12 use Scalar::Util 'weaken', 'reftype', 'blessed';
14 use Class::MOP::Class;
15 use Class::MOP::Attribute;
16 use Class::MOP::Method;
19 *IS_RUNNING_ON_5_10 = ($] < 5.009_005)
24 Carp::cluck("Class::MOP::HAVE_ISAREV is deprecated and will be removed in a future release. It has always returned 1 anyway.");
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 = '0.83';
33 our $XS_VERSION = $VERSION;
34 $VERSION = eval $VERSION;
35 our $AUTHORITY = 'cpan:STEVAN';
38 XSLoader::load( __PACKAGE__, $XS_VERSION );
42 # Metaclasses are singletons, so we cache them here.
43 # there is no need to worry about destruction though
44 # because they should die only when the program dies.
45 # After all, do package definitions even get reaped?
48 # means of accessing all the metaclasses that have
49 # been initialized thus far (for mugwumps obj browser)
50 sub get_all_metaclasses { %METAS }
51 sub get_all_metaclass_instances { values %METAS }
52 sub get_all_metaclass_names { keys %METAS }
53 sub get_metaclass_by_name { $METAS{$_[0]} }
54 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
55 sub weaken_metaclass { weaken($METAS{$_[0]}) }
56 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
57 sub remove_metaclass_by_name { $METAS{$_[0]} = undef }
59 # This handles instances as well as class names
61 return unless defined $_[0];
62 my $class = blessed($_[0]) || $_[0];
63 return $METAS{$class};
67 # We only cache metaclasses, meaning instances of
68 # Class::MOP::Class. We do not cache instance of
69 # Class::MOP::Package or Class::MOP::Module. Mostly
70 # because I don't yet see a good reason to do so.
73 sub _class_to_pmfile {
76 my $file = $class . '.pm';
82 sub load_first_existing_class {
86 foreach my $class (@classes) {
87 unless ( _is_valid_class_name($class) ) {
88 my $display = defined($class) ? $class : 'undef';
89 confess "Invalid class name ($display)";
95 for my $class (@classes) {
96 my $pmfile = _class_to_pmfile($class);
97 my $e = _try_load_one_class($class);
100 $exceptions{$class} = $e;
101 last if $e !~ /^Can't locate \Q$pmfile\E in \@INC/;
109 return $found if $found;
115 "Could not load class (%s) because : %s", $_,
120 exists $exceptions{$_}
125 sub _try_load_one_class {
128 return if is_class_loaded($class);
130 my $file = _class_to_pmfile($class);
134 eval { require($file) };
140 my $class = load_first_existing_class($_[0]);
141 return get_metaclass_by_name($class) || $class;
144 sub _is_valid_class_name {
147 return 0 if ref($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::Package
183 Class::MOP::Package->meta->add_attribute(
184 Class::MOP::Attribute->new('package' => (
186 # NOTE: we need to do this in order
187 # for the instance meta-object to
188 # not fall into meta-circular death
190 # we just alias the original method
191 # rather than re-produce it here
192 'name' => \&Class::MOP::Package::name
197 Class::MOP::Package->meta->add_attribute(
198 Class::MOP::Attribute->new('namespace' => (
201 # we just alias the original method
202 # rather than re-produce it here
203 'namespace' => \&Class::MOP::Package::namespace
206 default => sub { \undef }
210 ## --------------------------------------------------------
211 ## Class::MOP::Module
214 # yeah this is kind of stretching things a bit,
215 # but truthfully the version should be an attribute
216 # of the Module, the weirdness comes from having to
217 # stick to Perl 5 convention and store it in the
218 # $VERSION package variable. Basically if you just
219 # squint at it, it will look how you want it to look.
220 # Either as a package variable, or as a attribute of
221 # the metaclass, isn't abstraction great :)
223 Class::MOP::Module->meta->add_attribute(
224 Class::MOP::Attribute->new('version' => (
227 # we just alias the original method
228 # rather than re-produce it here
229 'version' => \&Class::MOP::Module::version
232 default => sub { \undef }
237 # By following the same conventions as version here,
238 # we are opening up the possibility that people can
239 # use the $AUTHORITY in non-Class::MOP modules as
242 Class::MOP::Module->meta->add_attribute(
243 Class::MOP::Attribute->new('authority' => (
246 # we just alias the original method
247 # rather than re-produce it here
248 'authority' => \&Class::MOP::Module::authority
251 default => sub { \undef }
255 ## --------------------------------------------------------
258 Class::MOP::Class->meta->add_attribute(
259 Class::MOP::Attribute->new('attributes' => (
261 # NOTE: we need to do this in order
262 # for the instance meta-object to
263 # not fall into meta-circular death
265 # we just alias the original method
266 # rather than re-produce it here
267 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
269 default => sub { {} }
273 Class::MOP::Class->meta->add_attribute(
274 Class::MOP::Attribute->new('methods' => (
277 # we just alias the original method
278 # rather than re-produce it here
279 'get_method_map' => \&Class::MOP::Class::get_method_map
281 default => sub { {} }
285 Class::MOP::Class->meta->add_attribute(
286 Class::MOP::Attribute->new('superclasses' => (
289 # we just alias the original method
290 # rather than re-produce it here
291 'superclasses' => \&Class::MOP::Class::superclasses
294 default => sub { \undef }
298 Class::MOP::Class->meta->add_attribute(
299 Class::MOP::Attribute->new('attribute_metaclass' => (
302 # we just alias the original method
303 # rather than re-produce it here
304 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
306 default => 'Class::MOP::Attribute',
310 Class::MOP::Class->meta->add_attribute(
311 Class::MOP::Attribute->new('method_metaclass' => (
314 # we just alias the original method
315 # rather than re-produce it here
316 'method_metaclass' => \&Class::MOP::Class::method_metaclass
318 default => 'Class::MOP::Method',
322 Class::MOP::Class->meta->add_attribute(
323 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
326 # we just alias the original method
327 # rather than re-produce it here
328 'wrapped_method_metaclass' => \&Class::MOP::Class::wrapped_method_metaclass
330 default => 'Class::MOP::Method::Wrapped',
334 Class::MOP::Class->meta->add_attribute(
335 Class::MOP::Attribute->new('instance_metaclass' => (
337 # NOTE: we need to do this in order
338 # for the instance meta-object to
339 # not fall into meta-circular death
341 # we just alias the original method
342 # rather than re-produce it here
343 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
345 default => 'Class::MOP::Instance',
349 Class::MOP::Class->meta->add_attribute(
350 Class::MOP::Attribute->new('immutable_trait' => (
352 'immutable_trait' => \&Class::MOP::Class::immutable_trait
354 default => "Class::MOP::Class::Immutable::Trait",
358 Class::MOP::Class->meta->add_attribute(
359 Class::MOP::Attribute->new('constructor_name' => (
361 'constructor_name' => \&Class::MOP::Class::constructor_name,
367 Class::MOP::Class->meta->add_attribute(
368 Class::MOP::Attribute->new('constructor_class' => (
370 'constructor_class' => \&Class::MOP::Class::constructor_class,
372 default => "Class::MOP::Method::Constructor",
377 Class::MOP::Class->meta->add_attribute(
378 Class::MOP::Attribute->new('destructor_class' => (
380 'destructor_class' => \&Class::MOP::Class::destructor_class,
386 # we don't actually need to tie the knot with
387 # Class::MOP::Class here, it is actually handled
388 # within Class::MOP::Class itself in the
389 # _construct_class_instance method.
391 ## --------------------------------------------------------
392 ## Class::MOP::Attribute
394 Class::MOP::Attribute->meta->add_attribute(
395 Class::MOP::Attribute->new('name' => (
397 # NOTE: we need to do this in order
398 # for the instance meta-object to
399 # not fall into meta-circular death
401 # we just alias the original method
402 # rather than re-produce it here
403 'name' => \&Class::MOP::Attribute::name
408 Class::MOP::Attribute->meta->add_attribute(
409 Class::MOP::Attribute->new('associated_class' => (
411 # NOTE: we need to do this in order
412 # for the instance meta-object to
413 # not fall into meta-circular death
415 # we just alias the original method
416 # rather than re-produce it here
417 'associated_class' => \&Class::MOP::Attribute::associated_class
422 Class::MOP::Attribute->meta->add_attribute(
423 Class::MOP::Attribute->new('accessor' => (
424 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
425 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
429 Class::MOP::Attribute->meta->add_attribute(
430 Class::MOP::Attribute->new('reader' => (
431 reader => { 'reader' => \&Class::MOP::Attribute::reader },
432 predicate => { 'has_reader' => \&Class::MOP::Attribute::has_reader },
436 Class::MOP::Attribute->meta->add_attribute(
437 Class::MOP::Attribute->new('initializer' => (
438 reader => { 'initializer' => \&Class::MOP::Attribute::initializer },
439 predicate => { 'has_initializer' => \&Class::MOP::Attribute::has_initializer },
443 Class::MOP::Attribute->meta->add_attribute(
444 Class::MOP::Attribute->new('definition_context' => (
445 reader => { 'definition_context' => \&Class::MOP::Attribute::definition_context },
449 Class::MOP::Attribute->meta->add_attribute(
450 Class::MOP::Attribute->new('writer' => (
451 reader => { 'writer' => \&Class::MOP::Attribute::writer },
452 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
456 Class::MOP::Attribute->meta->add_attribute(
457 Class::MOP::Attribute->new('predicate' => (
458 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
459 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
463 Class::MOP::Attribute->meta->add_attribute(
464 Class::MOP::Attribute->new('clearer' => (
465 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
466 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
470 Class::MOP::Attribute->meta->add_attribute(
471 Class::MOP::Attribute->new('builder' => (
472 reader => { 'builder' => \&Class::MOP::Attribute::builder },
473 predicate => { 'has_builder' => \&Class::MOP::Attribute::has_builder },
477 Class::MOP::Attribute->meta->add_attribute(
478 Class::MOP::Attribute->new('init_arg' => (
479 reader => { 'init_arg' => \&Class::MOP::Attribute::init_arg },
480 predicate => { 'has_init_arg' => \&Class::MOP::Attribute::has_init_arg },
484 Class::MOP::Attribute->meta->add_attribute(
485 Class::MOP::Attribute->new('default' => (
486 # default has a custom 'reader' method ...
487 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
491 Class::MOP::Attribute->meta->add_attribute(
492 Class::MOP::Attribute->new('associated_methods' => (
493 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
494 default => sub { [] }
498 Class::MOP::Attribute->meta->add_method('clone' => sub {
500 $self->meta->clone_object($self, @_);
503 ## --------------------------------------------------------
504 ## Class::MOP::Method
505 Class::MOP::Method->meta->add_attribute(
506 Class::MOP::Attribute->new('body' => (
507 reader => { 'body' => \&Class::MOP::Method::body },
511 Class::MOP::Method->meta->add_attribute(
512 Class::MOP::Attribute->new('associated_metaclass' => (
513 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
517 Class::MOP::Method->meta->add_attribute(
518 Class::MOP::Attribute->new('package_name' => (
519 reader => { 'package_name' => \&Class::MOP::Method::package_name },
523 Class::MOP::Method->meta->add_attribute(
524 Class::MOP::Attribute->new('name' => (
525 reader => { 'name' => \&Class::MOP::Method::name },
529 Class::MOP::Method->meta->add_attribute(
530 Class::MOP::Attribute->new('original_method' => (
531 reader => { 'original_method' => \&Class::MOP::Method::original_method },
532 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
536 Class::MOP::Method->meta->add_method('clone' => sub {
538 my $clone = $self->meta->clone_object($self, @_);
539 $clone->_set_original_method($self);
543 ## --------------------------------------------------------
544 ## Class::MOP::Method::Wrapped
547 # the way this item is initialized, this
548 # really does not follow the standard
549 # practices of attributes, but we put
550 # it here for completeness
551 Class::MOP::Method::Wrapped->meta->add_attribute(
552 Class::MOP::Attribute->new('modifier_table')
555 ## --------------------------------------------------------
556 ## Class::MOP::Method::Generated
558 Class::MOP::Method::Generated->meta->add_attribute(
559 Class::MOP::Attribute->new('is_inline' => (
560 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
565 Class::MOP::Method::Generated->meta->add_attribute(
566 Class::MOP::Attribute->new('definition_context' => (
567 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
572 ## --------------------------------------------------------
573 ## Class::MOP::Method::Inlined
575 Class::MOP::Method::Inlined->meta->add_attribute(
576 Class::MOP::Attribute->new('_expected_method_class' => (
577 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
581 ## --------------------------------------------------------
582 ## Class::MOP::Method::Accessor
584 Class::MOP::Method::Accessor->meta->add_attribute(
585 Class::MOP::Attribute->new('attribute' => (
587 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
592 Class::MOP::Method::Accessor->meta->add_attribute(
593 Class::MOP::Attribute->new('accessor_type' => (
594 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
598 ## --------------------------------------------------------
599 ## Class::MOP::Method::Constructor
601 Class::MOP::Method::Constructor->meta->add_attribute(
602 Class::MOP::Attribute->new('options' => (
604 'options' => \&Class::MOP::Method::Constructor::options
606 default => sub { +{} }
610 Class::MOP::Method::Constructor->meta->add_attribute(
611 Class::MOP::Attribute->new('associated_metaclass' => (
612 init_arg => "metaclass", # FIXME alias and rename
614 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
619 ## --------------------------------------------------------
620 ## Class::MOP::Instance
623 # these don't yet do much of anything, but are just
624 # included for completeness
626 Class::MOP::Instance->meta->add_attribute(
627 Class::MOP::Attribute->new('associated_metaclass',
628 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
632 Class::MOP::Instance->meta->add_attribute(
633 Class::MOP::Attribute->new('_class_name',
635 reader => { _class_name => \&Class::MOP::Instance::_class_name },
636 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
637 #default => sub { $_[0]->associated_metaclass->name },
641 Class::MOP::Instance->meta->add_attribute(
642 Class::MOP::Attribute->new('attributes',
643 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
647 Class::MOP::Instance->meta->add_attribute(
648 Class::MOP::Attribute->new('slots',
649 reader => { slots => \&Class::MOP::Instance::slots },
653 Class::MOP::Instance->meta->add_attribute(
654 Class::MOP::Attribute->new('slot_hash',
655 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
660 # we need the meta instance of the meta instance to be created now, in order
661 # for the constructor to be able to use it
662 Class::MOP::Instance->meta->get_meta_instance;
664 # pretend the add_method never happenned. it hasn't yet affected anything
665 undef Class::MOP::Instance->meta->{_package_cache_flag};
667 ## --------------------------------------------------------
668 ## Now close all the Class::MOP::* classes
670 # NOTE: we don't need to inline the the accessors this only lengthens
671 # the compile time of the MOP, and gives us no actual benefits.
673 # this is just nitpicking to ensure Class::MOP::Class->meta == ->meta->meta
674 Class::MOP::Class->meta->_immutable_metaclass;
675 $Class::MOP::Class::immutable_metaclass_cache{"Class::MOP::Class"}{"Class::MOP::Class::Immutable::Trait"} = Class::MOP::Class::Immutable::Class::MOP::Class->meta;
677 $_->meta->make_immutable(
678 inline_constructor => 1,
679 replace_constructor => 1,
680 constructor_name => "_new",
681 inline_accessors => 0,
686 Class::MOP::Class::Immutable::Trait
687 Class::MOP::Class::Immutable::Class::MOP::Class
689 Class::MOP::Attribute
695 Class::MOP::Method::Generated
696 Class::MOP::Method::Inlined
698 Class::MOP::Method::Accessor
699 Class::MOP::Method::Constructor
700 Class::MOP::Method::Wrapped
711 Class::MOP - A Meta Object Protocol for Perl 5
715 This module is a fully functioning meta object protocol for the
716 Perl 5 object system. It makes no attempt to change the behavior or
717 characteristics of the Perl 5 object system, only to create a
718 protocol for its manipulation and introspection.
720 That said, it does attempt to create the tools for building a rich set
721 of extensions to the Perl 5 object system. Every attempt has been made
722 to abide by the spirit of the Perl 5 object system that we all know
725 This documentation is sparse on conceptual details. We suggest looking
726 at the items listed in the L<SEE ALSO> section for more
727 information. In particular the book "The Art of the Meta Object
728 Protocol" was very influential in the development of this system.
730 =head2 What is a Meta Object Protocol?
732 A meta object protocol is an API to an object system.
734 To be more specific, it abstracts the components of an object system
735 (classes, object, methods, object attributes, etc.). These
736 abstractions can then be used to inspect and manipulate the object
737 system which they describe.
739 It can be said that there are two MOPs for any object system; the
740 implicit MOP and the explicit MOP. The implicit MOP handles things
741 like method dispatch or inheritance, which happen automatically as
742 part of how the object system works. The explicit MOP typically
743 handles the introspection/reflection features of the object system.
745 All object systems have implicit MOPs. Without one, they would not
746 work. Explicit MOPs are much less common, and depending on the
747 language can vary from restrictive (Reflection in Java or C#) to wide
748 open (CLOS is a perfect example).
750 =head2 Yet Another Class Builder! Why?
752 This is B<not> a class builder so much as a I<class builder
753 B<builder>>. The intent is that an end user will not use this module
754 directly, but instead this module is used by module authors to build
755 extensions and features onto the Perl 5 object system.
757 This system is used by L<Moose>, which supplies a powerful class
758 builder system built entirely on top of C<Class::MOP>.
760 =head2 Who is this module for?
762 This module is for anyone who has ever created or wanted to create a
763 module for the Class:: namespace. The tools which this module provides
764 make doing complex Perl 5 wizardry simpler, by removing such barriers
765 as the need to hack symbol tables, or understand the fine details of
768 =head2 What changes do I have to make to use this module?
770 This module was designed to be as unintrusive as possible. Many of its
771 features are accessible without B<any> change to your existing
772 code. It is meant to be a compliment to your existing code and not an
773 intrusion on your code base. Unlike many other B<Class::> modules,
774 this module B<does not> require you subclass it, or even that you
775 C<use> it in within your module's package.
777 The only features which requires additions to your code are the
778 attribute handling and instance construction features, and these are
779 both completely optional features. The only reason for this is because
780 Perl 5's object system does not actually have these features built
781 in. More information about this feature can be found below.
783 =head2 About Performance
785 It is a common misconception that explicit MOPs are a performance hit.
786 This is not a universal truth, it is a side-effect of some specific
787 implementations. For instance, using Java reflection is slow because
788 the JVM cannot take advantage of any compiler optimizations, and the
789 JVM has to deal with much more runtime type information as well.
791 Reflection in C# is marginally better as it was designed into the
792 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
793 Object System) was built to support an explicit MOP, and so
794 performance is tuned for it.
796 This library in particular does its absolute best to avoid putting
797 B<any> drain at all upon your code's performance. In fact, by itself
798 it does nothing to affect your existing code. So you only pay for what
801 =head2 About Metaclass compatibility
803 This module makes sure that all metaclasses created are both upwards
804 and downwards compatible. The topic of metaclass compatibility is
805 highly esoteric and is something only encountered when doing deep and
806 involved metaclass hacking. There are two basic kinds of metaclass
807 incompatibility; upwards and downwards.
809 Upwards metaclass compatibility means that the metaclass of a
810 given class is either the same as (or a subclass of) all of the
813 Downward metaclass compatibility means that the metaclasses of a
814 given class's ancestors are all either the same as (or a subclass
817 Here is a diagram showing a set of two classes (C<A> and C<B>) and
818 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
819 metaclass compatibility both upwards and downwards.
821 +---------+ +---------+
822 | Meta::A |<----| Meta::B | <....... (instance of )
823 +---------+ +---------+ <------- (inherits from)
826 +---------+ +---------+
828 +---------+ +---------+
830 As I said this is a highly esoteric topic and one you will only run
831 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
832 are interested in why this is an issue see the paper I<Uniform and
833 safe metaclass composition> linked to in the L<SEE ALSO> section of
836 =head2 Using custom metaclasses
838 Always use the L<metaclass> pragma when using a custom metaclass, this
839 will ensure the proper initialization order and not accidentally
840 create an incorrect type of metaclass for you. This is a very rare
841 problem, and one which can only occur if you are doing deep metaclass
842 programming. So in other words, don't worry about it.
844 Note that if you're using L<Moose> we encourage you to I<not> use
845 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
846 roles to a class's metaclasses. This topic is covered at length in
847 various L<Moose::Cookbook> recipes.
851 The meta-object protocol is divided into 4 main sub-protocols:
853 =head2 The Class protocol
855 This provides a means of manipulating and introspecting a Perl 5
856 class. It handles symbol table hacking for you, and provides a rich
857 set of methods that go beyond simple package introspection.
859 See L<Class::MOP::Class> for more details.
861 =head2 The Attribute protocol
863 This provides a consistent representation for an attribute of a Perl 5
864 class. Since there are so many ways to create and handle attributes in
865 Perl 5 OO, the Attribute protocol provide as much of a unified
866 approach as possible. Of course, you are always free to extend this
867 protocol by subclassing the appropriate classes.
869 See L<Class::MOP::Attribute> for more details.
871 =head2 The Method protocol
873 This provides a means of manipulating and introspecting methods in the
874 Perl 5 object system. As with attributes, there are many ways to
875 approach this topic, so we try to keep it pretty basic, while still
876 making it possible to extend the system in many ways.
878 See L<Class::MOP::Method> for more details.
880 =head2 The Instance protocol
882 This provides a layer of abstraction for creating object instances.
883 Since the other layers use this protocol, it is relatively easy to
884 change the type of your instances from the default hash reference to
885 some other type of reference. Several examples are provided in the
886 F<examples/> directory included in this distribution.
888 See L<Class::MOP::Instance> for more details.
892 Note that this module does not export any constants or functions.
898 =item I<Class::MOP::IS_RUNNING_ON_5_10>
900 We set this constant depending on what version perl we are on, this
901 allows us to take advantage of new 5.10 features and stay backwards
906 =head2 Utility functions
908 Note that these are all called as B<functions, not methods>.
912 =item B<Class::MOP::load_class($class_name)>
914 This will load the specified C<$class_name>. This function can be used
915 in place of tricks like C<eval "use $module"> or using C<require>
916 unconditionally. This will return the metaclass of C<$class_name>.
918 =item B<Class::MOP::is_class_loaded($class_name)>
920 Returns a boolean indicating whether or not C<$class_name> has been
923 This does a basic check of the symbol table to try and determine as
924 best it can if the C<$class_name> is loaded, it is probably correct
925 about 99% of the time, but it can be fooled into reporting false
928 =item B<Class::MOP::get_code_info($code)>
930 This function returns two values, the name of the package the C<$code>
931 is from and the name of the C<$code> itself. This is used by several
932 elements of the MOP to determine where a given C<$code> reference is
935 =item B<Class::MOP::class_of($instance_or_class_name)>
937 This will return the metaclass of the given instance or class name.
938 Even if the class lacks a metaclass, no metaclass will be initialized
939 and C<undef> will be returned.
941 =item B<Class::MOP::check_package_cache_flag($pkg)>
943 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
945 This will return an integer that is managed by L<Class::MOP::Class> to
946 determine if a module's symbol table has been altered.
948 In Perl 5.10 or greater, this flag is package specific. However in
949 versions prior to 5.10, this will use the C<PL_sub_generation>
950 variable which is not package specific.
952 =item B<Class::MOP::load_first_existing_class(@class_names)>
954 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
956 Given a list of class names, this function will attempt to load each
959 If it finds a class it can load, it will return that class' name. If
960 none of the classes can be loaded, it will throw an exception.
964 =head2 Metaclass cache functions
966 Class::MOP holds a cache of metaclasses. The following are functions
967 (B<not methods>) which can be used to access that cache. It is not
968 recommended that you mess with these. Bad things could happen, but if
969 you are brave and willing to risk it: go for it!
973 =item B<Class::MOP::get_all_metaclasses>
975 This will return a hash of all the metaclass instances that have
976 been cached by L<Class::MOP::Class>, keyed by the package name.
978 =item B<Class::MOP::get_all_metaclass_instances>
980 This will return a list of all the metaclass instances that have
981 been cached by L<Class::MOP::Class>.
983 =item B<Class::MOP::get_all_metaclass_names>
985 This will return a list of all the metaclass names that have
986 been cached by L<Class::MOP::Class>.
988 =item B<Class::MOP::get_metaclass_by_name($name)>
990 This will return a cached L<Class::MOP::Class> instance, or nothing
991 if no metaclass exists with that C<$name>.
993 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
995 This will store a metaclass in the cache at the supplied C<$key>.
997 =item B<Class::MOP::weaken_metaclass($name)>
999 In rare cases (e.g. anonymous metaclasses) it is desirable to
1000 store a weakened reference in the metaclass cache. This
1001 function will weaken the reference to the metaclass stored
1004 =item B<Class::MOP::does_metaclass_exist($name)>
1006 This will return true of there exists a metaclass stored in the
1007 C<$name> key, and return false otherwise.
1009 =item B<Class::MOP::remove_metaclass_by_name($name)>
1011 This will remove the metaclass stored in the C<$name> key.
1019 There are very few books out on Meta Object Protocols and Metaclasses
1020 because it is such an esoteric topic. The following books are really
1021 the only ones I have found. If you know of any more, B<I<please>>
1022 email me and let me know, I would love to hear about them.
1026 =item I<The Art of the Meta Object Protocol>
1028 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1030 =item I<Putting MetaClasses to Work>
1032 =item I<Smalltalk: The Language>
1040 =item "Uniform and safe metaclass composition"
1042 An excellent paper by the people who brought us the original Traits paper.
1043 This paper is on how Traits can be used to do safe metaclass composition,
1044 and offers an excellent introduction section which delves into the topic of
1045 metaclass compatibility.
1047 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1049 =item "Safe Metaclass Programming"
1051 This paper seems to precede the above paper, and propose a mix-in based
1052 approach as opposed to the Traits based approach. Both papers have similar
1053 information on the metaclass compatibility problem space.
1055 L<http://citeseer.ist.psu.edu/37617.html>
1063 =item The Perl 6 MetaModel work in the Pugs project
1067 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
1069 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1079 =item CPAN Module Review of Class::MOP
1081 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1085 =head1 SIMILAR MODULES
1087 As I have said above, this module is a class-builder-builder, so it is
1088 not the same thing as modules like L<Class::Accessor> and
1089 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1090 with similar goals to this module. The one I have found which is most
1091 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1092 creates are very different from this modules.
1096 All complex software has bugs lurking in it, and this module is no
1099 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1100 web interface at L<http://rt.cpan.org>.
1102 You can also discuss feature requests or possible bugs on the Moose
1103 mailing list (moose@perl.org) or on IRC at
1104 L<irc://irc.perl.org/#moose>.
1106 =head1 ACKNOWLEDGEMENTS
1112 Thanks to Rob for actually getting the development of this module kick-started.
1118 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1120 B<with contributions from:>
1122 Brandon (blblack) Black
1124 Florian (rafl) Ragwitz
1126 Guillermo (groditi) Roditi
1128 Dave (autarch) Rolsky
1132 Rob (robkinyon) Kinyon
1134 Yuval (nothingmuch) Kogman
1136 Scott (konobi) McWhirter
1138 =head1 COPYRIGHT AND LICENSE
1140 Copyright 2006-2009 by Infinity Interactive, Inc.
1142 L<http://www.iinteractive.com>
1144 This library is free software; you can redistribute it and/or modify
1145 it under the same terms as Perl itself.