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 $VERSION = '1.09';
33 our $XS_VERSION = $VERSION;
34 $VERSION = eval $VERSION;
35 our $AUTHORITY = 'cpan:STEVAN';
38 XSLoader::load( __PACKAGE__, $XS_VERSION );
41 # Metaclasses are singletons, so we cache them here.
42 # there is no need to worry about destruction though
43 # because they should die only when the program dies.
44 # After all, do package definitions even get reaped?
45 # Anonymous classes manage their own destruction.
48 sub get_all_metaclasses { %METAS }
49 sub get_all_metaclass_instances { values %METAS }
50 sub get_all_metaclass_names { keys %METAS }
51 sub get_metaclass_by_name { $METAS{$_[0]} }
52 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
53 sub weaken_metaclass { weaken($METAS{$_[0]}) }
54 sub metaclass_is_weak { isweak($METAS{$_[0]}) }
55 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
56 sub remove_metaclass_by_name { delete $METAS{$_[0]}; return }
58 # This handles instances as well as class names
60 return unless defined $_[0];
61 my $class = blessed($_[0]) || $_[0];
62 return $METAS{$class};
66 # We only cache metaclasses, meaning instances of
67 # Class::MOP::Class. We do not cache instance of
68 # Class::MOP::Package or Class::MOP::Module. Mostly
69 # because I don't yet see a good reason to do so.
72 sub _class_to_pmfile {
75 my $file = $class . '.pm';
81 sub load_first_existing_class {
82 my $classes = Data::OptList::mkopt(\@_)
85 foreach my $class (@{ $classes }) {
86 my $name = $class->[0];
87 unless ( _is_valid_class_name($name) ) {
88 my $display = defined($name) ? $name : 'undef';
89 confess "Invalid class name ($display)";
96 for my $class (@{ $classes }) {
97 my ($name, $options) = @{ $class };
100 return $name if is_class_loaded($name, $options);
101 if (is_class_loaded($name)) {
102 # we already know it's loaded and too old, but we call
103 # ->VERSION anyway to generate the exception for us
104 $name->VERSION($options->{-version});
108 return $name if is_class_loaded($name);
111 my $file = _class_to_pmfile($name);
112 return $name if try {
115 $name->VERSION($options->{-version})
116 if defined $options->{-version};
120 unless (/^Can't locate \Q$file\E in \@INC/) {
121 confess "Couldn't load class ($name) because: $_";
128 if ( @{ $classes } > 1 ) {
129 my @list = map { $_->[0] } @{ $classes };
130 confess "Can't locate any of @list in \@INC (\@INC contains: @INC).";
132 confess "Can't locate " . _class_to_pmfile($classes->[0]->[0]) . " in \@INC (\@INC contains: @INC).";
137 load_first_existing_class($_[0], ref $_[1] ? $_[1] : ());
139 # This is done to avoid breaking code which checked the return value. Said
140 # code is dumb. The return value was _always_ true, since it dies on
145 sub _is_valid_class_name {
148 return 0 if ref($class);
149 return 0 unless defined($class);
150 return 0 unless length($class);
152 return 1 if $class =~ /^\w+(?:::\w+)*$/;
157 ## ----------------------------------------------------------------------------
158 ## Setting up our environment ...
159 ## ----------------------------------------------------------------------------
160 ## Class::MOP needs to have a few things in the global perl environment so
161 ## that it can operate effectively. Those things are done here.
162 ## ----------------------------------------------------------------------------
164 # ... nothing yet actually ;)
166 ## ----------------------------------------------------------------------------
168 ## ----------------------------------------------------------------------------
169 ## The code below here is to bootstrap our MOP with itself. This is also
170 ## sometimes called "tying the knot". By doing this, we make it much easier
171 ## to extend the MOP through subclassing and such since now you can use the
172 ## MOP itself to extend itself.
174 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
175 ## ----------------------------------------------------------------------------
177 # We need to add in the meta-attributes here so that
178 # any subclass of Class::MOP::* will be able to
179 # inherit them using _construct_instance
181 ## --------------------------------------------------------
182 ## Class::MOP::Mixin::HasMethods
184 Class::MOP::Mixin::HasMethods->meta->add_attribute(
185 Class::MOP::Attribute->new('_methods' => (
188 # we just alias the original method
189 # rather than re-produce it here
190 '_full_method_map' => \&Class::MOP::Mixin::HasMethods::_full_method_map
192 default => sub { {} }
196 Class::MOP::Mixin::HasMethods->meta->add_attribute(
197 Class::MOP::Attribute->new('method_metaclass' => (
200 # we just alias the original method
201 # rather than re-produce it here
202 'method_metaclass' => \&Class::MOP::Mixin::HasMethods::method_metaclass
204 default => 'Class::MOP::Method',
208 Class::MOP::Mixin::HasMethods->meta->add_attribute(
209 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
212 # we just alias the original method
213 # rather than re-produce it here
214 'wrapped_method_metaclass' => \&Class::MOP::Mixin::HasMethods::wrapped_method_metaclass
216 default => 'Class::MOP::Method::Wrapped',
220 ## --------------------------------------------------------
221 ## Class::MOP::Mixin::HasMethods
223 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
224 Class::MOP::Attribute->new('attributes' => (
226 # NOTE: we need to do this in order
227 # for the instance meta-object to
228 # not fall into meta-circular death
230 # we just alias the original method
231 # rather than re-produce it here
232 '_attribute_map' => \&Class::MOP::Mixin::HasAttributes::_attribute_map
234 default => sub { {} }
238 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
239 Class::MOP::Attribute->new('attribute_metaclass' => (
242 # we just alias the original method
243 # rather than re-produce it here
244 'attribute_metaclass' => \&Class::MOP::Mixin::HasAttributes::attribute_metaclass
246 default => 'Class::MOP::Attribute',
250 ## --------------------------------------------------------
251 ## Class::MOP::Package
253 Class::MOP::Package->meta->add_attribute(
254 Class::MOP::Attribute->new('package' => (
256 # NOTE: we need to do this in order
257 # for the instance meta-object to
258 # not fall into meta-circular death
260 # we just alias the original method
261 # rather than re-produce it here
262 'name' => \&Class::MOP::Package::name
267 Class::MOP::Package->meta->add_attribute(
268 Class::MOP::Attribute->new('namespace' => (
271 # we just alias the original method
272 # rather than re-produce it here
273 'namespace' => \&Class::MOP::Package::namespace
276 default => sub { \undef }
280 ## --------------------------------------------------------
281 ## Class::MOP::Module
284 # yeah this is kind of stretching things a bit,
285 # but truthfully the version should be an attribute
286 # of the Module, the weirdness comes from having to
287 # stick to Perl 5 convention and store it in the
288 # $VERSION package variable. Basically if you just
289 # squint at it, it will look how you want it to look.
290 # Either as a package variable, or as a attribute of
291 # the metaclass, isn't abstraction great :)
293 Class::MOP::Module->meta->add_attribute(
294 Class::MOP::Attribute->new('version' => (
297 # we just alias the original method
298 # rather than re-produce it here
299 'version' => \&Class::MOP::Module::version
302 default => sub { \undef }
307 # By following the same conventions as version here,
308 # we are opening up the possibility that people can
309 # use the $AUTHORITY in non-Class::MOP modules as
312 Class::MOP::Module->meta->add_attribute(
313 Class::MOP::Attribute->new('authority' => (
316 # we just alias the original method
317 # rather than re-produce it here
318 'authority' => \&Class::MOP::Module::authority
321 default => sub { \undef }
325 ## --------------------------------------------------------
328 Class::MOP::Class->meta->add_attribute(
329 Class::MOP::Attribute->new('superclasses' => (
332 # we just alias the original method
333 # rather than re-produce it here
334 'superclasses' => \&Class::MOP::Class::superclasses
337 default => sub { \undef }
341 Class::MOP::Class->meta->add_attribute(
342 Class::MOP::Attribute->new('instance_metaclass' => (
344 # NOTE: we need to do this in order
345 # for the instance meta-object to
346 # not fall into meta-circular death
348 # we just alias the original method
349 # rather than re-produce it here
350 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
352 default => 'Class::MOP::Instance',
356 Class::MOP::Class->meta->add_attribute(
357 Class::MOP::Attribute->new('immutable_trait' => (
359 'immutable_trait' => \&Class::MOP::Class::immutable_trait
361 default => "Class::MOP::Class::Immutable::Trait",
365 Class::MOP::Class->meta->add_attribute(
366 Class::MOP::Attribute->new('constructor_name' => (
368 'constructor_name' => \&Class::MOP::Class::constructor_name,
374 Class::MOP::Class->meta->add_attribute(
375 Class::MOP::Attribute->new('constructor_class' => (
377 'constructor_class' => \&Class::MOP::Class::constructor_class,
379 default => "Class::MOP::Method::Constructor",
384 Class::MOP::Class->meta->add_attribute(
385 Class::MOP::Attribute->new('destructor_class' => (
387 'destructor_class' => \&Class::MOP::Class::destructor_class,
393 # we don't actually need to tie the knot with
394 # Class::MOP::Class here, it is actually handled
395 # within Class::MOP::Class itself in the
396 # _construct_class_instance method.
398 ## --------------------------------------------------------
399 ## Class::MOP::Mixin::AttributeCore
400 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
401 Class::MOP::Attribute->new('name' => (
403 # NOTE: we need to do this in order
404 # for the instance meta-object to
405 # not fall into meta-circular death
407 # we just alias the original method
408 # rather than re-produce it here
409 'name' => \&Class::MOP::Mixin::AttributeCore::name
414 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
415 Class::MOP::Attribute->new('accessor' => (
416 reader => { 'accessor' => \&Class::MOP::Mixin::AttributeCore::accessor },
417 predicate => { 'has_accessor' => \&Class::MOP::Mixin::AttributeCore::has_accessor },
421 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
422 Class::MOP::Attribute->new('reader' => (
423 reader => { 'reader' => \&Class::MOP::Mixin::AttributeCore::reader },
424 predicate => { 'has_reader' => \&Class::MOP::Mixin::AttributeCore::has_reader },
428 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
429 Class::MOP::Attribute->new('initializer' => (
430 reader => { 'initializer' => \&Class::MOP::Mixin::AttributeCore::initializer },
431 predicate => { 'has_initializer' => \&Class::MOP::Mixin::AttributeCore::has_initializer },
435 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
436 Class::MOP::Attribute->new('definition_context' => (
437 reader => { 'definition_context' => \&Class::MOP::Mixin::AttributeCore::definition_context },
441 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
442 Class::MOP::Attribute->new('writer' => (
443 reader => { 'writer' => \&Class::MOP::Mixin::AttributeCore::writer },
444 predicate => { 'has_writer' => \&Class::MOP::Mixin::AttributeCore::has_writer },
448 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
449 Class::MOP::Attribute->new('predicate' => (
450 reader => { 'predicate' => \&Class::MOP::Mixin::AttributeCore::predicate },
451 predicate => { 'has_predicate' => \&Class::MOP::Mixin::AttributeCore::has_predicate },
455 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
456 Class::MOP::Attribute->new('clearer' => (
457 reader => { 'clearer' => \&Class::MOP::Mixin::AttributeCore::clearer },
458 predicate => { 'has_clearer' => \&Class::MOP::Mixin::AttributeCore::has_clearer },
462 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
463 Class::MOP::Attribute->new('builder' => (
464 reader => { 'builder' => \&Class::MOP::Mixin::AttributeCore::builder },
465 predicate => { 'has_builder' => \&Class::MOP::Mixin::AttributeCore::has_builder },
469 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
470 Class::MOP::Attribute->new('init_arg' => (
471 reader => { 'init_arg' => \&Class::MOP::Mixin::AttributeCore::init_arg },
472 predicate => { 'has_init_arg' => \&Class::MOP::Mixin::AttributeCore::has_init_arg },
476 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
477 Class::MOP::Attribute->new('default' => (
478 # default has a custom 'reader' method ...
479 predicate => { 'has_default' => \&Class::MOP::Mixin::AttributeCore::has_default },
483 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
484 Class::MOP::Attribute->new('insertion_order' => (
485 reader => { 'insertion_order' => \&Class::MOP::Mixin::AttributeCore::insertion_order },
486 writer => { '_set_insertion_order' => \&Class::MOP::Mixin::AttributeCore::_set_insertion_order },
487 predicate => { 'has_insertion_order' => \&Class::MOP::Mixin::AttributeCore::has_insertion_order },
491 ## --------------------------------------------------------
492 ## Class::MOP::Attribute
493 Class::MOP::Attribute->meta->add_attribute(
494 Class::MOP::Attribute->new('associated_class' => (
496 # NOTE: we need to do this in order
497 # for the instance meta-object to
498 # not fall into meta-circular death
500 # we just alias the original method
501 # rather than re-produce it here
502 'associated_class' => \&Class::MOP::Attribute::associated_class
507 Class::MOP::Attribute->meta->add_attribute(
508 Class::MOP::Attribute->new('associated_methods' => (
509 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
510 default => sub { [] }
514 Class::MOP::Attribute->meta->add_method('clone' => sub {
516 $self->meta->clone_object($self, @_);
519 ## --------------------------------------------------------
520 ## Class::MOP::Method
521 Class::MOP::Method->meta->add_attribute(
522 Class::MOP::Attribute->new('body' => (
523 reader => { 'body' => \&Class::MOP::Method::body },
527 Class::MOP::Method->meta->add_attribute(
528 Class::MOP::Attribute->new('associated_metaclass' => (
529 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
533 Class::MOP::Method->meta->add_attribute(
534 Class::MOP::Attribute->new('package_name' => (
535 reader => { 'package_name' => \&Class::MOP::Method::package_name },
539 Class::MOP::Method->meta->add_attribute(
540 Class::MOP::Attribute->new('name' => (
541 reader => { 'name' => \&Class::MOP::Method::name },
545 Class::MOP::Method->meta->add_attribute(
546 Class::MOP::Attribute->new('original_method' => (
547 reader => { 'original_method' => \&Class::MOP::Method::original_method },
548 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
552 ## --------------------------------------------------------
553 ## Class::MOP::Method::Wrapped
556 # the way this item is initialized, this
557 # really does not follow the standard
558 # practices of attributes, but we put
559 # it here for completeness
560 Class::MOP::Method::Wrapped->meta->add_attribute(
561 Class::MOP::Attribute->new('modifier_table')
564 ## --------------------------------------------------------
565 ## Class::MOP::Method::Generated
567 Class::MOP::Method::Generated->meta->add_attribute(
568 Class::MOP::Attribute->new('is_inline' => (
569 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
574 Class::MOP::Method::Generated->meta->add_attribute(
575 Class::MOP::Attribute->new('definition_context' => (
576 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
581 ## --------------------------------------------------------
582 ## Class::MOP::Method::Inlined
584 Class::MOP::Method::Inlined->meta->add_attribute(
585 Class::MOP::Attribute->new('_expected_method_class' => (
586 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
590 ## --------------------------------------------------------
591 ## Class::MOP::Method::Accessor
593 Class::MOP::Method::Accessor->meta->add_attribute(
594 Class::MOP::Attribute->new('attribute' => (
596 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
601 Class::MOP::Method::Accessor->meta->add_attribute(
602 Class::MOP::Attribute->new('accessor_type' => (
603 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
607 ## --------------------------------------------------------
608 ## Class::MOP::Method::Constructor
610 Class::MOP::Method::Constructor->meta->add_attribute(
611 Class::MOP::Attribute->new('options' => (
613 'options' => \&Class::MOP::Method::Constructor::options
615 default => sub { +{} }
619 Class::MOP::Method::Constructor->meta->add_attribute(
620 Class::MOP::Attribute->new('associated_metaclass' => (
621 init_arg => "metaclass", # FIXME alias and rename
623 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
628 ## --------------------------------------------------------
629 ## Class::MOP::Instance
632 # these don't yet do much of anything, but are just
633 # included for completeness
635 Class::MOP::Instance->meta->add_attribute(
636 Class::MOP::Attribute->new('associated_metaclass',
637 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
641 Class::MOP::Instance->meta->add_attribute(
642 Class::MOP::Attribute->new('_class_name',
644 reader => { _class_name => \&Class::MOP::Instance::_class_name },
645 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
646 #default => sub { $_[0]->associated_metaclass->name },
650 Class::MOP::Instance->meta->add_attribute(
651 Class::MOP::Attribute->new('attributes',
652 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
656 Class::MOP::Instance->meta->add_attribute(
657 Class::MOP::Attribute->new('slots',
658 reader => { slots => \&Class::MOP::Instance::slots },
662 Class::MOP::Instance->meta->add_attribute(
663 Class::MOP::Attribute->new('slot_hash',
664 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
668 ## --------------------------------------------------------
669 ## Class::MOP::Object
671 # need to replace the meta method there with a real meta method object
672 Class::MOP::Object->meta->_add_meta_method('meta');
674 ## --------------------------------------------------------
677 # need to replace the meta method there with a real meta method object
678 Class::MOP::Mixin->meta->_add_meta_method('meta');
680 require Class::MOP::Deprecated unless our $no_deprecated;
682 # we need the meta instance of the meta instance to be created now, in order
683 # for the constructor to be able to use it
684 Class::MOP::Instance->meta->get_meta_instance;
686 # pretend the add_method never happenned. it hasn't yet affected anything
687 undef Class::MOP::Instance->meta->{_package_cache_flag};
689 ## --------------------------------------------------------
690 ## Now close all the Class::MOP::* classes
692 # NOTE: we don't need to inline the the accessors this only lengthens
693 # the compile time of the MOP, and gives us no actual benefits.
695 $_->meta->make_immutable(
696 inline_constructor => 0,
697 constructor_name => "_new",
698 inline_accessors => 0,
704 Class::MOP::Attribute
710 Class::MOP::Method::Generated
711 Class::MOP::Method::Inlined
713 Class::MOP::Method::Accessor
714 Class::MOP::Method::Constructor
715 Class::MOP::Method::Wrapped
717 Class::MOP::Method::Meta
720 $_->meta->make_immutable(
721 inline_constructor => 0,
722 constructor_name => undef,
723 inline_accessors => 0,
726 Class::MOP::Mixin::AttributeCore
727 Class::MOP::Mixin::HasAttributes
728 Class::MOP::Mixin::HasMethods
739 Class::MOP - A Meta Object Protocol for Perl 5
743 This module is a fully functioning meta object protocol for the
744 Perl 5 object system. It makes no attempt to change the behavior or
745 characteristics of the Perl 5 object system, only to create a
746 protocol for its manipulation and introspection.
748 That said, it does attempt to create the tools for building a rich set
749 of extensions to the Perl 5 object system. Every attempt has been made
750 to abide by the spirit of the Perl 5 object system that we all know
753 This documentation is sparse on conceptual details. We suggest looking
754 at the items listed in the L<SEE ALSO> section for more
755 information. In particular the book "The Art of the Meta Object
756 Protocol" was very influential in the development of this system.
758 =head2 What is a Meta Object Protocol?
760 A meta object protocol is an API to an object system.
762 To be more specific, it abstracts the components of an object system
763 (classes, object, methods, object attributes, etc.). These
764 abstractions can then be used to inspect and manipulate the object
765 system which they describe.
767 It can be said that there are two MOPs for any object system; the
768 implicit MOP and the explicit MOP. The implicit MOP handles things
769 like method dispatch or inheritance, which happen automatically as
770 part of how the object system works. The explicit MOP typically
771 handles the introspection/reflection features of the object system.
773 All object systems have implicit MOPs. Without one, they would not
774 work. Explicit MOPs are much less common, and depending on the
775 language can vary from restrictive (Reflection in Java or C#) to wide
776 open (CLOS is a perfect example).
778 =head2 Yet Another Class Builder! Why?
780 This is B<not> a class builder so much as a I<class builder
781 B<builder>>. The intent is that an end user will not use this module
782 directly, but instead this module is used by module authors to build
783 extensions and features onto the Perl 5 object system.
785 This system is used by L<Moose>, which supplies a powerful class
786 builder system built entirely on top of C<Class::MOP>.
788 =head2 Who is this module for?
790 This module is for anyone who has ever created or wanted to create a
791 module for the Class:: namespace. The tools which this module provides
792 make doing complex Perl 5 wizardry simpler, by removing such barriers
793 as the need to hack symbol tables, or understand the fine details of
796 =head2 What changes do I have to make to use this module?
798 This module was designed to be as unintrusive as possible. Many of its
799 features are accessible without B<any> change to your existing
800 code. It is meant to be a compliment to your existing code and not an
801 intrusion on your code base. Unlike many other B<Class::> modules,
802 this module B<does not> require you subclass it, or even that you
803 C<use> it in within your module's package.
805 The only features which requires additions to your code are the
806 attribute handling and instance construction features, and these are
807 both completely optional features. The only reason for this is because
808 Perl 5's object system does not actually have these features built
809 in. More information about this feature can be found below.
811 =head2 About Performance
813 It is a common misconception that explicit MOPs are a performance hit.
814 This is not a universal truth, it is a side-effect of some specific
815 implementations. For instance, using Java reflection is slow because
816 the JVM cannot take advantage of any compiler optimizations, and the
817 JVM has to deal with much more runtime type information as well.
819 Reflection in C# is marginally better as it was designed into the
820 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
821 Object System) was built to support an explicit MOP, and so
822 performance is tuned for it.
824 This library in particular does its absolute best to avoid putting
825 B<any> drain at all upon your code's performance. In fact, by itself
826 it does nothing to affect your existing code. So you only pay for what
829 =head2 About Metaclass compatibility
831 This module makes sure that all metaclasses created are both upwards
832 and downwards compatible. The topic of metaclass compatibility is
833 highly esoteric and is something only encountered when doing deep and
834 involved metaclass hacking. There are two basic kinds of metaclass
835 incompatibility; upwards and downwards.
837 Upwards metaclass compatibility means that the metaclass of a
838 given class is either the same as (or a subclass of) all of the
841 Downward metaclass compatibility means that the metaclasses of a
842 given class's ancestors are all either the same as (or a subclass
845 Here is a diagram showing a set of two classes (C<A> and C<B>) and
846 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
847 metaclass compatibility both upwards and downwards.
849 +---------+ +---------+
850 | Meta::A |<----| Meta::B | <....... (instance of )
851 +---------+ +---------+ <------- (inherits from)
854 +---------+ +---------+
856 +---------+ +---------+
858 In actuality, I<all> of a class's metaclasses must be compatible,
859 not just the class metaclass. That includes the instance, attribute,
860 and method metaclasses, as well as the constructor and destructor
863 C<Class::MOP> will attempt to fix some simple types of
864 incompatibilities. If all the metaclasses for the parent class are
865 I<subclasses> of the child's metaclasses then we can simply replace
866 the child's metaclasses with the parent's. In addition, if the child
867 is missing a metaclass that the parent has, we can also just make the
868 child use the parent's metaclass.
870 As I said this is a highly esoteric topic and one you will only run
871 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
872 are interested in why this is an issue see the paper I<Uniform and
873 safe metaclass composition> linked to in the L<SEE ALSO> section of
876 =head2 Using custom metaclasses
878 Always use the L<metaclass> pragma when using a custom metaclass, this
879 will ensure the proper initialization order and not accidentally
880 create an incorrect type of metaclass for you. This is a very rare
881 problem, and one which can only occur if you are doing deep metaclass
882 programming. So in other words, don't worry about it.
884 Note that if you're using L<Moose> we encourage you to I<not> use
885 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
886 roles to a class's metaclasses. This topic is covered at length in
887 various L<Moose::Cookbook> recipes.
891 The meta-object protocol is divided into 4 main sub-protocols:
893 =head2 The Class protocol
895 This provides a means of manipulating and introspecting a Perl 5
896 class. It handles symbol table hacking for you, and provides a rich
897 set of methods that go beyond simple package introspection.
899 See L<Class::MOP::Class> for more details.
901 =head2 The Attribute protocol
903 This provides a consistent representation for an attribute of a Perl 5
904 class. Since there are so many ways to create and handle attributes in
905 Perl 5 OO, the Attribute protocol provide as much of a unified
906 approach as possible. Of course, you are always free to extend this
907 protocol by subclassing the appropriate classes.
909 See L<Class::MOP::Attribute> for more details.
911 =head2 The Method protocol
913 This provides a means of manipulating and introspecting methods in the
914 Perl 5 object system. As with attributes, there are many ways to
915 approach this topic, so we try to keep it pretty basic, while still
916 making it possible to extend the system in many ways.
918 See L<Class::MOP::Method> for more details.
920 =head2 The Instance protocol
922 This provides a layer of abstraction for creating object instances.
923 Since the other layers use this protocol, it is relatively easy to
924 change the type of your instances from the default hash reference to
925 some other type of reference. Several examples are provided in the
926 F<examples/> directory included in this distribution.
928 See L<Class::MOP::Instance> for more details.
932 Note that this module does not export any constants or functions.
938 =item I<Class::MOP::IS_RUNNING_ON_5_10>
940 We set this constant depending on what version perl we are on, this
941 allows us to take advantage of new 5.10 features and stay backwards
946 =head2 Utility functions
948 Note that these are all called as B<functions, not methods>.
952 =item B<Class::MOP::load_class($class_name, \%options?)>
954 This will load the specified C<$class_name>, if it is not already
955 loaded (as reported by C<is_class_loaded>). This function can be used
956 in place of tricks like C<eval "use $module"> or using C<require>
959 If the module cannot be loaded, an exception is thrown.
961 You can pass a hash reference with options as second argument. The
962 only option currently recognised is C<-version>, which will ensure
963 that the loaded class has at least the required version.
965 See also L</Class Loading Options>.
967 For historical reasons, this function explicitly returns a true value.
969 =item B<Class::MOP::is_class_loaded($class_name, \%options?)>
971 Returns a boolean indicating whether or not C<$class_name> has been
974 This does a basic check of the symbol table to try and determine as
975 best it can if the C<$class_name> is loaded, it is probably correct
976 about 99% of the time, but it can be fooled into reporting false
977 positives. In particular, loading any of the core L<IO> modules will
978 cause most of the rest of the core L<IO> modules to falsely report
979 having been loaded, due to the way the base L<IO> module works.
981 You can pass a hash reference with options as second argument. The
982 only option currently recognised is C<-version>, which will ensure
983 that the loaded class has at least the required version.
985 See also L</Class Loading Options>.
987 =item B<Class::MOP::get_code_info($code)>
989 This function returns two values, the name of the package the C<$code>
990 is from and the name of the C<$code> itself. This is used by several
991 elements of the MOP to determine where a given C<$code> reference is
994 =item B<Class::MOP::class_of($instance_or_class_name)>
996 This will return the metaclass of the given instance or class name. If the
997 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
1000 =item B<Class::MOP::check_package_cache_flag($pkg)>
1002 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1004 This will return an integer that is managed by L<Class::MOP::Class> to
1005 determine if a module's symbol table has been altered.
1007 In Perl 5.10 or greater, this flag is package specific. However in
1008 versions prior to 5.10, this will use the C<PL_sub_generation>
1009 variable which is not package specific.
1011 =item B<Class::MOP::load_first_existing_class(@class_names)>
1013 =item B<Class::MOP::load_first_existing_class($classA, \%optionsA?, $classB, ...)>
1015 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1017 Given a list of class names, this function will attempt to load each
1020 If it finds a class it can load, it will return that class' name. If
1021 none of the classes can be loaded, it will throw an exception.
1023 Additionally, you can pass a hash reference with options after each
1024 class name. Currently, only C<-version> is recognised and will ensure
1025 that the loaded class has at least the required version. If the class
1026 version is not sufficient, an exception will be raised.
1028 See also L</Class Loading Options>.
1032 =head2 Metaclass cache functions
1034 Class::MOP holds a cache of metaclasses. The following are functions
1035 (B<not methods>) which can be used to access that cache. It is not
1036 recommended that you mess with these. Bad things could happen, but if
1037 you are brave and willing to risk it: go for it!
1041 =item B<Class::MOP::get_all_metaclasses>
1043 This will return a hash of all the metaclass instances that have
1044 been cached by L<Class::MOP::Class>, keyed by the package name.
1046 =item B<Class::MOP::get_all_metaclass_instances>
1048 This will return a list of all the metaclass instances that have
1049 been cached by L<Class::MOP::Class>.
1051 =item B<Class::MOP::get_all_metaclass_names>
1053 This will return a list of all the metaclass names that have
1054 been cached by L<Class::MOP::Class>.
1056 =item B<Class::MOP::get_metaclass_by_name($name)>
1058 This will return a cached L<Class::MOP::Class> instance, or nothing
1059 if no metaclass exists with that C<$name>.
1061 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
1063 This will store a metaclass in the cache at the supplied C<$key>.
1065 =item B<Class::MOP::weaken_metaclass($name)>
1067 In rare cases (e.g. anonymous metaclasses) it is desirable to
1068 store a weakened reference in the metaclass cache. This
1069 function will weaken the reference to the metaclass stored
1072 =item B<Class::MOP::metaclass_is_weak($name)>
1074 Returns true if the metaclass for C<$name> has been weakened
1075 (via C<weaken_metaclass>).
1077 =item B<Class::MOP::does_metaclass_exist($name)>
1079 This will return true of there exists a metaclass stored in the
1080 C<$name> key, and return false otherwise.
1082 =item B<Class::MOP::remove_metaclass_by_name($name)>
1084 This will remove the metaclass stored in the C<$name> key.
1088 =head2 Class Loading Options
1094 Can be used to pass a minimum required version that will be checked
1095 against the class version after it was loaded.
1103 There are very few books out on Meta Object Protocols and Metaclasses
1104 because it is such an esoteric topic. The following books are really
1105 the only ones I have found. If you know of any more, B<I<please>>
1106 email me and let me know, I would love to hear about them.
1110 =item I<The Art of the Meta Object Protocol>
1112 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1114 =item I<Putting MetaClasses to Work>
1116 =item I<Smalltalk: The Language>
1124 =item "Uniform and safe metaclass composition"
1126 An excellent paper by the people who brought us the original Traits paper.
1127 This paper is on how Traits can be used to do safe metaclass composition,
1128 and offers an excellent introduction section which delves into the topic of
1129 metaclass compatibility.
1131 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1133 =item "Safe Metaclass Programming"
1135 This paper seems to precede the above paper, and propose a mix-in based
1136 approach as opposed to the Traits based approach. Both papers have similar
1137 information on the metaclass compatibility problem space.
1139 L<http://citeseer.ist.psu.edu/37617.html>
1147 =item The Perl 6 MetaModel work in the Pugs project
1151 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1153 =item L<http://github.com/perl6/p5-modules/tree/master/Perl6-ObjectSpace/>
1163 =item CPAN Module Review of Class::MOP
1165 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1169 =head1 SIMILAR MODULES
1171 As I have said above, this module is a class-builder-builder, so it is
1172 not the same thing as modules like L<Class::Accessor> and
1173 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1174 with similar goals to this module. The one I have found which is most
1175 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1176 creates are very different from this modules.
1180 All complex software has bugs lurking in it, and this module is no
1183 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1184 web interface at L<http://rt.cpan.org>.
1186 You can also discuss feature requests or possible bugs on the Moose
1187 mailing list (moose@perl.org) or on IRC at
1188 L<irc://irc.perl.org/#moose>.
1190 =head1 ACKNOWLEDGEMENTS
1196 Thanks to Rob for actually getting the development of this module kick-started.
1202 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1204 B<with contributions from:>
1206 Brandon (blblack) Black
1208 Florian (rafl) Ragwitz
1210 Guillermo (groditi) Roditi
1212 Dave (autarch) Rolsky
1216 Rob (robkinyon) Kinyon
1218 Yuval (nothingmuch) Kogman
1220 Scott (konobi) McWhirter
1224 =head1 COPYRIGHT AND LICENSE
1226 Copyright 2006-2010 by Infinity Interactive, Inc.
1228 L<http://www.iinteractive.com>
1230 This library is free software; you can redistribute it and/or modify
1231 it under the same terms as Perl itself.