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 $Class::MOP::{VERSION} ? ${ $Class::MOP::{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 sub _definition_context {
156 @context{qw(package file line)} = caller(1);
159 definition_context => \%context,
163 ## ----------------------------------------------------------------------------
164 ## Setting up our environment ...
165 ## ----------------------------------------------------------------------------
166 ## Class::MOP needs to have a few things in the global perl environment so
167 ## that it can operate effectively. Those things are done here.
168 ## ----------------------------------------------------------------------------
170 # ... nothing yet actually ;)
172 ## ----------------------------------------------------------------------------
174 ## ----------------------------------------------------------------------------
175 ## The code below here is to bootstrap our MOP with itself. This is also
176 ## sometimes called "tying the knot". By doing this, we make it much easier
177 ## to extend the MOP through subclassing and such since now you can use the
178 ## MOP itself to extend itself.
180 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
181 ## ----------------------------------------------------------------------------
183 # We need to add in the meta-attributes here so that
184 # any subclass of Class::MOP::* will be able to
185 # inherit them using _construct_instance
187 ## --------------------------------------------------------
188 ## Class::MOP::Mixin::HasMethods
190 Class::MOP::Mixin::HasMethods->meta->add_attribute(
191 Class::MOP::Attribute->new('_methods' => (
194 # we just alias the original method
195 # rather than re-produce it here
196 '_method_map' => \&Class::MOP::Mixin::HasMethods::_method_map
198 default => sub { {} },
199 _definition_context(),
203 Class::MOP::Mixin::HasMethods->meta->add_attribute(
204 Class::MOP::Attribute->new('method_metaclass' => (
207 # we just alias the original method
208 # rather than re-produce it here
209 'method_metaclass' => \&Class::MOP::Mixin::HasMethods::method_metaclass
211 default => 'Class::MOP::Method',
212 _definition_context(),
216 Class::MOP::Mixin::HasMethods->meta->add_attribute(
217 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
220 # we just alias the original method
221 # rather than re-produce it here
222 'wrapped_method_metaclass' => \&Class::MOP::Mixin::HasMethods::wrapped_method_metaclass
224 default => 'Class::MOP::Method::Wrapped',
225 _definition_context(),
229 ## --------------------------------------------------------
230 ## Class::MOP::Mixin::HasMethods
232 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
233 Class::MOP::Attribute->new('attributes' => (
235 # NOTE: we need to do this in order
236 # for the instance meta-object to
237 # not fall into meta-circular death
239 # we just alias the original method
240 # rather than re-produce it here
241 '_attribute_map' => \&Class::MOP::Mixin::HasAttributes::_attribute_map
243 default => sub { {} },
244 _definition_context(),
248 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
249 Class::MOP::Attribute->new('attribute_metaclass' => (
252 # we just alias the original method
253 # rather than re-produce it here
254 'attribute_metaclass' => \&Class::MOP::Mixin::HasAttributes::attribute_metaclass
256 default => 'Class::MOP::Attribute',
257 _definition_context(),
261 ## --------------------------------------------------------
262 ## Class::MOP::Package
264 Class::MOP::Package->meta->add_attribute(
265 Class::MOP::Attribute->new('package' => (
267 # NOTE: we need to do this in order
268 # for the instance meta-object to
269 # not fall into meta-circular death
271 # we just alias the original method
272 # rather than re-produce it here
273 'name' => \&Class::MOP::Package::name
275 _definition_context(),
279 Class::MOP::Package->meta->add_attribute(
280 Class::MOP::Attribute->new('namespace' => (
283 # we just alias the original method
284 # rather than re-produce it here
285 'namespace' => \&Class::MOP::Package::namespace
288 default => sub { \undef },
289 _definition_context(),
293 ## --------------------------------------------------------
294 ## Class::MOP::Module
297 # yeah this is kind of stretching things a bit,
298 # but truthfully the version should be an attribute
299 # of the Module, the weirdness comes from having to
300 # stick to Perl 5 convention and store it in the
301 # $VERSION package variable. Basically if you just
302 # squint at it, it will look how you want it to look.
303 # Either as a package variable, or as a attribute of
304 # the metaclass, isn't abstraction great :)
306 Class::MOP::Module->meta->add_attribute(
307 Class::MOP::Attribute->new('version' => (
310 # we just alias the original method
311 # rather than re-produce it here
312 'version' => \&Class::MOP::Module::version
315 default => sub { \undef },
316 _definition_context(),
321 # By following the same conventions as version here,
322 # we are opening up the possibility that people can
323 # use the $AUTHORITY in non-Class::MOP modules as
326 Class::MOP::Module->meta->add_attribute(
327 Class::MOP::Attribute->new('authority' => (
330 # we just alias the original method
331 # rather than re-produce it here
332 'authority' => \&Class::MOP::Module::authority
335 default => sub { \undef },
336 _definition_context(),
340 ## --------------------------------------------------------
343 Class::MOP::Class->meta->add_attribute(
344 Class::MOP::Attribute->new('superclasses' => (
347 # we just alias the original method
348 # rather than re-produce it here
349 'superclasses' => \&Class::MOP::Class::superclasses
352 default => sub { \undef },
353 _definition_context(),
357 Class::MOP::Class->meta->add_attribute(
358 Class::MOP::Attribute->new('instance_metaclass' => (
360 # NOTE: we need to do this in order
361 # for the instance meta-object to
362 # not fall into meta-circular death
364 # we just alias the original method
365 # rather than re-produce it here
366 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
368 default => 'Class::MOP::Instance',
369 _definition_context(),
373 Class::MOP::Class->meta->add_attribute(
374 Class::MOP::Attribute->new('immutable_trait' => (
376 'immutable_trait' => \&Class::MOP::Class::immutable_trait
378 default => "Class::MOP::Class::Immutable::Trait",
379 _definition_context(),
383 Class::MOP::Class->meta->add_attribute(
384 Class::MOP::Attribute->new('constructor_name' => (
386 'constructor_name' => \&Class::MOP::Class::constructor_name,
389 _definition_context(),
393 Class::MOP::Class->meta->add_attribute(
394 Class::MOP::Attribute->new('constructor_class' => (
396 'constructor_class' => \&Class::MOP::Class::constructor_class,
398 default => "Class::MOP::Method::Constructor",
399 _definition_context(),
404 Class::MOP::Class->meta->add_attribute(
405 Class::MOP::Attribute->new('destructor_class' => (
407 'destructor_class' => \&Class::MOP::Class::destructor_class,
409 _definition_context(),
414 # we don't actually need to tie the knot with
415 # Class::MOP::Class here, it is actually handled
416 # within Class::MOP::Class itself in the
417 # _construct_class_instance method.
419 ## --------------------------------------------------------
420 ## Class::MOP::Mixin::AttributeCore
421 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
422 Class::MOP::Attribute->new('name' => (
424 # NOTE: we need to do this in order
425 # for the instance meta-object to
426 # not fall into meta-circular death
428 # we just alias the original method
429 # rather than re-produce it here
430 'name' => \&Class::MOP::Mixin::AttributeCore::name
432 _definition_context(),
436 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
437 Class::MOP::Attribute->new('accessor' => (
438 reader => { 'accessor' => \&Class::MOP::Mixin::AttributeCore::accessor },
439 predicate => { 'has_accessor' => \&Class::MOP::Mixin::AttributeCore::has_accessor },
440 _definition_context(),
444 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
445 Class::MOP::Attribute->new('reader' => (
446 reader => { 'reader' => \&Class::MOP::Mixin::AttributeCore::reader },
447 predicate => { 'has_reader' => \&Class::MOP::Mixin::AttributeCore::has_reader },
448 _definition_context(),
452 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
453 Class::MOP::Attribute->new('initializer' => (
454 reader => { 'initializer' => \&Class::MOP::Mixin::AttributeCore::initializer },
455 predicate => { 'has_initializer' => \&Class::MOP::Mixin::AttributeCore::has_initializer },
456 _definition_context(),
460 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
461 Class::MOP::Attribute->new('definition_context' => (
462 reader => { 'definition_context' => \&Class::MOP::Mixin::AttributeCore::definition_context },
463 _definition_context(),
467 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
468 Class::MOP::Attribute->new('writer' => (
469 reader => { 'writer' => \&Class::MOP::Mixin::AttributeCore::writer },
470 predicate => { 'has_writer' => \&Class::MOP::Mixin::AttributeCore::has_writer },
471 _definition_context(),
475 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
476 Class::MOP::Attribute->new('predicate' => (
477 reader => { 'predicate' => \&Class::MOP::Mixin::AttributeCore::predicate },
478 predicate => { 'has_predicate' => \&Class::MOP::Mixin::AttributeCore::has_predicate },
479 _definition_context(),
483 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
484 Class::MOP::Attribute->new('clearer' => (
485 reader => { 'clearer' => \&Class::MOP::Mixin::AttributeCore::clearer },
486 predicate => { 'has_clearer' => \&Class::MOP::Mixin::AttributeCore::has_clearer },
487 _definition_context(),
491 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
492 Class::MOP::Attribute->new('builder' => (
493 reader => { 'builder' => \&Class::MOP::Mixin::AttributeCore::builder },
494 predicate => { 'has_builder' => \&Class::MOP::Mixin::AttributeCore::has_builder },
495 _definition_context(),
499 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
500 Class::MOP::Attribute->new('init_arg' => (
501 reader => { 'init_arg' => \&Class::MOP::Mixin::AttributeCore::init_arg },
502 predicate => { 'has_init_arg' => \&Class::MOP::Mixin::AttributeCore::has_init_arg },
503 _definition_context(),
507 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
508 Class::MOP::Attribute->new('default' => (
509 # default has a custom 'reader' method ...
510 predicate => { 'has_default' => \&Class::MOP::Mixin::AttributeCore::has_default },
511 _definition_context(),
515 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
516 Class::MOP::Attribute->new('insertion_order' => (
517 reader => { 'insertion_order' => \&Class::MOP::Mixin::AttributeCore::insertion_order },
518 writer => { '_set_insertion_order' => \&Class::MOP::Mixin::AttributeCore::_set_insertion_order },
519 predicate => { 'has_insertion_order' => \&Class::MOP::Mixin::AttributeCore::has_insertion_order },
520 _definition_context(),
524 ## --------------------------------------------------------
525 ## Class::MOP::Attribute
526 Class::MOP::Attribute->meta->add_attribute(
527 Class::MOP::Attribute->new('associated_class' => (
529 # NOTE: we need to do this in order
530 # for the instance meta-object to
531 # not fall into meta-circular death
533 # we just alias the original method
534 # rather than re-produce it here
535 'associated_class' => \&Class::MOP::Attribute::associated_class
537 _definition_context(),
541 Class::MOP::Attribute->meta->add_attribute(
542 Class::MOP::Attribute->new('associated_methods' => (
543 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
544 default => sub { [] },
545 _definition_context(),
549 Class::MOP::Attribute->meta->add_method('clone' => sub {
551 $self->meta->clone_object($self, @_);
554 ## --------------------------------------------------------
555 ## Class::MOP::Method
556 Class::MOP::Method->meta->add_attribute(
557 Class::MOP::Attribute->new('body' => (
558 reader => { 'body' => \&Class::MOP::Method::body },
559 _definition_context(),
563 Class::MOP::Method->meta->add_attribute(
564 Class::MOP::Attribute->new('associated_metaclass' => (
565 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
566 _definition_context(),
570 Class::MOP::Method->meta->add_attribute(
571 Class::MOP::Attribute->new('package_name' => (
572 reader => { 'package_name' => \&Class::MOP::Method::package_name },
573 _definition_context(),
577 Class::MOP::Method->meta->add_attribute(
578 Class::MOP::Attribute->new('name' => (
579 reader => { 'name' => \&Class::MOP::Method::name },
580 _definition_context(),
584 Class::MOP::Method->meta->add_attribute(
585 Class::MOP::Attribute->new('original_method' => (
586 reader => { 'original_method' => \&Class::MOP::Method::original_method },
587 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
588 _definition_context(),
592 ## --------------------------------------------------------
593 ## Class::MOP::Method::Wrapped
596 # the way this item is initialized, this
597 # really does not follow the standard
598 # practices of attributes, but we put
599 # it here for completeness
600 Class::MOP::Method::Wrapped->meta->add_attribute(
601 Class::MOP::Attribute->new('modifier_table' => (
602 _definition_context(),
606 ## --------------------------------------------------------
607 ## Class::MOP::Method::Generated
609 Class::MOP::Method::Generated->meta->add_attribute(
610 Class::MOP::Attribute->new('is_inline' => (
611 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
613 _definition_context(),
617 Class::MOP::Method::Generated->meta->add_attribute(
618 Class::MOP::Attribute->new('definition_context' => (
619 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
620 _definition_context(),
625 ## --------------------------------------------------------
626 ## Class::MOP::Method::Inlined
628 Class::MOP::Method::Inlined->meta->add_attribute(
629 Class::MOP::Attribute->new('_expected_method_class' => (
630 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
631 _definition_context(),
635 ## --------------------------------------------------------
636 ## Class::MOP::Method::Accessor
638 Class::MOP::Method::Accessor->meta->add_attribute(
639 Class::MOP::Attribute->new('attribute' => (
641 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
643 _definition_context(),
647 Class::MOP::Method::Accessor->meta->add_attribute(
648 Class::MOP::Attribute->new('accessor_type' => (
649 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
650 _definition_context(),
654 ## --------------------------------------------------------
655 ## Class::MOP::Method::Constructor
657 Class::MOP::Method::Constructor->meta->add_attribute(
658 Class::MOP::Attribute->new('options' => (
660 'options' => \&Class::MOP::Method::Constructor::options
662 default => sub { +{} },
663 _definition_context(),
667 Class::MOP::Method::Constructor->meta->add_attribute(
668 Class::MOP::Attribute->new('associated_metaclass' => (
669 init_arg => "metaclass", # FIXME alias and rename
671 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
673 _definition_context(),
677 ## --------------------------------------------------------
678 ## Class::MOP::Instance
681 # these don't yet do much of anything, but are just
682 # included for completeness
684 Class::MOP::Instance->meta->add_attribute(
685 Class::MOP::Attribute->new('associated_metaclass',
686 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
687 _definition_context(),
691 Class::MOP::Instance->meta->add_attribute(
692 Class::MOP::Attribute->new('_class_name',
694 reader => { _class_name => \&Class::MOP::Instance::_class_name },
695 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
696 #default => sub { $_[0]->associated_metaclass->name },
697 _definition_context(),
701 Class::MOP::Instance->meta->add_attribute(
702 Class::MOP::Attribute->new('attributes',
703 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
704 _definition_context(),
708 Class::MOP::Instance->meta->add_attribute(
709 Class::MOP::Attribute->new('slots',
710 reader => { slots => \&Class::MOP::Instance::slots },
711 _definition_context(),
715 Class::MOP::Instance->meta->add_attribute(
716 Class::MOP::Attribute->new('slot_hash',
717 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
718 _definition_context(),
722 ## --------------------------------------------------------
723 ## Class::MOP::Object
725 # need to replace the meta method there with a real meta method object
726 Class::MOP::Object->meta->_add_meta_method('meta');
728 ## --------------------------------------------------------
731 # need to replace the meta method there with a real meta method object
732 Class::MOP::Mixin->meta->_add_meta_method('meta');
734 require Class::MOP::Deprecated unless our $no_deprecated;
736 # we need the meta instance of the meta instance to be created now, in order
737 # for the constructor to be able to use it
738 Class::MOP::Instance->meta->get_meta_instance;
740 # pretend the add_method never happenned. it hasn't yet affected anything
741 undef Class::MOP::Instance->meta->{_package_cache_flag};
743 ## --------------------------------------------------------
744 ## Now close all the Class::MOP::* classes
746 # NOTE: we don't need to inline the the accessors this only lengthens
747 # the compile time of the MOP, and gives us no actual benefits.
749 $_->meta->make_immutable(
750 inline_constructor => 0,
751 constructor_name => "_new",
752 inline_accessors => 0,
758 Class::MOP::Attribute
764 Class::MOP::Method::Generated
765 Class::MOP::Method::Inlined
767 Class::MOP::Method::Accessor
768 Class::MOP::Method::Constructor
769 Class::MOP::Method::Wrapped
771 Class::MOP::Method::Meta
774 $_->meta->make_immutable(
775 inline_constructor => 0,
776 constructor_name => undef,
777 inline_accessors => 0,
780 Class::MOP::Mixin::AttributeCore
781 Class::MOP::Mixin::HasAttributes
782 Class::MOP::Mixin::HasMethods
787 # ABSTRACT: A Meta Object Protocol for Perl 5
795 This module is a fully functioning meta object protocol for the
796 Perl 5 object system. It makes no attempt to change the behavior or
797 characteristics of the Perl 5 object system, only to create a
798 protocol for its manipulation and introspection.
800 That said, it does attempt to create the tools for building a rich set
801 of extensions to the Perl 5 object system. Every attempt has been made
802 to abide by the spirit of the Perl 5 object system that we all know
805 This documentation is sparse on conceptual details. We suggest looking
806 at the items listed in the L<SEE ALSO> section for more
807 information. In particular the book "The Art of the Meta Object
808 Protocol" was very influential in the development of this system.
810 =head2 What is a Meta Object Protocol?
812 A meta object protocol is an API to an object system.
814 To be more specific, it abstracts the components of an object system
815 (classes, object, methods, object attributes, etc.). These
816 abstractions can then be used to inspect and manipulate the object
817 system which they describe.
819 It can be said that there are two MOPs for any object system; the
820 implicit MOP and the explicit MOP. The implicit MOP handles things
821 like method dispatch or inheritance, which happen automatically as
822 part of how the object system works. The explicit MOP typically
823 handles the introspection/reflection features of the object system.
825 All object systems have implicit MOPs. Without one, they would not
826 work. Explicit MOPs are much less common, and depending on the
827 language can vary from restrictive (Reflection in Java or C#) to wide
828 open (CLOS is a perfect example).
830 =head2 Yet Another Class Builder! Why?
832 This is B<not> a class builder so much as a I<class builder
833 B<builder>>. The intent is that an end user will not use this module
834 directly, but instead this module is used by module authors to build
835 extensions and features onto the Perl 5 object system.
837 This system is used by L<Moose>, which supplies a powerful class
838 builder system built entirely on top of C<Class::MOP>.
840 =head2 Who is this module for?
842 This module is for anyone who has ever created or wanted to create a
843 module for the Class:: namespace. The tools which this module provides
844 make doing complex Perl 5 wizardry simpler, by removing such barriers
845 as the need to hack symbol tables, or understand the fine details of
848 =head2 What changes do I have to make to use this module?
850 This module was designed to be as unintrusive as possible. Many of its
851 features are accessible without B<any> change to your existing
852 code. It is meant to be a compliment to your existing code and not an
853 intrusion on your code base. Unlike many other B<Class::> modules,
854 this module B<does not> require you subclass it, or even that you
855 C<use> it in within your module's package.
857 The only features which requires additions to your code are the
858 attribute handling and instance construction features, and these are
859 both completely optional features. The only reason for this is because
860 Perl 5's object system does not actually have these features built
861 in. More information about this feature can be found below.
863 =head2 About Performance
865 It is a common misconception that explicit MOPs are a performance hit.
866 This is not a universal truth, it is a side-effect of some specific
867 implementations. For instance, using Java reflection is slow because
868 the JVM cannot take advantage of any compiler optimizations, and the
869 JVM has to deal with much more runtime type information as well.
871 Reflection in C# is marginally better as it was designed into the
872 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
873 Object System) was built to support an explicit MOP, and so
874 performance is tuned for it.
876 This library in particular does its absolute best to avoid putting
877 B<any> drain at all upon your code's performance. In fact, by itself
878 it does nothing to affect your existing code. So you only pay for what
881 =head2 About Metaclass compatibility
883 This module makes sure that all metaclasses created are both upwards
884 and downwards compatible. The topic of metaclass compatibility is
885 highly esoteric and is something only encountered when doing deep and
886 involved metaclass hacking. There are two basic kinds of metaclass
887 incompatibility; upwards and downwards.
889 Upwards metaclass compatibility means that the metaclass of a
890 given class is either the same as (or a subclass of) all of the
893 Downward metaclass compatibility means that the metaclasses of a
894 given class's ancestors are all either the same as (or a subclass
897 Here is a diagram showing a set of two classes (C<A> and C<B>) and
898 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
899 metaclass compatibility both upwards and downwards.
901 +---------+ +---------+
902 | Meta::A |<----| Meta::B | <....... (instance of )
903 +---------+ +---------+ <------- (inherits from)
906 +---------+ +---------+
908 +---------+ +---------+
910 In actuality, I<all> of a class's metaclasses must be compatible,
911 not just the class metaclass. That includes the instance, attribute,
912 and method metaclasses, as well as the constructor and destructor
915 C<Class::MOP> will attempt to fix some simple types of
916 incompatibilities. If all the metaclasses for the parent class are
917 I<subclasses> of the child's metaclasses then we can simply replace
918 the child's metaclasses with the parent's. In addition, if the child
919 is missing a metaclass that the parent has, we can also just make the
920 child use the parent's metaclass.
922 As I said this is a highly esoteric topic and one you will only run
923 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
924 are interested in why this is an issue see the paper I<Uniform and
925 safe metaclass composition> linked to in the L<SEE ALSO> section of
928 =head2 Using custom metaclasses
930 Always use the L<metaclass> pragma when using a custom metaclass, this
931 will ensure the proper initialization order and not accidentally
932 create an incorrect type of metaclass for you. This is a very rare
933 problem, and one which can only occur if you are doing deep metaclass
934 programming. So in other words, don't worry about it.
936 Note that if you're using L<Moose> we encourage you to I<not> use
937 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
938 roles to a class's metaclasses. This topic is covered at length in
939 various L<Moose::Cookbook> recipes.
943 The meta-object protocol is divided into 4 main sub-protocols:
945 =head2 The Class protocol
947 This provides a means of manipulating and introspecting a Perl 5
948 class. It handles symbol table hacking for you, and provides a rich
949 set of methods that go beyond simple package introspection.
951 See L<Class::MOP::Class> for more details.
953 =head2 The Attribute protocol
955 This provides a consistent representation for an attribute of a Perl 5
956 class. Since there are so many ways to create and handle attributes in
957 Perl 5 OO, the Attribute protocol provide as much of a unified
958 approach as possible. Of course, you are always free to extend this
959 protocol by subclassing the appropriate classes.
961 See L<Class::MOP::Attribute> for more details.
963 =head2 The Method protocol
965 This provides a means of manipulating and introspecting methods in the
966 Perl 5 object system. As with attributes, there are many ways to
967 approach this topic, so we try to keep it pretty basic, while still
968 making it possible to extend the system in many ways.
970 See L<Class::MOP::Method> for more details.
972 =head2 The Instance protocol
974 This provides a layer of abstraction for creating object instances.
975 Since the other layers use this protocol, it is relatively easy to
976 change the type of your instances from the default hash reference to
977 some other type of reference. Several examples are provided in the
978 F<examples/> directory included in this distribution.
980 See L<Class::MOP::Instance> for more details.
984 Note that this module does not export any constants or functions.
986 =head2 Utility functions
988 Note that these are all called as B<functions, not methods>.
992 =item B<Class::MOP::load_class($class_name, \%options?)>
994 This will load the specified C<$class_name>, if it is not already
995 loaded (as reported by C<is_class_loaded>). This function can be used
996 in place of tricks like C<eval "use $module"> or using C<require>
999 If the module cannot be loaded, an exception is thrown.
1001 You can pass a hash reference with options as second argument. The
1002 only option currently recognized is C<-version>, which will ensure
1003 that the loaded class has at least the required version.
1005 For historical reasons, this function explicitly returns a true value.
1007 =item B<Class::MOP::is_class_loaded($class_name, \%options?)>
1009 Returns a boolean indicating whether or not C<$class_name> has been
1012 This does a basic check of the symbol table to try and determine as
1013 best it can if the C<$class_name> is loaded, it is probably correct
1014 about 99% of the time, but it can be fooled into reporting false
1015 positives. In particular, loading any of the core L<IO> modules will
1016 cause most of the rest of the core L<IO> modules to falsely report
1017 having been loaded, due to the way the base L<IO> module works.
1019 You can pass a hash reference with options as second argument. The
1020 only option currently recognized is C<-version>, which will ensure
1021 that the loaded class has at least the required version.
1023 =item B<Class::MOP::get_code_info($code)>
1025 This function returns two values, the name of the package the C<$code>
1026 is from and the name of the C<$code> itself. This is used by several
1027 elements of the MOP to determine where a given C<$code> reference is
1030 =item B<Class::MOP::class_of($instance_or_class_name)>
1032 This will return the metaclass of the given instance or class name. If the
1033 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
1038 =head2 Metaclass cache functions
1040 Class::MOP holds a cache of metaclasses. The following are functions
1041 (B<not methods>) which can be used to access that cache. It is not
1042 recommended that you mess with these. Bad things could happen, but if
1043 you are brave and willing to risk it: go for it!
1047 =item B<Class::MOP::get_all_metaclasses>
1049 This will return a hash of all the metaclass instances that have
1050 been cached by L<Class::MOP::Class>, keyed by the package name.
1052 =item B<Class::MOP::get_all_metaclass_instances>
1054 This will return a list of all the metaclass instances that have
1055 been cached by L<Class::MOP::Class>.
1057 =item B<Class::MOP::get_all_metaclass_names>
1059 This will return a list of all the metaclass names that have
1060 been cached by L<Class::MOP::Class>.
1062 =item B<Class::MOP::get_metaclass_by_name($name)>
1064 This will return a cached L<Class::MOP::Class> instance, or nothing
1065 if no metaclass exists with that C<$name>.
1067 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
1069 This will store a metaclass in the cache at the supplied C<$key>.
1071 =item B<Class::MOP::weaken_metaclass($name)>
1073 In rare cases (e.g. anonymous metaclasses) it is desirable to
1074 store a weakened reference in the metaclass cache. This
1075 function will weaken the reference to the metaclass stored
1078 =item B<Class::MOP::metaclass_is_weak($name)>
1080 Returns true if the metaclass for C<$name> has been weakened
1081 (via C<weaken_metaclass>).
1083 =item B<Class::MOP::does_metaclass_exist($name)>
1085 This will return true of there exists a metaclass stored in the
1086 C<$name> key, and return false otherwise.
1088 =item B<Class::MOP::remove_metaclass_by_name($name)>
1090 This will remove the metaclass stored in the C<$name> key.
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 its 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.