12 use Scalar::Util 'weaken', 'isweak', 'reftype', 'blessed';
16 use Class::MOP::Mixin::AttributeCore;
17 use Class::MOP::Mixin::HasAttributes;
18 use Class::MOP::Mixin::HasMethods;
19 use Class::MOP::Class;
20 use Class::MOP::Attribute;
21 use Class::MOP::Method;
24 *IS_RUNNING_ON_5_10 = ($] < 5.009_005)
28 # this is either part of core or set up appropriately by MRO::Compat
29 *check_package_cache_flag = \&mro::get_pkg_gen;
32 our $AUTHORITY = 'cpan:STEVAN';
36 $Moose::{VERSION} ? $Moose::{VERSION} : ()
40 # Metaclasses are singletons, so we cache them here.
41 # there is no need to worry about destruction though
42 # because they should die only when the program dies.
43 # After all, do package definitions even get reaped?
44 # Anonymous classes manage their own destruction.
47 sub get_all_metaclasses { %METAS }
48 sub get_all_metaclass_instances { values %METAS }
49 sub get_all_metaclass_names { keys %METAS }
50 sub get_metaclass_by_name { $METAS{$_[0]} }
51 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
52 sub weaken_metaclass { weaken($METAS{$_[0]}) }
53 sub metaclass_is_weak { isweak($METAS{$_[0]}) }
54 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
55 sub remove_metaclass_by_name { delete $METAS{$_[0]}; return }
57 # This handles instances as well as class names
59 return unless defined $_[0];
60 my $class = blessed($_[0]) || $_[0];
61 return $METAS{$class};
65 # We only cache metaclasses, meaning instances of
66 # Class::MOP::Class. We do not cache instance of
67 # Class::MOP::Package or Class::MOP::Module. Mostly
68 # because I don't yet see a good reason to do so.
71 sub _class_to_pmfile {
74 my $file = $class . '.pm';
80 sub load_first_existing_class {
81 my $classes = Data::OptList::mkopt(\@_)
84 foreach my $class (@{ $classes }) {
85 my $name = $class->[0];
86 unless ( _is_valid_class_name($name) ) {
87 my $display = defined($name) ? $name : 'undef';
88 confess "Invalid class name ($display)";
95 for my $class (@{ $classes }) {
96 my ($name, $options) = @{ $class };
99 return $name if is_class_loaded($name, $options);
100 if (is_class_loaded($name)) {
101 # we already know it's loaded and too old, but we call
102 # ->VERSION anyway to generate the exception for us
103 $name->VERSION($options->{-version});
107 return $name if is_class_loaded($name);
110 my $file = _class_to_pmfile($name);
111 return $name if try {
114 $name->VERSION($options->{-version})
115 if defined $options->{-version};
119 unless (/^Can't locate \Q$file\E in \@INC/) {
120 confess "Couldn't load class ($name) because: $_";
127 if ( @{ $classes } > 1 ) {
128 my @list = map { $_->[0] } @{ $classes };
129 confess "Can't locate any of @list in \@INC (\@INC contains: @INC).";
131 confess "Can't locate " . _class_to_pmfile($classes->[0]->[0]) . " in \@INC (\@INC contains: @INC).";
136 load_first_existing_class($_[0], ref $_[1] ? $_[1] : ());
138 # This is done to avoid breaking code which checked the return value. Said
139 # code is dumb. The return value was _always_ true, since it dies on
144 sub _is_valid_class_name {
147 return 0 if ref($class);
148 return 0 unless defined($class);
149 return 0 unless length($class);
151 return 1 if $class =~ /^\w+(?:::\w+)*$/;
156 ## ----------------------------------------------------------------------------
157 ## Setting up our environment ...
158 ## ----------------------------------------------------------------------------
159 ## Class::MOP needs to have a few things in the global perl environment so
160 ## that it can operate effectively. Those things are done here.
161 ## ----------------------------------------------------------------------------
163 # ... nothing yet actually ;)
165 ## ----------------------------------------------------------------------------
167 ## ----------------------------------------------------------------------------
168 ## The code below here is to bootstrap our MOP with itself. This is also
169 ## sometimes called "tying the knot". By doing this, we make it much easier
170 ## to extend the MOP through subclassing and such since now you can use the
171 ## MOP itself to extend itself.
173 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
174 ## ----------------------------------------------------------------------------
176 # We need to add in the meta-attributes here so that
177 # any subclass of Class::MOP::* will be able to
178 # inherit them using _construct_instance
180 ## --------------------------------------------------------
181 ## Class::MOP::Mixin::HasMethods
183 Class::MOP::Mixin::HasMethods->meta->add_attribute(
184 Class::MOP::Attribute->new('_methods' => (
187 # we just alias the original method
188 # rather than re-produce it here
189 '_method_map' => \&Class::MOP::Mixin::HasMethods::_method_map
191 default => sub { {} }
195 Class::MOP::Mixin::HasMethods->meta->add_attribute(
196 Class::MOP::Attribute->new('method_metaclass' => (
199 # we just alias the original method
200 # rather than re-produce it here
201 'method_metaclass' => \&Class::MOP::Mixin::HasMethods::method_metaclass
203 default => 'Class::MOP::Method',
207 Class::MOP::Mixin::HasMethods->meta->add_attribute(
208 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
211 # we just alias the original method
212 # rather than re-produce it here
213 'wrapped_method_metaclass' => \&Class::MOP::Mixin::HasMethods::wrapped_method_metaclass
215 default => 'Class::MOP::Method::Wrapped',
219 ## --------------------------------------------------------
220 ## Class::MOP::Mixin::HasMethods
222 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
223 Class::MOP::Attribute->new('attributes' => (
225 # NOTE: we need to do this in order
226 # for the instance meta-object to
227 # not fall into meta-circular death
229 # we just alias the original method
230 # rather than re-produce it here
231 '_attribute_map' => \&Class::MOP::Mixin::HasAttributes::_attribute_map
233 default => sub { {} }
237 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
238 Class::MOP::Attribute->new('attribute_metaclass' => (
241 # we just alias the original method
242 # rather than re-produce it here
243 'attribute_metaclass' => \&Class::MOP::Mixin::HasAttributes::attribute_metaclass
245 default => 'Class::MOP::Attribute',
249 ## --------------------------------------------------------
250 ## Class::MOP::Package
252 Class::MOP::Package->meta->add_attribute(
253 Class::MOP::Attribute->new('package' => (
255 # NOTE: we need to do this in order
256 # for the instance meta-object to
257 # not fall into meta-circular death
259 # we just alias the original method
260 # rather than re-produce it here
261 'name' => \&Class::MOP::Package::name
266 Class::MOP::Package->meta->add_attribute(
267 Class::MOP::Attribute->new('namespace' => (
270 # we just alias the original method
271 # rather than re-produce it here
272 'namespace' => \&Class::MOP::Package::namespace
275 default => sub { \undef }
279 ## --------------------------------------------------------
280 ## Class::MOP::Module
283 # yeah this is kind of stretching things a bit,
284 # but truthfully the version should be an attribute
285 # of the Module, the weirdness comes from having to
286 # stick to Perl 5 convention and store it in the
287 # $VERSION package variable. Basically if you just
288 # squint at it, it will look how you want it to look.
289 # Either as a package variable, or as a attribute of
290 # the metaclass, isn't abstraction great :)
292 Class::MOP::Module->meta->add_attribute(
293 Class::MOP::Attribute->new('version' => (
296 # we just alias the original method
297 # rather than re-produce it here
298 'version' => \&Class::MOP::Module::version
301 default => sub { \undef }
306 # By following the same conventions as version here,
307 # we are opening up the possibility that people can
308 # use the $AUTHORITY in non-Class::MOP modules as
311 Class::MOP::Module->meta->add_attribute(
312 Class::MOP::Attribute->new('authority' => (
315 # we just alias the original method
316 # rather than re-produce it here
317 'authority' => \&Class::MOP::Module::authority
320 default => sub { \undef }
324 ## --------------------------------------------------------
327 Class::MOP::Class->meta->add_attribute(
328 Class::MOP::Attribute->new('superclasses' => (
331 # we just alias the original method
332 # rather than re-produce it here
333 'superclasses' => \&Class::MOP::Class::superclasses
336 default => sub { \undef }
340 Class::MOP::Class->meta->add_attribute(
341 Class::MOP::Attribute->new('instance_metaclass' => (
343 # NOTE: we need to do this in order
344 # for the instance meta-object to
345 # not fall into meta-circular death
347 # we just alias the original method
348 # rather than re-produce it here
349 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
351 default => 'Class::MOP::Instance',
355 Class::MOP::Class->meta->add_attribute(
356 Class::MOP::Attribute->new('immutable_trait' => (
358 'immutable_trait' => \&Class::MOP::Class::immutable_trait
360 default => "Class::MOP::Class::Immutable::Trait",
364 Class::MOP::Class->meta->add_attribute(
365 Class::MOP::Attribute->new('constructor_name' => (
367 'constructor_name' => \&Class::MOP::Class::constructor_name,
373 Class::MOP::Class->meta->add_attribute(
374 Class::MOP::Attribute->new('constructor_class' => (
376 'constructor_class' => \&Class::MOP::Class::constructor_class,
378 default => "Class::MOP::Method::Constructor",
383 Class::MOP::Class->meta->add_attribute(
384 Class::MOP::Attribute->new('destructor_class' => (
386 'destructor_class' => \&Class::MOP::Class::destructor_class,
392 # we don't actually need to tie the knot with
393 # Class::MOP::Class here, it is actually handled
394 # within Class::MOP::Class itself in the
395 # _construct_class_instance method.
397 ## --------------------------------------------------------
398 ## Class::MOP::Mixin::AttributeCore
399 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
400 Class::MOP::Attribute->new('name' => (
402 # NOTE: we need to do this in order
403 # for the instance meta-object to
404 # not fall into meta-circular death
406 # we just alias the original method
407 # rather than re-produce it here
408 'name' => \&Class::MOP::Mixin::AttributeCore::name
413 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
414 Class::MOP::Attribute->new('accessor' => (
415 reader => { 'accessor' => \&Class::MOP::Mixin::AttributeCore::accessor },
416 predicate => { 'has_accessor' => \&Class::MOP::Mixin::AttributeCore::has_accessor },
420 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
421 Class::MOP::Attribute->new('reader' => (
422 reader => { 'reader' => \&Class::MOP::Mixin::AttributeCore::reader },
423 predicate => { 'has_reader' => \&Class::MOP::Mixin::AttributeCore::has_reader },
427 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
428 Class::MOP::Attribute->new('initializer' => (
429 reader => { 'initializer' => \&Class::MOP::Mixin::AttributeCore::initializer },
430 predicate => { 'has_initializer' => \&Class::MOP::Mixin::AttributeCore::has_initializer },
434 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
435 Class::MOP::Attribute->new('definition_context' => (
436 reader => { 'definition_context' => \&Class::MOP::Mixin::AttributeCore::definition_context },
440 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
441 Class::MOP::Attribute->new('writer' => (
442 reader => { 'writer' => \&Class::MOP::Mixin::AttributeCore::writer },
443 predicate => { 'has_writer' => \&Class::MOP::Mixin::AttributeCore::has_writer },
447 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
448 Class::MOP::Attribute->new('predicate' => (
449 reader => { 'predicate' => \&Class::MOP::Mixin::AttributeCore::predicate },
450 predicate => { 'has_predicate' => \&Class::MOP::Mixin::AttributeCore::has_predicate },
454 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
455 Class::MOP::Attribute->new('clearer' => (
456 reader => { 'clearer' => \&Class::MOP::Mixin::AttributeCore::clearer },
457 predicate => { 'has_clearer' => \&Class::MOP::Mixin::AttributeCore::has_clearer },
461 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
462 Class::MOP::Attribute->new('builder' => (
463 reader => { 'builder' => \&Class::MOP::Mixin::AttributeCore::builder },
464 predicate => { 'has_builder' => \&Class::MOP::Mixin::AttributeCore::has_builder },
468 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
469 Class::MOP::Attribute->new('init_arg' => (
470 reader => { 'init_arg' => \&Class::MOP::Mixin::AttributeCore::init_arg },
471 predicate => { 'has_init_arg' => \&Class::MOP::Mixin::AttributeCore::has_init_arg },
475 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
476 Class::MOP::Attribute->new('default' => (
477 # default has a custom 'reader' method ...
478 predicate => { 'has_default' => \&Class::MOP::Mixin::AttributeCore::has_default },
482 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
483 Class::MOP::Attribute->new('insertion_order' => (
484 reader => { 'insertion_order' => \&Class::MOP::Mixin::AttributeCore::insertion_order },
485 writer => { '_set_insertion_order' => \&Class::MOP::Mixin::AttributeCore::_set_insertion_order },
486 predicate => { 'has_insertion_order' => \&Class::MOP::Mixin::AttributeCore::has_insertion_order },
490 ## --------------------------------------------------------
491 ## Class::MOP::Attribute
492 Class::MOP::Attribute->meta->add_attribute(
493 Class::MOP::Attribute->new('associated_class' => (
495 # NOTE: we need to do this in order
496 # for the instance meta-object to
497 # not fall into meta-circular death
499 # we just alias the original method
500 # rather than re-produce it here
501 'associated_class' => \&Class::MOP::Attribute::associated_class
506 Class::MOP::Attribute->meta->add_attribute(
507 Class::MOP::Attribute->new('associated_methods' => (
508 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
509 default => sub { [] }
513 Class::MOP::Attribute->meta->add_method('clone' => sub {
515 $self->meta->clone_object($self, @_);
518 ## --------------------------------------------------------
519 ## Class::MOP::Method
520 Class::MOP::Method->meta->add_attribute(
521 Class::MOP::Attribute->new('body' => (
522 reader => { 'body' => \&Class::MOP::Method::body },
526 Class::MOP::Method->meta->add_attribute(
527 Class::MOP::Attribute->new('associated_metaclass' => (
528 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
532 Class::MOP::Method->meta->add_attribute(
533 Class::MOP::Attribute->new('package_name' => (
534 reader => { 'package_name' => \&Class::MOP::Method::package_name },
538 Class::MOP::Method->meta->add_attribute(
539 Class::MOP::Attribute->new('name' => (
540 reader => { 'name' => \&Class::MOP::Method::name },
544 Class::MOP::Method->meta->add_attribute(
545 Class::MOP::Attribute->new('original_method' => (
546 reader => { 'original_method' => \&Class::MOP::Method::original_method },
547 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
551 ## --------------------------------------------------------
552 ## Class::MOP::Method::Wrapped
555 # the way this item is initialized, this
556 # really does not follow the standard
557 # practices of attributes, but we put
558 # it here for completeness
559 Class::MOP::Method::Wrapped->meta->add_attribute(
560 Class::MOP::Attribute->new('modifier_table')
563 ## --------------------------------------------------------
564 ## Class::MOP::Method::Generated
566 Class::MOP::Method::Generated->meta->add_attribute(
567 Class::MOP::Attribute->new('is_inline' => (
568 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
573 Class::MOP::Method::Generated->meta->add_attribute(
574 Class::MOP::Attribute->new('definition_context' => (
575 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
580 ## --------------------------------------------------------
581 ## Class::MOP::Method::Inlined
583 Class::MOP::Method::Inlined->meta->add_attribute(
584 Class::MOP::Attribute->new('_expected_method_class' => (
585 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
589 ## --------------------------------------------------------
590 ## Class::MOP::Method::Accessor
592 Class::MOP::Method::Accessor->meta->add_attribute(
593 Class::MOP::Attribute->new('attribute' => (
595 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
600 Class::MOP::Method::Accessor->meta->add_attribute(
601 Class::MOP::Attribute->new('accessor_type' => (
602 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
606 ## --------------------------------------------------------
607 ## Class::MOP::Method::Constructor
609 Class::MOP::Method::Constructor->meta->add_attribute(
610 Class::MOP::Attribute->new('options' => (
612 'options' => \&Class::MOP::Method::Constructor::options
614 default => sub { +{} }
618 Class::MOP::Method::Constructor->meta->add_attribute(
619 Class::MOP::Attribute->new('associated_metaclass' => (
620 init_arg => "metaclass", # FIXME alias and rename
622 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
627 ## --------------------------------------------------------
628 ## Class::MOP::Instance
631 # these don't yet do much of anything, but are just
632 # included for completeness
634 Class::MOP::Instance->meta->add_attribute(
635 Class::MOP::Attribute->new('associated_metaclass',
636 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
640 Class::MOP::Instance->meta->add_attribute(
641 Class::MOP::Attribute->new('_class_name',
643 reader => { _class_name => \&Class::MOP::Instance::_class_name },
644 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
645 #default => sub { $_[0]->associated_metaclass->name },
649 Class::MOP::Instance->meta->add_attribute(
650 Class::MOP::Attribute->new('attributes',
651 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
655 Class::MOP::Instance->meta->add_attribute(
656 Class::MOP::Attribute->new('slots',
657 reader => { slots => \&Class::MOP::Instance::slots },
661 Class::MOP::Instance->meta->add_attribute(
662 Class::MOP::Attribute->new('slot_hash',
663 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
667 ## --------------------------------------------------------
668 ## Class::MOP::Object
670 # need to replace the meta method there with a real meta method object
671 Class::MOP::Object->meta->_add_meta_method('meta');
673 ## --------------------------------------------------------
676 # need to replace the meta method there with a real meta method object
677 Class::MOP::Mixin->meta->_add_meta_method('meta');
679 require Class::MOP::Deprecated unless our $no_deprecated;
681 # we need the meta instance of the meta instance to be created now, in order
682 # for the constructor to be able to use it
683 Class::MOP::Instance->meta->get_meta_instance;
685 # pretend the add_method never happenned. it hasn't yet affected anything
686 undef Class::MOP::Instance->meta->{_package_cache_flag};
688 ## --------------------------------------------------------
689 ## Now close all the Class::MOP::* classes
691 # NOTE: we don't need to inline the the accessors this only lengthens
692 # the compile time of the MOP, and gives us no actual benefits.
694 $_->meta->make_immutable(
695 inline_constructor => 0,
696 constructor_name => "_new",
697 inline_accessors => 0,
703 Class::MOP::Attribute
709 Class::MOP::Method::Generated
710 Class::MOP::Method::Inlined
712 Class::MOP::Method::Accessor
713 Class::MOP::Method::Constructor
714 Class::MOP::Method::Wrapped
716 Class::MOP::Method::Meta
719 $_->meta->make_immutable(
720 inline_constructor => 0,
721 constructor_name => undef,
722 inline_accessors => 0,
725 Class::MOP::Mixin::AttributeCore
726 Class::MOP::Mixin::HasAttributes
727 Class::MOP::Mixin::HasMethods
732 # ABSTRACT: A Meta Object Protocol for Perl 5
740 This module is a fully functioning meta object protocol for the
741 Perl 5 object system. It makes no attempt to change the behavior or
742 characteristics of the Perl 5 object system, only to create a
743 protocol for its manipulation and introspection.
745 That said, it does attempt to create the tools for building a rich set
746 of extensions to the Perl 5 object system. Every attempt has been made
747 to abide by the spirit of the Perl 5 object system that we all know
750 This documentation is sparse on conceptual details. We suggest looking
751 at the items listed in the L<SEE ALSO> section for more
752 information. In particular the book "The Art of the Meta Object
753 Protocol" was very influential in the development of this system.
755 =head2 What is a Meta Object Protocol?
757 A meta object protocol is an API to an object system.
759 To be more specific, it abstracts the components of an object system
760 (classes, object, methods, object attributes, etc.). These
761 abstractions can then be used to inspect and manipulate the object
762 system which they describe.
764 It can be said that there are two MOPs for any object system; the
765 implicit MOP and the explicit MOP. The implicit MOP handles things
766 like method dispatch or inheritance, which happen automatically as
767 part of how the object system works. The explicit MOP typically
768 handles the introspection/reflection features of the object system.
770 All object systems have implicit MOPs. Without one, they would not
771 work. Explicit MOPs are much less common, and depending on the
772 language can vary from restrictive (Reflection in Java or C#) to wide
773 open (CLOS is a perfect example).
775 =head2 Yet Another Class Builder! Why?
777 This is B<not> a class builder so much as a I<class builder
778 B<builder>>. The intent is that an end user will not use this module
779 directly, but instead this module is used by module authors to build
780 extensions and features onto the Perl 5 object system.
782 This system is used by L<Moose>, which supplies a powerful class
783 builder system built entirely on top of C<Class::MOP>.
785 =head2 Who is this module for?
787 This module is for anyone who has ever created or wanted to create a
788 module for the Class:: namespace. The tools which this module provides
789 make doing complex Perl 5 wizardry simpler, by removing such barriers
790 as the need to hack symbol tables, or understand the fine details of
793 =head2 What changes do I have to make to use this module?
795 This module was designed to be as unintrusive as possible. Many of its
796 features are accessible without B<any> change to your existing
797 code. It is meant to be a compliment to your existing code and not an
798 intrusion on your code base. Unlike many other B<Class::> modules,
799 this module B<does not> require you subclass it, or even that you
800 C<use> it in within your module's package.
802 The only features which requires additions to your code are the
803 attribute handling and instance construction features, and these are
804 both completely optional features. The only reason for this is because
805 Perl 5's object system does not actually have these features built
806 in. More information about this feature can be found below.
808 =head2 About Performance
810 It is a common misconception that explicit MOPs are a performance hit.
811 This is not a universal truth, it is a side-effect of some specific
812 implementations. For instance, using Java reflection is slow because
813 the JVM cannot take advantage of any compiler optimizations, and the
814 JVM has to deal with much more runtime type information as well.
816 Reflection in C# is marginally better as it was designed into the
817 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
818 Object System) was built to support an explicit MOP, and so
819 performance is tuned for it.
821 This library in particular does its absolute best to avoid putting
822 B<any> drain at all upon your code's performance. In fact, by itself
823 it does nothing to affect your existing code. So you only pay for what
826 =head2 About Metaclass compatibility
828 This module makes sure that all metaclasses created are both upwards
829 and downwards compatible. The topic of metaclass compatibility is
830 highly esoteric and is something only encountered when doing deep and
831 involved metaclass hacking. There are two basic kinds of metaclass
832 incompatibility; upwards and downwards.
834 Upwards metaclass compatibility means that the metaclass of a
835 given class is either the same as (or a subclass of) all of the
838 Downward metaclass compatibility means that the metaclasses of a
839 given class's ancestors are all either the same as (or a subclass
842 Here is a diagram showing a set of two classes (C<A> and C<B>) and
843 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
844 metaclass compatibility both upwards and downwards.
846 +---------+ +---------+
847 | Meta::A |<----| Meta::B | <....... (instance of )
848 +---------+ +---------+ <------- (inherits from)
851 +---------+ +---------+
853 +---------+ +---------+
855 In actuality, I<all> of a class's metaclasses must be compatible,
856 not just the class metaclass. That includes the instance, attribute,
857 and method metaclasses, as well as the constructor and destructor
860 C<Class::MOP> will attempt to fix some simple types of
861 incompatibilities. If all the metaclasses for the parent class are
862 I<subclasses> of the child's metaclasses then we can simply replace
863 the child's metaclasses with the parent's. In addition, if the child
864 is missing a metaclass that the parent has, we can also just make the
865 child use the parent's metaclass.
867 As I said this is a highly esoteric topic and one you will only run
868 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
869 are interested in why this is an issue see the paper I<Uniform and
870 safe metaclass composition> linked to in the L<SEE ALSO> section of
873 =head2 Using custom metaclasses
875 Always use the L<metaclass> pragma when using a custom metaclass, this
876 will ensure the proper initialization order and not accidentally
877 create an incorrect type of metaclass for you. This is a very rare
878 problem, and one which can only occur if you are doing deep metaclass
879 programming. So in other words, don't worry about it.
881 Note that if you're using L<Moose> we encourage you to I<not> use
882 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
883 roles to a class's metaclasses. This topic is covered at length in
884 various L<Moose::Cookbook> recipes.
888 The meta-object protocol is divided into 4 main sub-protocols:
890 =head2 The Class protocol
892 This provides a means of manipulating and introspecting a Perl 5
893 class. It handles symbol table hacking for you, and provides a rich
894 set of methods that go beyond simple package introspection.
896 See L<Class::MOP::Class> for more details.
898 =head2 The Attribute protocol
900 This provides a consistent representation for an attribute of a Perl 5
901 class. Since there are so many ways to create and handle attributes in
902 Perl 5 OO, the Attribute protocol provide as much of a unified
903 approach as possible. Of course, you are always free to extend this
904 protocol by subclassing the appropriate classes.
906 See L<Class::MOP::Attribute> for more details.
908 =head2 The Method protocol
910 This provides a means of manipulating and introspecting methods in the
911 Perl 5 object system. As with attributes, there are many ways to
912 approach this topic, so we try to keep it pretty basic, while still
913 making it possible to extend the system in many ways.
915 See L<Class::MOP::Method> for more details.
917 =head2 The Instance protocol
919 This provides a layer of abstraction for creating object instances.
920 Since the other layers use this protocol, it is relatively easy to
921 change the type of your instances from the default hash reference to
922 some other type of reference. Several examples are provided in the
923 F<examples/> directory included in this distribution.
925 See L<Class::MOP::Instance> for more details.
929 Note that this module does not export any constants or functions.
935 =item I<Class::MOP::IS_RUNNING_ON_5_10>
937 We set this constant depending on what version perl we are on, this
938 allows us to take advantage of new 5.10 features and stay backwards
943 =head2 Utility functions
945 Note that these are all called as B<functions, not methods>.
949 =item B<Class::MOP::load_class($class_name, \%options?)>
951 This will load the specified C<$class_name>, if it is not already
952 loaded (as reported by C<is_class_loaded>). This function can be used
953 in place of tricks like C<eval "use $module"> or using C<require>
956 If the module cannot be loaded, an exception is thrown.
958 You can pass a hash reference with options as second argument. The
959 only option currently recognised is C<-version>, which will ensure
960 that the loaded class has at least the required version.
962 See also L</Class Loading Options>.
964 For historical reasons, this function explicitly returns a true value.
966 =item B<Class::MOP::is_class_loaded($class_name, \%options?)>
968 Returns a boolean indicating whether or not C<$class_name> has been
971 This does a basic check of the symbol table to try and determine as
972 best it can if the C<$class_name> is loaded, it is probably correct
973 about 99% of the time, but it can be fooled into reporting false
974 positives. In particular, loading any of the core L<IO> modules will
975 cause most of the rest of the core L<IO> modules to falsely report
976 having been loaded, due to the way the base L<IO> module works.
978 You can pass a hash reference with options as second argument. The
979 only option currently recognised is C<-version>, which will ensure
980 that the loaded class has at least the required version.
982 See also L</Class Loading Options>.
984 =item B<Class::MOP::get_code_info($code)>
986 This function returns two values, the name of the package the C<$code>
987 is from and the name of the C<$code> itself. This is used by several
988 elements of the MOP to determine where a given C<$code> reference is
991 =item B<Class::MOP::class_of($instance_or_class_name)>
993 This will return the metaclass of the given instance or class name. If the
994 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
997 =item B<Class::MOP::check_package_cache_flag($pkg)>
999 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1001 This will return an integer that is managed by L<Class::MOP::Class> to
1002 determine if a module's symbol table has been altered.
1004 In Perl 5.10 or greater, this flag is package specific. However in
1005 versions prior to 5.10, this will use the C<PL_sub_generation>
1006 variable which is not package specific.
1008 =item B<Class::MOP::load_first_existing_class(@class_names)>
1010 =item B<Class::MOP::load_first_existing_class($classA, \%optionsA?, $classB, ...)>
1012 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1014 Given a list of class names, this function will attempt to load each
1017 If it finds a class it can load, it will return that class' name. If
1018 none of the classes can be loaded, it will throw an exception.
1020 Additionally, you can pass a hash reference with options after each
1021 class name. Currently, only C<-version> is recognised and will ensure
1022 that the loaded class has at least the required version. If the class
1023 version is not sufficient, an exception will be raised.
1025 See also L</Class Loading Options>.
1029 =head2 Metaclass cache functions
1031 Class::MOP holds a cache of metaclasses. The following are functions
1032 (B<not methods>) which can be used to access that cache. It is not
1033 recommended that you mess with these. Bad things could happen, but if
1034 you are brave and willing to risk it: go for it!
1038 =item B<Class::MOP::get_all_metaclasses>
1040 This will return a hash of all the metaclass instances that have
1041 been cached by L<Class::MOP::Class>, keyed by the package name.
1043 =item B<Class::MOP::get_all_metaclass_instances>
1045 This will return a list of all the metaclass instances that have
1046 been cached by L<Class::MOP::Class>.
1048 =item B<Class::MOP::get_all_metaclass_names>
1050 This will return a list of all the metaclass names that have
1051 been cached by L<Class::MOP::Class>.
1053 =item B<Class::MOP::get_metaclass_by_name($name)>
1055 This will return a cached L<Class::MOP::Class> instance, or nothing
1056 if no metaclass exists with that C<$name>.
1058 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
1060 This will store a metaclass in the cache at the supplied C<$key>.
1062 =item B<Class::MOP::weaken_metaclass($name)>
1064 In rare cases (e.g. anonymous metaclasses) it is desirable to
1065 store a weakened reference in the metaclass cache. This
1066 function will weaken the reference to the metaclass stored
1069 =item B<Class::MOP::metaclass_is_weak($name)>
1071 Returns true if the metaclass for C<$name> has been weakened
1072 (via C<weaken_metaclass>).
1074 =item B<Class::MOP::does_metaclass_exist($name)>
1076 This will return true of there exists a metaclass stored in the
1077 C<$name> key, and return false otherwise.
1079 =item B<Class::MOP::remove_metaclass_by_name($name)>
1081 This will remove the metaclass stored in the C<$name> key.
1085 =head2 Class Loading Options
1091 Can be used to pass a minimum required version that will be checked
1092 against the class version after it was loaded.
1100 There are very few books out on Meta Object Protocols and Metaclasses
1101 because it is such an esoteric topic. The following books are really
1102 the only ones I have found. If you know of any more, B<I<please>>
1103 email me and let me know, I would love to hear about them.
1107 =item I<The Art of the Meta Object Protocol>
1109 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1111 =item I<Putting MetaClasses to Work>
1113 =item I<Smalltalk: The Language>
1121 =item "Uniform and safe metaclass composition"
1123 An excellent paper by the people who brought us the original Traits paper.
1124 This paper is on how Traits can be used to do safe metaclass composition,
1125 and offers an excellent introduction section which delves into the topic of
1126 metaclass compatibility.
1128 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1130 =item "Safe Metaclass Programming"
1132 This paper seems to precede the above paper, and propose a mix-in based
1133 approach as opposed to the Traits based approach. Both papers have similar
1134 information on the metaclass compatibility problem space.
1136 L<http://citeseer.ist.psu.edu/37617.html>
1144 =item The Perl 6 MetaModel work in the Pugs project
1148 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1150 =item L<http://github.com/perl6/p5-modules/tree/master/Perl6-ObjectSpace/>
1160 =item CPAN Module Review of Class::MOP
1162 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1166 =head1 SIMILAR MODULES
1168 As I have said above, this module is a class-builder-builder, so it is
1169 not the same thing as modules like L<Class::Accessor> and
1170 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1171 with similar goals to this module. The one I have found which is most
1172 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1173 creates are very different from this modules.
1177 All complex software has bugs lurking in it, and this module is no
1180 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1181 web interface at L<http://rt.cpan.org>.
1183 You can also discuss feature requests or possible bugs on the Moose
1184 mailing list (moose@perl.org) or on IRC at
1185 L<irc://irc.perl.org/#moose>.
1187 =head1 ACKNOWLEDGEMENTS
1193 Thanks to Rob for actually getting the development of this module kick-started.