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 # 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 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 '_full_method_map' => \&Class::MOP::Mixin::HasMethods::_full_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;
673 ## --------------------------------------------------------
676 # need to replace the meta method there with a real meta method object
677 Class::MOP::Mixin->meta->_add_meta_method;
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
738 Class::MOP - A Meta Object Protocol for Perl 5
742 This module is a fully functioning meta object protocol for the
743 Perl 5 object system. It makes no attempt to change the behavior or
744 characteristics of the Perl 5 object system, only to create a
745 protocol for its manipulation and introspection.
747 That said, it does attempt to create the tools for building a rich set
748 of extensions to the Perl 5 object system. Every attempt has been made
749 to abide by the spirit of the Perl 5 object system that we all know
752 This documentation is sparse on conceptual details. We suggest looking
753 at the items listed in the L<SEE ALSO> section for more
754 information. In particular the book "The Art of the Meta Object
755 Protocol" was very influential in the development of this system.
757 =head2 What is a Meta Object Protocol?
759 A meta object protocol is an API to an object system.
761 To be more specific, it abstracts the components of an object system
762 (classes, object, methods, object attributes, etc.). These
763 abstractions can then be used to inspect and manipulate the object
764 system which they describe.
766 It can be said that there are two MOPs for any object system; the
767 implicit MOP and the explicit MOP. The implicit MOP handles things
768 like method dispatch or inheritance, which happen automatically as
769 part of how the object system works. The explicit MOP typically
770 handles the introspection/reflection features of the object system.
772 All object systems have implicit MOPs. Without one, they would not
773 work. Explicit MOPs are much less common, and depending on the
774 language can vary from restrictive (Reflection in Java or C#) to wide
775 open (CLOS is a perfect example).
777 =head2 Yet Another Class Builder! Why?
779 This is B<not> a class builder so much as a I<class builder
780 B<builder>>. The intent is that an end user will not use this module
781 directly, but instead this module is used by module authors to build
782 extensions and features onto the Perl 5 object system.
784 This system is used by L<Moose>, which supplies a powerful class
785 builder system built entirely on top of C<Class::MOP>.
787 =head2 Who is this module for?
789 This module is for anyone who has ever created or wanted to create a
790 module for the Class:: namespace. The tools which this module provides
791 make doing complex Perl 5 wizardry simpler, by removing such barriers
792 as the need to hack symbol tables, or understand the fine details of
795 =head2 What changes do I have to make to use this module?
797 This module was designed to be as unintrusive as possible. Many of its
798 features are accessible without B<any> change to your existing
799 code. It is meant to be a compliment to your existing code and not an
800 intrusion on your code base. Unlike many other B<Class::> modules,
801 this module B<does not> require you subclass it, or even that you
802 C<use> it in within your module's package.
804 The only features which requires additions to your code are the
805 attribute handling and instance construction features, and these are
806 both completely optional features. The only reason for this is because
807 Perl 5's object system does not actually have these features built
808 in. More information about this feature can be found below.
810 =head2 About Performance
812 It is a common misconception that explicit MOPs are a performance hit.
813 This is not a universal truth, it is a side-effect of some specific
814 implementations. For instance, using Java reflection is slow because
815 the JVM cannot take advantage of any compiler optimizations, and the
816 JVM has to deal with much more runtime type information as well.
818 Reflection in C# is marginally better as it was designed into the
819 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
820 Object System) was built to support an explicit MOP, and so
821 performance is tuned for it.
823 This library in particular does its absolute best to avoid putting
824 B<any> drain at all upon your code's performance. In fact, by itself
825 it does nothing to affect your existing code. So you only pay for what
828 =head2 About Metaclass compatibility
830 This module makes sure that all metaclasses created are both upwards
831 and downwards compatible. The topic of metaclass compatibility is
832 highly esoteric and is something only encountered when doing deep and
833 involved metaclass hacking. There are two basic kinds of metaclass
834 incompatibility; upwards and downwards.
836 Upwards metaclass compatibility means that the metaclass of a
837 given class is either the same as (or a subclass of) all of the
840 Downward metaclass compatibility means that the metaclasses of a
841 given class's ancestors are all either the same as (or a subclass
844 Here is a diagram showing a set of two classes (C<A> and C<B>) and
845 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
846 metaclass compatibility both upwards and downwards.
848 +---------+ +---------+
849 | Meta::A |<----| Meta::B | <....... (instance of )
850 +---------+ +---------+ <------- (inherits from)
853 +---------+ +---------+
855 +---------+ +---------+
857 In actuality, I<all> of a class's metaclasses must be compatible,
858 not just the class metaclass. That includes the instance, attribute,
859 and method metaclasses, as well as the constructor and destructor
862 C<Class::MOP> will attempt to fix some simple types of
863 incompatibilities. If all the metaclasses for the parent class are
864 I<subclasses> of the child's metaclasses then we can simply replace
865 the child's metaclasses with the parent's. In addition, if the child
866 is missing a metaclass that the parent has, we can also just make the
867 child use the parent's metaclass.
869 As I said this is a highly esoteric topic and one you will only run
870 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
871 are interested in why this is an issue see the paper I<Uniform and
872 safe metaclass composition> linked to in the L<SEE ALSO> section of
875 =head2 Using custom metaclasses
877 Always use the L<metaclass> pragma when using a custom metaclass, this
878 will ensure the proper initialization order and not accidentally
879 create an incorrect type of metaclass for you. This is a very rare
880 problem, and one which can only occur if you are doing deep metaclass
881 programming. So in other words, don't worry about it.
883 Note that if you're using L<Moose> we encourage you to I<not> use
884 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
885 roles to a class's metaclasses. This topic is covered at length in
886 various L<Moose::Cookbook> recipes.
890 The meta-object protocol is divided into 4 main sub-protocols:
892 =head2 The Class protocol
894 This provides a means of manipulating and introspecting a Perl 5
895 class. It handles symbol table hacking for you, and provides a rich
896 set of methods that go beyond simple package introspection.
898 See L<Class::MOP::Class> for more details.
900 =head2 The Attribute protocol
902 This provides a consistent representation for an attribute of a Perl 5
903 class. Since there are so many ways to create and handle attributes in
904 Perl 5 OO, the Attribute protocol provide as much of a unified
905 approach as possible. Of course, you are always free to extend this
906 protocol by subclassing the appropriate classes.
908 See L<Class::MOP::Attribute> for more details.
910 =head2 The Method protocol
912 This provides a means of manipulating and introspecting methods in the
913 Perl 5 object system. As with attributes, there are many ways to
914 approach this topic, so we try to keep it pretty basic, while still
915 making it possible to extend the system in many ways.
917 See L<Class::MOP::Method> for more details.
919 =head2 The Instance protocol
921 This provides a layer of abstraction for creating object instances.
922 Since the other layers use this protocol, it is relatively easy to
923 change the type of your instances from the default hash reference to
924 some other type of reference. Several examples are provided in the
925 F<examples/> directory included in this distribution.
927 See L<Class::MOP::Instance> for more details.
931 Note that this module does not export any constants or functions.
937 =item I<Class::MOP::IS_RUNNING_ON_5_10>
939 We set this constant depending on what version perl we are on, this
940 allows us to take advantage of new 5.10 features and stay backwards
945 =head2 Utility functions
947 Note that these are all called as B<functions, not methods>.
951 =item B<Class::MOP::load_class($class_name, \%options?)>
953 This will load the specified C<$class_name>, if it is not already
954 loaded (as reported by C<is_class_loaded>). This function can be used
955 in place of tricks like C<eval "use $module"> or using C<require>
958 If the module cannot be loaded, an exception is thrown.
960 You can pass a hash reference with options as second argument. The
961 only option currently recognised is C<-version>, which will ensure
962 that the loaded class has at least the required version.
964 See also L</Class Loading Options>.
966 For historical reasons, this function explicitly returns a true value.
968 =item B<Class::MOP::is_class_loaded($class_name, \%options?)>
970 Returns a boolean indicating whether or not C<$class_name> has been
973 This does a basic check of the symbol table to try and determine as
974 best it can if the C<$class_name> is loaded, it is probably correct
975 about 99% of the time, but it can be fooled into reporting false
976 positives. In particular, loading any of the core L<IO> modules will
977 cause most of the rest of the core L<IO> modules to falsely report
978 having been loaded, due to the way the base L<IO> module works.
980 You can pass a hash reference with options as second argument. The
981 only option currently recognised is C<-version>, which will ensure
982 that the loaded class has at least the required version.
984 See also L</Class Loading Options>.
986 =item B<Class::MOP::get_code_info($code)>
988 This function returns two values, the name of the package the C<$code>
989 is from and the name of the C<$code> itself. This is used by several
990 elements of the MOP to determine where a given C<$code> reference is
993 =item B<Class::MOP::class_of($instance_or_class_name)>
995 This will return the metaclass of the given instance or class name. If the
996 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
999 =item B<Class::MOP::check_package_cache_flag($pkg)>
1001 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1003 This will return an integer that is managed by L<Class::MOP::Class> to
1004 determine if a module's symbol table has been altered.
1006 In Perl 5.10 or greater, this flag is package specific. However in
1007 versions prior to 5.10, this will use the C<PL_sub_generation>
1008 variable which is not package specific.
1010 =item B<Class::MOP::load_first_existing_class(@class_names)>
1012 =item B<Class::MOP::load_first_existing_class($classA, \%optionsA?, $classB, ...)>
1014 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1016 Given a list of class names, this function will attempt to load each
1019 If it finds a class it can load, it will return that class' name. If
1020 none of the classes can be loaded, it will throw an exception.
1022 Additionally, you can pass a hash reference with options after each
1023 class name. Currently, only C<-version> is recognised and will ensure
1024 that the loaded class has at least the required version. If the class
1025 version is not sufficient, an exception will be raised.
1027 See also L</Class Loading Options>.
1031 =head2 Metaclass cache functions
1033 Class::MOP holds a cache of metaclasses. The following are functions
1034 (B<not methods>) which can be used to access that cache. It is not
1035 recommended that you mess with these. Bad things could happen, but if
1036 you are brave and willing to risk it: go for it!
1040 =item B<Class::MOP::get_all_metaclasses>
1042 This will return a hash of all the metaclass instances that have
1043 been cached by L<Class::MOP::Class>, keyed by the package name.
1045 =item B<Class::MOP::get_all_metaclass_instances>
1047 This will return a list of all the metaclass instances that have
1048 been cached by L<Class::MOP::Class>.
1050 =item B<Class::MOP::get_all_metaclass_names>
1052 This will return a list of all the metaclass names that have
1053 been cached by L<Class::MOP::Class>.
1055 =item B<Class::MOP::get_metaclass_by_name($name)>
1057 This will return a cached L<Class::MOP::Class> instance, or nothing
1058 if no metaclass exists with that C<$name>.
1060 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
1062 This will store a metaclass in the cache at the supplied C<$key>.
1064 =item B<Class::MOP::weaken_metaclass($name)>
1066 In rare cases (e.g. anonymous metaclasses) it is desirable to
1067 store a weakened reference in the metaclass cache. This
1068 function will weaken the reference to the metaclass stored
1071 =item B<Class::MOP::does_metaclass_exist($name)>
1073 This will return true of there exists a metaclass stored in the
1074 C<$name> key, and return false otherwise.
1076 =item B<Class::MOP::remove_metaclass_by_name($name)>
1078 This will remove the metaclass stored in the C<$name> key.
1082 =head2 Class Loading Options
1088 Can be used to pass a minimum required version that will be checked
1089 against the class version after it was loaded.
1097 There are very few books out on Meta Object Protocols and Metaclasses
1098 because it is such an esoteric topic. The following books are really
1099 the only ones I have found. If you know of any more, B<I<please>>
1100 email me and let me know, I would love to hear about them.
1104 =item I<The Art of the Meta Object Protocol>
1106 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1108 =item I<Putting MetaClasses to Work>
1110 =item I<Smalltalk: The Language>
1118 =item "Uniform and safe metaclass composition"
1120 An excellent paper by the people who brought us the original Traits paper.
1121 This paper is on how Traits can be used to do safe metaclass composition,
1122 and offers an excellent introduction section which delves into the topic of
1123 metaclass compatibility.
1125 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1127 =item "Safe Metaclass Programming"
1129 This paper seems to precede the above paper, and propose a mix-in based
1130 approach as opposed to the Traits based approach. Both papers have similar
1131 information on the metaclass compatibility problem space.
1133 L<http://citeseer.ist.psu.edu/37617.html>
1141 =item The Perl 6 MetaModel work in the Pugs project
1145 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1147 =item L<http://github.com/perl6/p5-modules/tree/master/Perl6-ObjectSpace/>
1157 =item CPAN Module Review of Class::MOP
1159 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1163 =head1 SIMILAR MODULES
1165 As I have said above, this module is a class-builder-builder, so it is
1166 not the same thing as modules like L<Class::Accessor> and
1167 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1168 with similar goals to this module. The one I have found which is most
1169 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1170 creates are very different from this modules.
1174 All complex software has bugs lurking in it, and this module is no
1177 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1178 web interface at L<http://rt.cpan.org>.
1180 You can also discuss feature requests or possible bugs on the Moose
1181 mailing list (moose@perl.org) or on IRC at
1182 L<irc://irc.perl.org/#moose>.
1184 =head1 ACKNOWLEDGEMENTS
1190 Thanks to Rob for actually getting the development of this module kick-started.
1196 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1198 B<with contributions from:>
1200 Brandon (blblack) Black
1202 Florian (rafl) Ragwitz
1204 Guillermo (groditi) Roditi
1206 Dave (autarch) Rolsky
1210 Rob (robkinyon) Kinyon
1212 Yuval (nothingmuch) Kogman
1214 Scott (konobi) McWhirter
1218 =head1 COPYRIGHT AND LICENSE
1220 Copyright 2006-2010 by Infinity Interactive, Inc.
1222 L<http://www.iinteractive.com>
1224 This library is free software; you can redistribute it and/or modify
1225 it under the same terms as Perl itself.