12 use Devel::GlobalDestruction qw( in_global_destruction );
13 use Scalar::Util 'weaken', 'reftype', 'blessed';
14 use Sub::Name qw( subname );
16 use Class::MOP::Class;
17 use Class::MOP::Attribute;
18 use Class::MOP::Method;
20 use Class::MOP::Immutable;
23 *IS_RUNNING_ON_5_10 = ($] < 5.009_005)
27 *HAVE_ISAREV = defined(&mro::get_isarev)
31 # this is either part of core or set up appropriately by MRO::Compat
32 *check_package_cache_flag = \&mro::get_pkg_gen;
35 our $VERSION = '0.79';
36 our $XS_VERSION = $VERSION;
37 $VERSION = eval $VERSION;
38 our $AUTHORITY = 'cpan:STEVAN';
41 XSLoader::load( __PACKAGE__, $XS_VERSION );
45 # Metaclasses are singletons, so we cache them here.
46 # there is no need to worry about destruction though
47 # because they should die only when the program dies.
48 # After all, do package definitions even get reaped?
51 # means of accessing all the metaclasses that have
52 # been initialized thus far (for mugwumps obj browser)
53 sub get_all_metaclasses { %METAS }
54 sub get_all_metaclass_instances { values %METAS }
55 sub get_all_metaclass_names { keys %METAS }
56 sub get_metaclass_by_name { $METAS{$_[0]} }
57 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
58 sub weaken_metaclass { weaken($METAS{$_[0]}) }
59 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
60 sub remove_metaclass_by_name { $METAS{$_[0]} = undef }
63 # We only cache metaclasses, meaning instances of
64 # Class::MOP::Class. We do not cache instance of
65 # Class::MOP::Package or Class::MOP::Module. Mostly
66 # because I don't yet see a good reason to do so.
69 sub load_first_existing_class {
73 foreach my $class (@classes) {
74 unless ( _is_valid_class_name($class) ) {
75 my $display = defined($class) ? $class : 'undef';
76 confess "Invalid class name ($display)";
82 for my $class (@classes) {
83 my $e = _try_load_one_class($class);
86 $exceptions{$class} = $e;
94 return $found if $found;
100 "Could not load class (%s) because : %s", $_,
107 sub _try_load_one_class {
110 return if is_class_loaded($class);
112 my $file = $class . '.pm';
117 eval { require($file) };
123 my $class = load_first_existing_class($_[0]);
124 return get_metaclass_by_name($class) || $class;
127 sub _is_valid_class_name {
130 return 0 if ref($class);
131 return 0 unless defined($class);
132 return 0 unless length($class);
134 return 1 if $class =~ /^\w+(?:::\w+)*$/;
140 my $class = blessed($_[0]) || $_[0];
142 return undef if !defined($class);
143 return get_metaclass_by_name($class);
146 ## ----------------------------------------------------------------------------
147 ## Setting up our environment ...
148 ## ----------------------------------------------------------------------------
149 ## Class::MOP needs to have a few things in the global perl environment so
150 ## that it can operate effectively. Those things are done here.
151 ## ----------------------------------------------------------------------------
153 # ... nothing yet actually ;)
155 ## ----------------------------------------------------------------------------
157 ## ----------------------------------------------------------------------------
158 ## The code below here is to bootstrap our MOP with itself. This is also
159 ## sometimes called "tying the knot". By doing this, we make it much easier
160 ## to extend the MOP through subclassing and such since now you can use the
161 ## MOP itself to extend itself.
163 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
164 ## ----------------------------------------------------------------------------
166 # We need to add in the meta-attributes here so that
167 # any subclass of Class::MOP::* will be able to
168 # inherit them using &construct_instance
170 ## --------------------------------------------------------
171 ## Class::MOP::Package
173 Class::MOP::Package->meta->add_attribute(
174 Class::MOP::Attribute->new('package' => (
176 # NOTE: we need to do this in order
177 # for the instance meta-object to
178 # not fall into meta-circular death
180 # we just alias the original method
181 # rather than re-produce it here
182 'name' => \&Class::MOP::Package::name
187 Class::MOP::Package->meta->add_attribute(
188 Class::MOP::Attribute->new('namespace' => (
191 # we just alias the original method
192 # rather than re-produce it here
193 'namespace' => \&Class::MOP::Package::namespace
196 default => sub { \undef }
200 ## --------------------------------------------------------
201 ## Class::MOP::Module
204 # yeah this is kind of stretching things a bit,
205 # but truthfully the version should be an attribute
206 # of the Module, the weirdness comes from having to
207 # stick to Perl 5 convention and store it in the
208 # $VERSION package variable. Basically if you just
209 # squint at it, it will look how you want it to look.
210 # Either as a package variable, or as a attribute of
211 # the metaclass, isn't abstraction great :)
213 Class::MOP::Module->meta->add_attribute(
214 Class::MOP::Attribute->new('version' => (
217 # we just alias the original method
218 # rather than re-produce it here
219 'version' => \&Class::MOP::Module::version
222 default => sub { \undef }
227 # By following the same conventions as version here,
228 # we are opening up the possibility that people can
229 # use the $AUTHORITY in non-Class::MOP modules as
232 Class::MOP::Module->meta->add_attribute(
233 Class::MOP::Attribute->new('authority' => (
236 # we just alias the original method
237 # rather than re-produce it here
238 'authority' => \&Class::MOP::Module::authority
241 default => sub { \undef }
245 ## --------------------------------------------------------
248 Class::MOP::Class->meta->add_attribute(
249 Class::MOP::Attribute->new('attributes' => (
251 # NOTE: we need to do this in order
252 # for the instance meta-object to
253 # not fall into meta-circular death
255 # we just alias the original method
256 # rather than re-produce it here
257 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
259 default => sub { {} }
263 Class::MOP::Class->meta->add_attribute(
264 Class::MOP::Attribute->new('methods' => (
267 # we just alias the original method
268 # rather than re-produce it here
269 'get_method_map' => \&Class::MOP::Class::get_method_map
271 default => sub { {} }
275 Class::MOP::Class->meta->add_attribute(
276 Class::MOP::Attribute->new('superclasses' => (
279 # we just alias the original method
280 # rather than re-produce it here
281 'superclasses' => \&Class::MOP::Class::superclasses
284 default => sub { \undef }
288 Class::MOP::Class->meta->add_attribute(
289 Class::MOP::Attribute->new('attribute_metaclass' => (
292 # we just alias the original method
293 # rather than re-produce it here
294 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
296 default => 'Class::MOP::Attribute',
300 Class::MOP::Class->meta->add_attribute(
301 Class::MOP::Attribute->new('method_metaclass' => (
304 # we just alias the original method
305 # rather than re-produce it here
306 'method_metaclass' => \&Class::MOP::Class::method_metaclass
308 default => 'Class::MOP::Method',
312 Class::MOP::Class->meta->add_attribute(
313 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
316 # we just alias the original method
317 # rather than re-produce it here
318 'wrapped_method_metaclass' => \&Class::MOP::Class::wrapped_method_metaclass
320 default => 'Class::MOP::Method::Wrapped',
324 Class::MOP::Class->meta->add_attribute(
325 Class::MOP::Attribute->new('instance_metaclass' => (
327 # NOTE: we need to do this in order
328 # for the instance meta-object to
329 # not fall into meta-circular death
331 # we just alias the original method
332 # rather than re-produce it here
333 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
335 default => 'Class::MOP::Instance',
339 Class::MOP::Class->meta->add_attribute(
340 Class::MOP::Attribute->new('immutable_transformer' => (
342 'immutable_transformer' => \&Class::MOP::Class::immutable_transformer
345 '_set_immutable_transformer' => \&Class::MOP::Class::_set_immutable_transformer
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::Attribute
359 Class::MOP::Attribute->meta->add_attribute(
360 Class::MOP::Attribute->new('name' => (
362 # NOTE: we need to do this in order
363 # for the instance meta-object to
364 # not fall into meta-circular death
366 # we just alias the original method
367 # rather than re-produce it here
368 'name' => \&Class::MOP::Attribute::name
373 Class::MOP::Attribute->meta->add_attribute(
374 Class::MOP::Attribute->new('associated_class' => (
376 # NOTE: we need to do this in order
377 # for the instance meta-object to
378 # not fall into meta-circular death
380 # we just alias the original method
381 # rather than re-produce it here
382 'associated_class' => \&Class::MOP::Attribute::associated_class
387 Class::MOP::Attribute->meta->add_attribute(
388 Class::MOP::Attribute->new('accessor' => (
389 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
390 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
394 Class::MOP::Attribute->meta->add_attribute(
395 Class::MOP::Attribute->new('reader' => (
396 reader => { 'reader' => \&Class::MOP::Attribute::reader },
397 predicate => { 'has_reader' => \&Class::MOP::Attribute::has_reader },
401 Class::MOP::Attribute->meta->add_attribute(
402 Class::MOP::Attribute->new('initializer' => (
403 reader => { 'initializer' => \&Class::MOP::Attribute::initializer },
404 predicate => { 'has_initializer' => \&Class::MOP::Attribute::has_initializer },
408 Class::MOP::Attribute->meta->add_attribute(
409 Class::MOP::Attribute->new('definition_context' => (
410 reader => { 'definition_context' => \&Class::MOP::Attribute::definition_context },
414 Class::MOP::Attribute->meta->add_attribute(
415 Class::MOP::Attribute->new('writer' => (
416 reader => { 'writer' => \&Class::MOP::Attribute::writer },
417 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
421 Class::MOP::Attribute->meta->add_attribute(
422 Class::MOP::Attribute->new('predicate' => (
423 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
424 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
428 Class::MOP::Attribute->meta->add_attribute(
429 Class::MOP::Attribute->new('clearer' => (
430 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
431 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
435 Class::MOP::Attribute->meta->add_attribute(
436 Class::MOP::Attribute->new('builder' => (
437 reader => { 'builder' => \&Class::MOP::Attribute::builder },
438 predicate => { 'has_builder' => \&Class::MOP::Attribute::has_builder },
442 Class::MOP::Attribute->meta->add_attribute(
443 Class::MOP::Attribute->new('init_arg' => (
444 reader => { 'init_arg' => \&Class::MOP::Attribute::init_arg },
445 predicate => { 'has_init_arg' => \&Class::MOP::Attribute::has_init_arg },
449 Class::MOP::Attribute->meta->add_attribute(
450 Class::MOP::Attribute->new('default' => (
451 # default has a custom 'reader' method ...
452 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
456 Class::MOP::Attribute->meta->add_attribute(
457 Class::MOP::Attribute->new('associated_methods' => (
458 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
459 default => sub { [] }
463 Class::MOP::Attribute->meta->add_method('clone' => sub {
465 $self->meta->clone_object($self, @_);
468 ## --------------------------------------------------------
469 ## Class::MOP::Method
470 Class::MOP::Method->meta->add_attribute(
471 Class::MOP::Attribute->new('body' => (
472 reader => { 'body' => \&Class::MOP::Method::body },
476 Class::MOP::Method->meta->add_attribute(
477 Class::MOP::Attribute->new('associated_metaclass' => (
478 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
482 Class::MOP::Method->meta->add_attribute(
483 Class::MOP::Attribute->new('package_name' => (
484 reader => { 'package_name' => \&Class::MOP::Method::package_name },
488 Class::MOP::Method->meta->add_attribute(
489 Class::MOP::Attribute->new('name' => (
490 reader => { 'name' => \&Class::MOP::Method::name },
494 Class::MOP::Method->meta->add_attribute(
495 Class::MOP::Attribute->new('original_method' => (
496 reader => { 'original_method' => \&Class::MOP::Method::original_method },
497 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
501 Class::MOP::Method->meta->add_method('clone' => sub {
503 my $clone = $self->meta->clone_object($self, @_);
504 $clone->_set_original_method($self);
508 ## --------------------------------------------------------
509 ## Class::MOP::Method::Wrapped
512 # the way this item is initialized, this
513 # really does not follow the standard
514 # practices of attributes, but we put
515 # it here for completeness
516 Class::MOP::Method::Wrapped->meta->add_attribute(
517 Class::MOP::Attribute->new('modifier_table')
520 ## --------------------------------------------------------
521 ## Class::MOP::Method::Generated
523 Class::MOP::Method::Generated->meta->add_attribute(
524 Class::MOP::Attribute->new('is_inline' => (
525 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
530 Class::MOP::Method::Generated->meta->add_attribute(
531 Class::MOP::Attribute->new('definition_context' => (
532 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
536 ## --------------------------------------------------------
537 ## Class::MOP::Method::Accessor
539 Class::MOP::Method::Accessor->meta->add_attribute(
540 Class::MOP::Attribute->new('attribute' => (
542 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
547 Class::MOP::Method::Accessor->meta->add_attribute(
548 Class::MOP::Attribute->new('accessor_type' => (
549 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
553 ## --------------------------------------------------------
554 ## Class::MOP::Method::Constructor
556 Class::MOP::Method::Constructor->meta->add_attribute(
557 Class::MOP::Attribute->new('options' => (
559 'options' => \&Class::MOP::Method::Constructor::options
561 default => sub { +{} }
565 Class::MOP::Method::Constructor->meta->add_attribute(
566 Class::MOP::Attribute->new('associated_metaclass' => (
567 init_arg => "metaclass", # FIXME alias and rename
569 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
574 ## --------------------------------------------------------
575 ## Class::MOP::Instance
578 # these don't yet do much of anything, but are just
579 # included for completeness
581 Class::MOP::Instance->meta->add_attribute(
582 Class::MOP::Attribute->new('associated_metaclass',
583 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
587 Class::MOP::Instance->meta->add_attribute(
588 Class::MOP::Attribute->new('_class_name',
590 reader => { _class_name => \&Class::MOP::Instance::_class_name },
591 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
592 #default => sub { $_[0]->associated_metaclass->name },
596 Class::MOP::Instance->meta->add_attribute(
597 Class::MOP::Attribute->new('attributes',
598 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
602 Class::MOP::Instance->meta->add_attribute(
603 Class::MOP::Attribute->new('slots',
604 reader => { slots => \&Class::MOP::Instance::slots },
608 Class::MOP::Instance->meta->add_attribute(
609 Class::MOP::Attribute->new('slot_hash',
610 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
615 # we need the meta instance of the meta instance to be created now, in order
616 # for the constructor to be able to use it
617 Class::MOP::Instance->meta->get_meta_instance;
619 # pretend the add_method never happenned. it hasn't yet affected anything
620 undef Class::MOP::Instance->meta->{_package_cache_flag};
622 ## --------------------------------------------------------
623 ## Now close all the Class::MOP::* classes
625 # NOTE: we don't need to inline the the accessors this only lengthens
626 # the compile time of the MOP, and gives us no actual benefits.
628 $_->meta->make_immutable(
629 inline_constructor => 1,
630 replace_constructor => 1,
631 constructor_name => "_new",
632 inline_accessors => 0,
638 Class::MOP::Attribute
644 Class::MOP::Method::Generated
646 Class::MOP::Method::Accessor
647 Class::MOP::Method::Constructor
648 Class::MOP::Method::Wrapped
659 Class::MOP - A Meta Object Protocol for Perl 5
663 This module is a fully functioning meta object protocol for the
664 Perl 5 object system. It makes no attempt to change the behavior or
665 characteristics of the Perl 5 object system, only to create a
666 protocol for its manipulation and introspection.
668 That said, it does attempt to create the tools for building a rich set
669 of extensions to the Perl 5 object system. Every attempt has been made
670 to abide by the spirit of the Perl 5 object system that we all know
673 This documentation is sparse on conceptual details. We suggest looking
674 at the items listed in the L<SEE ALSO> section for more
675 information. In particular the book "The Art of the Meta Object
676 Protocol" was very influential in the development of this system.
678 =head2 What is a Meta Object Protocol?
680 A meta object protocol is an API to an object system.
682 To be more specific, it abstracts the components of an object system
683 (classes, object, methods, object attributes, etc.). These
684 abstractions can then be used to inspect and manipulate the object
685 system which they describe.
687 It can be said that there are two MOPs for any object system; the
688 implicit MOP and the explicit MOP. The implicit MOP handles things
689 like method dispatch or inheritance, which happen automatically as
690 part of how the object system works. The explicit MOP typically
691 handles the introspection/reflection features of the object system.
693 All object systems have implicit MOPs. Without one, they would not
694 work. Explicit MOPs are much less common, and depending on the
695 language can vary from restrictive (Reflection in Java or C#) to wide
696 open (CLOS is a perfect example).
698 =head2 Yet Another Class Builder! Why?
700 This is B<not> a class builder so much as a I<class builder
701 B<builder>>. The intent is that an end user will not use this module
702 directly, but instead this module is used by module authors to build
703 extensions and features onto the Perl 5 object system.
705 This system is used by L<Moose>, which supplies a powerful class
706 builder system built entirely on top of C<Class::MOP>.
708 =head2 Who is this module for?
710 This module is for anyone who has ever created or wanted to create a
711 module for the Class:: namespace. The tools which this module provides
712 make doing complex Perl 5 wizardry simpler, by removing such barriers
713 as the need to hack symbol tables, or understand the fine details of
716 =head2 What changes do I have to make to use this module?
718 This module was designed to be as unintrusive as possible. Many of its
719 features are accessible without B<any> change to your existing
720 code. It is meant to be a compliment to your existing code and not an
721 intrusion on your code base. Unlike many other B<Class::> modules,
722 this module B<does not> require you subclass it, or even that you
723 C<use> it in within your module's package.
725 The only features which requires additions to your code are the
726 attribute handling and instance construction features, and these are
727 both completely optional features. The only reason for this is because
728 Perl 5's object system does not actually have these features built
729 in. More information about this feature can be found below.
731 =head2 About Performance
733 It is a common misconception that explicit MOPs are a performance hit.
734 This is not a universal truth, it is a side-effect of some specific
735 implementations. For instance, using Java reflection is slow because
736 the JVM cannot take advantage of any compiler optimizations, and the
737 JVM has to deal with much more runtime type information as well.
739 Reflection in C# is marginally better as it was designed into the
740 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
741 Object System) was built to support an explicit MOP, and so
742 performance is tuned for it.
744 This library in particular does its absolute best to avoid putting
745 B<any> drain at all upon your code's performance. In fact, by itself
746 it does nothing to affect your existing code. So you only pay for what
749 =head2 About Metaclass compatibility
751 This module makes sure that all metaclasses created are both upwards
752 and downwards compatible. The topic of metaclass compatibility is
753 highly esoteric and is something only encountered when doing deep and
754 involved metaclass hacking. There are two basic kinds of metaclass
755 incompatibility; upwards and downwards.
757 Upwards metaclass compatibility means that the metaclass of a
758 given class is either the same as (or a subclass of) all of the
761 Downward metaclass compatibility means that the metaclasses of a
762 given class's ancestors are all either the same as (or a subclass
765 Here is a diagram showing a set of two classes (C<A> and C<B>) and
766 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
767 metaclass compatibility both upwards and downwards.
769 +---------+ +---------+
770 | Meta::A |<----| Meta::B | <....... (instance of )
771 +---------+ +---------+ <------- (inherits from)
774 +---------+ +---------+
776 +---------+ +---------+
778 As I said this is a highly esoteric topic and one you will only run
779 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
780 are interested in why this is an issue see the paper I<Uniform and
781 safe metaclass composition> linked to in the L<SEE ALSO> section of
784 =head2 Using custom metaclasses
786 Always use the L<metaclass> pragma when using a custom metaclass, this
787 will ensure the proper initialization order and not accidentally
788 create an incorrect type of metaclass for you. This is a very rare
789 problem, and one which can only occur if you are doing deep metaclass
790 programming. So in other words, don't worry about it.
792 Note that if you're using L<Moose> we encourage you to I<not> use
793 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
794 roles to a class's metaclasses. This topic is covered at length in
795 various L<Moose::Cookbook> recipes.
799 The meta-object protocol is divided into 4 main sub-protocols:
801 =head2 The Class protocol
803 This provides a means of manipulating and introspecting a Perl 5
804 class. It handles symbol table hacking for you, and provides a rich
805 set of methods that go beyond simple package introspection.
807 See L<Class::MOP::Class> for more details.
809 =head2 The Attribute protocol
811 This provides a consistent representation for an attribute of a Perl 5
812 class. Since there are so many ways to create and handle attributes in
813 Perl 5 OO, the Attribute protocol provide as much of a unified
814 approach as possible. Of course, you are always free to extend this
815 protocol by subclassing the appropriate classes.
817 See L<Class::MOP::Attribute> for more details.
819 =head2 The Method protocol
821 This provides a means of manipulating and introspecting methods in the
822 Perl 5 object system. As with attributes, there are many ways to
823 approach this topic, so we try to keep it pretty basic, while still
824 making it possible to extend the system in many ways.
826 See L<Class::MOP::Method> for more details.
828 =head2 The Instance protocol
830 This provides a layer of abstraction for creating object instances.
831 Since the other layers use this protocol, it is relatively easy to
832 change the type of your instances from the default hash reference to
833 some other type of reference. Several examples are provided in the
834 F<examples/> directory included in this distribution.
836 See L<Class::MOP::Instance> for more details.
840 Note that this module does not export any constants or functions.
846 =item I<Class::MOP::IS_RUNNING_ON_5_10>
848 We set this constant depending on what version perl we are on, this
849 allows us to take advantage of new 5.10 features and stay backwards
852 =item I<Class::MOP::HAVE_ISAREV>
854 Whether or not the L<mro> pragma provides C<get_isarev>, a much faster
855 way to get all the subclasses of a certain class.
859 =head2 Utility functions
861 Note that these are all called as B<functions, not methods>.
865 =item B<Class::MOP::load_class($class_name)>
867 This will load the specified C<$class_name>. This function can be used
868 in place of tricks like C<eval "use $module"> or using C<require>
871 =item B<Class::MOP::is_class_loaded($class_name)>
873 Returns a boolean indicating whether or not C<$class_name> has been
876 This does a basic check of the symbol table to try and determine as
877 best it can if the C<$class_name> is loaded, it is probably correct
878 about 99% of the time, but it can be fooled into reporting false
881 =item B<Class::MOP::get_code_info($code)>
883 This function returns two values, the name of the package the C<$code>
884 is from and the name of the C<$code> itself. This is used by several
885 elements of the MOP to determine where a given C<$code> reference is
888 =item B<Class::MOP::class_of($instance_or_class_name)>
890 This will return the metaclass of the given instance or class name.
891 Even if the class lacks a metaclass, no metaclass will be initialized
892 and C<undef> will be returned.
894 =item B<Class::MOP::check_package_cache_flag($pkg)>
896 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
898 This will return an integer that is managed by L<Class::MOP::Class> to
899 determine if a module's symbol table has been altered.
901 In Perl 5.10 or greater, this flag is package specific. However in
902 versions prior to 5.10, this will use the C<PL_sub_generation>
903 variable which is not package specific.
905 =item B<Class::MOP::load_first_existing_class(@class_names)>
907 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
909 Given a list of class names, this function will attempt to load each
912 If it finds a class it can load, it will return that class' name. If
913 none of the classes can be loaded, it will throw an exception.
917 =head2 Metaclass cache functions
919 Class::MOP holds a cache of metaclasses. The following are functions
920 (B<not methods>) which can be used to access that cache. It is not
921 recommended that you mess with these. Bad things could happen, but if
922 you are brave and willing to risk it: go for it!
926 =item B<Class::MOP::get_all_metaclasses>
928 This will return a hash of all the metaclass instances that have
929 been cached by L<Class::MOP::Class>, keyed by the package name.
931 =item B<Class::MOP::get_all_metaclass_instances>
933 This will return a list of all the metaclass instances that have
934 been cached by L<Class::MOP::Class>.
936 =item B<Class::MOP::get_all_metaclass_names>
938 This will return a list of all the metaclass names that have
939 been cached by L<Class::MOP::Class>.
941 =item B<Class::MOP::get_metaclass_by_name($name)>
943 This will return a cached L<Class::MOP::Class> instance, or nothing
944 if no metaclass exists with that C<$name>.
946 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
948 This will store a metaclass in the cache at the supplied C<$key>.
950 =item B<Class::MOP::weaken_metaclass($name)>
952 In rare cases (e.g. anonymous metaclasses) it is desirable to
953 store a weakened reference in the metaclass cache. This
954 function will weaken the reference to the metaclass stored
957 =item B<Class::MOP::does_metaclass_exist($name)>
959 This will return true of there exists a metaclass stored in the
960 C<$name> key, and return false otherwise.
962 =item B<Class::MOP::remove_metaclass_by_name($name)>
964 This will remove the metaclass stored in the C<$name> key.
972 There are very few books out on Meta Object Protocols and Metaclasses
973 because it is such an esoteric topic. The following books are really
974 the only ones I have found. If you know of any more, B<I<please>>
975 email me and let me know, I would love to hear about them.
979 =item I<The Art of the Meta Object Protocol>
981 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
983 =item I<Putting MetaClasses to Work>
985 =item I<Smalltalk: The Language>
993 =item "Uniform and safe metaclass composition"
995 An excellent paper by the people who brought us the original Traits paper.
996 This paper is on how Traits can be used to do safe metaclass composition,
997 and offers an excellent introduction section which delves into the topic of
998 metaclass compatibility.
1000 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1002 =item "Safe Metaclass Programming"
1004 This paper seems to precede the above paper, and propose a mix-in based
1005 approach as opposed to the Traits based approach. Both papers have similar
1006 information on the metaclass compatibility problem space.
1008 L<http://citeseer.ist.psu.edu/37617.html>
1016 =item The Perl 6 MetaModel work in the Pugs project
1020 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
1022 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1032 =item CPAN Module Review of Class::MOP
1034 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1038 =head1 SIMILAR MODULES
1040 As I have said above, this module is a class-builder-builder, so it is
1041 not the same thing as modules like L<Class::Accessor> and
1042 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1043 with similar goals to this module. The one I have found which is most
1044 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1045 creates are very different from this modules.
1049 All complex software has bugs lurking in it, and this module is no
1050 exception. If you find a bug please either email me, or add the bug
1053 =head1 ACKNOWLEDGEMENTS
1059 Thanks to Rob for actually getting the development of this module kick-started.
1065 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1067 B<with contributions from:>
1069 Brandon (blblack) Black
1071 Florian (rafl) Ragwitz
1073 Guillermo (groditi) Roditi
1077 Rob (robkinyon) Kinyon
1079 Yuval (nothingmuch) Kogman
1081 Scott (konobi) McWhirter
1083 =head1 COPYRIGHT AND LICENSE
1085 Copyright 2006-2009 by Infinity Interactive, Inc.
1087 L<http://www.iinteractive.com>
1089 This library is free software; you can redistribute it and/or modify
1090 it under the same terms as Perl itself.