12 use Scalar::Util 'weaken', 'isweak', 'reftype', 'blessed';
16 use Class::MOP::Mixin::AttributeCore;
17 use Class::MOP::Mixin::HasAttributes;
18 use Class::MOP::Mixin::HasMethods;
19 use Class::MOP::Class;
20 use Class::MOP::Attribute;
21 use Class::MOP::Method;
24 *IS_RUNNING_ON_5_10 = ($] < 5.009_005)
28 # this is either part of core or set up appropriately by MRO::Compat
29 *check_package_cache_flag = \&mro::get_pkg_gen;
32 our $AUTHORITY = 'cpan:STEVAN';
36 $Moose::{VERSION} ? $Moose::{VERSION}
37 : $ENV{_XS_VERSION} ? $ENV{_XS_VERSION}
42 # Metaclasses are singletons, so we cache them here.
43 # there is no need to worry about destruction though
44 # because they should die only when the program dies.
45 # After all, do package definitions even get reaped?
46 # Anonymous classes manage their own destruction.
49 sub get_all_metaclasses { %METAS }
50 sub get_all_metaclass_instances { values %METAS }
51 sub get_all_metaclass_names { keys %METAS }
52 sub get_metaclass_by_name { $METAS{$_[0]} }
53 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
54 sub weaken_metaclass { weaken($METAS{$_[0]}) }
55 sub metaclass_is_weak { isweak($METAS{$_[0]}) }
56 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
57 sub remove_metaclass_by_name { delete $METAS{$_[0]}; return }
59 # This handles instances as well as class names
61 return unless defined $_[0];
62 my $class = blessed($_[0]) || $_[0];
63 return $METAS{$class};
67 # We only cache metaclasses, meaning instances of
68 # Class::MOP::Class. We do not cache instance of
69 # Class::MOP::Package or Class::MOP::Module. Mostly
70 # because I don't yet see a good reason to do so.
73 sub _class_to_pmfile {
76 my $file = $class . '.pm';
82 sub load_first_existing_class {
83 my $classes = Data::OptList::mkopt(\@_)
86 foreach my $class (@{ $classes }) {
87 my $name = $class->[0];
88 unless ( _is_valid_class_name($name) ) {
89 my $display = defined($name) ? $name : 'undef';
90 confess "Invalid class name ($display)";
97 for my $class (@{ $classes }) {
98 my ($name, $options) = @{ $class };
101 return $name if is_class_loaded($name, $options);
102 if (is_class_loaded($name)) {
103 # we already know it's loaded and too old, but we call
104 # ->VERSION anyway to generate the exception for us
105 $name->VERSION($options->{-version});
109 return $name if is_class_loaded($name);
112 my $file = _class_to_pmfile($name);
113 return $name if try {
116 $name->VERSION($options->{-version})
117 if defined $options->{-version};
121 unless (/^Can't locate \Q$file\E in \@INC/) {
122 confess "Couldn't load class ($name) because: $_";
129 if ( @{ $classes } > 1 ) {
130 my @list = map { $_->[0] } @{ $classes };
131 confess "Can't locate any of @list in \@INC (\@INC contains: @INC).";
133 confess "Can't locate " . _class_to_pmfile($classes->[0]->[0]) . " in \@INC (\@INC contains: @INC).";
138 load_first_existing_class($_[0], ref $_[1] ? $_[1] : ());
140 # This is done to avoid breaking code which checked the return value. Said
141 # code is dumb. The return value was _always_ true, since it dies on
146 sub _is_valid_class_name {
149 return 0 if ref($class);
150 return 0 unless defined($class);
151 return 0 unless length($class);
153 return 1 if $class =~ /^\w+(?:::\w+)*$/;
158 ## ----------------------------------------------------------------------------
159 ## Setting up our environment ...
160 ## ----------------------------------------------------------------------------
161 ## Class::MOP needs to have a few things in the global perl environment so
162 ## that it can operate effectively. Those things are done here.
163 ## ----------------------------------------------------------------------------
165 # ... nothing yet actually ;)
167 ## ----------------------------------------------------------------------------
169 ## ----------------------------------------------------------------------------
170 ## The code below here is to bootstrap our MOP with itself. This is also
171 ## sometimes called "tying the knot". By doing this, we make it much easier
172 ## to extend the MOP through subclassing and such since now you can use the
173 ## MOP itself to extend itself.
175 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
176 ## ----------------------------------------------------------------------------
178 # We need to add in the meta-attributes here so that
179 # any subclass of Class::MOP::* will be able to
180 # inherit them using _construct_instance
182 ## --------------------------------------------------------
183 ## Class::MOP::Mixin::HasMethods
185 Class::MOP::Mixin::HasMethods->meta->add_attribute(
186 Class::MOP::Attribute->new('_methods' => (
189 # we just alias the original method
190 # rather than re-produce it here
191 '_method_map' => \&Class::MOP::Mixin::HasMethods::_method_map
193 default => sub { {} }
197 Class::MOP::Mixin::HasMethods->meta->add_attribute(
198 Class::MOP::Attribute->new('method_metaclass' => (
201 # we just alias the original method
202 # rather than re-produce it here
203 'method_metaclass' => \&Class::MOP::Mixin::HasMethods::method_metaclass
205 default => 'Class::MOP::Method',
209 Class::MOP::Mixin::HasMethods->meta->add_attribute(
210 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
213 # we just alias the original method
214 # rather than re-produce it here
215 'wrapped_method_metaclass' => \&Class::MOP::Mixin::HasMethods::wrapped_method_metaclass
217 default => 'Class::MOP::Method::Wrapped',
221 ## --------------------------------------------------------
222 ## Class::MOP::Mixin::HasMethods
224 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
225 Class::MOP::Attribute->new('attributes' => (
227 # NOTE: we need to do this in order
228 # for the instance meta-object to
229 # not fall into meta-circular death
231 # we just alias the original method
232 # rather than re-produce it here
233 '_attribute_map' => \&Class::MOP::Mixin::HasAttributes::_attribute_map
235 default => sub { {} }
239 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
240 Class::MOP::Attribute->new('attribute_metaclass' => (
243 # we just alias the original method
244 # rather than re-produce it here
245 'attribute_metaclass' => \&Class::MOP::Mixin::HasAttributes::attribute_metaclass
247 default => 'Class::MOP::Attribute',
251 ## --------------------------------------------------------
252 ## Class::MOP::Package
254 Class::MOP::Package->meta->add_attribute(
255 Class::MOP::Attribute->new('package' => (
257 # NOTE: we need to do this in order
258 # for the instance meta-object to
259 # not fall into meta-circular death
261 # we just alias the original method
262 # rather than re-produce it here
263 'name' => \&Class::MOP::Package::name
268 Class::MOP::Package->meta->add_attribute(
269 Class::MOP::Attribute->new('namespace' => (
272 # we just alias the original method
273 # rather than re-produce it here
274 'namespace' => \&Class::MOP::Package::namespace
277 default => sub { \undef }
281 ## --------------------------------------------------------
282 ## Class::MOP::Module
285 # yeah this is kind of stretching things a bit,
286 # but truthfully the version should be an attribute
287 # of the Module, the weirdness comes from having to
288 # stick to Perl 5 convention and store it in the
289 # $VERSION package variable. Basically if you just
290 # squint at it, it will look how you want it to look.
291 # Either as a package variable, or as a attribute of
292 # the metaclass, isn't abstraction great :)
294 Class::MOP::Module->meta->add_attribute(
295 Class::MOP::Attribute->new('version' => (
298 # we just alias the original method
299 # rather than re-produce it here
300 'version' => \&Class::MOP::Module::version
303 default => sub { \undef }
308 # By following the same conventions as version here,
309 # we are opening up the possibility that people can
310 # use the $AUTHORITY in non-Class::MOP modules as
313 Class::MOP::Module->meta->add_attribute(
314 Class::MOP::Attribute->new('authority' => (
317 # we just alias the original method
318 # rather than re-produce it here
319 'authority' => \&Class::MOP::Module::authority
322 default => sub { \undef }
326 ## --------------------------------------------------------
329 Class::MOP::Class->meta->add_attribute(
330 Class::MOP::Attribute->new('superclasses' => (
333 # we just alias the original method
334 # rather than re-produce it here
335 'superclasses' => \&Class::MOP::Class::superclasses
338 default => sub { \undef }
342 Class::MOP::Class->meta->add_attribute(
343 Class::MOP::Attribute->new('instance_metaclass' => (
345 # NOTE: we need to do this in order
346 # for the instance meta-object to
347 # not fall into meta-circular death
349 # we just alias the original method
350 # rather than re-produce it here
351 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
353 default => 'Class::MOP::Instance',
357 Class::MOP::Class->meta->add_attribute(
358 Class::MOP::Attribute->new('immutable_trait' => (
360 'immutable_trait' => \&Class::MOP::Class::immutable_trait
362 default => "Class::MOP::Class::Immutable::Trait",
366 Class::MOP::Class->meta->add_attribute(
367 Class::MOP::Attribute->new('constructor_name' => (
369 'constructor_name' => \&Class::MOP::Class::constructor_name,
375 Class::MOP::Class->meta->add_attribute(
376 Class::MOP::Attribute->new('constructor_class' => (
378 'constructor_class' => \&Class::MOP::Class::constructor_class,
380 default => "Class::MOP::Method::Constructor",
385 Class::MOP::Class->meta->add_attribute(
386 Class::MOP::Attribute->new('destructor_class' => (
388 'destructor_class' => \&Class::MOP::Class::destructor_class,
394 # we don't actually need to tie the knot with
395 # Class::MOP::Class here, it is actually handled
396 # within Class::MOP::Class itself in the
397 # _construct_class_instance method.
399 ## --------------------------------------------------------
400 ## Class::MOP::Mixin::AttributeCore
401 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
402 Class::MOP::Attribute->new('name' => (
404 # NOTE: we need to do this in order
405 # for the instance meta-object to
406 # not fall into meta-circular death
408 # we just alias the original method
409 # rather than re-produce it here
410 'name' => \&Class::MOP::Mixin::AttributeCore::name
415 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
416 Class::MOP::Attribute->new('accessor' => (
417 reader => { 'accessor' => \&Class::MOP::Mixin::AttributeCore::accessor },
418 predicate => { 'has_accessor' => \&Class::MOP::Mixin::AttributeCore::has_accessor },
422 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
423 Class::MOP::Attribute->new('reader' => (
424 reader => { 'reader' => \&Class::MOP::Mixin::AttributeCore::reader },
425 predicate => { 'has_reader' => \&Class::MOP::Mixin::AttributeCore::has_reader },
429 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
430 Class::MOP::Attribute->new('initializer' => (
431 reader => { 'initializer' => \&Class::MOP::Mixin::AttributeCore::initializer },
432 predicate => { 'has_initializer' => \&Class::MOP::Mixin::AttributeCore::has_initializer },
436 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
437 Class::MOP::Attribute->new('definition_context' => (
438 reader => { 'definition_context' => \&Class::MOP::Mixin::AttributeCore::definition_context },
442 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
443 Class::MOP::Attribute->new('writer' => (
444 reader => { 'writer' => \&Class::MOP::Mixin::AttributeCore::writer },
445 predicate => { 'has_writer' => \&Class::MOP::Mixin::AttributeCore::has_writer },
449 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
450 Class::MOP::Attribute->new('predicate' => (
451 reader => { 'predicate' => \&Class::MOP::Mixin::AttributeCore::predicate },
452 predicate => { 'has_predicate' => \&Class::MOP::Mixin::AttributeCore::has_predicate },
456 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
457 Class::MOP::Attribute->new('clearer' => (
458 reader => { 'clearer' => \&Class::MOP::Mixin::AttributeCore::clearer },
459 predicate => { 'has_clearer' => \&Class::MOP::Mixin::AttributeCore::has_clearer },
463 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
464 Class::MOP::Attribute->new('builder' => (
465 reader => { 'builder' => \&Class::MOP::Mixin::AttributeCore::builder },
466 predicate => { 'has_builder' => \&Class::MOP::Mixin::AttributeCore::has_builder },
470 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
471 Class::MOP::Attribute->new('init_arg' => (
472 reader => { 'init_arg' => \&Class::MOP::Mixin::AttributeCore::init_arg },
473 predicate => { 'has_init_arg' => \&Class::MOP::Mixin::AttributeCore::has_init_arg },
477 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
478 Class::MOP::Attribute->new('default' => (
479 # default has a custom 'reader' method ...
480 predicate => { 'has_default' => \&Class::MOP::Mixin::AttributeCore::has_default },
484 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
485 Class::MOP::Attribute->new('insertion_order' => (
486 reader => { 'insertion_order' => \&Class::MOP::Mixin::AttributeCore::insertion_order },
487 writer => { '_set_insertion_order' => \&Class::MOP::Mixin::AttributeCore::_set_insertion_order },
488 predicate => { 'has_insertion_order' => \&Class::MOP::Mixin::AttributeCore::has_insertion_order },
492 ## --------------------------------------------------------
493 ## Class::MOP::Attribute
494 Class::MOP::Attribute->meta->add_attribute(
495 Class::MOP::Attribute->new('associated_class' => (
497 # NOTE: we need to do this in order
498 # for the instance meta-object to
499 # not fall into meta-circular death
501 # we just alias the original method
502 # rather than re-produce it here
503 'associated_class' => \&Class::MOP::Attribute::associated_class
508 Class::MOP::Attribute->meta->add_attribute(
509 Class::MOP::Attribute->new('associated_methods' => (
510 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
511 default => sub { [] }
515 Class::MOP::Attribute->meta->add_method('clone' => sub {
517 $self->meta->clone_object($self, @_);
520 ## --------------------------------------------------------
521 ## Class::MOP::Method
522 Class::MOP::Method->meta->add_attribute(
523 Class::MOP::Attribute->new('body' => (
524 reader => { 'body' => \&Class::MOP::Method::body },
528 Class::MOP::Method->meta->add_attribute(
529 Class::MOP::Attribute->new('associated_metaclass' => (
530 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
534 Class::MOP::Method->meta->add_attribute(
535 Class::MOP::Attribute->new('package_name' => (
536 reader => { 'package_name' => \&Class::MOP::Method::package_name },
540 Class::MOP::Method->meta->add_attribute(
541 Class::MOP::Attribute->new('name' => (
542 reader => { 'name' => \&Class::MOP::Method::name },
546 Class::MOP::Method->meta->add_attribute(
547 Class::MOP::Attribute->new('original_method' => (
548 reader => { 'original_method' => \&Class::MOP::Method::original_method },
549 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
553 ## --------------------------------------------------------
554 ## Class::MOP::Method::Wrapped
557 # the way this item is initialized, this
558 # really does not follow the standard
559 # practices of attributes, but we put
560 # it here for completeness
561 Class::MOP::Method::Wrapped->meta->add_attribute(
562 Class::MOP::Attribute->new('modifier_table')
565 ## --------------------------------------------------------
566 ## Class::MOP::Method::Generated
568 Class::MOP::Method::Generated->meta->add_attribute(
569 Class::MOP::Attribute->new('is_inline' => (
570 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
575 Class::MOP::Method::Generated->meta->add_attribute(
576 Class::MOP::Attribute->new('definition_context' => (
577 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
582 ## --------------------------------------------------------
583 ## Class::MOP::Method::Inlined
585 Class::MOP::Method::Inlined->meta->add_attribute(
586 Class::MOP::Attribute->new('_expected_method_class' => (
587 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
591 ## --------------------------------------------------------
592 ## Class::MOP::Method::Accessor
594 Class::MOP::Method::Accessor->meta->add_attribute(
595 Class::MOP::Attribute->new('attribute' => (
597 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
602 Class::MOP::Method::Accessor->meta->add_attribute(
603 Class::MOP::Attribute->new('accessor_type' => (
604 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
608 ## --------------------------------------------------------
609 ## Class::MOP::Method::Constructor
611 Class::MOP::Method::Constructor->meta->add_attribute(
612 Class::MOP::Attribute->new('options' => (
614 'options' => \&Class::MOP::Method::Constructor::options
616 default => sub { +{} }
620 Class::MOP::Method::Constructor->meta->add_attribute(
621 Class::MOP::Attribute->new('associated_metaclass' => (
622 init_arg => "metaclass", # FIXME alias and rename
624 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
629 ## --------------------------------------------------------
630 ## Class::MOP::Instance
633 # these don't yet do much of anything, but are just
634 # included for completeness
636 Class::MOP::Instance->meta->add_attribute(
637 Class::MOP::Attribute->new('associated_metaclass',
638 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
642 Class::MOP::Instance->meta->add_attribute(
643 Class::MOP::Attribute->new('_class_name',
645 reader => { _class_name => \&Class::MOP::Instance::_class_name },
646 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
647 #default => sub { $_[0]->associated_metaclass->name },
651 Class::MOP::Instance->meta->add_attribute(
652 Class::MOP::Attribute->new('attributes',
653 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
657 Class::MOP::Instance->meta->add_attribute(
658 Class::MOP::Attribute->new('slots',
659 reader => { slots => \&Class::MOP::Instance::slots },
663 Class::MOP::Instance->meta->add_attribute(
664 Class::MOP::Attribute->new('slot_hash',
665 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
669 ## --------------------------------------------------------
670 ## Class::MOP::Object
672 # need to replace the meta method there with a real meta method object
673 Class::MOP::Object->meta->_add_meta_method('meta');
675 ## --------------------------------------------------------
678 # need to replace the meta method there with a real meta method object
679 Class::MOP::Mixin->meta->_add_meta_method('meta');
681 require Class::MOP::Deprecated unless our $no_deprecated;
683 # we need the meta instance of the meta instance to be created now, in order
684 # for the constructor to be able to use it
685 Class::MOP::Instance->meta->get_meta_instance;
687 # pretend the add_method never happenned. it hasn't yet affected anything
688 undef Class::MOP::Instance->meta->{_package_cache_flag};
690 ## --------------------------------------------------------
691 ## Now close all the Class::MOP::* classes
693 # NOTE: we don't need to inline the the accessors this only lengthens
694 # the compile time of the MOP, and gives us no actual benefits.
696 $_->meta->make_immutable(
697 inline_constructor => 0,
698 constructor_name => "_new",
699 inline_accessors => 0,
705 Class::MOP::Attribute
711 Class::MOP::Method::Generated
712 Class::MOP::Method::Inlined
714 Class::MOP::Method::Accessor
715 Class::MOP::Method::Constructor
716 Class::MOP::Method::Wrapped
718 Class::MOP::Method::Meta
721 $_->meta->make_immutable(
722 inline_constructor => 0,
723 constructor_name => undef,
724 inline_accessors => 0,
727 Class::MOP::Mixin::AttributeCore
728 Class::MOP::Mixin::HasAttributes
729 Class::MOP::Mixin::HasMethods
734 # ABSTRACT: A Meta Object Protocol for Perl 5
742 This module is a fully functioning meta object protocol for the
743 Perl 5 object system. It makes no attempt to change the behavior or
744 characteristics of the Perl 5 object system, only to create a
745 protocol for its manipulation and introspection.
747 That said, it does attempt to create the tools for building a rich set
748 of extensions to the Perl 5 object system. Every attempt has been made
749 to abide by the spirit of the Perl 5 object system that we all know
752 This documentation is sparse on conceptual details. We suggest looking
753 at the items listed in the L<SEE ALSO> section for more
754 information. In particular the book "The Art of the Meta Object
755 Protocol" was very influential in the development of this system.
757 =head2 What is a Meta Object Protocol?
759 A meta object protocol is an API to an object system.
761 To be more specific, it abstracts the components of an object system
762 (classes, object, methods, object attributes, etc.). These
763 abstractions can then be used to inspect and manipulate the object
764 system which they describe.
766 It can be said that there are two MOPs for any object system; the
767 implicit MOP and the explicit MOP. The implicit MOP handles things
768 like method dispatch or inheritance, which happen automatically as
769 part of how the object system works. The explicit MOP typically
770 handles the introspection/reflection features of the object system.
772 All object systems have implicit MOPs. Without one, they would not
773 work. Explicit MOPs are much less common, and depending on the
774 language can vary from restrictive (Reflection in Java or C#) to wide
775 open (CLOS is a perfect example).
777 =head2 Yet Another Class Builder! Why?
779 This is B<not> a class builder so much as a I<class builder
780 B<builder>>. The intent is that an end user will not use this module
781 directly, but instead this module is used by module authors to build
782 extensions and features onto the Perl 5 object system.
784 This system is used by L<Moose>, which supplies a powerful class
785 builder system built entirely on top of C<Class::MOP>.
787 =head2 Who is this module for?
789 This module is for anyone who has ever created or wanted to create a
790 module for the Class:: namespace. The tools which this module provides
791 make doing complex Perl 5 wizardry simpler, by removing such barriers
792 as the need to hack symbol tables, or understand the fine details of
795 =head2 What changes do I have to make to use this module?
797 This module was designed to be as unintrusive as possible. Many of its
798 features are accessible without B<any> change to your existing
799 code. It is meant to be a compliment to your existing code and not an
800 intrusion on your code base. Unlike many other B<Class::> modules,
801 this module B<does not> require you subclass it, or even that you
802 C<use> it in within your module's package.
804 The only features which requires additions to your code are the
805 attribute handling and instance construction features, and these are
806 both completely optional features. The only reason for this is because
807 Perl 5's object system does not actually have these features built
808 in. More information about this feature can be found below.
810 =head2 About Performance
812 It is a common misconception that explicit MOPs are a performance hit.
813 This is not a universal truth, it is a side-effect of some specific
814 implementations. For instance, using Java reflection is slow because
815 the JVM cannot take advantage of any compiler optimizations, and the
816 JVM has to deal with much more runtime type information as well.
818 Reflection in C# is marginally better as it was designed into the
819 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
820 Object System) was built to support an explicit MOP, and so
821 performance is tuned for it.
823 This library in particular does its absolute best to avoid putting
824 B<any> drain at all upon your code's performance. In fact, by itself
825 it does nothing to affect your existing code. So you only pay for what
828 =head2 About Metaclass compatibility
830 This module makes sure that all metaclasses created are both upwards
831 and downwards compatible. The topic of metaclass compatibility is
832 highly esoteric and is something only encountered when doing deep and
833 involved metaclass hacking. There are two basic kinds of metaclass
834 incompatibility; upwards and downwards.
836 Upwards metaclass compatibility means that the metaclass of a
837 given class is either the same as (or a subclass of) all of the
840 Downward metaclass compatibility means that the metaclasses of a
841 given class's ancestors are all either the same as (or a subclass
844 Here is a diagram showing a set of two classes (C<A> and C<B>) and
845 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
846 metaclass compatibility both upwards and downwards.
848 +---------+ +---------+
849 | Meta::A |<----| Meta::B | <....... (instance of )
850 +---------+ +---------+ <------- (inherits from)
853 +---------+ +---------+
855 +---------+ +---------+
857 In actuality, I<all> of a class's metaclasses must be compatible,
858 not just the class metaclass. That includes the instance, attribute,
859 and method metaclasses, as well as the constructor and destructor
862 C<Class::MOP> will attempt to fix some simple types of
863 incompatibilities. If all the metaclasses for the parent class are
864 I<subclasses> of the child's metaclasses then we can simply replace
865 the child's metaclasses with the parent's. In addition, if the child
866 is missing a metaclass that the parent has, we can also just make the
867 child use the parent's metaclass.
869 As I said this is a highly esoteric topic and one you will only run
870 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
871 are interested in why this is an issue see the paper I<Uniform and
872 safe metaclass composition> linked to in the L<SEE ALSO> section of
875 =head2 Using custom metaclasses
877 Always use the L<metaclass> pragma when using a custom metaclass, this
878 will ensure the proper initialization order and not accidentally
879 create an incorrect type of metaclass for you. This is a very rare
880 problem, and one which can only occur if you are doing deep metaclass
881 programming. So in other words, don't worry about it.
883 Note that if you're using L<Moose> we encourage you to I<not> use
884 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
885 roles to a class's metaclasses. This topic is covered at length in
886 various L<Moose::Cookbook> recipes.
890 The meta-object protocol is divided into 4 main sub-protocols:
892 =head2 The Class protocol
894 This provides a means of manipulating and introspecting a Perl 5
895 class. It handles symbol table hacking for you, and provides a rich
896 set of methods that go beyond simple package introspection.
898 See L<Class::MOP::Class> for more details.
900 =head2 The Attribute protocol
902 This provides a consistent representation for an attribute of a Perl 5
903 class. Since there are so many ways to create and handle attributes in
904 Perl 5 OO, the Attribute protocol provide as much of a unified
905 approach as possible. Of course, you are always free to extend this
906 protocol by subclassing the appropriate classes.
908 See L<Class::MOP::Attribute> for more details.
910 =head2 The Method protocol
912 This provides a means of manipulating and introspecting methods in the
913 Perl 5 object system. As with attributes, there are many ways to
914 approach this topic, so we try to keep it pretty basic, while still
915 making it possible to extend the system in many ways.
917 See L<Class::MOP::Method> for more details.
919 =head2 The Instance protocol
921 This provides a layer of abstraction for creating object instances.
922 Since the other layers use this protocol, it is relatively easy to
923 change the type of your instances from the default hash reference to
924 some other type of reference. Several examples are provided in the
925 F<examples/> directory included in this distribution.
927 See L<Class::MOP::Instance> for more details.
931 Note that this module does not export any constants or functions.
937 =item I<Class::MOP::IS_RUNNING_ON_5_10>
939 We set this constant depending on what version perl we are on, this
940 allows us to take advantage of new 5.10 features and stay backwards
945 =head2 Utility functions
947 Note that these are all called as B<functions, not methods>.
951 =item B<Class::MOP::load_class($class_name, \%options?)>
953 This will load the specified C<$class_name>, if it is not already
954 loaded (as reported by C<is_class_loaded>). This function can be used
955 in place of tricks like C<eval "use $module"> or using C<require>
958 If the module cannot be loaded, an exception is thrown.
960 You can pass a hash reference with options as second argument. The
961 only option currently recognised is C<-version>, which will ensure
962 that the loaded class has at least the required version.
964 See also L</Class Loading Options>.
966 For historical reasons, this function explicitly returns a true value.
968 =item B<Class::MOP::is_class_loaded($class_name, \%options?)>
970 Returns a boolean indicating whether or not C<$class_name> has been
973 This does a basic check of the symbol table to try and determine as
974 best it can if the C<$class_name> is loaded, it is probably correct
975 about 99% of the time, but it can be fooled into reporting false
976 positives. In particular, loading any of the core L<IO> modules will
977 cause most of the rest of the core L<IO> modules to falsely report
978 having been loaded, due to the way the base L<IO> module works.
980 You can pass a hash reference with options as second argument. The
981 only option currently recognised is C<-version>, which will ensure
982 that the loaded class has at least the required version.
984 See also L</Class Loading Options>.
986 =item B<Class::MOP::get_code_info($code)>
988 This function returns two values, the name of the package the C<$code>
989 is from and the name of the C<$code> itself. This is used by several
990 elements of the MOP to determine where a given C<$code> reference is
993 =item B<Class::MOP::class_of($instance_or_class_name)>
995 This will return the metaclass of the given instance or class name. If the
996 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
999 =item B<Class::MOP::check_package_cache_flag($pkg)>
1001 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1003 This will return an integer that is managed by L<Class::MOP::Class> to
1004 determine if a module's symbol table has been altered.
1006 In Perl 5.10 or greater, this flag is package specific. However in
1007 versions prior to 5.10, this will use the C<PL_sub_generation>
1008 variable which is not package specific.
1010 =item B<Class::MOP::load_first_existing_class(@class_names)>
1012 =item B<Class::MOP::load_first_existing_class($classA, \%optionsA?, $classB, ...)>
1014 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1016 Given a list of class names, this function will attempt to load each
1019 If it finds a class it can load, it will return that class' name. If
1020 none of the classes can be loaded, it will throw an exception.
1022 Additionally, you can pass a hash reference with options after each
1023 class name. Currently, only C<-version> is recognised and will ensure
1024 that the loaded class has at least the required version. If the class
1025 version is not sufficient, an exception will be raised.
1027 See also L</Class Loading Options>.
1031 =head2 Metaclass cache functions
1033 Class::MOP holds a cache of metaclasses. The following are functions
1034 (B<not methods>) which can be used to access that cache. It is not
1035 recommended that you mess with these. Bad things could happen, but if
1036 you are brave and willing to risk it: go for it!
1040 =item B<Class::MOP::get_all_metaclasses>
1042 This will return a hash of all the metaclass instances that have
1043 been cached by L<Class::MOP::Class>, keyed by the package name.
1045 =item B<Class::MOP::get_all_metaclass_instances>
1047 This will return a list of all the metaclass instances that have
1048 been cached by L<Class::MOP::Class>.
1050 =item B<Class::MOP::get_all_metaclass_names>
1052 This will return a list of all the metaclass names that have
1053 been cached by L<Class::MOP::Class>.
1055 =item B<Class::MOP::get_metaclass_by_name($name)>
1057 This will return a cached L<Class::MOP::Class> instance, or nothing
1058 if no metaclass exists with that C<$name>.
1060 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
1062 This will store a metaclass in the cache at the supplied C<$key>.
1064 =item B<Class::MOP::weaken_metaclass($name)>
1066 In rare cases (e.g. anonymous metaclasses) it is desirable to
1067 store a weakened reference in the metaclass cache. This
1068 function will weaken the reference to the metaclass stored
1071 =item B<Class::MOP::metaclass_is_weak($name)>
1073 Returns true if the metaclass for C<$name> has been weakened
1074 (via C<weaken_metaclass>).
1076 =item B<Class::MOP::does_metaclass_exist($name)>
1078 This will return true of there exists a metaclass stored in the
1079 C<$name> key, and return false otherwise.
1081 =item B<Class::MOP::remove_metaclass_by_name($name)>
1083 This will remove the metaclass stored in the C<$name> key.
1087 =head2 Class Loading Options
1093 Can be used to pass a minimum required version that will be checked
1094 against the class version after it was loaded.
1102 There are very few books out on Meta Object Protocols and Metaclasses
1103 because it is such an esoteric topic. The following books are really
1104 the only ones I have found. If you know of any more, B<I<please>>
1105 email me and let me know, I would love to hear about them.
1109 =item I<The Art of the Meta Object Protocol>
1111 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1113 =item I<Putting MetaClasses to Work>
1115 =item I<Smalltalk: The Language>
1123 =item "Uniform and safe metaclass composition"
1125 An excellent paper by the people who brought us the original Traits paper.
1126 This paper is on how Traits can be used to do safe metaclass composition,
1127 and offers an excellent introduction section which delves into the topic of
1128 metaclass compatibility.
1130 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1132 =item "Safe Metaclass Programming"
1134 This paper seems to precede the above paper, and propose a mix-in based
1135 approach as opposed to the Traits based approach. Both papers have similar
1136 information on the metaclass compatibility problem space.
1138 L<http://citeseer.ist.psu.edu/37617.html>
1146 =item The Perl 6 MetaModel work in the Pugs project
1150 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1152 =item L<http://github.com/perl6/p5-modules/tree/master/Perl6-ObjectSpace/>
1162 =item CPAN Module Review of Class::MOP
1164 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1168 =head1 SIMILAR MODULES
1170 As I have said above, this module is a class-builder-builder, so it is
1171 not the same thing as modules like L<Class::Accessor> and
1172 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1173 with similar goals to this module. The one I have found which is most
1174 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1175 creates are very different from this modules.
1179 All complex software has bugs lurking in it, and this module is no
1182 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1183 web interface at L<http://rt.cpan.org>.
1185 You can also discuss feature requests or possible bugs on the Moose
1186 mailing list (moose@perl.org) or on IRC at
1187 L<irc://irc.perl.org/#moose>.
1189 =head1 ACKNOWLEDGEMENTS
1195 Thanks to Rob for actually getting the development of this module kick-started.