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
702 $_->meta->make_immutable(
703 inline_constructor => 0,
704 constructor_name => undef,
705 inline_accessors => 0,
708 Class::MOP::Mixin::AttributeCore
709 Class::MOP::Mixin::HasAttributes
710 Class::MOP::Mixin::HasMethods
715 # ABSTRACT: A Meta Object Protocol for Perl 5
723 This module is a fully functioning meta object protocol for the
724 Perl 5 object system. It makes no attempt to change the behavior or
725 characteristics of the Perl 5 object system, only to create a
726 protocol for its manipulation and introspection.
728 That said, it does attempt to create the tools for building a rich set
729 of extensions to the Perl 5 object system. Every attempt has been made
730 to abide by the spirit of the Perl 5 object system that we all know
733 This documentation is sparse on conceptual details. We suggest looking
734 at the items listed in the L<SEE ALSO> section for more
735 information. In particular the book "The Art of the Meta Object
736 Protocol" was very influential in the development of this system.
738 =head2 What is a Meta Object Protocol?
740 A meta object protocol is an API to an object system.
742 To be more specific, it abstracts the components of an object system
743 (classes, object, methods, object attributes, etc.). These
744 abstractions can then be used to inspect and manipulate the object
745 system which they describe.
747 It can be said that there are two MOPs for any object system; the
748 implicit MOP and the explicit MOP. The implicit MOP handles things
749 like method dispatch or inheritance, which happen automatically as
750 part of how the object system works. The explicit MOP typically
751 handles the introspection/reflection features of the object system.
753 All object systems have implicit MOPs. Without one, they would not
754 work. Explicit MOPs are much less common, and depending on the
755 language can vary from restrictive (Reflection in Java or C#) to wide
756 open (CLOS is a perfect example).
758 =head2 Yet Another Class Builder! Why?
760 This is B<not> a class builder so much as a I<class builder
761 B<builder>>. The intent is that an end user will not use this module
762 directly, but instead this module is used by module authors to build
763 extensions and features onto the Perl 5 object system.
765 This system is used by L<Moose>, which supplies a powerful class
766 builder system built entirely on top of C<Class::MOP>.
768 =head2 Who is this module for?
770 This module is for anyone who has ever created or wanted to create a
771 module for the Class:: namespace. The tools which this module provides
772 make doing complex Perl 5 wizardry simpler, by removing such barriers
773 as the need to hack symbol tables, or understand the fine details of
776 =head2 What changes do I have to make to use this module?
778 This module was designed to be as unintrusive as possible. Many of its
779 features are accessible without B<any> change to your existing
780 code. It is meant to be a complement to your existing code and not an
781 intrusion on your code base. Unlike many other B<Class::> modules,
782 this module B<does not> require you subclass it, or even that you
783 C<use> it in within your module's package.
785 The only features which require additions to your code are the
786 attribute handling and instance construction features, and these are
787 both completely optional features. The only reason for this is because
788 Perl 5's object system does not actually have these features built
789 in. More information about this feature can be found below.
791 =head2 About Performance
793 It is a common misconception that explicit MOPs are a performance hit.
794 This is not a universal truth, it is a side-effect of some specific
795 implementations. For instance, using Java reflection is slow because
796 the JVM cannot take advantage of any compiler optimizations, and the
797 JVM has to deal with much more runtime type information as well.
799 Reflection in C# is marginally better as it was designed into the
800 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
801 Object System) was built to support an explicit MOP, and so
802 performance is tuned for it.
804 This library in particular does its absolute best to avoid putting
805 B<any> drain at all upon your code's performance. In fact, by itself
806 it does nothing to affect your existing code. So you only pay for what
809 =head2 About Metaclass compatibility
811 This module makes sure that all metaclasses created are both upwards
812 and downwards compatible. The topic of metaclass compatibility is
813 highly esoteric and is something only encountered when doing deep and
814 involved metaclass hacking. There are two basic kinds of metaclass
815 incompatibility; upwards and downwards.
817 Upwards metaclass compatibility means that the metaclass of a
818 given class is either the same as (or a subclass of) all of the
821 Downward metaclass compatibility means that the metaclasses of a
822 given class's ancestors are all the same as (or a subclass of) that
825 Here is a diagram showing a set of two classes (C<A> and C<B>) and
826 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
827 metaclass compatibility both upwards and downwards.
829 +---------+ +---------+
830 | Meta::A |<----| Meta::B | <....... (instance of )
831 +---------+ +---------+ <------- (inherits from)
834 +---------+ +---------+
836 +---------+ +---------+
838 In actuality, I<all> of a class's metaclasses must be compatible,
839 not just the class metaclass. That includes the instance, attribute,
840 and method metaclasses, as well as the constructor and destructor
843 C<Class::MOP> will attempt to fix some simple types of
844 incompatibilities. If all the metaclasses for the parent class are
845 I<subclasses> of the child's metaclasses then we can simply replace
846 the child's metaclasses with the parent's. In addition, if the child
847 is missing a metaclass that the parent has, we can also just make the
848 child use the parent's metaclass.
850 As I said this is a highly esoteric topic and one you will only run
851 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
852 are interested in why this is an issue see the paper I<Uniform and
853 safe metaclass composition> linked to in the L<SEE ALSO> section of
856 =head2 Using custom metaclasses
858 Always use the L<metaclass> pragma when using a custom metaclass, this
859 will ensure the proper initialization order and not accidentally
860 create an incorrect type of metaclass for you. This is a very rare
861 problem, and one which can only occur if you are doing deep metaclass
862 programming. So in other words, don't worry about it.
864 Note that if you're using L<Moose> we encourage you to I<not> use the
865 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
866 roles to a class's metaclasses. This topic is covered at length in
867 various L<Moose::Cookbook> recipes.
871 The meta-object protocol is divided into 4 main sub-protocols:
873 =head2 The Class protocol
875 This provides a means of manipulating and introspecting a Perl 5
876 class. It handles symbol table hacking for you, and provides a rich
877 set of methods that go beyond simple package introspection.
879 See L<Class::MOP::Class> for more details.
881 =head2 The Attribute protocol
883 This provides a consistent representation for an attribute of a Perl 5
884 class. Since there are so many ways to create and handle attributes in
885 Perl 5 OO, the Attribute protocol provide as much of a unified
886 approach as possible. Of course, you are always free to extend this
887 protocol by subclassing the appropriate classes.
889 See L<Class::MOP::Attribute> for more details.
891 =head2 The Method protocol
893 This provides a means of manipulating and introspecting methods in the
894 Perl 5 object system. As with attributes, there are many ways to
895 approach this topic, so we try to keep it pretty basic, while still
896 making it possible to extend the system in many ways.
898 See L<Class::MOP::Method> for more details.
900 =head2 The Instance protocol
902 This provides a layer of abstraction for creating object instances.
903 Since the other layers use this protocol, it is relatively easy to
904 change the type of your instances from the default hash reference to
905 some other type of reference. Several examples are provided in the
906 F<examples/> directory included in this distribution.
908 See L<Class::MOP::Instance> for more details.
912 Note that this module does not export any constants or functions.
914 =head2 Utility functions
916 Note that these are all called as B<functions, not methods>.
920 =item B<Class::MOP::get_code_info($code)>
922 This function returns two values, the name of the package the C<$code>
923 is from and the name of the C<$code> itself. This is used by several
924 elements of the MOP to determine where a given C<$code> reference is
927 =item B<Class::MOP::class_of($instance_or_class_name)>
929 This will return the metaclass of the given instance or class name. If the
930 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
935 =head2 Metaclass cache functions
937 C<Class::MOP> holds a cache of metaclasses. The following are functions
938 (B<not methods>) which can be used to access that cache. It is not
939 recommended that you mess with these. Bad things could happen, but if
940 you are brave and willing to risk it: go for it!
944 =item B<Class::MOP::get_all_metaclasses>
946 This will return a hash of all the metaclass instances that have
947 been cached by L<Class::MOP::Class>, keyed by the package name.
949 =item B<Class::MOP::get_all_metaclass_instances>
951 This will return a list of all the metaclass instances that have
952 been cached by L<Class::MOP::Class>.
954 =item B<Class::MOP::get_all_metaclass_names>
956 This will return a list of all the metaclass names that have
957 been cached by L<Class::MOP::Class>.
959 =item B<Class::MOP::get_metaclass_by_name($name)>
961 This will return a cached L<Class::MOP::Class> instance, or nothing
962 if no metaclass exists with that C<$name>.
964 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
966 This will store a metaclass in the cache at the supplied C<$key>.
968 =item B<Class::MOP::weaken_metaclass($name)>
970 In rare cases (e.g. anonymous metaclasses) it is desirable to
971 store a weakened reference in the metaclass cache. This
972 function will weaken the reference to the metaclass stored
975 =item B<Class::MOP::metaclass_is_weak($name)>
977 Returns true if the metaclass for C<$name> has been weakened
978 (via C<weaken_metaclass>).
980 =item B<Class::MOP::does_metaclass_exist($name)>
982 This will return true of there exists a metaclass stored in the
983 C<$name> key, and return false otherwise.
985 =item B<Class::MOP::remove_metaclass_by_name($name)>
987 This will remove the metaclass stored in the C<$name> key.
991 Some utility functions (such as C<Class::MOP::load_class>) that were
992 previously defined in C<Class::MOP> regarding loading of classes have been
993 extracted to L<Class::Load>. Please see L<Class::Load> for documentation.
999 There are very few books out on Meta Object Protocols and Metaclasses
1000 because it is such an esoteric topic. The following books are really
1001 the only ones I have found. If you know of any more, B<I<please>>
1002 email me and let me know, I would love to hear about them.
1006 =item I<The Art of the Meta Object Protocol>
1008 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1010 =item I<Putting MetaClasses to Work>
1012 =item I<Smalltalk: The Language>
1020 =item "Uniform and safe metaclass composition"
1022 An excellent paper by the people who brought us the original Traits paper.
1023 This paper is on how Traits can be used to do safe metaclass composition,
1024 and offers an excellent introduction section which delves into the topic of
1025 metaclass compatibility.
1027 L<http://scg.unibe.ch/archive/papers/Duca05ySafeMetaclassTrait.pdf>
1029 =item "Safe Metaclass Programming"
1031 This paper seems to precede the above paper, and propose a mix-in based
1032 approach as opposed to the Traits based approach. Both papers have similar
1033 information on the metaclass compatibility problem space.
1035 L<http://citeseer.ist.psu.edu/37617.html>
1043 =item The Perl 6 MetaModel work in the Pugs project
1047 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1049 =item L<http://github.com/perl6/p5-modules/tree/master/Perl6-ObjectSpace/>
1059 =item CPAN Module Review of Class::MOP
1061 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1065 =head1 SIMILAR MODULES
1067 As I have said above, this module is a class-builder-builder, so it is
1068 not the same thing as modules like L<Class::Accessor> and
1069 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1070 with similar goals to this module. The one I have found which is most
1071 like this module is L<Class::Meta>, although its philosophy and the MOP it
1072 creates are very different from this modules.
1076 All complex software has bugs lurking in it, and this module is no
1079 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1080 web interface at L<http://rt.cpan.org>.
1082 You can also discuss feature requests or possible bugs on the Moose
1083 mailing list (moose@perl.org) or on IRC at
1084 L<irc://irc.perl.org/#moose>.
1086 =head1 ACKNOWLEDGEMENTS
1092 Thanks to Rob for actually getting the development of this module kick-started.