11 use Scalar::Util 'weaken', 'reftype', 'blessed';
14 use Class::MOP::Mixin::AttributeCore;
15 use Class::MOP::Mixin::HasAttributes;
16 use Class::MOP::Mixin::HasMethods;
17 use Class::MOP::Class;
18 use Class::MOP::Attribute;
19 use Class::MOP::Method;
22 *IS_RUNNING_ON_5_10 = ($] < 5.009_005)
26 # this is either part of core or set up appropriately by MRO::Compat
27 *check_package_cache_flag = \&mro::get_pkg_gen;
30 our $VERSION = '0.98';
31 our $XS_VERSION = $VERSION;
32 $VERSION = eval $VERSION;
33 our $AUTHORITY = 'cpan:STEVAN';
36 XSLoader::load( __PACKAGE__, $XS_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 does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
54 sub remove_metaclass_by_name { delete $METAS{$_[0]}; return }
56 # This handles instances as well as class names
58 return unless defined $_[0];
59 my $class = blessed($_[0]) || $_[0];
60 return $METAS{$class};
64 # We only cache metaclasses, meaning instances of
65 # Class::MOP::Class. We do not cache instance of
66 # Class::MOP::Package or Class::MOP::Module. Mostly
67 # because I don't yet see a good reason to do so.
70 sub _class_to_pmfile {
73 my $file = $class . '.pm';
79 sub load_first_existing_class {
83 foreach my $class (@classes) {
84 unless ( _is_valid_class_name($class) ) {
85 my $display = defined($class) ? $class : 'undef';
86 confess "Invalid class name ($display)";
91 for my $class (@classes) {
93 return $class if is_class_loaded($class);;
95 my $file = _class_to_pmfile($class);
101 return [$class, $file];
104 unless (/^Can't locate \Q$file\E in \@INC/) {
105 confess "Couldn't load class ($class) because: $_";
110 return $class if $file_loaded && is_class_loaded($class);
114 confess "$file_loaded->[0] is empty or missing in file $file_loaded->[1]";
116 elsif ( @classes > 1 ) {
117 confess "Can't locate any of @classes in \@INC (\@INC contains: @INC).";
120 confess "Can't locate " . _class_to_pmfile($classes[0]) . " in \@INC (\@INC contains: @INC).";
125 load_first_existing_class($_[0]);
127 # This is done to avoid breaking code which checked the return value. Said
128 # code is dumb. The return value was _always_ true, since it dies on
133 sub _is_valid_class_name {
136 return 0 if ref($class);
137 return 0 unless defined($class);
138 return 0 unless length($class);
140 return 1 if $class =~ /^\w+(?:::\w+)*$/;
145 ## ----------------------------------------------------------------------------
146 ## Setting up our environment ...
147 ## ----------------------------------------------------------------------------
148 ## Class::MOP needs to have a few things in the global perl environment so
149 ## that it can operate effectively. Those things are done here.
150 ## ----------------------------------------------------------------------------
152 # ... nothing yet actually ;)
154 ## ----------------------------------------------------------------------------
156 ## ----------------------------------------------------------------------------
157 ## The code below here is to bootstrap our MOP with itself. This is also
158 ## sometimes called "tying the knot". By doing this, we make it much easier
159 ## to extend the MOP through subclassing and such since now you can use the
160 ## MOP itself to extend itself.
162 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
163 ## ----------------------------------------------------------------------------
165 # We need to add in the meta-attributes here so that
166 # any subclass of Class::MOP::* will be able to
167 # inherit them using _construct_instance
169 ## --------------------------------------------------------
170 ## Class::MOP::Mixin::HasMethods
172 Class::MOP::Mixin::HasMethods->meta->add_attribute(
173 Class::MOP::Attribute->new('_methods' => (
176 # we just alias the original method
177 # rather than re-produce it here
178 '_full_method_map' => \&Class::MOP::Mixin::HasMethods::_full_method_map
180 default => sub { {} }
184 Class::MOP::Mixin::HasMethods->meta->add_attribute(
185 Class::MOP::Attribute->new('method_metaclass' => (
188 # we just alias the original method
189 # rather than re-produce it here
190 'method_metaclass' => \&Class::MOP::Mixin::HasMethods::method_metaclass
192 default => 'Class::MOP::Method',
196 Class::MOP::Mixin::HasMethods->meta->add_attribute(
197 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
200 # we just alias the original method
201 # rather than re-produce it here
202 'wrapped_method_metaclass' => \&Class::MOP::Mixin::HasMethods::wrapped_method_metaclass
204 default => 'Class::MOP::Method::Wrapped',
208 ## --------------------------------------------------------
209 ## Class::MOP::Mixin::HasMethods
211 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
212 Class::MOP::Attribute->new('attributes' => (
214 # NOTE: we need to do this in order
215 # for the instance meta-object to
216 # not fall into meta-circular death
218 # we just alias the original method
219 # rather than re-produce it here
220 '_attribute_map' => \&Class::MOP::Mixin::HasAttributes::_attribute_map
222 default => sub { {} }
226 Class::MOP::Mixin::HasAttributes->meta->add_attribute(
227 Class::MOP::Attribute->new('attribute_metaclass' => (
230 # we just alias the original method
231 # rather than re-produce it here
232 'attribute_metaclass' => \&Class::MOP::Mixin::HasAttributes::attribute_metaclass
234 default => 'Class::MOP::Attribute',
238 ## --------------------------------------------------------
239 ## Class::MOP::Package
241 Class::MOP::Package->meta->add_attribute(
242 Class::MOP::Attribute->new('package' => (
244 # NOTE: we need to do this in order
245 # for the instance meta-object to
246 # not fall into meta-circular death
248 # we just alias the original method
249 # rather than re-produce it here
250 'name' => \&Class::MOP::Package::name
255 Class::MOP::Package->meta->add_attribute(
256 Class::MOP::Attribute->new('namespace' => (
259 # we just alias the original method
260 # rather than re-produce it here
261 'namespace' => \&Class::MOP::Package::namespace
264 default => sub { \undef }
268 ## --------------------------------------------------------
269 ## Class::MOP::Module
272 # yeah this is kind of stretching things a bit,
273 # but truthfully the version should be an attribute
274 # of the Module, the weirdness comes from having to
275 # stick to Perl 5 convention and store it in the
276 # $VERSION package variable. Basically if you just
277 # squint at it, it will look how you want it to look.
278 # Either as a package variable, or as a attribute of
279 # the metaclass, isn't abstraction great :)
281 Class::MOP::Module->meta->add_attribute(
282 Class::MOP::Attribute->new('version' => (
285 # we just alias the original method
286 # rather than re-produce it here
287 'version' => \&Class::MOP::Module::version
290 default => sub { \undef }
295 # By following the same conventions as version here,
296 # we are opening up the possibility that people can
297 # use the $AUTHORITY in non-Class::MOP modules as
300 Class::MOP::Module->meta->add_attribute(
301 Class::MOP::Attribute->new('authority' => (
304 # we just alias the original method
305 # rather than re-produce it here
306 'authority' => \&Class::MOP::Module::authority
309 default => sub { \undef }
313 ## --------------------------------------------------------
316 Class::MOP::Class->meta->add_attribute(
317 Class::MOP::Attribute->new('superclasses' => (
320 # we just alias the original method
321 # rather than re-produce it here
322 'superclasses' => \&Class::MOP::Class::superclasses
325 default => sub { \undef }
329 Class::MOP::Class->meta->add_attribute(
330 Class::MOP::Attribute->new('instance_metaclass' => (
332 # NOTE: we need to do this in order
333 # for the instance meta-object to
334 # not fall into meta-circular death
336 # we just alias the original method
337 # rather than re-produce it here
338 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
340 default => 'Class::MOP::Instance',
344 Class::MOP::Class->meta->add_attribute(
345 Class::MOP::Attribute->new('immutable_trait' => (
347 'immutable_trait' => \&Class::MOP::Class::immutable_trait
349 default => "Class::MOP::Class::Immutable::Trait",
353 Class::MOP::Class->meta->add_attribute(
354 Class::MOP::Attribute->new('constructor_name' => (
356 'constructor_name' => \&Class::MOP::Class::constructor_name,
362 Class::MOP::Class->meta->add_attribute(
363 Class::MOP::Attribute->new('constructor_class' => (
365 'constructor_class' => \&Class::MOP::Class::constructor_class,
367 default => "Class::MOP::Method::Constructor",
372 Class::MOP::Class->meta->add_attribute(
373 Class::MOP::Attribute->new('destructor_class' => (
375 'destructor_class' => \&Class::MOP::Class::destructor_class,
381 # we don't actually need to tie the knot with
382 # Class::MOP::Class here, it is actually handled
383 # within Class::MOP::Class itself in the
384 # _construct_class_instance method.
386 ## --------------------------------------------------------
387 ## Class::MOP::Mixin::AttributeCore
388 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
389 Class::MOP::Attribute->new('name' => (
391 # NOTE: we need to do this in order
392 # for the instance meta-object to
393 # not fall into meta-circular death
395 # we just alias the original method
396 # rather than re-produce it here
397 'name' => \&Class::MOP::Mixin::AttributeCore::name
402 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
403 Class::MOP::Attribute->new('accessor' => (
404 reader => { 'accessor' => \&Class::MOP::Mixin::AttributeCore::accessor },
405 predicate => { 'has_accessor' => \&Class::MOP::Mixin::AttributeCore::has_accessor },
409 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
410 Class::MOP::Attribute->new('reader' => (
411 reader => { 'reader' => \&Class::MOP::Mixin::AttributeCore::reader },
412 predicate => { 'has_reader' => \&Class::MOP::Mixin::AttributeCore::has_reader },
416 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
417 Class::MOP::Attribute->new('initializer' => (
418 reader => { 'initializer' => \&Class::MOP::Mixin::AttributeCore::initializer },
419 predicate => { 'has_initializer' => \&Class::MOP::Mixin::AttributeCore::has_initializer },
423 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
424 Class::MOP::Attribute->new('definition_context' => (
425 reader => { 'definition_context' => \&Class::MOP::Mixin::AttributeCore::definition_context },
429 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
430 Class::MOP::Attribute->new('writer' => (
431 reader => { 'writer' => \&Class::MOP::Mixin::AttributeCore::writer },
432 predicate => { 'has_writer' => \&Class::MOP::Mixin::AttributeCore::has_writer },
436 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
437 Class::MOP::Attribute->new('predicate' => (
438 reader => { 'predicate' => \&Class::MOP::Mixin::AttributeCore::predicate },
439 predicate => { 'has_predicate' => \&Class::MOP::Mixin::AttributeCore::has_predicate },
443 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
444 Class::MOP::Attribute->new('clearer' => (
445 reader => { 'clearer' => \&Class::MOP::Mixin::AttributeCore::clearer },
446 predicate => { 'has_clearer' => \&Class::MOP::Mixin::AttributeCore::has_clearer },
450 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
451 Class::MOP::Attribute->new('builder' => (
452 reader => { 'builder' => \&Class::MOP::Mixin::AttributeCore::builder },
453 predicate => { 'has_builder' => \&Class::MOP::Mixin::AttributeCore::has_builder },
457 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
458 Class::MOP::Attribute->new('init_arg' => (
459 reader => { 'init_arg' => \&Class::MOP::Mixin::AttributeCore::init_arg },
460 predicate => { 'has_init_arg' => \&Class::MOP::Mixin::AttributeCore::has_init_arg },
464 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
465 Class::MOP::Attribute->new('default' => (
466 # default has a custom 'reader' method ...
467 predicate => { 'has_default' => \&Class::MOP::Mixin::AttributeCore::has_default },
471 Class::MOP::Mixin::AttributeCore->meta->add_attribute(
472 Class::MOP::Attribute->new('insertion_order' => (
473 reader => { 'insertion_order' => \&Class::MOP::Mixin::AttributeCore::insertion_order },
474 writer => { '_set_insertion_order' => \&Class::MOP::Mixin::AttributeCore::_set_insertion_order },
475 predicate => { 'has_insertion_order' => \&Class::MOP::Mixin::AttributeCore::has_insertion_order },
479 ## --------------------------------------------------------
480 ## Class::MOP::Attribute
481 Class::MOP::Attribute->meta->add_attribute(
482 Class::MOP::Attribute->new('associated_class' => (
484 # NOTE: we need to do this in order
485 # for the instance meta-object to
486 # not fall into meta-circular death
488 # we just alias the original method
489 # rather than re-produce it here
490 'associated_class' => \&Class::MOP::Attribute::associated_class
495 Class::MOP::Attribute->meta->add_attribute(
496 Class::MOP::Attribute->new('associated_methods' => (
497 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
498 default => sub { [] }
502 Class::MOP::Attribute->meta->add_method('clone' => sub {
504 $self->meta->clone_object($self, @_);
507 ## --------------------------------------------------------
508 ## Class::MOP::Method
509 Class::MOP::Method->meta->add_attribute(
510 Class::MOP::Attribute->new('body' => (
511 reader => { 'body' => \&Class::MOP::Method::body },
515 Class::MOP::Method->meta->add_attribute(
516 Class::MOP::Attribute->new('associated_metaclass' => (
517 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
521 Class::MOP::Method->meta->add_attribute(
522 Class::MOP::Attribute->new('package_name' => (
523 reader => { 'package_name' => \&Class::MOP::Method::package_name },
527 Class::MOP::Method->meta->add_attribute(
528 Class::MOP::Attribute->new('name' => (
529 reader => { 'name' => \&Class::MOP::Method::name },
533 Class::MOP::Method->meta->add_attribute(
534 Class::MOP::Attribute->new('original_method' => (
535 reader => { 'original_method' => \&Class::MOP::Method::original_method },
536 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
540 Class::MOP::Method->meta->add_method('clone' => sub {
542 my $clone = $self->meta->clone_object($self, @_);
543 $clone->_set_original_method($self);
547 ## --------------------------------------------------------
548 ## Class::MOP::Method::Wrapped
551 # the way this item is initialized, this
552 # really does not follow the standard
553 # practices of attributes, but we put
554 # it here for completeness
555 Class::MOP::Method::Wrapped->meta->add_attribute(
556 Class::MOP::Attribute->new('modifier_table')
559 ## --------------------------------------------------------
560 ## Class::MOP::Method::Generated
562 Class::MOP::Method::Generated->meta->add_attribute(
563 Class::MOP::Attribute->new('is_inline' => (
564 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
569 Class::MOP::Method::Generated->meta->add_attribute(
570 Class::MOP::Attribute->new('definition_context' => (
571 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
576 ## --------------------------------------------------------
577 ## Class::MOP::Method::Inlined
579 Class::MOP::Method::Inlined->meta->add_attribute(
580 Class::MOP::Attribute->new('_expected_method_class' => (
581 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
585 ## --------------------------------------------------------
586 ## Class::MOP::Method::Accessor
588 Class::MOP::Method::Accessor->meta->add_attribute(
589 Class::MOP::Attribute->new('attribute' => (
591 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
596 Class::MOP::Method::Accessor->meta->add_attribute(
597 Class::MOP::Attribute->new('accessor_type' => (
598 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
602 ## --------------------------------------------------------
603 ## Class::MOP::Method::Constructor
605 Class::MOP::Method::Constructor->meta->add_attribute(
606 Class::MOP::Attribute->new('options' => (
608 'options' => \&Class::MOP::Method::Constructor::options
610 default => sub { +{} }
614 Class::MOP::Method::Constructor->meta->add_attribute(
615 Class::MOP::Attribute->new('associated_metaclass' => (
616 init_arg => "metaclass", # FIXME alias and rename
618 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
623 ## --------------------------------------------------------
624 ## Class::MOP::Instance
627 # these don't yet do much of anything, but are just
628 # included for completeness
630 Class::MOP::Instance->meta->add_attribute(
631 Class::MOP::Attribute->new('associated_metaclass',
632 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
636 Class::MOP::Instance->meta->add_attribute(
637 Class::MOP::Attribute->new('_class_name',
639 reader => { _class_name => \&Class::MOP::Instance::_class_name },
640 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
641 #default => sub { $_[0]->associated_metaclass->name },
645 Class::MOP::Instance->meta->add_attribute(
646 Class::MOP::Attribute->new('attributes',
647 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
651 Class::MOP::Instance->meta->add_attribute(
652 Class::MOP::Attribute->new('slots',
653 reader => { slots => \&Class::MOP::Instance::slots },
657 Class::MOP::Instance->meta->add_attribute(
658 Class::MOP::Attribute->new('slot_hash',
659 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
663 require Class::MOP::Deprecated unless our $no_deprecated;
665 # we need the meta instance of the meta instance to be created now, in order
666 # for the constructor to be able to use it
667 Class::MOP::Instance->meta->get_meta_instance;
669 # pretend the add_method never happenned. it hasn't yet affected anything
670 undef Class::MOP::Instance->meta->{_package_cache_flag};
672 ## --------------------------------------------------------
673 ## Now close all the Class::MOP::* classes
675 # NOTE: we don't need to inline the the accessors this only lengthens
676 # the compile time of the MOP, and gives us no actual benefits.
678 $_->meta->make_immutable(
679 inline_constructor => 0,
680 constructor_name => "_new",
681 inline_accessors => 0,
687 Class::MOP::Attribute
693 Class::MOP::Method::Generated
694 Class::MOP::Method::Inlined
696 Class::MOP::Method::Accessor
697 Class::MOP::Method::Constructor
698 Class::MOP::Method::Wrapped
701 $_->meta->make_immutable(
702 inline_constructor => 0,
703 constructor_name => undef,
704 inline_accessors => 0,
707 Class::MOP::Mixin::AttributeCore
708 Class::MOP::Mixin::HasAttributes
709 Class::MOP::Mixin::HasMethods
720 Class::MOP - A Meta Object Protocol for Perl 5
724 This module is a fully functioning meta object protocol for the
725 Perl 5 object system. It makes no attempt to change the behavior or
726 characteristics of the Perl 5 object system, only to create a
727 protocol for its manipulation and introspection.
729 That said, it does attempt to create the tools for building a rich set
730 of extensions to the Perl 5 object system. Every attempt has been made
731 to abide by the spirit of the Perl 5 object system that we all know
734 This documentation is sparse on conceptual details. We suggest looking
735 at the items listed in the L<SEE ALSO> section for more
736 information. In particular the book "The Art of the Meta Object
737 Protocol" was very influential in the development of this system.
739 =head2 What is a Meta Object Protocol?
741 A meta object protocol is an API to an object system.
743 To be more specific, it abstracts the components of an object system
744 (classes, object, methods, object attributes, etc.). These
745 abstractions can then be used to inspect and manipulate the object
746 system which they describe.
748 It can be said that there are two MOPs for any object system; the
749 implicit MOP and the explicit MOP. The implicit MOP handles things
750 like method dispatch or inheritance, which happen automatically as
751 part of how the object system works. The explicit MOP typically
752 handles the introspection/reflection features of the object system.
754 All object systems have implicit MOPs. Without one, they would not
755 work. Explicit MOPs are much less common, and depending on the
756 language can vary from restrictive (Reflection in Java or C#) to wide
757 open (CLOS is a perfect example).
759 =head2 Yet Another Class Builder! Why?
761 This is B<not> a class builder so much as a I<class builder
762 B<builder>>. The intent is that an end user will not use this module
763 directly, but instead this module is used by module authors to build
764 extensions and features onto the Perl 5 object system.
766 This system is used by L<Moose>, which supplies a powerful class
767 builder system built entirely on top of C<Class::MOP>.
769 =head2 Who is this module for?
771 This module is for anyone who has ever created or wanted to create a
772 module for the Class:: namespace. The tools which this module provides
773 make doing complex Perl 5 wizardry simpler, by removing such barriers
774 as the need to hack symbol tables, or understand the fine details of
777 =head2 What changes do I have to make to use this module?
779 This module was designed to be as unintrusive as possible. Many of its
780 features are accessible without B<any> change to your existing
781 code. It is meant to be a compliment to your existing code and not an
782 intrusion on your code base. Unlike many other B<Class::> modules,
783 this module B<does not> require you subclass it, or even that you
784 C<use> it in within your module's package.
786 The only features which requires additions to your code are the
787 attribute handling and instance construction features, and these are
788 both completely optional features. The only reason for this is because
789 Perl 5's object system does not actually have these features built
790 in. More information about this feature can be found below.
792 =head2 About Performance
794 It is a common misconception that explicit MOPs are a performance hit.
795 This is not a universal truth, it is a side-effect of some specific
796 implementations. For instance, using Java reflection is slow because
797 the JVM cannot take advantage of any compiler optimizations, and the
798 JVM has to deal with much more runtime type information as well.
800 Reflection in C# is marginally better as it was designed into the
801 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
802 Object System) was built to support an explicit MOP, and so
803 performance is tuned for it.
805 This library in particular does its absolute best to avoid putting
806 B<any> drain at all upon your code's performance. In fact, by itself
807 it does nothing to affect your existing code. So you only pay for what
810 =head2 About Metaclass compatibility
812 This module makes sure that all metaclasses created are both upwards
813 and downwards compatible. The topic of metaclass compatibility is
814 highly esoteric and is something only encountered when doing deep and
815 involved metaclass hacking. There are two basic kinds of metaclass
816 incompatibility; upwards and downwards.
818 Upwards metaclass compatibility means that the metaclass of a
819 given class is either the same as (or a subclass of) all of the
822 Downward metaclass compatibility means that the metaclasses of a
823 given class's ancestors are all either the same as (or a subclass
826 Here is a diagram showing a set of two classes (C<A> and C<B>) and
827 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
828 metaclass compatibility both upwards and downwards.
830 +---------+ +---------+
831 | Meta::A |<----| Meta::B | <....... (instance of )
832 +---------+ +---------+ <------- (inherits from)
835 +---------+ +---------+
837 +---------+ +---------+
839 As I said this is a highly esoteric topic and one you will only run
840 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
841 are interested in why this is an issue see the paper I<Uniform and
842 safe metaclass composition> linked to in the L<SEE ALSO> section of
845 =head2 Using custom metaclasses
847 Always use the L<metaclass> pragma when using a custom metaclass, this
848 will ensure the proper initialization order and not accidentally
849 create an incorrect type of metaclass for you. This is a very rare
850 problem, and one which can only occur if you are doing deep metaclass
851 programming. So in other words, don't worry about it.
853 Note that if you're using L<Moose> we encourage you to I<not> use
854 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
855 roles to a class's metaclasses. This topic is covered at length in
856 various L<Moose::Cookbook> recipes.
860 The meta-object protocol is divided into 4 main sub-protocols:
862 =head2 The Class protocol
864 This provides a means of manipulating and introspecting a Perl 5
865 class. It handles symbol table hacking for you, and provides a rich
866 set of methods that go beyond simple package introspection.
868 See L<Class::MOP::Class> for more details.
870 =head2 The Attribute protocol
872 This provides a consistent representation for an attribute of a Perl 5
873 class. Since there are so many ways to create and handle attributes in
874 Perl 5 OO, the Attribute protocol provide as much of a unified
875 approach as possible. Of course, you are always free to extend this
876 protocol by subclassing the appropriate classes.
878 See L<Class::MOP::Attribute> for more details.
880 =head2 The Method protocol
882 This provides a means of manipulating and introspecting methods in the
883 Perl 5 object system. As with attributes, there are many ways to
884 approach this topic, so we try to keep it pretty basic, while still
885 making it possible to extend the system in many ways.
887 See L<Class::MOP::Method> for more details.
889 =head2 The Instance protocol
891 This provides a layer of abstraction for creating object instances.
892 Since the other layers use this protocol, it is relatively easy to
893 change the type of your instances from the default hash reference to
894 some other type of reference. Several examples are provided in the
895 F<examples/> directory included in this distribution.
897 See L<Class::MOP::Instance> for more details.
901 Note that this module does not export any constants or functions.
907 =item I<Class::MOP::IS_RUNNING_ON_5_10>
909 We set this constant depending on what version perl we are on, this
910 allows us to take advantage of new 5.10 features and stay backwards
915 =head2 Utility functions
917 Note that these are all called as B<functions, not methods>.
921 =item B<Class::MOP::load_class($class_name)>
923 This will load the specified C<$class_name>, if it is not already
924 loaded (as reported by C<is_class_loaded>). This function can be used
925 in place of tricks like C<eval "use $module"> or using C<require>
928 If the module cannot be loaded, an exception is thrown.
930 For historical reasons, this function explicitly returns a true value.
932 =item B<Class::MOP::is_class_loaded($class_name)>
934 Returns a boolean indicating whether or not C<$class_name> has been
937 This does a basic check of the symbol table to try and determine as
938 best it can if the C<$class_name> is loaded, it is probably correct
939 about 99% of the time, but it can be fooled into reporting false
940 positives. In particular, loading any of the core L<IO> modules will
941 cause most of the rest of the core L<IO> modules to falsely report
942 having been loaded, due to the way the base L<IO> module works.
944 =item B<Class::MOP::get_code_info($code)>
946 This function returns two values, the name of the package the C<$code>
947 is from and the name of the C<$code> itself. This is used by several
948 elements of the MOP to determine where a given C<$code> reference is
951 =item B<Class::MOP::class_of($instance_or_class_name)>
953 This will return the metaclass of the given instance or class name. If the
954 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
957 =item B<Class::MOP::check_package_cache_flag($pkg)>
959 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
961 This will return an integer that is managed by L<Class::MOP::Class> to
962 determine if a module's symbol table has been altered.
964 In Perl 5.10 or greater, this flag is package specific. However in
965 versions prior to 5.10, this will use the C<PL_sub_generation>
966 variable which is not package specific.
968 =item B<Class::MOP::load_first_existing_class(@class_names)>
970 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
972 Given a list of class names, this function will attempt to load each
975 If it finds a class it can load, it will return that class' name. If
976 none of the classes can be loaded, it will throw an exception.
980 =head2 Metaclass cache functions
982 Class::MOP holds a cache of metaclasses. The following are functions
983 (B<not methods>) which can be used to access that cache. It is not
984 recommended that you mess with these. Bad things could happen, but if
985 you are brave and willing to risk it: go for it!
989 =item B<Class::MOP::get_all_metaclasses>
991 This will return a hash of all the metaclass instances that have
992 been cached by L<Class::MOP::Class>, keyed by the package name.
994 =item B<Class::MOP::get_all_metaclass_instances>
996 This will return a list of all the metaclass instances that have
997 been cached by L<Class::MOP::Class>.
999 =item B<Class::MOP::get_all_metaclass_names>
1001 This will return a list of all the metaclass names that have
1002 been cached by L<Class::MOP::Class>.
1004 =item B<Class::MOP::get_metaclass_by_name($name)>
1006 This will return a cached L<Class::MOP::Class> instance, or nothing
1007 if no metaclass exists with that C<$name>.
1009 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
1011 This will store a metaclass in the cache at the supplied C<$key>.
1013 =item B<Class::MOP::weaken_metaclass($name)>
1015 In rare cases (e.g. anonymous metaclasses) it is desirable to
1016 store a weakened reference in the metaclass cache. This
1017 function will weaken the reference to the metaclass stored
1020 =item B<Class::MOP::does_metaclass_exist($name)>
1022 This will return true of there exists a metaclass stored in the
1023 C<$name> key, and return false otherwise.
1025 =item B<Class::MOP::remove_metaclass_by_name($name)>
1027 This will remove the metaclass stored in the C<$name> key.
1035 There are very few books out on Meta Object Protocols and Metaclasses
1036 because it is such an esoteric topic. The following books are really
1037 the only ones I have found. If you know of any more, B<I<please>>
1038 email me and let me know, I would love to hear about them.
1042 =item I<The Art of the Meta Object Protocol>
1044 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1046 =item I<Putting MetaClasses to Work>
1048 =item I<Smalltalk: The Language>
1056 =item "Uniform and safe metaclass composition"
1058 An excellent paper by the people who brought us the original Traits paper.
1059 This paper is on how Traits can be used to do safe metaclass composition,
1060 and offers an excellent introduction section which delves into the topic of
1061 metaclass compatibility.
1063 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1065 =item "Safe Metaclass Programming"
1067 This paper seems to precede the above paper, and propose a mix-in based
1068 approach as opposed to the Traits based approach. Both papers have similar
1069 information on the metaclass compatibility problem space.
1071 L<http://citeseer.ist.psu.edu/37617.html>
1079 =item The Perl 6 MetaModel work in the Pugs project
1083 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1085 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1095 =item CPAN Module Review of Class::MOP
1097 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1101 =head1 SIMILAR MODULES
1103 As I have said above, this module is a class-builder-builder, so it is
1104 not the same thing as modules like L<Class::Accessor> and
1105 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1106 with similar goals to this module. The one I have found which is most
1107 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1108 creates are very different from this modules.
1112 All complex software has bugs lurking in it, and this module is no
1115 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1116 web interface at L<http://rt.cpan.org>.
1118 You can also discuss feature requests or possible bugs on the Moose
1119 mailing list (moose@perl.org) or on IRC at
1120 L<irc://irc.perl.org/#moose>.
1122 =head1 ACKNOWLEDGEMENTS
1128 Thanks to Rob for actually getting the development of this module kick-started.
1134 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1136 B<with contributions from:>
1138 Brandon (blblack) Black
1140 Florian (rafl) Ragwitz
1142 Guillermo (groditi) Roditi
1144 Dave (autarch) Rolsky
1148 Rob (robkinyon) Kinyon
1150 Yuval (nothingmuch) Kogman
1152 Scott (konobi) McWhirter
1156 =head1 COPYRIGHT AND LICENSE
1158 Copyright 2006-2010 by Infinity Interactive, Inc.
1160 L<http://www.iinteractive.com>
1162 This library is free software; you can redistribute it and/or modify
1163 it under the same terms as Perl itself.