12 use Scalar::Util 'weaken', 'reftype', 'blessed';
15 use Class::MOP::Mixin::AttributeCore;
16 use Class::MOP::Mixin::HasAttributes;
17 use Class::MOP::Mixin::HasMethods;
18 use Class::MOP::Class;
19 use Class::MOP::Attribute;
20 use Class::MOP::Method;
23 *IS_RUNNING_ON_5_10 = ($] < 5.009_005)
27 # this is either part of core or set up appropriately by MRO::Compat
28 *check_package_cache_flag = \&mro::get_pkg_gen;
31 our $VERSION = '0.98';
32 our $XS_VERSION = $VERSION;
33 $VERSION = eval $VERSION;
34 our $AUTHORITY = 'cpan:STEVAN';
37 XSLoader::load( __PACKAGE__, $XS_VERSION );
41 # Metaclasses are singletons, so we cache them here.
42 # there is no need to worry about destruction though
43 # because they should die only when the program dies.
44 # After all, do package definitions even get reaped?
45 # Anonymous classes manage their own destruction.
48 sub get_all_metaclasses { %METAS }
49 sub get_all_metaclass_instances { values %METAS }
50 sub get_all_metaclass_names { keys %METAS }
51 sub get_metaclass_by_name { $METAS{$_[0]} }
52 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
53 sub weaken_metaclass { weaken($METAS{$_[0]}) }
54 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
55 sub remove_metaclass_by_name { delete $METAS{$_[0]}; return }
57 # This handles instances as well as class names
59 return unless defined $_[0];
60 my $class = blessed($_[0]) || $_[0];
61 return $METAS{$class};
65 # We only cache metaclasses, meaning instances of
66 # Class::MOP::Class. We do not cache instance of
67 # Class::MOP::Package or Class::MOP::Module. Mostly
68 # because I don't yet see a good reason to do so.
71 sub _class_to_pmfile {
74 my $file = $class . '.pm';
80 sub load_first_existing_class {
84 foreach my $class (@classes) {
85 unless ( _is_valid_class_name($class) ) {
86 my $display = defined($class) ? $class : 'undef';
87 confess "Invalid class name ($display)";
94 for my $class (@classes) {
95 my $file = _class_to_pmfile($class);
97 return $class if is_class_loaded($class);;
99 return $class if try {
105 unless (/^Can't locate \Q$file\E in \@INC/) {
106 confess "Couldn't load class ($class) because: $_";
113 if ( @classes > 1 ) {
114 confess "Can't locate any of @classes in \@INC (\@INC contains: @INC).";
116 confess "Can't locate " . _class_to_pmfile($classes[0]) . " in \@INC (\@INC contains: @INC).";
121 load_first_existing_class($_[0]);
123 # This is done to avoid breaking code which checked the return value. Said
124 # code is dumb. The return value was _always_ true, since it dies on
129 sub _is_valid_class_name {
132 return 0 if ref($class);
133 return 0 unless defined($class);
134 return 0 unless length($class);
136 return 1 if $class =~ /^\w+(?:::\w+)*$/;
141 ## ----------------------------------------------------------------------------
142 ## Setting up our environment ...
143 ## ----------------------------------------------------------------------------
144 ## Class::MOP needs to have a few things in the global perl environment so
145 ## that it can operate effectively. Those things are done here.
146 ## ----------------------------------------------------------------------------
148 # ... nothing yet actually ;)
150 ## ----------------------------------------------------------------------------
152 ## ----------------------------------------------------------------------------
153 ## The code below here is to bootstrap our MOP with itself. This is also
154 ## sometimes called "tying the knot". By doing this, we make it much easier
155 ## to extend the MOP through subclassing and such since now you can use the
156 ## MOP itself to extend itself.
158 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
159 ## ----------------------------------------------------------------------------
161 # We need to add in the meta-attributes here so that
162 # any subclass of Class::MOP::* will be able to
163 # inherit them using _construct_instance
165 ## --------------------------------------------------------
166 ## Class::MOP::Mixin::HasMethods
168 Class::MOP::Mixin::HasMethods->meta->add_attribute(
169 Class::MOP::Attribute->new('_methods' => (
172 # we just alias the original method
173 # rather than re-produce it here
174 '_full_method_map' => \&Class::MOP::Mixin::HasMethods::_full_method_map
176 default => sub { {} }
180 Class::MOP::Mixin::HasMethods->meta->add_attribute(
181 Class::MOP::Attribute->new('method_metaclass' => (
184 # we just alias the original method
185 # rather than re-produce it here
186 'method_metaclass' => \&Class::MOP::Mixin::HasMethods::method_metaclass
188 default => 'Class::MOP::Method',
192 Class::MOP::Mixin::HasMethods->meta->add_attribute(
193 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
196 # we just alias the original method
197 # rather than re-produce it here
198 'wrapped_method_metaclass' => \&Class::MOP::Mixin::HasMethods::wrapped_method_metaclass
200 default => 'Class::MOP::Method::Wrapped',
204 ## --------------------------------------------------------
205 ## Class::MOP::Mixin::HasMethods
207 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
208 Class::MOP::Attribute->new('attributes' => (
210 # NOTE: we need to do this in order
211 # for the instance meta-object to
212 # not fall into meta-circular death
214 # we just alias the original method
215 # rather than re-produce it here
216 '_attribute_map' => \&Class::MOP::Mixin::HasAttributes::_attribute_map
218 default => sub { {} }
222 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
223 Class::MOP::Attribute->new('attribute_metaclass' => (
226 # we just alias the original method
227 # rather than re-produce it here
228 'attribute_metaclass' => \&Class::MOP::Mixin::HasAttributes::attribute_metaclass
230 default => 'Class::MOP::Attribute',
234 ## --------------------------------------------------------
235 ## Class::MOP::Package
237 Class::MOP::Package->meta->add_attribute(
238 Class::MOP::Attribute->new('package' => (
240 # NOTE: we need to do this in order
241 # for the instance meta-object to
242 # not fall into meta-circular death
244 # we just alias the original method
245 # rather than re-produce it here
246 'name' => \&Class::MOP::Package::name
251 Class::MOP::Package->meta->add_attribute(
252 Class::MOP::Attribute->new('namespace' => (
255 # we just alias the original method
256 # rather than re-produce it here
257 'namespace' => \&Class::MOP::Package::namespace
260 default => sub { \undef }
264 ## --------------------------------------------------------
265 ## Class::MOP::Module
268 # yeah this is kind of stretching things a bit,
269 # but truthfully the version should be an attribute
270 # of the Module, the weirdness comes from having to
271 # stick to Perl 5 convention and store it in the
272 # $VERSION package variable. Basically if you just
273 # squint at it, it will look how you want it to look.
274 # Either as a package variable, or as a attribute of
275 # the metaclass, isn't abstraction great :)
277 Class::MOP::Module->meta->add_attribute(
278 Class::MOP::Attribute->new('version' => (
281 # we just alias the original method
282 # rather than re-produce it here
283 'version' => \&Class::MOP::Module::version
286 default => sub { \undef }
291 # By following the same conventions as version here,
292 # we are opening up the possibility that people can
293 # use the $AUTHORITY in non-Class::MOP modules as
296 Class::MOP::Module->meta->add_attribute(
297 Class::MOP::Attribute->new('authority' => (
300 # we just alias the original method
301 # rather than re-produce it here
302 'authority' => \&Class::MOP::Module::authority
305 default => sub { \undef }
309 ## --------------------------------------------------------
312 Class::MOP::Class->meta->add_attribute(
313 Class::MOP::Attribute->new('superclasses' => (
316 # we just alias the original method
317 # rather than re-produce it here
318 'superclasses' => \&Class::MOP::Class::superclasses
321 default => sub { \undef }
325 Class::MOP::Class->meta->add_attribute(
326 Class::MOP::Attribute->new('instance_metaclass' => (
328 # NOTE: we need to do this in order
329 # for the instance meta-object to
330 # not fall into meta-circular death
332 # we just alias the original method
333 # rather than re-produce it here
334 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
336 default => 'Class::MOP::Instance',
340 Class::MOP::Class->meta->add_attribute(
341 Class::MOP::Attribute->new('immutable_trait' => (
343 'immutable_trait' => \&Class::MOP::Class::immutable_trait
345 default => "Class::MOP::Class::Immutable::Trait",
349 Class::MOP::Class->meta->add_attribute(
350 Class::MOP::Attribute->new('constructor_name' => (
352 'constructor_name' => \&Class::MOP::Class::constructor_name,
358 Class::MOP::Class->meta->add_attribute(
359 Class::MOP::Attribute->new('constructor_class' => (
361 'constructor_class' => \&Class::MOP::Class::constructor_class,
363 default => "Class::MOP::Method::Constructor",
368 Class::MOP::Class->meta->add_attribute(
369 Class::MOP::Attribute->new('destructor_class' => (
371 'destructor_class' => \&Class::MOP::Class::destructor_class,
377 # we don't actually need to tie the knot with
378 # Class::MOP::Class here, it is actually handled
379 # within Class::MOP::Class itself in the
380 # _construct_class_instance method.
382 ## --------------------------------------------------------
383 ## Class::MOP::Mixin::AttributeCore
384 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
385 Class::MOP::Attribute->new('name' => (
387 # NOTE: we need to do this in order
388 # for the instance meta-object to
389 # not fall into meta-circular death
391 # we just alias the original method
392 # rather than re-produce it here
393 'name' => \&Class::MOP::Mixin::AttributeCore::name
398 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
399 Class::MOP::Attribute->new('accessor' => (
400 reader => { 'accessor' => \&Class::MOP::Mixin::AttributeCore::accessor },
401 predicate => { 'has_accessor' => \&Class::MOP::Mixin::AttributeCore::has_accessor },
405 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
406 Class::MOP::Attribute->new('reader' => (
407 reader => { 'reader' => \&Class::MOP::Mixin::AttributeCore::reader },
408 predicate => { 'has_reader' => \&Class::MOP::Mixin::AttributeCore::has_reader },
412 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
413 Class::MOP::Attribute->new('initializer' => (
414 reader => { 'initializer' => \&Class::MOP::Mixin::AttributeCore::initializer },
415 predicate => { 'has_initializer' => \&Class::MOP::Mixin::AttributeCore::has_initializer },
419 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
420 Class::MOP::Attribute->new('definition_context' => (
421 reader => { 'definition_context' => \&Class::MOP::Mixin::AttributeCore::definition_context },
425 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
426 Class::MOP::Attribute->new('writer' => (
427 reader => { 'writer' => \&Class::MOP::Mixin::AttributeCore::writer },
428 predicate => { 'has_writer' => \&Class::MOP::Mixin::AttributeCore::has_writer },
432 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
433 Class::MOP::Attribute->new('predicate' => (
434 reader => { 'predicate' => \&Class::MOP::Mixin::AttributeCore::predicate },
435 predicate => { 'has_predicate' => \&Class::MOP::Mixin::AttributeCore::has_predicate },
439 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
440 Class::MOP::Attribute->new('clearer' => (
441 reader => { 'clearer' => \&Class::MOP::Mixin::AttributeCore::clearer },
442 predicate => { 'has_clearer' => \&Class::MOP::Mixin::AttributeCore::has_clearer },
446 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
447 Class::MOP::Attribute->new('builder' => (
448 reader => { 'builder' => \&Class::MOP::Mixin::AttributeCore::builder },
449 predicate => { 'has_builder' => \&Class::MOP::Mixin::AttributeCore::has_builder },
453 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
454 Class::MOP::Attribute->new('init_arg' => (
455 reader => { 'init_arg' => \&Class::MOP::Mixin::AttributeCore::init_arg },
456 predicate => { 'has_init_arg' => \&Class::MOP::Mixin::AttributeCore::has_init_arg },
460 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
461 Class::MOP::Attribute->new('default' => (
462 # default has a custom 'reader' method ...
463 predicate => { 'has_default' => \&Class::MOP::Mixin::AttributeCore::has_default },
467 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
468 Class::MOP::Attribute->new('insertion_order' => (
469 reader => { 'insertion_order' => \&Class::MOP::Mixin::AttributeCore::insertion_order },
470 writer => { '_set_insertion_order' => \&Class::MOP::Mixin::AttributeCore::_set_insertion_order },
471 predicate => { 'has_insertion_order' => \&Class::MOP::Mixin::AttributeCore::has_insertion_order },
475 ## --------------------------------------------------------
476 ## Class::MOP::Attribute
477 Class::MOP::Attribute->meta->add_attribute(
478 Class::MOP::Attribute->new('associated_class' => (
480 # NOTE: we need to do this in order
481 # for the instance meta-object to
482 # not fall into meta-circular death
484 # we just alias the original method
485 # rather than re-produce it here
486 'associated_class' => \&Class::MOP::Attribute::associated_class
491 Class::MOP::Attribute->meta->add_attribute(
492 Class::MOP::Attribute->new('associated_methods' => (
493 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
494 default => sub { [] }
498 Class::MOP::Attribute->meta->add_method('clone' => sub {
500 $self->meta->clone_object($self, @_);
503 ## --------------------------------------------------------
504 ## Class::MOP::Method
505 Class::MOP::Method->meta->add_attribute(
506 Class::MOP::Attribute->new('body' => (
507 reader => { 'body' => \&Class::MOP::Method::body },
511 Class::MOP::Method->meta->add_attribute(
512 Class::MOP::Attribute->new('associated_metaclass' => (
513 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
517 Class::MOP::Method->meta->add_attribute(
518 Class::MOP::Attribute->new('package_name' => (
519 reader => { 'package_name' => \&Class::MOP::Method::package_name },
523 Class::MOP::Method->meta->add_attribute(
524 Class::MOP::Attribute->new('name' => (
525 reader => { 'name' => \&Class::MOP::Method::name },
529 Class::MOP::Method->meta->add_attribute(
530 Class::MOP::Attribute->new('original_method' => (
531 reader => { 'original_method' => \&Class::MOP::Method::original_method },
532 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
536 Class::MOP::Method->meta->add_method('clone' => sub {
538 my $clone = $self->meta->clone_object($self, @_);
539 $clone->_set_original_method($self);
543 ## --------------------------------------------------------
544 ## Class::MOP::Method::Wrapped
547 # the way this item is initialized, this
548 # really does not follow the standard
549 # practices of attributes, but we put
550 # it here for completeness
551 Class::MOP::Method::Wrapped->meta->add_attribute(
552 Class::MOP::Attribute->new('modifier_table')
555 ## --------------------------------------------------------
556 ## Class::MOP::Method::Generated
558 Class::MOP::Method::Generated->meta->add_attribute(
559 Class::MOP::Attribute->new('is_inline' => (
560 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
565 Class::MOP::Method::Generated->meta->add_attribute(
566 Class::MOP::Attribute->new('definition_context' => (
567 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
572 ## --------------------------------------------------------
573 ## Class::MOP::Method::Inlined
575 Class::MOP::Method::Inlined->meta->add_attribute(
576 Class::MOP::Attribute->new('_expected_method_class' => (
577 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
581 ## --------------------------------------------------------
582 ## Class::MOP::Method::Accessor
584 Class::MOP::Method::Accessor->meta->add_attribute(
585 Class::MOP::Attribute->new('attribute' => (
587 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
592 Class::MOP::Method::Accessor->meta->add_attribute(
593 Class::MOP::Attribute->new('accessor_type' => (
594 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
598 ## --------------------------------------------------------
599 ## Class::MOP::Method::Constructor
601 Class::MOP::Method::Constructor->meta->add_attribute(
602 Class::MOP::Attribute->new('options' => (
604 'options' => \&Class::MOP::Method::Constructor::options
606 default => sub { +{} }
610 Class::MOP::Method::Constructor->meta->add_attribute(
611 Class::MOP::Attribute->new('associated_metaclass' => (
612 init_arg => "metaclass", # FIXME alias and rename
614 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
619 ## --------------------------------------------------------
620 ## Class::MOP::Instance
623 # these don't yet do much of anything, but are just
624 # included for completeness
626 Class::MOP::Instance->meta->add_attribute(
627 Class::MOP::Attribute->new('associated_metaclass',
628 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
632 Class::MOP::Instance->meta->add_attribute(
633 Class::MOP::Attribute->new('_class_name',
635 reader => { _class_name => \&Class::MOP::Instance::_class_name },
636 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
637 #default => sub { $_[0]->associated_metaclass->name },
641 Class::MOP::Instance->meta->add_attribute(
642 Class::MOP::Attribute->new('attributes',
643 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
647 Class::MOP::Instance->meta->add_attribute(
648 Class::MOP::Attribute->new('slots',
649 reader => { slots => \&Class::MOP::Instance::slots },
653 Class::MOP::Instance->meta->add_attribute(
654 Class::MOP::Attribute->new('slot_hash',
655 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
659 require Class::MOP::Deprecated unless our $no_deprecated;
661 # we need the meta instance of the meta instance to be created now, in order
662 # for the constructor to be able to use it
663 Class::MOP::Instance->meta->get_meta_instance;
665 # pretend the add_method never happenned. it hasn't yet affected anything
666 undef Class::MOP::Instance->meta->{_package_cache_flag};
668 ## --------------------------------------------------------
669 ## Now close all the Class::MOP::* classes
671 # NOTE: we don't need to inline the the accessors this only lengthens
672 # the compile time of the MOP, and gives us no actual benefits.
674 $_->meta->make_immutable(
675 inline_constructor => 0,
676 constructor_name => "_new",
677 inline_accessors => 0,
683 Class::MOP::Attribute
689 Class::MOP::Method::Generated
690 Class::MOP::Method::Inlined
692 Class::MOP::Method::Accessor
693 Class::MOP::Method::Constructor
694 Class::MOP::Method::Wrapped
697 $_->meta->make_immutable(
698 inline_constructor => 0,
699 constructor_name => undef,
700 inline_accessors => 0,
703 Class::MOP::Mixin::AttributeCore
704 Class::MOP::Mixin::HasAttributes
705 Class::MOP::Mixin::HasMethods
716 Class::MOP - A Meta Object Protocol for Perl 5
720 This module is a fully functioning meta object protocol for the
721 Perl 5 object system. It makes no attempt to change the behavior or
722 characteristics of the Perl 5 object system, only to create a
723 protocol for its manipulation and introspection.
725 That said, it does attempt to create the tools for building a rich set
726 of extensions to the Perl 5 object system. Every attempt has been made
727 to abide by the spirit of the Perl 5 object system that we all know
730 This documentation is sparse on conceptual details. We suggest looking
731 at the items listed in the L<SEE ALSO> section for more
732 information. In particular the book "The Art of the Meta Object
733 Protocol" was very influential in the development of this system.
735 =head2 What is a Meta Object Protocol?
737 A meta object protocol is an API to an object system.
739 To be more specific, it abstracts the components of an object system
740 (classes, object, methods, object attributes, etc.). These
741 abstractions can then be used to inspect and manipulate the object
742 system which they describe.
744 It can be said that there are two MOPs for any object system; the
745 implicit MOP and the explicit MOP. The implicit MOP handles things
746 like method dispatch or inheritance, which happen automatically as
747 part of how the object system works. The explicit MOP typically
748 handles the introspection/reflection features of the object system.
750 All object systems have implicit MOPs. Without one, they would not
751 work. Explicit MOPs are much less common, and depending on the
752 language can vary from restrictive (Reflection in Java or C#) to wide
753 open (CLOS is a perfect example).
755 =head2 Yet Another Class Builder! Why?
757 This is B<not> a class builder so much as a I<class builder
758 B<builder>>. The intent is that an end user will not use this module
759 directly, but instead this module is used by module authors to build
760 extensions and features onto the Perl 5 object system.
762 This system is used by L<Moose>, which supplies a powerful class
763 builder system built entirely on top of C<Class::MOP>.
765 =head2 Who is this module for?
767 This module is for anyone who has ever created or wanted to create a
768 module for the Class:: namespace. The tools which this module provides
769 make doing complex Perl 5 wizardry simpler, by removing such barriers
770 as the need to hack symbol tables, or understand the fine details of
773 =head2 What changes do I have to make to use this module?
775 This module was designed to be as unintrusive as possible. Many of its
776 features are accessible without B<any> change to your existing
777 code. It is meant to be a compliment to your existing code and not an
778 intrusion on your code base. Unlike many other B<Class::> modules,
779 this module B<does not> require you subclass it, or even that you
780 C<use> it in within your module's package.
782 The only features which requires additions to your code are the
783 attribute handling and instance construction features, and these are
784 both completely optional features. The only reason for this is because
785 Perl 5's object system does not actually have these features built
786 in. More information about this feature can be found below.
788 =head2 About Performance
790 It is a common misconception that explicit MOPs are a performance hit.
791 This is not a universal truth, it is a side-effect of some specific
792 implementations. For instance, using Java reflection is slow because
793 the JVM cannot take advantage of any compiler optimizations, and the
794 JVM has to deal with much more runtime type information as well.
796 Reflection in C# is marginally better as it was designed into the
797 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
798 Object System) was built to support an explicit MOP, and so
799 performance is tuned for it.
801 This library in particular does its absolute best to avoid putting
802 B<any> drain at all upon your code's performance. In fact, by itself
803 it does nothing to affect your existing code. So you only pay for what
806 =head2 About Metaclass compatibility
808 This module makes sure that all metaclasses created are both upwards
809 and downwards compatible. The topic of metaclass compatibility is
810 highly esoteric and is something only encountered when doing deep and
811 involved metaclass hacking. There are two basic kinds of metaclass
812 incompatibility; upwards and downwards.
814 Upwards metaclass compatibility means that the metaclass of a
815 given class is either the same as (or a subclass of) all of the
818 Downward metaclass compatibility means that the metaclasses of a
819 given class's ancestors are all either the same as (or a subclass
822 Here is a diagram showing a set of two classes (C<A> and C<B>) and
823 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
824 metaclass compatibility both upwards and downwards.
826 +---------+ +---------+
827 | Meta::A |<----| Meta::B | <....... (instance of )
828 +---------+ +---------+ <------- (inherits from)
831 +---------+ +---------+
833 +---------+ +---------+
835 As I said this is a highly esoteric topic and one you will only run
836 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
837 are interested in why this is an issue see the paper I<Uniform and
838 safe metaclass composition> linked to in the L<SEE ALSO> section of
841 =head2 Using custom metaclasses
843 Always use the L<metaclass> pragma when using a custom metaclass, this
844 will ensure the proper initialization order and not accidentally
845 create an incorrect type of metaclass for you. This is a very rare
846 problem, and one which can only occur if you are doing deep metaclass
847 programming. So in other words, don't worry about it.
849 Note that if you're using L<Moose> we encourage you to I<not> use
850 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
851 roles to a class's metaclasses. This topic is covered at length in
852 various L<Moose::Cookbook> recipes.
856 The meta-object protocol is divided into 4 main sub-protocols:
858 =head2 The Class protocol
860 This provides a means of manipulating and introspecting a Perl 5
861 class. It handles symbol table hacking for you, and provides a rich
862 set of methods that go beyond simple package introspection.
864 See L<Class::MOP::Class> for more details.
866 =head2 The Attribute protocol
868 This provides a consistent representation for an attribute of a Perl 5
869 class. Since there are so many ways to create and handle attributes in
870 Perl 5 OO, the Attribute protocol provide as much of a unified
871 approach as possible. Of course, you are always free to extend this
872 protocol by subclassing the appropriate classes.
874 See L<Class::MOP::Attribute> for more details.
876 =head2 The Method protocol
878 This provides a means of manipulating and introspecting methods in the
879 Perl 5 object system. As with attributes, there are many ways to
880 approach this topic, so we try to keep it pretty basic, while still
881 making it possible to extend the system in many ways.
883 See L<Class::MOP::Method> for more details.
885 =head2 The Instance protocol
887 This provides a layer of abstraction for creating object instances.
888 Since the other layers use this protocol, it is relatively easy to
889 change the type of your instances from the default hash reference to
890 some other type of reference. Several examples are provided in the
891 F<examples/> directory included in this distribution.
893 See L<Class::MOP::Instance> for more details.
897 Note that this module does not export any constants or functions.
903 =item I<Class::MOP::IS_RUNNING_ON_5_10>
905 We set this constant depending on what version perl we are on, this
906 allows us to take advantage of new 5.10 features and stay backwards
911 =head2 Utility functions
913 Note that these are all called as B<functions, not methods>.
917 =item B<Class::MOP::load_class($class_name)>
919 This will load the specified C<$class_name>, if it is not already
920 loaded (as reported by C<is_class_loaded>). This function can be used
921 in place of tricks like C<eval "use $module"> or using C<require>
924 If the module cannot be loaded, an exception is thrown.
926 For historical reasons, this function explicitly returns a true value.
928 =item B<Class::MOP::is_class_loaded($class_name)>
930 Returns a boolean indicating whether or not C<$class_name> has been
933 This does a basic check of the symbol table to try and determine as
934 best it can if the C<$class_name> is loaded, it is probably correct
935 about 99% of the time, but it can be fooled into reporting false
936 positives. In particular, loading any of the core L<IO> modules will
937 cause most of the rest of the core L<IO> modules to falsely report
938 having been loaded, due to the way the base L<IO> module works.
940 =item B<Class::MOP::get_code_info($code)>
942 This function returns two values, the name of the package the C<$code>
943 is from and the name of the C<$code> itself. This is used by several
944 elements of the MOP to determine where a given C<$code> reference is
947 =item B<Class::MOP::class_of($instance_or_class_name)>
949 This will return the metaclass of the given instance or class name. If the
950 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
953 =item B<Class::MOP::check_package_cache_flag($pkg)>
955 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
957 This will return an integer that is managed by L<Class::MOP::Class> to
958 determine if a module's symbol table has been altered.
960 In Perl 5.10 or greater, this flag is package specific. However in
961 versions prior to 5.10, this will use the C<PL_sub_generation>
962 variable which is not package specific.
964 =item B<Class::MOP::load_first_existing_class(@class_names)>
966 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
968 Given a list of class names, this function will attempt to load each
971 If it finds a class it can load, it will return that class' name. If
972 none of the classes can be loaded, it will throw an exception.
976 =head2 Metaclass cache functions
978 Class::MOP holds a cache of metaclasses. The following are functions
979 (B<not methods>) which can be used to access that cache. It is not
980 recommended that you mess with these. Bad things could happen, but if
981 you are brave and willing to risk it: go for it!
985 =item B<Class::MOP::get_all_metaclasses>
987 This will return a hash of all the metaclass instances that have
988 been cached by L<Class::MOP::Class>, keyed by the package name.
990 =item B<Class::MOP::get_all_metaclass_instances>
992 This will return a list of all the metaclass instances that have
993 been cached by L<Class::MOP::Class>.
995 =item B<Class::MOP::get_all_metaclass_names>
997 This will return a list of all the metaclass names that have
998 been cached by L<Class::MOP::Class>.
1000 =item B<Class::MOP::get_metaclass_by_name($name)>
1002 This will return a cached L<Class::MOP::Class> instance, or nothing
1003 if no metaclass exists with that C<$name>.
1005 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
1007 This will store a metaclass in the cache at the supplied C<$key>.
1009 =item B<Class::MOP::weaken_metaclass($name)>
1011 In rare cases (e.g. anonymous metaclasses) it is desirable to
1012 store a weakened reference in the metaclass cache. This
1013 function will weaken the reference to the metaclass stored
1016 =item B<Class::MOP::does_metaclass_exist($name)>
1018 This will return true of there exists a metaclass stored in the
1019 C<$name> key, and return false otherwise.
1021 =item B<Class::MOP::remove_metaclass_by_name($name)>
1023 This will remove the metaclass stored in the C<$name> key.
1031 There are very few books out on Meta Object Protocols and Metaclasses
1032 because it is such an esoteric topic. The following books are really
1033 the only ones I have found. If you know of any more, B<I<please>>
1034 email me and let me know, I would love to hear about them.
1038 =item I<The Art of the Meta Object Protocol>
1040 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1042 =item I<Putting MetaClasses to Work>
1044 =item I<Smalltalk: The Language>
1052 =item "Uniform and safe metaclass composition"
1054 An excellent paper by the people who brought us the original Traits paper.
1055 This paper is on how Traits can be used to do safe metaclass composition,
1056 and offers an excellent introduction section which delves into the topic of
1057 metaclass compatibility.
1059 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1061 =item "Safe Metaclass Programming"
1063 This paper seems to precede the above paper, and propose a mix-in based
1064 approach as opposed to the Traits based approach. Both papers have similar
1065 information on the metaclass compatibility problem space.
1067 L<http://citeseer.ist.psu.edu/37617.html>
1075 =item The Perl 6 MetaModel work in the Pugs project
1079 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1081 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1091 =item CPAN Module Review of Class::MOP
1093 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1097 =head1 SIMILAR MODULES
1099 As I have said above, this module is a class-builder-builder, so it is
1100 not the same thing as modules like L<Class::Accessor> and
1101 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1102 with similar goals to this module. The one I have found which is most
1103 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1104 creates are very different from this modules.
1108 All complex software has bugs lurking in it, and this module is no
1111 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1112 web interface at L<http://rt.cpan.org>.
1114 You can also discuss feature requests or possible bugs on the Moose
1115 mailing list (moose@perl.org) or on IRC at
1116 L<irc://irc.perl.org/#moose>.
1118 =head1 ACKNOWLEDGEMENTS
1124 Thanks to Rob for actually getting the development of this module kick-started.
1130 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1132 B<with contributions from:>
1134 Brandon (blblack) Black
1136 Florian (rafl) Ragwitz
1138 Guillermo (groditi) Roditi
1140 Dave (autarch) Rolsky
1144 Rob (robkinyon) Kinyon
1146 Yuval (nothingmuch) Kogman
1148 Scott (konobi) McWhirter
1152 =head1 COPYRIGHT AND LICENSE
1154 Copyright 2006-2010 by Infinity Interactive, Inc.
1156 L<http://www.iinteractive.com>
1158 This library is free software; you can redistribute it and/or modify
1159 it under the same terms as Perl itself.