and destructor
844 C<Class::MOP> will attempt to fix some simple types of
845 incompatibilities. If all the metaclasses for the parent class are
846 I<subclasses> of the child's metaclasses then we can simply replace
847 the child's metaclasses with the parent's. In addition, if the child
848 is missing a metaclass that the parent has, we can also just make the
849 child use the parent's metaclass.
851 As I said this is a highly esoteric topic and one you will only run
852 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
853 are interested in why this is an issue see the paper I<Uniform and
854 safe metaclass composition> linked to in the L<SEE ALSO> section of
857 =head2 Using custom metaclasses
859 Always use the L<metaclass> pragma when using a custom metaclass, this
860 will ensure the proper initialization order and not accidentally
861 create an incorrect type of metaclass for you. This is a very rare
862 problem, and one which can only occur if you are doing deep metaclass
863 programming. So in other words, don't worry about it.
865 Note that if you're using L<Moose> we encourage you to I<not> use the
866 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
867 roles to a class's metaclasses. This topic is covered at length in
868 various L<Moose::Cookbook> recipes.
872 The meta-object protocol is divided into 4 main sub-protocols:
874 =head2 The Class protocol
876 This provides a means of manipulating and introspecting a Perl 5
877 class. It handles symbol table hacking for you, and provides a rich
878 set of methods that go beyond simple package introspection.
880 See L<Class::MOP::Class> for more details.
882 =head2 The Attribute protocol
884 This provides a consistent representation for an attribute of a Perl 5
885 class. Since there are so many ways to create and handle attributes in
886 Perl 5 OO, the Attribute protocol provide as much of a unified
887 approach as possible. Of course, you are always free to extend this
888 protocol by subclassing the appropriate classes.
890 See L<Class::MOP::Attribute> for more details.
892 =head2 The Method protocol
894 This provides a means of manipulating and introspecting methods in the
895 Perl 5 object system. As with attributes, there are many ways to
896 approach this topic, so we try to keep it pretty basic, while still
897 making it possible to extend the system in many ways.
899 See L<Class::MOP::Method> for more details.
901 =head2 The Instance protocol
903 This provides a layer of abstraction for creating object instances.
904 Since the other layers use this protocol, it is relatively easy to
905 change the type of your instances from the default hash reference to
906 some other type of reference. Several examples are provided in the
907 F<examples/> directory included in this distribution.
909 See L<Class::MOP::Instance> for more details.
913 Note that this module does not export any constants or functions.
915 =head2 Utility functions
917 Note that these are all called as B<functions, not methods>.
921 =item B<Class::MOP::get_code_info($code)>
923 This function returns two values, the name of the package the C<$code>
924 is from and the name of the C<$code> itself. This is used by several
925 elements of the MOP to determine where a given C<$code> reference is
928 =item B<Class::MOP::class_of($instance_or_class_name)>
930 This will return the metaclass of the given instance or class name. If the
931 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
936 =head2 Metaclass cache functions
938 C<Class::MOP> holds a cache of metaclasses. The following are functions
939 (B<not methods>) which can be used to access that cache. It is not
940 recommended that you mess with these. Bad things could happen, but if
941 you are brave and willing to risk it: go for it!
945 =item B<Class::MOP::get_all_metaclasses>
947 This will return a hash of all the metaclass instances that have
948 been cached by L<Class::MOP::Class>, keyed by the package name.
950 =item B<Class::MOP::get_all_metaclass_instances>
952 This will return a list of all the metaclass instances that have
953 been cached by L<Class::MOP::Class>.
955 =item B<Class::MOP::get_all_metaclass_names>
957 This will return a list of all the metaclass names that have
958 been cached by L<Class::MOP::Class>.
960 =item B<Class::MOP::get_metaclass_by_name($name)>
962 This will return a cached L<Class::MOP::Class> instance, or nothing
963 if no metaclass exists with that C<$name>.
965 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
967 This will store a metaclass in the cache at the supplied C<$key>.
969 =item B<Class::MOP::weaken_metaclass($name)>
971 In rare cases (e.g. anonymous metaclasses) it is desirable to
972 store a weakened reference in the metaclass cache. This
973 function will weaken the reference to the metaclass stored
976 =item B<Class::MOP::metaclass_is_weak($name)>
978 Returns true if the metaclass for C<$name> has been weakened
979 (via C<weaken_metaclass>).
981 =item B<Class::MOP::does_metaclass_exist($name)>
983 This will return true of there exists a metaclass stored in the
984 C<$name> key, and return false otherwise.
986 =item B<Class::MOP::remove_metaclass_by_name($name)>
988 This will remove the metaclass stored in the C<$name> key.
992 Some utility functions (such as C<Class::MOP::load_class>) that were
993 previously defined in C<Class::MOP> regarding loading of classes have been
994 extracted to L<Class::Load>. Please see L<Class::Load> for documentation.
1000 There are very few books out on Meta Object Protocols and Metaclasses
1001 because it is such an esoteric topic. The following books are really
1002 the only ones I have found. If you know of any more, B<I<please>>
1003 email me and let me know, I would love to hear about them.
1007 =item I<The Art of the Meta Object Protocol>
1009 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1011 =item I<Putting MetaClasses to Work>
1013 =item I<Smalltalk: The Language>
1021 =item "Uniform and safe metaclass composition"
1023 An excellent paper by the people who brought us the original Traits paper.
1024 This paper is on how Traits can be used to do safe metaclass composition,
1025 and offers an excellent introduction section which delves into the topic of
1026 metaclass compatibility.
1028 L<http://scg.unibe.ch/archive/papers/Duca05ySafeMetaclassTrait.pdf>
1030 =item "Safe Metaclass Programming"
1032 This paper seems to precede the above paper, and propose a mix-in based
1033 approach as opposed to the Traits based approach. Both papers have similar
1034 information on the metaclass compatibility problem space.
1036 L<http://citeseer.ist.psu.edu/37617.html>
1044 =item The Perl 6 MetaModel work in the Pugs project
1048 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1050 =item L<http://github.com/perl6/p5-modules/tree/master/Perl6-ObjectSpace/>
1060 =item CPAN Module Review of Class::MOP
1062 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1066 =head1 SIMILAR MODULES
1068 As I have said above, this module is a class-builder-builder, so it is
1069 not the same thing as modules like L<Class::Accessor> and
1070 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1071 with similar goals to this module. The one I have found which is most
1072 like this module is L<Class::Meta>, although its philosophy and the MOP it
1073 creates are very different from this modules.
1077 All complex software has bugs lurking in it, and this module is no
1080 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1081 web interface at L<http://rt.cpan.org>.
1083 You can also discuss feature requests or possible bugs on the Moose
1084 mailing list (moose@perl.org) or on IRC at
1085 L<irc://irc.perl.org/#moose>.
1087 =head1 ACKNOWLEDGEMENTS
1093 Thanks to Rob for actually getting the development of this module kick-started.