12 use Scalar::Util 'weaken', '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 = ($] < 5.009_005)
28 *DEBUG_NO_META = ($ENV{DEBUG_NO_META})
32 # this is either part of core or set up appropriately by MRO::Compat
33 *check_package_cache_flag = \&mro::get_pkg_gen;
36 our $VERSION = '1.08';
37 our $XS_VERSION = $VERSION;
38 $VERSION = eval $VERSION;
39 our $AUTHORITY = 'cpan:STEVAN';
42 XSLoader::load( __PACKAGE__, $XS_VERSION );
45 # Metaclasses are singletons, so we cache them here.
46 # there is no need to worry about destruction though
47 # because they should die only when the program dies.
48 # After all, do package definitions even get reaped?
49 # Anonymous classes manage their own destruction.
52 sub get_all_metaclasses { %METAS }
53 sub get_all_metaclass_instances { values %METAS }
54 sub get_all_metaclass_names { keys %METAS }
55 sub get_metaclass_by_name { $METAS{$_[0]} }
56 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
57 sub weaken_metaclass { weaken($METAS{$_[0]}) }
58 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
59 sub remove_metaclass_by_name { delete $METAS{$_[0]}; return }
61 # This handles instances as well as class names
63 return unless defined $_[0];
64 my $class = blessed($_[0]) || $_[0];
65 return $METAS{$class};
69 # We only cache metaclasses, meaning instances of
70 # Class::MOP::Class. We do not cache instance of
71 # Class::MOP::Package or Class::MOP::Module. Mostly
72 # because I don't yet see a good reason to do so.
75 sub _class_to_pmfile {
78 my $file = $class . '.pm';
84 sub load_first_existing_class {
85 my $classes = Data::OptList::mkopt(\@_)
88 foreach my $class (@{ $classes }) {
89 my $name = $class->[0];
90 unless ( _is_valid_class_name($name) ) {
91 my $display = defined($name) ? $name : 'undef';
92 confess "Invalid class name ($display)";
99 for my $class (@{ $classes }) {
100 my ($name, $options) = @{ $class };
103 return $name if is_class_loaded($name, $options);
104 if (is_class_loaded($name)) {
105 # we already know it's loaded and too old, but we call
106 # ->VERSION anyway to generate the exception for us
107 $name->VERSION($options->{-version});
111 return $name if is_class_loaded($name);
114 my $file = _class_to_pmfile($name);
115 return $name if try {
118 $name->VERSION($options->{-version})
119 if defined $options->{-version};
123 unless (/^Can't locate \Q$file\E in \@INC/) {
124 confess "Couldn't load class ($name) because: $_";
131 if ( @{ $classes } > 1 ) {
132 my @list = map { $_->[0] } @{ $classes };
133 confess "Can't locate any of @list in \@INC (\@INC contains: @INC).";
135 confess "Can't locate " . _class_to_pmfile($classes->[0]->[0]) . " in \@INC (\@INC contains: @INC).";
140 load_first_existing_class($_[0], ref $_[1] ? $_[1] : ());
142 # This is done to avoid breaking code which checked the return value. Said
143 # code is dumb. The return value was _always_ true, since it dies on
148 sub _is_valid_class_name {
151 return 0 if ref($class);
152 return 0 unless defined($class);
153 return 0 unless length($class);
155 return 1 if $class =~ /^\w+(?:::\w+)*$/;
160 ## ----------------------------------------------------------------------------
161 ## Setting up our environment ...
162 ## ----------------------------------------------------------------------------
163 ## Class::MOP needs to have a few things in the global perl environment so
164 ## that it can operate effectively. Those things are done here.
165 ## ----------------------------------------------------------------------------
167 # ... nothing yet actually ;)
169 ## ----------------------------------------------------------------------------
171 ## ----------------------------------------------------------------------------
172 ## The code below here is to bootstrap our MOP with itself. This is also
173 ## sometimes called "tying the knot". By doing this, we make it much easier
174 ## to extend the MOP through subclassing and such since now you can use the
175 ## MOP itself to extend itself.
177 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
178 ## ----------------------------------------------------------------------------
180 # We need to add in the meta-attributes here so that
181 # any subclass of Class::MOP::* will be able to
182 # inherit them using _construct_instance
184 ## --------------------------------------------------------
185 ## Class::MOP::Mixin::HasMethods
187 Class::MOP::Mixin::HasMethods->meta->add_attribute(
188 Class::MOP::Attribute->new('_methods' => (
191 # we just alias the original method
192 # rather than re-produce it here
193 '_full_method_map' => \&Class::MOP::Mixin::HasMethods::_full_method_map
195 default => sub { {} }
199 Class::MOP::Mixin::HasMethods->meta->add_attribute(
200 Class::MOP::Attribute->new('method_metaclass' => (
203 # we just alias the original method
204 # rather than re-produce it here
205 'method_metaclass' => \&Class::MOP::Mixin::HasMethods::method_metaclass
207 default => 'Class::MOP::Method',
211 Class::MOP::Mixin::HasMethods->meta->add_attribute(
212 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
215 # we just alias the original method
216 # rather than re-produce it here
217 'wrapped_method_metaclass' => \&Class::MOP::Mixin::HasMethods::wrapped_method_metaclass
219 default => 'Class::MOP::Method::Wrapped',
223 ## --------------------------------------------------------
224 ## Class::MOP::Mixin::HasMethods
226 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
227 Class::MOP::Attribute->new('attributes' => (
229 # NOTE: we need to do this in order
230 # for the instance meta-object to
231 # not fall into meta-circular death
233 # we just alias the original method
234 # rather than re-produce it here
235 '_attribute_map' => \&Class::MOP::Mixin::HasAttributes::_attribute_map
237 default => sub { {} }
241 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
242 Class::MOP::Attribute->new('attribute_metaclass' => (
245 # we just alias the original method
246 # rather than re-produce it here
247 'attribute_metaclass' => \&Class::MOP::Mixin::HasAttributes::attribute_metaclass
249 default => 'Class::MOP::Attribute',
253 ## --------------------------------------------------------
254 ## Class::MOP::Package
256 Class::MOP::Package->meta->add_attribute(
257 Class::MOP::Attribute->new('package' => (
259 # NOTE: we need to do this in order
260 # for the instance meta-object to
261 # not fall into meta-circular death
263 # we just alias the original method
264 # rather than re-produce it here
265 'name' => \&Class::MOP::Package::name
270 Class::MOP::Package->meta->add_attribute(
271 Class::MOP::Attribute->new('namespace' => (
274 # we just alias the original method
275 # rather than re-produce it here
276 'namespace' => \&Class::MOP::Package::namespace
279 default => sub { \undef }
283 ## --------------------------------------------------------
284 ## Class::MOP::Module
287 # yeah this is kind of stretching things a bit,
288 # but truthfully the version should be an attribute
289 # of the Module, the weirdness comes from having to
290 # stick to Perl 5 convention and store it in the
291 # $VERSION package variable. Basically if you just
292 # squint at it, it will look how you want it to look.
293 # Either as a package variable, or as a attribute of
294 # the metaclass, isn't abstraction great :)
296 Class::MOP::Module->meta->add_attribute(
297 Class::MOP::Attribute->new('version' => (
300 # we just alias the original method
301 # rather than re-produce it here
302 'version' => \&Class::MOP::Module::version
305 default => sub { \undef }
310 # By following the same conventions as version here,
311 # we are opening up the possibility that people can
312 # use the $AUTHORITY in non-Class::MOP modules as
315 Class::MOP::Module->meta->add_attribute(
316 Class::MOP::Attribute->new('authority' => (
319 # we just alias the original method
320 # rather than re-produce it here
321 'authority' => \&Class::MOP::Module::authority
324 default => sub { \undef }
328 ## --------------------------------------------------------
331 Class::MOP::Class->meta->add_attribute(
332 Class::MOP::Attribute->new('superclasses' => (
335 # we just alias the original method
336 # rather than re-produce it here
337 'superclasses' => \&Class::MOP::Class::superclasses
340 default => sub { \undef }
344 Class::MOP::Class->meta->add_attribute(
345 Class::MOP::Attribute->new('instance_metaclass' => (
347 # NOTE: we need to do this in order
348 # for the instance meta-object to
349 # not fall into meta-circular death
351 # we just alias the original method
352 # rather than re-produce it here
353 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
355 default => 'Class::MOP::Instance',
359 Class::MOP::Class->meta->add_attribute(
360 Class::MOP::Attribute->new('immutable_trait' => (
362 'immutable_trait' => \&Class::MOP::Class::immutable_trait
364 default => "Class::MOP::Class::Immutable::Trait",
368 Class::MOP::Class->meta->add_attribute(
369 Class::MOP::Attribute->new('constructor_name' => (
371 'constructor_name' => \&Class::MOP::Class::constructor_name,
377 Class::MOP::Class->meta->add_attribute(
378 Class::MOP::Attribute->new('constructor_class' => (
380 'constructor_class' => \&Class::MOP::Class::constructor_class,
382 default => "Class::MOP::Method::Constructor",
387 Class::MOP::Class->meta->add_attribute(
388 Class::MOP::Attribute->new('destructor_class' => (
390 'destructor_class' => \&Class::MOP::Class::destructor_class,
396 # we don't actually need to tie the knot with
397 # Class::MOP::Class here, it is actually handled
398 # within Class::MOP::Class itself in the
399 # _construct_class_instance method.
401 ## --------------------------------------------------------
402 ## Class::MOP::Mixin::AttributeCore
403 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
404 Class::MOP::Attribute->new('name' => (
406 # NOTE: we need to do this in order
407 # for the instance meta-object to
408 # not fall into meta-circular death
410 # we just alias the original method
411 # rather than re-produce it here
412 'name' => \&Class::MOP::Mixin::AttributeCore::name
417 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
418 Class::MOP::Attribute->new('accessor' => (
419 reader => { 'accessor' => \&Class::MOP::Mixin::AttributeCore::accessor },
420 predicate => { 'has_accessor' => \&Class::MOP::Mixin::AttributeCore::has_accessor },
424 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
425 Class::MOP::Attribute->new('reader' => (
426 reader => { 'reader' => \&Class::MOP::Mixin::AttributeCore::reader },
427 predicate => { 'has_reader' => \&Class::MOP::Mixin::AttributeCore::has_reader },
431 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
432 Class::MOP::Attribute->new('initializer' => (
433 reader => { 'initializer' => \&Class::MOP::Mixin::AttributeCore::initializer },
434 predicate => { 'has_initializer' => \&Class::MOP::Mixin::AttributeCore::has_initializer },
438 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
439 Class::MOP::Attribute->new('definition_context' => (
440 reader => { 'definition_context' => \&Class::MOP::Mixin::AttributeCore::definition_context },
444 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
445 Class::MOP::Attribute->new('writer' => (
446 reader => { 'writer' => \&Class::MOP::Mixin::AttributeCore::writer },
447 predicate => { 'has_writer' => \&Class::MOP::Mixin::AttributeCore::has_writer },
451 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
452 Class::MOP::Attribute->new('predicate' => (
453 reader => { 'predicate' => \&Class::MOP::Mixin::AttributeCore::predicate },
454 predicate => { 'has_predicate' => \&Class::MOP::Mixin::AttributeCore::has_predicate },
458 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
459 Class::MOP::Attribute->new('clearer' => (
460 reader => { 'clearer' => \&Class::MOP::Mixin::AttributeCore::clearer },
461 predicate => { 'has_clearer' => \&Class::MOP::Mixin::AttributeCore::has_clearer },
465 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
466 Class::MOP::Attribute->new('builder' => (
467 reader => { 'builder' => \&Class::MOP::Mixin::AttributeCore::builder },
468 predicate => { 'has_builder' => \&Class::MOP::Mixin::AttributeCore::has_builder },
472 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
473 Class::MOP::Attribute->new('init_arg' => (
474 reader => { 'init_arg' => \&Class::MOP::Mixin::AttributeCore::init_arg },
475 predicate => { 'has_init_arg' => \&Class::MOP::Mixin::AttributeCore::has_init_arg },
479 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
480 Class::MOP::Attribute->new('default' => (
481 # default has a custom 'reader' method ...
482 predicate => { 'has_default' => \&Class::MOP::Mixin::AttributeCore::has_default },
486 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
487 Class::MOP::Attribute->new('insertion_order' => (
488 reader => { 'insertion_order' => \&Class::MOP::Mixin::AttributeCore::insertion_order },
489 writer => { '_set_insertion_order' => \&Class::MOP::Mixin::AttributeCore::_set_insertion_order },
490 predicate => { 'has_insertion_order' => \&Class::MOP::Mixin::AttributeCore::has_insertion_order },
494 ## --------------------------------------------------------
495 ## Class::MOP::Attribute
496 Class::MOP::Attribute->meta->add_attribute(
497 Class::MOP::Attribute->new('associated_class' => (
499 # NOTE: we need to do this in order
500 # for the instance meta-object to
501 # not fall into meta-circular death
503 # we just alias the original method
504 # rather than re-produce it here
505 'associated_class' => \&Class::MOP::Attribute::associated_class
510 Class::MOP::Attribute->meta->add_attribute(
511 Class::MOP::Attribute->new('associated_methods' => (
512 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
513 default => sub { [] }
517 Class::MOP::Attribute->meta->add_method('clone' => sub {
519 $self->meta->clone_object($self, @_);
522 ## --------------------------------------------------------
523 ## Class::MOP::Method
524 Class::MOP::Method->meta->add_attribute(
525 Class::MOP::Attribute->new('body' => (
526 reader => { 'body' => \&Class::MOP::Method::body },
530 Class::MOP::Method->meta->add_attribute(
531 Class::MOP::Attribute->new('associated_metaclass' => (
532 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
536 Class::MOP::Method->meta->add_attribute(
537 Class::MOP::Attribute->new('package_name' => (
538 reader => { 'package_name' => \&Class::MOP::Method::package_name },
542 Class::MOP::Method->meta->add_attribute(
543 Class::MOP::Attribute->new('name' => (
544 reader => { 'name' => \&Class::MOP::Method::name },
548 Class::MOP::Method->meta->add_attribute(
549 Class::MOP::Attribute->new('original_method' => (
550 reader => { 'original_method' => \&Class::MOP::Method::original_method },
551 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
555 ## --------------------------------------------------------
556 ## Class::MOP::Method::Wrapped
559 # the way this item is initialized, this
560 # really does not follow the standard
561 # practices of attributes, but we put
562 # it here for completeness
563 Class::MOP::Method::Wrapped->meta->add_attribute(
564 Class::MOP::Attribute->new('modifier_table')
567 ## --------------------------------------------------------
568 ## Class::MOP::Method::Generated
570 Class::MOP::Method::Generated->meta->add_attribute(
571 Class::MOP::Attribute->new('is_inline' => (
572 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
577 Class::MOP::Method::Generated->meta->add_attribute(
578 Class::MOP::Attribute->new('definition_context' => (
579 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
584 ## --------------------------------------------------------
585 ## Class::MOP::Method::Inlined
587 Class::MOP::Method::Inlined->meta->add_attribute(
588 Class::MOP::Attribute->new('_expected_method_class' => (
589 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
593 ## --------------------------------------------------------
594 ## Class::MOP::Method::Accessor
596 Class::MOP::Method::Accessor->meta->add_attribute(
597 Class::MOP::Attribute->new('attribute' => (
599 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
604 Class::MOP::Method::Accessor->meta->add_attribute(
605 Class::MOP::Attribute->new('accessor_type' => (
606 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
610 ## --------------------------------------------------------
611 ## Class::MOP::Method::Constructor
613 Class::MOP::Method::Constructor->meta->add_attribute(
614 Class::MOP::Attribute->new('options' => (
616 'options' => \&Class::MOP::Method::Constructor::options
618 default => sub { +{} }
622 Class::MOP::Method::Constructor->meta->add_attribute(
623 Class::MOP::Attribute->new('associated_metaclass' => (
624 init_arg => "metaclass", # FIXME alias and rename
626 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
631 ## --------------------------------------------------------
632 ## Class::MOP::Instance
635 # these don't yet do much of anything, but are just
636 # included for completeness
638 Class::MOP::Instance->meta->add_attribute(
639 Class::MOP::Attribute->new('associated_metaclass',
640 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
644 Class::MOP::Instance->meta->add_attribute(
645 Class::MOP::Attribute->new('_class_name',
647 reader => { _class_name => \&Class::MOP::Instance::_class_name },
648 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
649 #default => sub { $_[0]->associated_metaclass->name },
653 Class::MOP::Instance->meta->add_attribute(
654 Class::MOP::Attribute->new('attributes',
655 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
659 Class::MOP::Instance->meta->add_attribute(
660 Class::MOP::Attribute->new('slots',
661 reader => { slots => \&Class::MOP::Instance::slots },
665 Class::MOP::Instance->meta->add_attribute(
666 Class::MOP::Attribute->new('slot_hash',
667 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
671 require Class::MOP::Deprecated unless our $no_deprecated;
673 # we need the meta instance of the meta instance to be created now, in order
674 # for the constructor to be able to use it
675 Class::MOP::Instance->meta->get_meta_instance;
677 # pretend the add_method never happenned. it hasn't yet affected anything
678 undef Class::MOP::Instance->meta->{_package_cache_flag};
680 ## --------------------------------------------------------
681 ## Now close all the Class::MOP::* classes
683 # NOTE: we don't need to inline the the accessors this only lengthens
684 # the compile time of the MOP, and gives us no actual benefits.
686 $_->meta->make_immutable(
687 inline_constructor => 0,
688 constructor_name => "_new",
689 inline_accessors => 0,
695 Class::MOP::Attribute
701 Class::MOP::Method::Generated
702 Class::MOP::Method::Inlined
704 Class::MOP::Method::Accessor
705 Class::MOP::Method::Constructor
706 Class::MOP::Method::Wrapped
709 $_->meta->make_immutable(
710 inline_constructor => 0,
711 constructor_name => undef,
712 inline_accessors => 0,
715 Class::MOP::Mixin::AttributeCore
716 Class::MOP::Mixin::HasAttributes
717 Class::MOP::Mixin::HasMethods
728 Class::MOP - A Meta Object Protocol for Perl 5
732 This module is a fully functioning meta object protocol for the
733 Perl 5 object system. It makes no attempt to change the behavior or
734 characteristics of the Perl 5 object system, only to create a
735 protocol for its manipulation and introspection.
737 That said, it does attempt to create the tools for building a rich set
738 of extensions to the Perl 5 object system. Every attempt has been made
739 to abide by the spirit of the Perl 5 object system that we all know
742 This documentation is sparse on conceptual details. We suggest looking
743 at the items listed in the L<SEE ALSO> section for more
744 information. In particular the book "The Art of the Meta Object
745 Protocol" was very influential in the development of this system.
747 =head2 What is a Meta Object Protocol?
749 A meta object protocol is an API to an object system.
751 To be more specific, it abstracts the components of an object system
752 (classes, object, methods, object attributes, etc.). These
753 abstractions can then be used to inspect and manipulate the object
754 system which they describe.
756 It can be said that there are two MOPs for any object system; the
757 implicit MOP and the explicit MOP. The implicit MOP handles things
758 like method dispatch or inheritance, which happen automatically as
759 part of how the object system works. The explicit MOP typically
760 handles the introspection/reflection features of the object system.
762 All object systems have implicit MOPs. Without one, they would not
763 work. Explicit MOPs are much less common, and depending on the
764 language can vary from restrictive (Reflection in Java or C#) to wide
765 open (CLOS is a perfect example).
767 =head2 Yet Another Class Builder! Why?
769 This is B<not> a class builder so much as a I<class builder
770 B<builder>>. The intent is that an end user will not use this module
771 directly, but instead this module is used by module authors to build
772 extensions and features onto the Perl 5 object system.
774 This system is used by L<Moose>, which supplies a powerful class
775 builder system built entirely on top of C<Class::MOP>.
777 =head2 Who is this module for?
779 This module is for anyone who has ever created or wanted to create a
780 module for the Class:: namespace. The tools which this module provides
781 make doing complex Perl 5 wizardry simpler, by removing such barriers
782 as the need to hack symbol tables, or understand the fine details of
785 =head2 What changes do I have to make to use this module?
787 This module was designed to be as unintrusive as possible. Many of its
788 features are accessible without B<any> change to your existing
789 code. It is meant to be a compliment to your existing code and not an
790 intrusion on your code base. Unlike many other B<Class::> modules,
791 this module B<does not> require you subclass it, or even that you
792 C<use> it in within your module's package.
794 The only features which requires additions to your code are the
795 attribute handling and instance construction features, and these are
796 both completely optional features. The only reason for this is because
797 Perl 5's object system does not actually have these features built
798 in. More information about this feature can be found below.
800 =head2 About Performance
802 It is a common misconception that explicit MOPs are a performance hit.
803 This is not a universal truth, it is a side-effect of some specific
804 implementations. For instance, using Java reflection is slow because
805 the JVM cannot take advantage of any compiler optimizations, and the
806 JVM has to deal with much more runtime type information as well.
808 Reflection in C# is marginally better as it was designed into the
809 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
810 Object System) was built to support an explicit MOP, and so
811 performance is tuned for it.
813 This library in particular does its absolute best to avoid putting
814 B<any> drain at all upon your code's performance. In fact, by itself
815 it does nothing to affect your existing code. So you only pay for what
818 =head2 About Metaclass compatibility
820 This module makes sure that all metaclasses created are both upwards
821 and downwards compatible. The topic of metaclass compatibility is
822 highly esoteric and is something only encountered when doing deep and
823 involved metaclass hacking. There are two basic kinds of metaclass
824 incompatibility; upwards and downwards.
826 Upwards metaclass compatibility means that the metaclass of a
827 given class is either the same as (or a subclass of) all of the
830 Downward metaclass compatibility means that the metaclasses of a
831 given class's ancestors are all either the same as (or a subclass
834 Here is a diagram showing a set of two classes (C<A> and C<B>) and
835 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
836 metaclass compatibility both upwards and downwards.
838 +---------+ +---------+
839 | Meta::A |<----| Meta::B | <....... (instance of )
840 +---------+ +---------+ <------- (inherits from)
843 +---------+ +---------+
845 +---------+ +---------+
847 In actuality, I<all> of a class's metaclasses must be compatible,
848 not just the class metaclass. That includes the instance, attribute,
849 and method metaclasses, as well as the constructor and destructor
852 C<Class::MOP> will attempt to fix some simple types of
853 incompatibilities. If all the metaclasses for the parent class are
854 I<subclasses> of the child's metaclasses then we can simply replace
855 the child's metaclasses with the parent's. In addition, if the child
856 is missing a metaclass that the parent has, we can also just make the
857 child use the parent's metaclass.
859 As I said this is a highly esoteric topic and one you will only run
860 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
861 are interested in why this is an issue see the paper I<Uniform and
862 safe metaclass composition> linked to in the L<SEE ALSO> section of
865 =head2 Using custom metaclasses
867 Always use the L<metaclass> pragma when using a custom metaclass, this
868 will ensure the proper initialization order and not accidentally
869 create an incorrect type of metaclass for you. This is a very rare
870 problem, and one which can only occur if you are doing deep metaclass
871 programming. So in other words, don't worry about it.
873 Note that if you're using L<Moose> we encourage you to I<not> use
874 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
875 roles to a class's metaclasses. This topic is covered at length in
876 various L<Moose::Cookbook> recipes.
880 The meta-object protocol is divided into 4 main sub-protocols:
882 =head2 The Class protocol
884 This provides a means of manipulating and introspecting a Perl 5
885 class. It handles symbol table hacking for you, and provides a rich
886 set of methods that go beyond simple package introspection.
888 See L<Class::MOP::Class> for more details.
890 =head2 The Attribute protocol
892 This provides a consistent representation for an attribute of a Perl 5
893 class. Since there are so many ways to create and handle attributes in
894 Perl 5 OO, the Attribute protocol provide as much of a unified
895 approach as possible. Of course, you are always free to extend this
896 protocol by subclassing the appropriate classes.
898 See L<Class::MOP::Attribute> for more details.
900 =head2 The Method protocol
902 This provides a means of manipulating and introspecting methods in the
903 Perl 5 object system. As with attributes, there are many ways to
904 approach this topic, so we try to keep it pretty basic, while still
905 making it possible to extend the system in many ways.
907 See L<Class::MOP::Method> for more details.
909 =head2 The Instance protocol
911 This provides a layer of abstraction for creating object instances.
912 Since the other layers use this protocol, it is relatively easy to
913 change the type of your instances from the default hash reference to
914 some other type of reference. Several examples are provided in the
915 F<examples/> directory included in this distribution.
917 See L<Class::MOP::Instance> for more details.
921 Note that this module does not export any constants or functions.
927 =item I<Class::MOP::IS_RUNNING_ON_5_10>
929 We set this constant depending on what version perl we are on, this
930 allows us to take advantage of new 5.10 features and stay backwards
935 =head2 Utility functions
937 Note that these are all called as B<functions, not methods>.
941 =item B<Class::MOP::load_class($class_name, \%options?)>
943 This will load the specified C<$class_name>, if it is not already
944 loaded (as reported by C<is_class_loaded>). This function can be used
945 in place of tricks like C<eval "use $module"> or using C<require>
948 If the module cannot be loaded, an exception is thrown.
950 You can pass a hash reference with options as second argument. The
951 only option currently recognised is C<-version>, which will ensure
952 that the loaded class has at least the required version.
954 See also L</Class Loading Options>.
956 For historical reasons, this function explicitly returns a true value.
958 =item B<Class::MOP::is_class_loaded($class_name, \%options?)>
960 Returns a boolean indicating whether or not C<$class_name> has been
963 This does a basic check of the symbol table to try and determine as
964 best it can if the C<$class_name> is loaded, it is probably correct
965 about 99% of the time, but it can be fooled into reporting false
966 positives. In particular, loading any of the core L<IO> modules will
967 cause most of the rest of the core L<IO> modules to falsely report
968 having been loaded, due to the way the base L<IO> module works.
970 You can pass a hash reference with options as second argument. The
971 only option currently recognised is C<-version>, which will ensure
972 that the loaded class has at least the required version.
974 See also L</Class Loading Options>.
976 =item B<Class::MOP::get_code_info($code)>
978 This function returns two values, the name of the package the C<$code>
979 is from and the name of the C<$code> itself. This is used by several
980 elements of the MOP to determine where a given C<$code> reference is
983 =item B<Class::MOP::class_of($instance_or_class_name)>
985 This will return the metaclass of the given instance or class name. If the
986 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
989 =item B<Class::MOP::check_package_cache_flag($pkg)>
991 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
993 This will return an integer that is managed by L<Class::MOP::Class> to
994 determine if a module's symbol table has been altered.
996 In Perl 5.10 or greater, this flag is package specific. However in
997 versions prior to 5.10, this will use the C<PL_sub_generation>
998 variable which is not package specific.
1000 =item B<Class::MOP::load_first_existing_class(@class_names)>
1002 =item B<Class::MOP::load_first_existing_class($classA, \%optionsA?, $classB, ...)>
1004 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1006 Given a list of class names, this function will attempt to load each
1009 If it finds a class it can load, it will return that class' name. If
1010 none of the classes can be loaded, it will throw an exception.
1012 Additionally, you can pass a hash reference with options after each
1013 class name. Currently, only C<-version> is recognised and will ensure
1014 that the loaded class has at least the required version. If the class
1015 version is not sufficient, an exception will be raised.
1017 See also L</Class Loading Options>.
1021 =head2 Metaclass cache functions
1023 Class::MOP holds a cache of metaclasses. The following are functions
1024 (B<not methods>) which can be used to access that cache. It is not
1025 recommended that you mess with these. Bad things could happen, but if
1026 you are brave and willing to risk it: go for it!
1030 =item B<Class::MOP::get_all_metaclasses>
1032 This will return a hash of all the metaclass instances that have
1033 been cached by L<Class::MOP::Class>, keyed by the package name.
1035 =item B<Class::MOP::get_all_metaclass_instances>
1037 This will return a list of all the metaclass instances that have
1038 been cached by L<Class::MOP::Class>.
1040 =item B<Class::MOP::get_all_metaclass_names>
1042 This will return a list of all the metaclass names that have
1043 been cached by L<Class::MOP::Class>.
1045 =item B<Class::MOP::get_metaclass_by_name($name)>
1047 This will return a cached L<Class::MOP::Class> instance, or nothing
1048 if no metaclass exists with that C<$name>.
1050 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
1052 This will store a metaclass in the cache at the supplied C<$key>.
1054 =item B<Class::MOP::weaken_metaclass($name)>
1056 In rare cases (e.g. anonymous metaclasses) it is desirable to
1057 store a weakened reference in the metaclass cache. This
1058 function will weaken the reference to the metaclass stored
1061 =item B<Class::MOP::does_metaclass_exist($name)>
1063 This will return true of there exists a metaclass stored in the
1064 C<$name> key, and return false otherwise.
1066 =item B<Class::MOP::remove_metaclass_by_name($name)>
1068 This will remove the metaclass stored in the C<$name> key.
1072 =head2 Class Loading Options
1078 Can be used to pass a minimum required version that will be checked
1079 against the class version after it was loaded.
1087 There are very few books out on Meta Object Protocols and Metaclasses
1088 because it is such an esoteric topic. The following books are really
1089 the only ones I have found. If you know of any more, B<I<please>>
1090 email me and let me know, I would love to hear about them.
1094 =item I<The Art of the Meta Object Protocol>
1096 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1098 =item I<Putting MetaClasses to Work>
1100 =item I<Smalltalk: The Language>
1108 =item "Uniform and safe metaclass composition"
1110 An excellent paper by the people who brought us the original Traits paper.
1111 This paper is on how Traits can be used to do safe metaclass composition,
1112 and offers an excellent introduction section which delves into the topic of
1113 metaclass compatibility.
1115 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1117 =item "Safe Metaclass Programming"
1119 This paper seems to precede the above paper, and propose a mix-in based
1120 approach as opposed to the Traits based approach. Both papers have similar
1121 information on the metaclass compatibility problem space.
1123 L<http://citeseer.ist.psu.edu/37617.html>
1131 =item The Perl 6 MetaModel work in the Pugs project
1135 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1137 =item L<http://github.com/perl6/p5-modules/tree/master/Perl6-ObjectSpace/>
1147 =item CPAN Module Review of Class::MOP
1149 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1153 =head1 SIMILAR MODULES
1155 As I have said above, this module is a class-builder-builder, so it is
1156 not the same thing as modules like L<Class::Accessor> and
1157 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1158 with similar goals to this module. The one I have found which is most
1159 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1160 creates are very different from this modules.
1164 All complex software has bugs lurking in it, and this module is no
1167 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1168 web interface at L<http://rt.cpan.org>.
1170 You can also discuss feature requests or possible bugs on the Moose
1171 mailing list (moose@perl.org) or on IRC at
1172 L<irc://irc.perl.org/#moose>.
1174 =head1 ACKNOWLEDGEMENTS
1180 Thanks to Rob for actually getting the development of this module kick-started.
1186 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1188 B<with contributions from:>
1190 Brandon (blblack) Black
1192 Florian (rafl) Ragwitz
1194 Guillermo (groditi) Roditi
1196 Dave (autarch) Rolsky
1200 Rob (robkinyon) Kinyon
1202 Yuval (nothingmuch) Kogman
1204 Scott (konobi) McWhirter
1208 =head1 COPYRIGHT AND LICENSE
1210 Copyright 2006-2010 by Infinity Interactive, Inc.
1212 L<http://www.iinteractive.com>
1214 This library is free software; you can redistribute it and/or modify
1215 it under the same terms as Perl itself.