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)
23 # this is either part of core or set up appropriately by MRO::Compat
24 *check_package_cache_flag = \&mro::get_pkg_gen;
27 our $VERSION = '0.94';
28 our $XS_VERSION = $VERSION;
29 $VERSION = eval $VERSION;
30 our $AUTHORITY = 'cpan:STEVAN';
33 XSLoader::load( __PACKAGE__, $XS_VERSION );
37 # Metaclasses are singletons, so we cache them here.
38 # there is no need to worry about destruction though
39 # because they should die only when the program dies.
40 # After all, do package definitions even get reaped?
41 # Anonymous classes manage their own destruction.
44 sub get_all_metaclasses { %METAS }
45 sub get_all_metaclass_instances { values %METAS }
46 sub get_all_metaclass_names { keys %METAS }
47 sub get_metaclass_by_name { $METAS{$_[0]} }
48 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
49 sub weaken_metaclass { weaken($METAS{$_[0]}) }
50 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
51 sub remove_metaclass_by_name { delete $METAS{$_[0]}; return }
53 # This handles instances as well as class names
55 return unless defined $_[0];
56 my $class = blessed($_[0]) || $_[0];
57 return $METAS{$class};
61 # We only cache metaclasses, meaning instances of
62 # Class::MOP::Class. We do not cache instance of
63 # Class::MOP::Package or Class::MOP::Module. Mostly
64 # because I don't yet see a good reason to do so.
67 sub _class_to_pmfile {
70 my $file = $class . '.pm';
76 sub load_first_existing_class {
80 foreach my $class (@classes) {
81 unless ( _is_valid_class_name($class) ) {
82 my $display = defined($class) ? $class : 'undef';
83 confess "Invalid class name ($display)";
89 for my $class (@classes) {
90 my $e = _try_load_one_class($class);
93 my $pmfile = _class_to_pmfile($class);
94 $exceptions{$class} = $e;
95 last if $e !~ /^Can't locate \Q$pmfile\E in \@INC/;
103 return $found if $found;
109 "Could not load class (%s) because : %s", $_,
114 exists $exceptions{$_}
119 sub _try_load_one_class {
122 return if is_class_loaded($class);
124 my $file = _class_to_pmfile($class);
129 eval { require($file) };
135 load_first_existing_class($_[0]);
137 # This is done to avoid breaking code which checked the return value. Said
138 # code is dumb. The return value was _always_ true, since it dies on
143 sub _is_valid_class_name {
146 return 0 if ref($class);
147 return 0 unless defined($class);
148 return 0 unless length($class);
150 return 1 if $class =~ /^\w+(?:::\w+)*$/;
155 ## ----------------------------------------------------------------------------
156 ## Setting up our environment ...
157 ## ----------------------------------------------------------------------------
158 ## Class::MOP needs to have a few things in the global perl environment so
159 ## that it can operate effectively. Those things are done here.
160 ## ----------------------------------------------------------------------------
162 # ... nothing yet actually ;)
164 ## ----------------------------------------------------------------------------
166 ## ----------------------------------------------------------------------------
167 ## The code below here is to bootstrap our MOP with itself. This is also
168 ## sometimes called "tying the knot". By doing this, we make it much easier
169 ## to extend the MOP through subclassing and such since now you can use the
170 ## MOP itself to extend itself.
172 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
173 ## ----------------------------------------------------------------------------
175 # We need to add in the meta-attributes here so that
176 # any subclass of Class::MOP::* will be able to
177 # inherit them using _construct_instance
179 ## --------------------------------------------------------
180 ## Class::MOP::Package
182 Class::MOP::Package->meta->add_attribute(
183 Class::MOP::Attribute->new('package' => (
185 # NOTE: we need to do this in order
186 # for the instance meta-object to
187 # not fall into meta-circular death
189 # we just alias the original method
190 # rather than re-produce it here
191 'name' => \&Class::MOP::Package::name
196 Class::MOP::Package->meta->add_attribute(
197 Class::MOP::Attribute->new('namespace' => (
200 # we just alias the original method
201 # rather than re-produce it here
202 'namespace' => \&Class::MOP::Package::namespace
205 default => sub { \undef }
209 Class::MOP::Package->meta->add_attribute(
210 Class::MOP::Attribute->new('_methods' => (
213 # we just alias the original method
214 # rather than re-produce it here
215 '_full_method_map' => \&Class::MOP::Package::_full_method_map
217 default => sub { {} }
221 Class::MOP::Package->meta->add_attribute(
222 Class::MOP::Attribute->new('method_metaclass' => (
225 # we just alias the original method
226 # rather than re-produce it here
227 'method_metaclass' => \&Class::MOP::Package::method_metaclass
229 default => 'Class::MOP::Method',
233 Class::MOP::Package->meta->add_attribute(
234 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
237 # we just alias the original method
238 # rather than re-produce it here
239 'wrapped_method_metaclass' => \&Class::MOP::Package::wrapped_method_metaclass
241 default => 'Class::MOP::Method::Wrapped',
245 ## --------------------------------------------------------
246 ## Class::MOP::Module
249 # yeah this is kind of stretching things a bit,
250 # but truthfully the version should be an attribute
251 # of the Module, the weirdness comes from having to
252 # stick to Perl 5 convention and store it in the
253 # $VERSION package variable. Basically if you just
254 # squint at it, it will look how you want it to look.
255 # Either as a package variable, or as a attribute of
256 # the metaclass, isn't abstraction great :)
258 Class::MOP::Module->meta->add_attribute(
259 Class::MOP::Attribute->new('version' => (
262 # we just alias the original method
263 # rather than re-produce it here
264 'version' => \&Class::MOP::Module::version
267 default => sub { \undef }
272 # By following the same conventions as version here,
273 # we are opening up the possibility that people can
274 # use the $AUTHORITY in non-Class::MOP modules as
277 Class::MOP::Module->meta->add_attribute(
278 Class::MOP::Attribute->new('authority' => (
281 # we just alias the original method
282 # rather than re-produce it here
283 'authority' => \&Class::MOP::Module::authority
286 default => sub { \undef }
290 ## --------------------------------------------------------
293 Class::MOP::Class->meta->add_attribute(
294 Class::MOP::Attribute->new('attributes' => (
296 # NOTE: we need to do this in order
297 # for the instance meta-object to
298 # not fall into meta-circular death
300 # we just alias the original method
301 # rather than re-produce it here
302 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
304 default => sub { {} }
308 Class::MOP::Class->meta->add_attribute(
309 Class::MOP::Attribute->new('superclasses' => (
312 # we just alias the original method
313 # rather than re-produce it here
314 'superclasses' => \&Class::MOP::Class::superclasses
317 default => sub { \undef }
321 Class::MOP::Class->meta->add_attribute(
322 Class::MOP::Attribute->new('attribute_metaclass' => (
325 # we just alias the original method
326 # rather than re-produce it here
327 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
329 default => 'Class::MOP::Attribute',
333 Class::MOP::Class->meta->add_attribute(
334 Class::MOP::Attribute->new('instance_metaclass' => (
336 # NOTE: we need to do this in order
337 # for the instance meta-object to
338 # not fall into meta-circular death
340 # we just alias the original method
341 # rather than re-produce it here
342 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
344 default => 'Class::MOP::Instance',
348 Class::MOP::Class->meta->add_attribute(
349 Class::MOP::Attribute->new('immutable_trait' => (
351 'immutable_trait' => \&Class::MOP::Class::immutable_trait
353 default => "Class::MOP::Class::Immutable::Trait",
357 Class::MOP::Class->meta->add_attribute(
358 Class::MOP::Attribute->new('constructor_name' => (
360 'constructor_name' => \&Class::MOP::Class::constructor_name,
366 Class::MOP::Class->meta->add_attribute(
367 Class::MOP::Attribute->new('constructor_class' => (
369 'constructor_class' => \&Class::MOP::Class::constructor_class,
371 default => "Class::MOP::Method::Constructor",
376 Class::MOP::Class->meta->add_attribute(
377 Class::MOP::Attribute->new('destructor_class' => (
379 'destructor_class' => \&Class::MOP::Class::destructor_class,
385 # we don't actually need to tie the knot with
386 # Class::MOP::Class here, it is actually handled
387 # within Class::MOP::Class itself in the
388 # _construct_class_instance method.
390 ## --------------------------------------------------------
391 ## Class::MOP::Attribute
393 Class::MOP::Attribute->meta->add_attribute(
394 Class::MOP::Attribute->new('name' => (
396 # NOTE: we need to do this in order
397 # for the instance meta-object to
398 # not fall into meta-circular death
400 # we just alias the original method
401 # rather than re-produce it here
402 'name' => \&Class::MOP::Attribute::name
407 Class::MOP::Attribute->meta->add_attribute(
408 Class::MOP::Attribute->new('associated_class' => (
410 # NOTE: we need to do this in order
411 # for the instance meta-object to
412 # not fall into meta-circular death
414 # we just alias the original method
415 # rather than re-produce it here
416 'associated_class' => \&Class::MOP::Attribute::associated_class
421 Class::MOP::Attribute->meta->add_attribute(
422 Class::MOP::Attribute->new('accessor' => (
423 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
424 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
428 Class::MOP::Attribute->meta->add_attribute(
429 Class::MOP::Attribute->new('reader' => (
430 reader => { 'reader' => \&Class::MOP::Attribute::reader },
431 predicate => { 'has_reader' => \&Class::MOP::Attribute::has_reader },
435 Class::MOP::Attribute->meta->add_attribute(
436 Class::MOP::Attribute->new('initializer' => (
437 reader => { 'initializer' => \&Class::MOP::Attribute::initializer },
438 predicate => { 'has_initializer' => \&Class::MOP::Attribute::has_initializer },
442 Class::MOP::Attribute->meta->add_attribute(
443 Class::MOP::Attribute->new('definition_context' => (
444 reader => { 'definition_context' => \&Class::MOP::Attribute::definition_context },
448 Class::MOP::Attribute->meta->add_attribute(
449 Class::MOP::Attribute->new('writer' => (
450 reader => { 'writer' => \&Class::MOP::Attribute::writer },
451 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
455 Class::MOP::Attribute->meta->add_attribute(
456 Class::MOP::Attribute->new('predicate' => (
457 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
458 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
462 Class::MOP::Attribute->meta->add_attribute(
463 Class::MOP::Attribute->new('clearer' => (
464 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
465 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
469 Class::MOP::Attribute->meta->add_attribute(
470 Class::MOP::Attribute->new('builder' => (
471 reader => { 'builder' => \&Class::MOP::Attribute::builder },
472 predicate => { 'has_builder' => \&Class::MOP::Attribute::has_builder },
476 Class::MOP::Attribute->meta->add_attribute(
477 Class::MOP::Attribute->new('init_arg' => (
478 reader => { 'init_arg' => \&Class::MOP::Attribute::init_arg },
479 predicate => { 'has_init_arg' => \&Class::MOP::Attribute::has_init_arg },
483 Class::MOP::Attribute->meta->add_attribute(
484 Class::MOP::Attribute->new('lazy' => (
485 reader => { 'is_lazy' => \&Class::MOP::Attribute::is_lazy },
489 Class::MOP::Attribute->meta->add_attribute(
490 Class::MOP::Attribute->new('default' => (
491 # default has a custom 'reader' method ...
492 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
496 Class::MOP::Attribute->meta->add_attribute(
497 Class::MOP::Attribute->new('associated_methods' => (
498 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
499 default => sub { [] }
503 Class::MOP::Attribute->meta->add_attribute(
504 Class::MOP::Attribute->new('insertion_order' => (
505 reader => { 'insertion_order' => \&Class::MOP::Attribute::insertion_order },
506 writer => { '_set_insertion_order' => \&Class::MOP::Attribute::_set_insertion_order },
507 predicate => { 'has_insertion_order' => \&Class::MOP::Attribute::has_insertion_order },
511 Class::MOP::Attribute->meta->add_method('clone' => sub {
513 $self->meta->clone_object($self, @_);
516 ## --------------------------------------------------------
517 ## Class::MOP::Method
518 Class::MOP::Method->meta->add_attribute(
519 Class::MOP::Attribute->new('body' => (
521 default => \&Class::MOP::Method::body,
525 Class::MOP::Method->meta->add_attribute(
526 Class::MOP::Attribute->new('associated_metaclass' => (
527 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
531 Class::MOP::Method->meta->add_attribute(
532 Class::MOP::Attribute->new('package_name' => (
533 reader => { 'package_name' => \&Class::MOP::Method::package_name },
537 Class::MOP::Method->meta->add_attribute(
538 Class::MOP::Attribute->new('name' => (
539 reader => { 'name' => \&Class::MOP::Method::name },
543 Class::MOP::Method->meta->add_attribute(
544 Class::MOP::Attribute->new('original_method' => (
545 reader => { 'original_method' => \&Class::MOP::Method::original_method },
546 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
550 Class::MOP::Method->meta->add_method('clone' => sub {
552 my $clone = $self->meta->clone_object($self, @_);
553 $clone->_set_original_method($self);
557 ## --------------------------------------------------------
558 ## Class::MOP::Method::Wrapped
561 # the way this item is initialized, this
562 # really does not follow the standard
563 # practices of attributes, but we put
564 # it here for completeness
565 Class::MOP::Method::Wrapped->meta->add_attribute(
566 Class::MOP::Attribute->new('modifier_table')
569 ## --------------------------------------------------------
570 ## Class::MOP::Method::Generated
572 Class::MOP::Method::Generated->meta->add_attribute(
573 Class::MOP::Attribute->new('is_inline' => (
574 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
579 Class::MOP::Method::Generated->meta->add_attribute(
580 Class::MOP::Attribute->new('definition_context' => (
581 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
586 ## --------------------------------------------------------
587 ## Class::MOP::Method::Inlined
589 Class::MOP::Method::Inlined->meta->add_attribute(
590 Class::MOP::Attribute->new('_expected_method_class' => (
591 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
595 ## --------------------------------------------------------
596 ## Class::MOP::Method::Attribute
598 Class::MOP::Method::Attribute->meta->add_attribute(
599 Class::MOP::Attribute->new('attribute' => (
601 'associated_attribute' => \&Class::MOP::Method::Attribute::associated_attribute
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 require Class::MOP::Deprecated unless our $no_deprecated;
669 # we need the meta instance of the meta instance to be created now, in order
670 # for the constructor to be able to use it
671 Class::MOP::Instance->meta->get_meta_instance;
673 # pretend the add_method never happenned. it hasn't yet affected anything
674 undef Class::MOP::Instance->meta->{_package_cache_flag};
676 ## --------------------------------------------------------
677 ## Now close all the Class::MOP::* classes
679 # NOTE: we don't need to inline the the accessors this only lengthens
680 # the compile time of the MOP, and gives us no actual benefits.
682 $_->meta->make_immutable(
683 inline_constructor => 0,
684 constructor_name => "_new",
685 inline_accessors => 0,
691 Class::MOP::Attribute
697 Class::MOP::Method::Generated
698 Class::MOP::Method::Inlined
700 Class::MOP::Method::Accessor
701 Class::MOP::Method::Attribute
702 Class::MOP::Method::Clearer
703 Class::MOP::Method::Constructor
704 Class::MOP::Method::Predicate
705 Class::MOP::Method::Reader
706 Class::MOP::Method::Wrapped
707 Class::MOP::Method::Writer
718 Class::MOP - A Meta Object Protocol for Perl 5
722 This module is a fully functioning meta object protocol for the
723 Perl 5 object system. It makes no attempt to change the behavior or
724 characteristics of the Perl 5 object system, only to create a
725 protocol for its manipulation and introspection.
727 That said, it does attempt to create the tools for building a rich set
728 of extensions to the Perl 5 object system. Every attempt has been made
729 to abide by the spirit of the Perl 5 object system that we all know
732 This documentation is sparse on conceptual details. We suggest looking
733 at the items listed in the L<SEE ALSO> section for more
734 information. In particular the book "The Art of the Meta Object
735 Protocol" was very influential in the development of this system.
737 =head2 What is a Meta Object Protocol?
739 A meta object protocol is an API to an object system.
741 To be more specific, it abstracts the components of an object system
742 (classes, object, methods, object attributes, etc.). These
743 abstractions can then be used to inspect and manipulate the object
744 system which they describe.
746 It can be said that there are two MOPs for any object system; the
747 implicit MOP and the explicit MOP. The implicit MOP handles things
748 like method dispatch or inheritance, which happen automatically as
749 part of how the object system works. The explicit MOP typically
750 handles the introspection/reflection features of the object system.
752 All object systems have implicit MOPs. Without one, they would not
753 work. Explicit MOPs are much less common, and depending on the
754 language can vary from restrictive (Reflection in Java or C#) to wide
755 open (CLOS is a perfect example).
757 =head2 Yet Another Class Builder! Why?
759 This is B<not> a class builder so much as a I<class builder
760 B<builder>>. The intent is that an end user will not use this module
761 directly, but instead this module is used by module authors to build
762 extensions and features onto the Perl 5 object system.
764 This system is used by L<Moose>, which supplies a powerful class
765 builder system built entirely on top of C<Class::MOP>.
767 =head2 Who is this module for?
769 This module is for anyone who has ever created or wanted to create a
770 module for the Class:: namespace. The tools which this module provides
771 make doing complex Perl 5 wizardry simpler, by removing such barriers
772 as the need to hack symbol tables, or understand the fine details of
775 =head2 What changes do I have to make to use this module?
777 This module was designed to be as unintrusive as possible. Many of its
778 features are accessible without B<any> change to your existing
779 code. It is meant to be a compliment to your existing code and not an
780 intrusion on your code base. Unlike many other B<Class::> modules,
781 this module B<does not> require you subclass it, or even that you
782 C<use> it in within your module's package.
784 The only features which requires additions to your code are the
785 attribute handling and instance construction features, and these are
786 both completely optional features. The only reason for this is because
787 Perl 5's object system does not actually have these features built
788 in. More information about this feature can be found below.
790 =head2 About Performance
792 It is a common misconception that explicit MOPs are a performance hit.
793 This is not a universal truth, it is a side-effect of some specific
794 implementations. For instance, using Java reflection is slow because
795 the JVM cannot take advantage of any compiler optimizations, and the
796 JVM has to deal with much more runtime type information as well.
798 Reflection in C# is marginally better as it was designed into the
799 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
800 Object System) was built to support an explicit MOP, and so
801 performance is tuned for it.
803 This library in particular does its absolute best to avoid putting
804 B<any> drain at all upon your code's performance. In fact, by itself
805 it does nothing to affect your existing code. So you only pay for what
808 =head2 About Metaclass compatibility
810 This module makes sure that all metaclasses created are both upwards
811 and downwards compatible. The topic of metaclass compatibility is
812 highly esoteric and is something only encountered when doing deep and
813 involved metaclass hacking. There are two basic kinds of metaclass
814 incompatibility; upwards and downwards.
816 Upwards metaclass compatibility means that the metaclass of a
817 given class is either the same as (or a subclass of) all of the
820 Downward metaclass compatibility means that the metaclasses of a
821 given class's ancestors are all either the same as (or a subclass
824 Here is a diagram showing a set of two classes (C<A> and C<B>) and
825 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
826 metaclass compatibility both upwards and downwards.
828 +---------+ +---------+
829 | Meta::A |<----| Meta::B | <....... (instance of )
830 +---------+ +---------+ <------- (inherits from)
833 +---------+ +---------+
835 +---------+ +---------+
837 As I said this is a highly esoteric topic and one you will only run
838 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
839 are interested in why this is an issue see the paper I<Uniform and
840 safe metaclass composition> linked to in the L<SEE ALSO> section of
843 =head2 Using custom metaclasses
845 Always use the L<metaclass> pragma when using a custom metaclass, this
846 will ensure the proper initialization order and not accidentally
847 create an incorrect type of metaclass for you. This is a very rare
848 problem, and one which can only occur if you are doing deep metaclass
849 programming. So in other words, don't worry about it.
851 Note that if you're using L<Moose> we encourage you to I<not> use
852 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
853 roles to a class's metaclasses. This topic is covered at length in
854 various L<Moose::Cookbook> recipes.
858 The meta-object protocol is divided into 4 main sub-protocols:
860 =head2 The Class protocol
862 This provides a means of manipulating and introspecting a Perl 5
863 class. It handles symbol table hacking for you, and provides a rich
864 set of methods that go beyond simple package introspection.
866 See L<Class::MOP::Class> for more details.
868 =head2 The Attribute protocol
870 This provides a consistent representation for an attribute of a Perl 5
871 class. Since there are so many ways to create and handle attributes in
872 Perl 5 OO, the Attribute protocol provide as much of a unified
873 approach as possible. Of course, you are always free to extend this
874 protocol by subclassing the appropriate classes.
876 See L<Class::MOP::Attribute> for more details.
878 =head2 The Method protocol
880 This provides a means of manipulating and introspecting methods in the
881 Perl 5 object system. As with attributes, there are many ways to
882 approach this topic, so we try to keep it pretty basic, while still
883 making it possible to extend the system in many ways.
885 See L<Class::MOP::Method> for more details.
887 =head2 The Instance protocol
889 This provides a layer of abstraction for creating object instances.
890 Since the other layers use this protocol, it is relatively easy to
891 change the type of your instances from the default hash reference to
892 some other type of reference. Several examples are provided in the
893 F<examples/> directory included in this distribution.
895 See L<Class::MOP::Instance> for more details.
899 Note that this module does not export any constants or functions.
905 =item I<Class::MOP::IS_RUNNING_ON_5_10>
907 We set this constant depending on what version perl we are on, this
908 allows us to take advantage of new 5.10 features and stay backwards
913 =head2 Utility functions
915 Note that these are all called as B<functions, not methods>.
919 =item B<Class::MOP::load_class($class_name)>
921 This will load the specified C<$class_name>, if it is not already
922 loaded (as reported by C<is_class_loaded>). This function can be used
923 in place of tricks like C<eval "use $module"> or using C<require>
926 If the module cannot be loaded, an exception is thrown.
928 For historical reasons, this function returns explicitly returns a true value.
930 =item B<Class::MOP::is_class_loaded($class_name)>
932 Returns a boolean indicating whether or not C<$class_name> has been
935 This does a basic check of the symbol table to try and determine as
936 best it can if the C<$class_name> is loaded, it is probably correct
937 about 99% of the time, but it can be fooled into reporting false
938 positives. In particular, loading any of the core L<IO> modules will
939 cause most of the rest of the core L<IO> modules to falsely report
940 having been loaded, due to the way the base L<IO> module works.
942 =item B<Class::MOP::get_code_info($code)>
944 This function returns two values, the name of the package the C<$code>
945 is from and the name of the C<$code> itself. This is used by several
946 elements of the MOP to determine where a given C<$code> reference is
949 =item B<Class::MOP::class_of($instance_or_class_name)>
951 This will return the metaclass of the given instance or class name. If the
952 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
955 =item B<Class::MOP::check_package_cache_flag($pkg)>
957 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
959 This will return an integer that is managed by L<Class::MOP::Class> to
960 determine if a module's symbol table has been altered.
962 In Perl 5.10 or greater, this flag is package specific. However in
963 versions prior to 5.10, this will use the C<PL_sub_generation>
964 variable which is not package specific.
966 =item B<Class::MOP::load_first_existing_class(@class_names)>
968 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
970 Given a list of class names, this function will attempt to load each
973 If it finds a class it can load, it will return that class' name. If
974 none of the classes can be loaded, it will throw an exception.
978 =head2 Metaclass cache functions
980 Class::MOP holds a cache of metaclasses. The following are functions
981 (B<not methods>) which can be used to access that cache. It is not
982 recommended that you mess with these. Bad things could happen, but if
983 you are brave and willing to risk it: go for it!
987 =item B<Class::MOP::get_all_metaclasses>
989 This will return a hash of all the metaclass instances that have
990 been cached by L<Class::MOP::Class>, keyed by the package name.
992 =item B<Class::MOP::get_all_metaclass_instances>
994 This will return a list of all the metaclass instances that have
995 been cached by L<Class::MOP::Class>.
997 =item B<Class::MOP::get_all_metaclass_names>
999 This will return a list of all the metaclass names that have
1000 been cached by L<Class::MOP::Class>.
1002 =item B<Class::MOP::get_metaclass_by_name($name)>
1004 This will return a cached L<Class::MOP::Class> instance, or nothing
1005 if no metaclass exists with that C<$name>.
1007 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
1009 This will store a metaclass in the cache at the supplied C<$key>.
1011 =item B<Class::MOP::weaken_metaclass($name)>
1013 In rare cases (e.g. anonymous metaclasses) it is desirable to
1014 store a weakened reference in the metaclass cache. This
1015 function will weaken the reference to the metaclass stored
1018 =item B<Class::MOP::does_metaclass_exist($name)>
1020 This will return true of there exists a metaclass stored in the
1021 C<$name> key, and return false otherwise.
1023 =item B<Class::MOP::remove_metaclass_by_name($name)>
1025 This will remove the metaclass stored in the C<$name> key.
1033 There are very few books out on Meta Object Protocols and Metaclasses
1034 because it is such an esoteric topic. The following books are really
1035 the only ones I have found. If you know of any more, B<I<please>>
1036 email me and let me know, I would love to hear about them.
1040 =item I<The Art of the Meta Object Protocol>
1042 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1044 =item I<Putting MetaClasses to Work>
1046 =item I<Smalltalk: The Language>
1054 =item "Uniform and safe metaclass composition"
1056 An excellent paper by the people who brought us the original Traits paper.
1057 This paper is on how Traits can be used to do safe metaclass composition,
1058 and offers an excellent introduction section which delves into the topic of
1059 metaclass compatibility.
1061 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1063 =item "Safe Metaclass Programming"
1065 This paper seems to precede the above paper, and propose a mix-in based
1066 approach as opposed to the Traits based approach. Both papers have similar
1067 information on the metaclass compatibility problem space.
1069 L<http://citeseer.ist.psu.edu/37617.html>
1077 =item The Perl 6 MetaModel work in the Pugs project
1081 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1083 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1093 =item CPAN Module Review of Class::MOP
1095 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1099 =head1 SIMILAR MODULES
1101 As I have said above, this module is a class-builder-builder, so it is
1102 not the same thing as modules like L<Class::Accessor> and
1103 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1104 with similar goals to this module. The one I have found which is most
1105 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1106 creates are very different from this modules.
1110 All complex software has bugs lurking in it, and this module is no
1113 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1114 web interface at L<http://rt.cpan.org>.
1116 You can also discuss feature requests or possible bugs on the Moose
1117 mailing list (moose@perl.org) or on IRC at
1118 L<irc://irc.perl.org/#moose>.
1120 =head1 ACKNOWLEDGEMENTS
1126 Thanks to Rob for actually getting the development of this module kick-started.
1132 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1134 B<with contributions from:>
1136 Brandon (blblack) Black
1138 Florian (rafl) Ragwitz
1140 Guillermo (groditi) Roditi
1142 Dave (autarch) Rolsky
1146 Rob (robkinyon) Kinyon
1148 Yuval (nothingmuch) Kogman
1150 Scott (konobi) McWhirter
1154 =head1 COPYRIGHT AND LICENSE
1156 Copyright 2006-2009 by Infinity Interactive, Inc.
1158 L<http://www.iinteractive.com>
1160 This library is free software; you can redistribute it and/or modify
1161 it under the same terms as Perl itself.