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;
34 $Moose::{VERSION} ? $Moose::{VERSION} : ()
38 # Metaclasses are singletons, so we cache them here.
39 # there is no need to worry about destruction though
40 # because they should die only when the program dies.
41 # After all, do package definitions even get reaped?
42 # Anonymous classes manage their own destruction.
45 sub get_all_metaclasses { %METAS }
46 sub get_all_metaclass_instances { values %METAS }
47 sub get_all_metaclass_names { keys %METAS }
48 sub get_metaclass_by_name { $METAS{$_[0]} }
49 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
50 sub weaken_metaclass { weaken($METAS{$_[0]}) }
51 sub metaclass_is_weak { isweak($METAS{$_[0]}) }
52 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
53 sub remove_metaclass_by_name { delete $METAS{$_[0]}; return }
55 # This handles instances as well as class names
57 return unless defined $_[0];
58 my $class = blessed($_[0]) || $_[0];
59 return $METAS{$class};
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 _class_to_pmfile {
72 my $file = $class . '.pm';
78 sub load_first_existing_class {
79 my $classes = Data::OptList::mkopt(\@_)
82 foreach my $class (@{ $classes }) {
83 my $name = $class->[0];
84 unless ( _is_valid_class_name($name) ) {
85 my $display = defined($name) ? $name : 'undef';
86 confess "Invalid class name ($display)";
93 for my $class (@{ $classes }) {
94 my ($name, $options) = @{ $class };
97 return $name if is_class_loaded($name, $options);
98 if (is_class_loaded($name)) {
99 # we already know it's loaded and too old, but we call
100 # ->VERSION anyway to generate the exception for us
101 $name->VERSION($options->{-version});
105 return $name if is_class_loaded($name);
108 my $file = _class_to_pmfile($name);
109 return $name if try {
112 $name->VERSION($options->{-version})
113 if defined $options->{-version};
117 unless (/^Can't locate \Q$file\E in \@INC/) {
118 confess "Couldn't load class ($name) because: $_";
125 if ( @{ $classes } > 1 ) {
126 my @list = map { $_->[0] } @{ $classes };
127 confess "Can't locate any of @list in \@INC (\@INC contains: @INC).";
129 confess "Can't locate " . _class_to_pmfile($classes->[0]->[0]) . " in \@INC (\@INC contains: @INC).";
134 load_first_existing_class($_[0], ref $_[1] ? $_[1] : ());
136 # This is done to avoid breaking code which checked the return value. Said
137 # code is dumb. The return value was _always_ true, since it dies on
142 sub _is_valid_class_name {
145 return 0 if ref($class);
146 return 0 unless defined($class);
147 return 0 unless length($class);
149 return 1 if $class =~ /^\w+(?:::\w+)*$/;
154 ## ----------------------------------------------------------------------------
155 ## Setting up our environment ...
156 ## ----------------------------------------------------------------------------
157 ## Class::MOP needs to have a few things in the global perl environment so
158 ## that it can operate effectively. Those things are done here.
159 ## ----------------------------------------------------------------------------
161 # ... nothing yet actually ;)
163 ## ----------------------------------------------------------------------------
165 ## ----------------------------------------------------------------------------
166 ## The code below here is to bootstrap our MOP with itself. This is also
167 ## sometimes called "tying the knot". By doing this, we make it much easier
168 ## to extend the MOP through subclassing and such since now you can use the
169 ## MOP itself to extend itself.
171 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
172 ## ----------------------------------------------------------------------------
174 # We need to add in the meta-attributes here so that
175 # any subclass of Class::MOP::* will be able to
176 # inherit them using _construct_instance
178 ## --------------------------------------------------------
179 ## Class::MOP::Mixin::HasMethods
181 Class::MOP::Mixin::HasMethods->meta->add_attribute(
182 Class::MOP::Attribute->new('_methods' => (
185 # we just alias the original method
186 # rather than re-produce it here
187 '_method_map' => \&Class::MOP::Mixin::HasMethods::_method_map
189 default => sub { {} }
193 Class::MOP::Mixin::HasMethods->meta->add_attribute(
194 Class::MOP::Attribute->new('method_metaclass' => (
197 # we just alias the original method
198 # rather than re-produce it here
199 'method_metaclass' => \&Class::MOP::Mixin::HasMethods::method_metaclass
201 default => 'Class::MOP::Method',
205 Class::MOP::Mixin::HasMethods->meta->add_attribute(
206 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
209 # we just alias the original method
210 # rather than re-produce it here
211 'wrapped_method_metaclass' => \&Class::MOP::Mixin::HasMethods::wrapped_method_metaclass
213 default => 'Class::MOP::Method::Wrapped',
217 ## --------------------------------------------------------
218 ## Class::MOP::Mixin::HasMethods
220 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
221 Class::MOP::Attribute->new('attributes' => (
223 # NOTE: we need to do this in order
224 # for the instance meta-object to
225 # not fall into meta-circular death
227 # we just alias the original method
228 # rather than re-produce it here
229 '_attribute_map' => \&Class::MOP::Mixin::HasAttributes::_attribute_map
231 default => sub { {} }
235 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
236 Class::MOP::Attribute->new('attribute_metaclass' => (
239 # we just alias the original method
240 # rather than re-produce it here
241 'attribute_metaclass' => \&Class::MOP::Mixin::HasAttributes::attribute_metaclass
243 default => 'Class::MOP::Attribute',
247 ## --------------------------------------------------------
248 ## Class::MOP::Package
250 Class::MOP::Package->meta->add_attribute(
251 Class::MOP::Attribute->new('package' => (
253 # NOTE: we need to do this in order
254 # for the instance meta-object to
255 # not fall into meta-circular death
257 # we just alias the original method
258 # rather than re-produce it here
259 'name' => \&Class::MOP::Package::name
264 Class::MOP::Package->meta->add_attribute(
265 Class::MOP::Attribute->new('namespace' => (
268 # we just alias the original method
269 # rather than re-produce it here
270 'namespace' => \&Class::MOP::Package::namespace
273 default => sub { \undef }
277 ## --------------------------------------------------------
278 ## Class::MOP::Module
281 # yeah this is kind of stretching things a bit,
282 # but truthfully the version should be an attribute
283 # of the Module, the weirdness comes from having to
284 # stick to Perl 5 convention and store it in the
285 # $VERSION package variable. Basically if you just
286 # squint at it, it will look how you want it to look.
287 # Either as a package variable, or as a attribute of
288 # the metaclass, isn't abstraction great :)
290 Class::MOP::Module->meta->add_attribute(
291 Class::MOP::Attribute->new('version' => (
294 # we just alias the original method
295 # rather than re-produce it here
296 'version' => \&Class::MOP::Module::version
299 default => sub { \undef }
304 # By following the same conventions as version here,
305 # we are opening up the possibility that people can
306 # use the $AUTHORITY in non-Class::MOP modules as
309 Class::MOP::Module->meta->add_attribute(
310 Class::MOP::Attribute->new('authority' => (
313 # we just alias the original method
314 # rather than re-produce it here
315 'authority' => \&Class::MOP::Module::authority
318 default => sub { \undef }
322 ## --------------------------------------------------------
325 Class::MOP::Class->meta->add_attribute(
326 Class::MOP::Attribute->new('superclasses' => (
329 # we just alias the original method
330 # rather than re-produce it here
331 'superclasses' => \&Class::MOP::Class::superclasses
334 default => sub { \undef }
338 Class::MOP::Class->meta->add_attribute(
339 Class::MOP::Attribute->new('instance_metaclass' => (
341 # NOTE: we need to do this in order
342 # for the instance meta-object to
343 # not fall into meta-circular death
345 # we just alias the original method
346 # rather than re-produce it here
347 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
349 default => 'Class::MOP::Instance',
353 Class::MOP::Class->meta->add_attribute(
354 Class::MOP::Attribute->new('immutable_trait' => (
356 'immutable_trait' => \&Class::MOP::Class::immutable_trait
358 default => "Class::MOP::Class::Immutable::Trait",
362 Class::MOP::Class->meta->add_attribute(
363 Class::MOP::Attribute->new('constructor_name' => (
365 'constructor_name' => \&Class::MOP::Class::constructor_name,
371 Class::MOP::Class->meta->add_attribute(
372 Class::MOP::Attribute->new('constructor_class' => (
374 'constructor_class' => \&Class::MOP::Class::constructor_class,
376 default => "Class::MOP::Method::Constructor",
381 Class::MOP::Class->meta->add_attribute(
382 Class::MOP::Attribute->new('destructor_class' => (
384 'destructor_class' => \&Class::MOP::Class::destructor_class,
390 # we don't actually need to tie the knot with
391 # Class::MOP::Class here, it is actually handled
392 # within Class::MOP::Class itself in the
393 # _construct_class_instance method.
395 ## --------------------------------------------------------
396 ## Class::MOP::Mixin::AttributeCore
397 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
398 Class::MOP::Attribute->new('name' => (
400 # NOTE: we need to do this in order
401 # for the instance meta-object to
402 # not fall into meta-circular death
404 # we just alias the original method
405 # rather than re-produce it here
406 'name' => \&Class::MOP::Mixin::AttributeCore::name
411 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
412 Class::MOP::Attribute->new('accessor' => (
413 reader => { 'accessor' => \&Class::MOP::Mixin::AttributeCore::accessor },
414 predicate => { 'has_accessor' => \&Class::MOP::Mixin::AttributeCore::has_accessor },
418 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
419 Class::MOP::Attribute->new('reader' => (
420 reader => { 'reader' => \&Class::MOP::Mixin::AttributeCore::reader },
421 predicate => { 'has_reader' => \&Class::MOP::Mixin::AttributeCore::has_reader },
425 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
426 Class::MOP::Attribute->new('initializer' => (
427 reader => { 'initializer' => \&Class::MOP::Mixin::AttributeCore::initializer },
428 predicate => { 'has_initializer' => \&Class::MOP::Mixin::AttributeCore::has_initializer },
432 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
433 Class::MOP::Attribute->new('definition_context' => (
434 reader => { 'definition_context' => \&Class::MOP::Mixin::AttributeCore::definition_context },
438 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
439 Class::MOP::Attribute->new('writer' => (
440 reader => { 'writer' => \&Class::MOP::Mixin::AttributeCore::writer },
441 predicate => { 'has_writer' => \&Class::MOP::Mixin::AttributeCore::has_writer },
445 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
446 Class::MOP::Attribute->new('predicate' => (
447 reader => { 'predicate' => \&Class::MOP::Mixin::AttributeCore::predicate },
448 predicate => { 'has_predicate' => \&Class::MOP::Mixin::AttributeCore::has_predicate },
452 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
453 Class::MOP::Attribute->new('clearer' => (
454 reader => { 'clearer' => \&Class::MOP::Mixin::AttributeCore::clearer },
455 predicate => { 'has_clearer' => \&Class::MOP::Mixin::AttributeCore::has_clearer },
459 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
460 Class::MOP::Attribute->new('builder' => (
461 reader => { 'builder' => \&Class::MOP::Mixin::AttributeCore::builder },
462 predicate => { 'has_builder' => \&Class::MOP::Mixin::AttributeCore::has_builder },
466 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
467 Class::MOP::Attribute->new('init_arg' => (
468 reader => { 'init_arg' => \&Class::MOP::Mixin::AttributeCore::init_arg },
469 predicate => { 'has_init_arg' => \&Class::MOP::Mixin::AttributeCore::has_init_arg },
473 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
474 Class::MOP::Attribute->new('default' => (
475 # default has a custom 'reader' method ...
476 predicate => { 'has_default' => \&Class::MOP::Mixin::AttributeCore::has_default },
480 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
481 Class::MOP::Attribute->new('insertion_order' => (
482 reader => { 'insertion_order' => \&Class::MOP::Mixin::AttributeCore::insertion_order },
483 writer => { '_set_insertion_order' => \&Class::MOP::Mixin::AttributeCore::_set_insertion_order },
484 predicate => { 'has_insertion_order' => \&Class::MOP::Mixin::AttributeCore::has_insertion_order },
488 ## --------------------------------------------------------
489 ## Class::MOP::Attribute
490 Class::MOP::Attribute->meta->add_attribute(
491 Class::MOP::Attribute->new('associated_class' => (
493 # NOTE: we need to do this in order
494 # for the instance meta-object to
495 # not fall into meta-circular death
497 # we just alias the original method
498 # rather than re-produce it here
499 'associated_class' => \&Class::MOP::Attribute::associated_class
504 Class::MOP::Attribute->meta->add_attribute(
505 Class::MOP::Attribute->new('associated_methods' => (
506 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
507 default => sub { [] }
511 Class::MOP::Attribute->meta->add_method('clone' => sub {
513 $self->meta->clone_object($self, @_);
516 ## --------------------------------------------------------
517 ## Class::MOP::Method
518 Class::MOP::Method->meta->add_attribute(
519 Class::MOP::Attribute->new('body' => (
520 reader => { 'body' => \&Class::MOP::Method::body },
524 Class::MOP::Method->meta->add_attribute(
525 Class::MOP::Attribute->new('associated_metaclass' => (
526 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
530 Class::MOP::Method->meta->add_attribute(
531 Class::MOP::Attribute->new('package_name' => (
532 reader => { 'package_name' => \&Class::MOP::Method::package_name },
536 Class::MOP::Method->meta->add_attribute(
537 Class::MOP::Attribute->new('name' => (
538 reader => { 'name' => \&Class::MOP::Method::name },
542 Class::MOP::Method->meta->add_attribute(
543 Class::MOP::Attribute->new('original_method' => (
544 reader => { 'original_method' => \&Class::MOP::Method::original_method },
545 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
549 ## --------------------------------------------------------
550 ## Class::MOP::Method::Wrapped
553 # the way this item is initialized, this
554 # really does not follow the standard
555 # practices of attributes, but we put
556 # it here for completeness
557 Class::MOP::Method::Wrapped->meta->add_attribute(
558 Class::MOP::Attribute->new('modifier_table')
561 ## --------------------------------------------------------
562 ## Class::MOP::Method::Generated
564 Class::MOP::Method::Generated->meta->add_attribute(
565 Class::MOP::Attribute->new('is_inline' => (
566 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
571 Class::MOP::Method::Generated->meta->add_attribute(
572 Class::MOP::Attribute->new('definition_context' => (
573 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
578 ## --------------------------------------------------------
579 ## Class::MOP::Method::Inlined
581 Class::MOP::Method::Inlined->meta->add_attribute(
582 Class::MOP::Attribute->new('_expected_method_class' => (
583 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
587 ## --------------------------------------------------------
588 ## Class::MOP::Method::Accessor
590 Class::MOP::Method::Accessor->meta->add_attribute(
591 Class::MOP::Attribute->new('attribute' => (
593 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
598 Class::MOP::Method::Accessor->meta->add_attribute(
599 Class::MOP::Attribute->new('accessor_type' => (
600 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
604 ## --------------------------------------------------------
605 ## Class::MOP::Method::Constructor
607 Class::MOP::Method::Constructor->meta->add_attribute(
608 Class::MOP::Attribute->new('options' => (
610 'options' => \&Class::MOP::Method::Constructor::options
612 default => sub { +{} }
616 Class::MOP::Method::Constructor->meta->add_attribute(
617 Class::MOP::Attribute->new('associated_metaclass' => (
618 init_arg => "metaclass", # FIXME alias and rename
620 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
625 ## --------------------------------------------------------
626 ## Class::MOP::Instance
629 # these don't yet do much of anything, but are just
630 # included for completeness
632 Class::MOP::Instance->meta->add_attribute(
633 Class::MOP::Attribute->new('associated_metaclass',
634 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
638 Class::MOP::Instance->meta->add_attribute(
639 Class::MOP::Attribute->new('_class_name',
641 reader => { _class_name => \&Class::MOP::Instance::_class_name },
642 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
643 #default => sub { $_[0]->associated_metaclass->name },
647 Class::MOP::Instance->meta->add_attribute(
648 Class::MOP::Attribute->new('attributes',
649 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
653 Class::MOP::Instance->meta->add_attribute(
654 Class::MOP::Attribute->new('slots',
655 reader => { slots => \&Class::MOP::Instance::slots },
659 Class::MOP::Instance->meta->add_attribute(
660 Class::MOP::Attribute->new('slot_hash',
661 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
665 ## --------------------------------------------------------
666 ## Class::MOP::Object
668 # need to replace the meta method there with a real meta method object
669 Class::MOP::Object->meta->_add_meta_method('meta');
671 ## --------------------------------------------------------
674 # need to replace the meta method there with a real meta method object
675 Class::MOP::Mixin->meta->_add_meta_method('meta');
677 require Class::MOP::Deprecated unless our $no_deprecated;
679 # we need the meta instance of the meta instance to be created now, in order
680 # for the constructor to be able to use it
681 Class::MOP::Instance->meta->get_meta_instance;
683 # pretend the add_method never happenned. it hasn't yet affected anything
684 undef Class::MOP::Instance->meta->{_package_cache_flag};
686 ## --------------------------------------------------------
687 ## Now close all the Class::MOP::* classes
689 # NOTE: we don't need to inline the the accessors this only lengthens
690 # the compile time of the MOP, and gives us no actual benefits.
692 $_->meta->make_immutable(
693 inline_constructor => 0,
694 constructor_name => "_new",
695 inline_accessors => 0,
701 Class::MOP::Attribute
707 Class::MOP::Method::Generated
708 Class::MOP::Method::Inlined
710 Class::MOP::Method::Accessor
711 Class::MOP::Method::Constructor
712 Class::MOP::Method::Wrapped
714 Class::MOP::Method::Meta
717 $_->meta->make_immutable(
718 inline_constructor => 0,
719 constructor_name => undef,
720 inline_accessors => 0,
723 Class::MOP::Mixin::AttributeCore
724 Class::MOP::Mixin::HasAttributes
725 Class::MOP::Mixin::HasMethods
730 # ABSTRACT: A Meta Object Protocol for Perl 5
738 This module is a fully functioning meta object protocol for the
739 Perl 5 object system. It makes no attempt to change the behavior or
740 characteristics of the Perl 5 object system, only to create a
741 protocol for its manipulation and introspection.
743 That said, it does attempt to create the tools for building a rich set
744 of extensions to the Perl 5 object system. Every attempt has been made
745 to abide by the spirit of the Perl 5 object system that we all know
748 This documentation is sparse on conceptual details. We suggest looking
749 at the items listed in the L<SEE ALSO> section for more
750 information. In particular the book "The Art of the Meta Object
751 Protocol" was very influential in the development of this system.
753 =head2 What is a Meta Object Protocol?
755 A meta object protocol is an API to an object system.
757 To be more specific, it abstracts the components of an object system
758 (classes, object, methods, object attributes, etc.). These
759 abstractions can then be used to inspect and manipulate the object
760 system which they describe.
762 It can be said that there are two MOPs for any object system; the
763 implicit MOP and the explicit MOP. The implicit MOP handles things
764 like method dispatch or inheritance, which happen automatically as
765 part of how the object system works. The explicit MOP typically
766 handles the introspection/reflection features of the object system.
768 All object systems have implicit MOPs. Without one, they would not
769 work. Explicit MOPs are much less common, and depending on the
770 language can vary from restrictive (Reflection in Java or C#) to wide
771 open (CLOS is a perfect example).
773 =head2 Yet Another Class Builder! Why?
775 This is B<not> a class builder so much as a I<class builder
776 B<builder>>. The intent is that an end user will not use this module
777 directly, but instead this module is used by module authors to build
778 extensions and features onto the Perl 5 object system.
780 This system is used by L<Moose>, which supplies a powerful class
781 builder system built entirely on top of C<Class::MOP>.
783 =head2 Who is this module for?
785 This module is for anyone who has ever created or wanted to create a
786 module for the Class:: namespace. The tools which this module provides
787 make doing complex Perl 5 wizardry simpler, by removing such barriers
788 as the need to hack symbol tables, or understand the fine details of
791 =head2 What changes do I have to make to use this module?
793 This module was designed to be as unintrusive as possible. Many of its
794 features are accessible without B<any> change to your existing
795 code. It is meant to be a compliment to your existing code and not an
796 intrusion on your code base. Unlike many other B<Class::> modules,
797 this module B<does not> require you subclass it, or even that you
798 C<use> it in within your module's package.
800 The only features which requires additions to your code are the
801 attribute handling and instance construction features, and these are
802 both completely optional features. The only reason for this is because
803 Perl 5's object system does not actually have these features built
804 in. More information about this feature can be found below.
806 =head2 About Performance
808 It is a common misconception that explicit MOPs are a performance hit.
809 This is not a universal truth, it is a side-effect of some specific
810 implementations. For instance, using Java reflection is slow because
811 the JVM cannot take advantage of any compiler optimizations, and the
812 JVM has to deal with much more runtime type information as well.
814 Reflection in C# is marginally better as it was designed into the
815 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
816 Object System) was built to support an explicit MOP, and so
817 performance is tuned for it.
819 This library in particular does its absolute best to avoid putting
820 B<any> drain at all upon your code's performance. In fact, by itself
821 it does nothing to affect your existing code. So you only pay for what
824 =head2 About Metaclass compatibility
826 This module makes sure that all metaclasses created are both upwards
827 and downwards compatible. The topic of metaclass compatibility is
828 highly esoteric and is something only encountered when doing deep and
829 involved metaclass hacking. There are two basic kinds of metaclass
830 incompatibility; upwards and downwards.
832 Upwards metaclass compatibility means that the metaclass of a
833 given class is either the same as (or a subclass of) all of the
836 Downward metaclass compatibility means that the metaclasses of a
837 given class's ancestors are all either the same as (or a subclass
840 Here is a diagram showing a set of two classes (C<A> and C<B>) and
841 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
842 metaclass compatibility both upwards and downwards.
844 +---------+ +---------+
845 | Meta::A |<----| Meta::B | <....... (instance of )
846 +---------+ +---------+ <------- (inherits from)
849 +---------+ +---------+
851 +---------+ +---------+
853 In actuality, I<all> of a class's metaclasses must be compatible,
854 not just the class metaclass. That includes the instance, attribute,
855 and method metaclasses, as well as the constructor and destructor
858 C<Class::MOP> will attempt to fix some simple types of
859 incompatibilities. If all the metaclasses for the parent class are
860 I<subclasses> of the child's metaclasses then we can simply replace
861 the child's metaclasses with the parent's. In addition, if the child
862 is missing a metaclass that the parent has, we can also just make the
863 child use the parent's metaclass.
865 As I said this is a highly esoteric topic and one you will only run
866 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
867 are interested in why this is an issue see the paper I<Uniform and
868 safe metaclass composition> linked to in the L<SEE ALSO> section of
871 =head2 Using custom metaclasses
873 Always use the L<metaclass> pragma when using a custom metaclass, this
874 will ensure the proper initialization order and not accidentally
875 create an incorrect type of metaclass for you. This is a very rare
876 problem, and one which can only occur if you are doing deep metaclass
877 programming. So in other words, don't worry about it.
879 Note that if you're using L<Moose> we encourage you to I<not> use
880 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
881 roles to a class's metaclasses. This topic is covered at length in
882 various L<Moose::Cookbook> recipes.
886 The meta-object protocol is divided into 4 main sub-protocols:
888 =head2 The Class protocol
890 This provides a means of manipulating and introspecting a Perl 5
891 class. It handles symbol table hacking for you, and provides a rich
892 set of methods that go beyond simple package introspection.
894 See L<Class::MOP::Class> for more details.
896 =head2 The Attribute protocol
898 This provides a consistent representation for an attribute of a Perl 5
899 class. Since there are so many ways to create and handle attributes in
900 Perl 5 OO, the Attribute protocol provide as much of a unified
901 approach as possible. Of course, you are always free to extend this
902 protocol by subclassing the appropriate classes.
904 See L<Class::MOP::Attribute> for more details.
906 =head2 The Method protocol
908 This provides a means of manipulating and introspecting methods in the
909 Perl 5 object system. As with attributes, there are many ways to
910 approach this topic, so we try to keep it pretty basic, while still
911 making it possible to extend the system in many ways.
913 See L<Class::MOP::Method> for more details.
915 =head2 The Instance protocol
917 This provides a layer of abstraction for creating object instances.
918 Since the other layers use this protocol, it is relatively easy to
919 change the type of your instances from the default hash reference to
920 some other type of reference. Several examples are provided in the
921 F<examples/> directory included in this distribution.
923 See L<Class::MOP::Instance> for more details.
927 Note that this module does not export any constants or functions.
933 =item I<Class::MOP::IS_RUNNING_ON_5_10>
935 We set this constant depending on what version perl we are on, this
936 allows us to take advantage of new 5.10 features and stay backwards
941 =head2 Utility functions
943 Note that these are all called as B<functions, not methods>.
947 =item B<Class::MOP::load_class($class_name, \%options?)>
949 This will load the specified C<$class_name>, if it is not already
950 loaded (as reported by C<is_class_loaded>). This function can be used
951 in place of tricks like C<eval "use $module"> or using C<require>
954 If the module cannot be loaded, an exception is thrown.
956 You can pass a hash reference with options as second argument. The
957 only option currently recognised is C<-version>, which will ensure
958 that the loaded class has at least the required version.
960 See also L</Class Loading Options>.
962 For historical reasons, this function explicitly returns a true value.
964 =item B<Class::MOP::is_class_loaded($class_name, \%options?)>
966 Returns a boolean indicating whether or not C<$class_name> has been
969 This does a basic check of the symbol table to try and determine as
970 best it can if the C<$class_name> is loaded, it is probably correct
971 about 99% of the time, but it can be fooled into reporting false
972 positives. In particular, loading any of the core L<IO> modules will
973 cause most of the rest of the core L<IO> modules to falsely report
974 having been loaded, due to the way the base L<IO> module works.
976 You can pass a hash reference with options as second argument. The
977 only option currently recognised is C<-version>, which will ensure
978 that the loaded class has at least the required version.
980 See also L</Class Loading Options>.
982 =item B<Class::MOP::get_code_info($code)>
984 This function returns two values, the name of the package the C<$code>
985 is from and the name of the C<$code> itself. This is used by several
986 elements of the MOP to determine where a given C<$code> reference is
989 =item B<Class::MOP::class_of($instance_or_class_name)>
991 This will return the metaclass of the given instance or class name. If the
992 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
995 =item B<Class::MOP::check_package_cache_flag($pkg)>
997 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
999 This will return an integer that is managed by L<Class::MOP::Class> to
1000 determine if a module's symbol table has been altered.
1002 In Perl 5.10 or greater, this flag is package specific. However in
1003 versions prior to 5.10, this will use the C<PL_sub_generation>
1004 variable which is not package specific.
1006 =item B<Class::MOP::load_first_existing_class(@class_names)>
1008 =item B<Class::MOP::load_first_existing_class($classA, \%optionsA?, $classB, ...)>
1010 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1012 Given a list of class names, this function will attempt to load each
1015 If it finds a class it can load, it will return that class' name. If
1016 none of the classes can be loaded, it will throw an exception.
1018 Additionally, you can pass a hash reference with options after each
1019 class name. Currently, only C<-version> is recognised and will ensure
1020 that the loaded class has at least the required version. If the class
1021 version is not sufficient, an exception will be raised.
1023 See also L</Class Loading Options>.
1027 =head2 Metaclass cache functions
1029 Class::MOP holds a cache of metaclasses. The following are functions
1030 (B<not methods>) which can be used to access that cache. It is not
1031 recommended that you mess with these. Bad things could happen, but if
1032 you are brave and willing to risk it: go for it!
1036 =item B<Class::MOP::get_all_metaclasses>
1038 This will return a hash of all the metaclass instances that have
1039 been cached by L<Class::MOP::Class>, keyed by the package name.
1041 =item B<Class::MOP::get_all_metaclass_instances>
1043 This will return a list of all the metaclass instances that have
1044 been cached by L<Class::MOP::Class>.
1046 =item B<Class::MOP::get_all_metaclass_names>
1048 This will return a list of all the metaclass names that have
1049 been cached by L<Class::MOP::Class>.
1051 =item B<Class::MOP::get_metaclass_by_name($name)>
1053 This will return a cached L<Class::MOP::Class> instance, or nothing
1054 if no metaclass exists with that C<$name>.
1056 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
1058 This will store a metaclass in the cache at the supplied C<$key>.
1060 =item B<Class::MOP::weaken_metaclass($name)>
1062 In rare cases (e.g. anonymous metaclasses) it is desirable to
1063 store a weakened reference in the metaclass cache. This
1064 function will weaken the reference to the metaclass stored
1067 =item B<Class::MOP::metaclass_is_weak($name)>
1069 Returns true if the metaclass for C<$name> has been weakened
1070 (via C<weaken_metaclass>).
1072 =item B<Class::MOP::does_metaclass_exist($name)>
1074 This will return true of there exists a metaclass stored in the
1075 C<$name> key, and return false otherwise.
1077 =item B<Class::MOP::remove_metaclass_by_name($name)>
1079 This will remove the metaclass stored in the C<$name> key.
1083 =head2 Class Loading Options
1089 Can be used to pass a minimum required version that will be checked
1090 against the class version after it was loaded.
1098 There are very few books out on Meta Object Protocols and Metaclasses
1099 because it is such an esoteric topic. The following books are really
1100 the only ones I have found. If you know of any more, B<I<please>>
1101 email me and let me know, I would love to hear about them.
1105 =item I<The Art of the Meta Object Protocol>
1107 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1109 =item I<Putting MetaClasses to Work>
1111 =item I<Smalltalk: The Language>
1119 =item "Uniform and safe metaclass composition"
1121 An excellent paper by the people who brought us the original Traits paper.
1122 This paper is on how Traits can be used to do safe metaclass composition,
1123 and offers an excellent introduction section which delves into the topic of
1124 metaclass compatibility.
1126 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1128 =item "Safe Metaclass Programming"
1130 This paper seems to precede the above paper, and propose a mix-in based
1131 approach as opposed to the Traits based approach. Both papers have similar
1132 information on the metaclass compatibility problem space.
1134 L<http://citeseer.ist.psu.edu/37617.html>
1142 =item The Perl 6 MetaModel work in the Pugs project
1146 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1148 =item L<http://github.com/perl6/p5-modules/tree/master/Perl6-ObjectSpace/>
1158 =item CPAN Module Review of Class::MOP
1160 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1164 =head1 SIMILAR MODULES
1166 As I have said above, this module is a class-builder-builder, so it is
1167 not the same thing as modules like L<Class::Accessor> and
1168 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1169 with similar goals to this module. The one I have found which is most
1170 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1171 creates are very different from this modules.
1175 All complex software has bugs lurking in it, and this module is no
1178 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1179 web interface at L<http://rt.cpan.org>.
1181 You can also discuss feature requests or possible bugs on the Moose
1182 mailing list (moose@perl.org) or on IRC at
1183 L<irc://irc.perl.org/#moose>.
1185 =head1 ACKNOWLEDGEMENTS
1191 Thanks to Rob for actually getting the development of this module kick-started.