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 = '0.92_01';
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('method_metaclass' => (
213 # we just alias the original method
214 # rather than re-produce it here
215 'method_metaclass' => \&Class::MOP::Package::method_metaclass
217 default => 'Class::MOP::Method',
221 Class::MOP::Package->meta->add_attribute(
222 Class::MOP::Attribute->new('wrapped_method_metaclass' => (
225 # we just alias the original method
226 # rather than re-produce it here
227 'wrapped_method_metaclass' => \&Class::MOP::Package::wrapped_method_metaclass
229 default => 'Class::MOP::Method::Wrapped',
233 ## --------------------------------------------------------
234 ## Class::MOP::Module
237 # yeah this is kind of stretching things a bit,
238 # but truthfully the version should be an attribute
239 # of the Module, the weirdness comes from having to
240 # stick to Perl 5 convention and store it in the
241 # $VERSION package variable. Basically if you just
242 # squint at it, it will look how you want it to look.
243 # Either as a package variable, or as a attribute of
244 # the metaclass, isn't abstraction great :)
246 Class::MOP::Module->meta->add_attribute(
247 Class::MOP::Attribute->new('version' => (
250 # we just alias the original method
251 # rather than re-produce it here
252 'version' => \&Class::MOP::Module::version
255 default => sub { \undef }
260 # By following the same conventions as version here,
261 # we are opening up the possibility that people can
262 # use the $AUTHORITY in non-Class::MOP modules as
265 Class::MOP::Module->meta->add_attribute(
266 Class::MOP::Attribute->new('authority' => (
269 # we just alias the original method
270 # rather than re-produce it here
271 'authority' => \&Class::MOP::Module::authority
274 default => sub { \undef }
278 ## --------------------------------------------------------
281 Class::MOP::Class->meta->add_attribute(
282 Class::MOP::Attribute->new('attributes' => (
284 # NOTE: we need to do this in order
285 # for the instance meta-object to
286 # not fall into meta-circular death
288 # we just alias the original method
289 # rather than re-produce it here
290 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
292 default => sub { {} }
296 Class::MOP::Class->meta->add_attribute(
297 Class::MOP::Attribute->new('superclasses' => (
300 # we just alias the original method
301 # rather than re-produce it here
302 'superclasses' => \&Class::MOP::Class::superclasses
305 default => sub { \undef }
309 Class::MOP::Class->meta->add_attribute(
310 Class::MOP::Attribute->new('attribute_metaclass' => (
313 # we just alias the original method
314 # rather than re-produce it here
315 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
317 default => 'Class::MOP::Attribute',
321 Class::MOP::Class->meta->add_attribute(
322 Class::MOP::Attribute->new('instance_metaclass' => (
324 # NOTE: we need to do this in order
325 # for the instance meta-object to
326 # not fall into meta-circular death
328 # we just alias the original method
329 # rather than re-produce it here
330 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
332 default => 'Class::MOP::Instance',
336 Class::MOP::Class->meta->add_attribute(
337 Class::MOP::Attribute->new('immutable_trait' => (
339 'immutable_trait' => \&Class::MOP::Class::immutable_trait
341 default => "Class::MOP::Class::Immutable::Trait",
345 Class::MOP::Class->meta->add_attribute(
346 Class::MOP::Attribute->new('constructor_name' => (
348 'constructor_name' => \&Class::MOP::Class::constructor_name,
354 Class::MOP::Class->meta->add_attribute(
355 Class::MOP::Attribute->new('constructor_class' => (
357 'constructor_class' => \&Class::MOP::Class::constructor_class,
359 default => "Class::MOP::Method::Constructor",
364 Class::MOP::Class->meta->add_attribute(
365 Class::MOP::Attribute->new('destructor_class' => (
367 'destructor_class' => \&Class::MOP::Class::destructor_class,
373 # we don't actually need to tie the knot with
374 # Class::MOP::Class here, it is actually handled
375 # within Class::MOP::Class itself in the
376 # _construct_class_instance method.
378 ## --------------------------------------------------------
379 ## Class::MOP::Attribute
381 Class::MOP::Attribute->meta->add_attribute(
382 Class::MOP::Attribute->new('name' => (
384 # NOTE: we need to do this in order
385 # for the instance meta-object to
386 # not fall into meta-circular death
388 # we just alias the original method
389 # rather than re-produce it here
390 'name' => \&Class::MOP::Attribute::name
395 Class::MOP::Attribute->meta->add_attribute(
396 Class::MOP::Attribute->new('associated_class' => (
398 # NOTE: we need to do this in order
399 # for the instance meta-object to
400 # not fall into meta-circular death
402 # we just alias the original method
403 # rather than re-produce it here
404 'associated_class' => \&Class::MOP::Attribute::associated_class
409 Class::MOP::Attribute->meta->add_attribute(
410 Class::MOP::Attribute->new('accessor' => (
411 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
412 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
416 Class::MOP::Attribute->meta->add_attribute(
417 Class::MOP::Attribute->new('reader' => (
418 reader => { 'reader' => \&Class::MOP::Attribute::reader },
419 predicate => { 'has_reader' => \&Class::MOP::Attribute::has_reader },
423 Class::MOP::Attribute->meta->add_attribute(
424 Class::MOP::Attribute->new('initializer' => (
425 reader => { 'initializer' => \&Class::MOP::Attribute::initializer },
426 predicate => { 'has_initializer' => \&Class::MOP::Attribute::has_initializer },
430 Class::MOP::Attribute->meta->add_attribute(
431 Class::MOP::Attribute->new('definition_context' => (
432 reader => { 'definition_context' => \&Class::MOP::Attribute::definition_context },
436 Class::MOP::Attribute->meta->add_attribute(
437 Class::MOP::Attribute->new('writer' => (
438 reader => { 'writer' => \&Class::MOP::Attribute::writer },
439 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
443 Class::MOP::Attribute->meta->add_attribute(
444 Class::MOP::Attribute->new('predicate' => (
445 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
446 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
450 Class::MOP::Attribute->meta->add_attribute(
451 Class::MOP::Attribute->new('clearer' => (
452 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
453 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
457 Class::MOP::Attribute->meta->add_attribute(
458 Class::MOP::Attribute->new('builder' => (
459 reader => { 'builder' => \&Class::MOP::Attribute::builder },
460 predicate => { 'has_builder' => \&Class::MOP::Attribute::has_builder },
464 Class::MOP::Attribute->meta->add_attribute(
465 Class::MOP::Attribute->new('init_arg' => (
466 reader => { 'init_arg' => \&Class::MOP::Attribute::init_arg },
467 predicate => { 'has_init_arg' => \&Class::MOP::Attribute::has_init_arg },
471 Class::MOP::Attribute->meta->add_attribute(
472 Class::MOP::Attribute->new('default' => (
473 # default has a custom 'reader' method ...
474 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
478 Class::MOP::Attribute->meta->add_attribute(
479 Class::MOP::Attribute->new('associated_methods' => (
480 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
481 default => sub { [] }
485 Class::MOP::Attribute->meta->add_attribute(
486 Class::MOP::Attribute->new('insertion_order' => (
487 reader => { 'insertion_order' => \&Class::MOP::Attribute::insertion_order },
488 writer => { '_set_insertion_order' => \&Class::MOP::Attribute::_set_insertion_order },
489 predicate => { 'has_insertion_order' => \&Class::MOP::Attribute::has_insertion_order },
493 Class::MOP::Attribute->meta->add_method('clone' => sub {
495 $self->meta->clone_object($self, @_);
498 ## --------------------------------------------------------
499 ## Class::MOP::Method
500 Class::MOP::Method->meta->add_attribute(
501 Class::MOP::Attribute->new('body' => (
502 reader => { 'body' => \&Class::MOP::Method::body },
506 Class::MOP::Method->meta->add_attribute(
507 Class::MOP::Attribute->new('associated_metaclass' => (
508 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
512 Class::MOP::Method->meta->add_attribute(
513 Class::MOP::Attribute->new('package_name' => (
514 reader => { 'package_name' => \&Class::MOP::Method::package_name },
518 Class::MOP::Method->meta->add_attribute(
519 Class::MOP::Attribute->new('name' => (
520 reader => { 'name' => \&Class::MOP::Method::name },
524 Class::MOP::Method->meta->add_attribute(
525 Class::MOP::Attribute->new('original_method' => (
526 reader => { 'original_method' => \&Class::MOP::Method::original_method },
527 writer => { '_set_original_method' => \&Class::MOP::Method::_set_original_method },
531 Class::MOP::Method->meta->add_method('clone' => sub {
533 my $clone = $self->meta->clone_object($self, @_);
534 $clone->_set_original_method($self);
538 ## --------------------------------------------------------
539 ## Class::MOP::Method::Wrapped
542 # the way this item is initialized, this
543 # really does not follow the standard
544 # practices of attributes, but we put
545 # it here for completeness
546 Class::MOP::Method::Wrapped->meta->add_attribute(
547 Class::MOP::Attribute->new('modifier_table')
550 ## --------------------------------------------------------
551 ## Class::MOP::Method::Generated
553 Class::MOP::Method::Generated->meta->add_attribute(
554 Class::MOP::Attribute->new('is_inline' => (
555 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
560 Class::MOP::Method::Generated->meta->add_attribute(
561 Class::MOP::Attribute->new('definition_context' => (
562 reader => { 'definition_context' => \&Class::MOP::Method::Generated::definition_context },
567 ## --------------------------------------------------------
568 ## Class::MOP::Method::Inlined
570 Class::MOP::Method::Inlined->meta->add_attribute(
571 Class::MOP::Attribute->new('_expected_method_class' => (
572 reader => { '_expected_method_class' => \&Class::MOP::Method::Inlined::_expected_method_class },
576 ## --------------------------------------------------------
577 ## Class::MOP::Method::Accessor
579 Class::MOP::Method::Accessor->meta->add_attribute(
580 Class::MOP::Attribute->new('attribute' => (
582 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
587 Class::MOP::Method::Accessor->meta->add_attribute(
588 Class::MOP::Attribute->new('accessor_type' => (
589 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
593 ## --------------------------------------------------------
594 ## Class::MOP::Method::Constructor
596 Class::MOP::Method::Constructor->meta->add_attribute(
597 Class::MOP::Attribute->new('options' => (
599 'options' => \&Class::MOP::Method::Constructor::options
601 default => sub { +{} }
605 Class::MOP::Method::Constructor->meta->add_attribute(
606 Class::MOP::Attribute->new('associated_metaclass' => (
607 init_arg => "metaclass", # FIXME alias and rename
609 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
614 ## --------------------------------------------------------
615 ## Class::MOP::Instance
618 # these don't yet do much of anything, but are just
619 # included for completeness
621 Class::MOP::Instance->meta->add_attribute(
622 Class::MOP::Attribute->new('associated_metaclass',
623 reader => { associated_metaclass => \&Class::MOP::Instance::associated_metaclass },
627 Class::MOP::Instance->meta->add_attribute(
628 Class::MOP::Attribute->new('_class_name',
630 reader => { _class_name => \&Class::MOP::Instance::_class_name },
631 #lazy => 1, # not yet supported by Class::MOP but out our version does it anyway
632 #default => sub { $_[0]->associated_metaclass->name },
636 Class::MOP::Instance->meta->add_attribute(
637 Class::MOP::Attribute->new('attributes',
638 reader => { attributes => \&Class::MOP::Instance::get_all_attributes },
642 Class::MOP::Instance->meta->add_attribute(
643 Class::MOP::Attribute->new('slots',
644 reader => { slots => \&Class::MOP::Instance::slots },
648 Class::MOP::Instance->meta->add_attribute(
649 Class::MOP::Attribute->new('slot_hash',
650 reader => { slot_hash => \&Class::MOP::Instance::slot_hash },
654 require Class::MOP::Deprecated unless our $no_deprecated;
656 # we need the meta instance of the meta instance to be created now, in order
657 # for the constructor to be able to use it
658 Class::MOP::Instance->meta->get_meta_instance;
660 # pretend the add_method never happenned. it hasn't yet affected anything
661 undef Class::MOP::Instance->meta->{_package_cache_flag};
663 ## --------------------------------------------------------
664 ## Now close all the Class::MOP::* classes
666 # NOTE: we don't need to inline the the accessors this only lengthens
667 # the compile time of the MOP, and gives us no actual benefits.
669 $_->meta->make_immutable(
670 inline_constructor => 0,
671 constructor_name => "_new",
672 inline_accessors => 0,
678 Class::MOP::Attribute
684 Class::MOP::Method::Generated
685 Class::MOP::Method::Inlined
687 Class::MOP::Method::Accessor
688 Class::MOP::Method::Constructor
689 Class::MOP::Method::Wrapped
700 Class::MOP - A Meta Object Protocol for Perl 5
704 This module is a fully functioning meta object protocol for the
705 Perl 5 object system. It makes no attempt to change the behavior or
706 characteristics of the Perl 5 object system, only to create a
707 protocol for its manipulation and introspection.
709 That said, it does attempt to create the tools for building a rich set
710 of extensions to the Perl 5 object system. Every attempt has been made
711 to abide by the spirit of the Perl 5 object system that we all know
714 This documentation is sparse on conceptual details. We suggest looking
715 at the items listed in the L<SEE ALSO> section for more
716 information. In particular the book "The Art of the Meta Object
717 Protocol" was very influential in the development of this system.
719 =head2 What is a Meta Object Protocol?
721 A meta object protocol is an API to an object system.
723 To be more specific, it abstracts the components of an object system
724 (classes, object, methods, object attributes, etc.). These
725 abstractions can then be used to inspect and manipulate the object
726 system which they describe.
728 It can be said that there are two MOPs for any object system; the
729 implicit MOP and the explicit MOP. The implicit MOP handles things
730 like method dispatch or inheritance, which happen automatically as
731 part of how the object system works. The explicit MOP typically
732 handles the introspection/reflection features of the object system.
734 All object systems have implicit MOPs. Without one, they would not
735 work. Explicit MOPs are much less common, and depending on the
736 language can vary from restrictive (Reflection in Java or C#) to wide
737 open (CLOS is a perfect example).
739 =head2 Yet Another Class Builder! Why?
741 This is B<not> a class builder so much as a I<class builder
742 B<builder>>. The intent is that an end user will not use this module
743 directly, but instead this module is used by module authors to build
744 extensions and features onto the Perl 5 object system.
746 This system is used by L<Moose>, which supplies a powerful class
747 builder system built entirely on top of C<Class::MOP>.
749 =head2 Who is this module for?
751 This module is for anyone who has ever created or wanted to create a
752 module for the Class:: namespace. The tools which this module provides
753 make doing complex Perl 5 wizardry simpler, by removing such barriers
754 as the need to hack symbol tables, or understand the fine details of
757 =head2 What changes do I have to make to use this module?
759 This module was designed to be as unintrusive as possible. Many of its
760 features are accessible without B<any> change to your existing
761 code. It is meant to be a compliment to your existing code and not an
762 intrusion on your code base. Unlike many other B<Class::> modules,
763 this module B<does not> require you subclass it, or even that you
764 C<use> it in within your module's package.
766 The only features which requires additions to your code are the
767 attribute handling and instance construction features, and these are
768 both completely optional features. The only reason for this is because
769 Perl 5's object system does not actually have these features built
770 in. More information about this feature can be found below.
772 =head2 About Performance
774 It is a common misconception that explicit MOPs are a performance hit.
775 This is not a universal truth, it is a side-effect of some specific
776 implementations. For instance, using Java reflection is slow because
777 the JVM cannot take advantage of any compiler optimizations, and the
778 JVM has to deal with much more runtime type information as well.
780 Reflection in C# is marginally better as it was designed into the
781 language and runtime (the CLR). In contrast, CLOS (the Common Lisp
782 Object System) was built to support an explicit MOP, and so
783 performance is tuned for it.
785 This library in particular does its absolute best to avoid putting
786 B<any> drain at all upon your code's performance. In fact, by itself
787 it does nothing to affect your existing code. So you only pay for what
790 =head2 About Metaclass compatibility
792 This module makes sure that all metaclasses created are both upwards
793 and downwards compatible. The topic of metaclass compatibility is
794 highly esoteric and is something only encountered when doing deep and
795 involved metaclass hacking. There are two basic kinds of metaclass
796 incompatibility; upwards and downwards.
798 Upwards metaclass compatibility means that the metaclass of a
799 given class is either the same as (or a subclass of) all of the
802 Downward metaclass compatibility means that the metaclasses of a
803 given class's ancestors are all either the same as (or a subclass
806 Here is a diagram showing a set of two classes (C<A> and C<B>) and
807 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
808 metaclass compatibility both upwards and downwards.
810 +---------+ +---------+
811 | Meta::A |<----| Meta::B | <....... (instance of )
812 +---------+ +---------+ <------- (inherits from)
815 +---------+ +---------+
817 +---------+ +---------+
819 As I said this is a highly esoteric topic and one you will only run
820 into if you do a lot of subclassing of L<Class::MOP::Class>. If you
821 are interested in why this is an issue see the paper I<Uniform and
822 safe metaclass composition> linked to in the L<SEE ALSO> section of
825 =head2 Using custom metaclasses
827 Always use the L<metaclass> pragma when using a custom metaclass, this
828 will ensure the proper initialization order and not accidentally
829 create an incorrect type of metaclass for you. This is a very rare
830 problem, and one which can only occur if you are doing deep metaclass
831 programming. So in other words, don't worry about it.
833 Note that if you're using L<Moose> we encourage you to I<not> use
834 L<metaclass> pragma, and instead use L<Moose::Util::MetaRole> to apply
835 roles to a class's metaclasses. This topic is covered at length in
836 various L<Moose::Cookbook> recipes.
840 The meta-object protocol is divided into 4 main sub-protocols:
842 =head2 The Class protocol
844 This provides a means of manipulating and introspecting a Perl 5
845 class. It handles symbol table hacking for you, and provides a rich
846 set of methods that go beyond simple package introspection.
848 See L<Class::MOP::Class> for more details.
850 =head2 The Attribute protocol
852 This provides a consistent representation for an attribute of a Perl 5
853 class. Since there are so many ways to create and handle attributes in
854 Perl 5 OO, the Attribute protocol provide as much of a unified
855 approach as possible. Of course, you are always free to extend this
856 protocol by subclassing the appropriate classes.
858 See L<Class::MOP::Attribute> for more details.
860 =head2 The Method protocol
862 This provides a means of manipulating and introspecting methods in the
863 Perl 5 object system. As with attributes, there are many ways to
864 approach this topic, so we try to keep it pretty basic, while still
865 making it possible to extend the system in many ways.
867 See L<Class::MOP::Method> for more details.
869 =head2 The Instance protocol
871 This provides a layer of abstraction for creating object instances.
872 Since the other layers use this protocol, it is relatively easy to
873 change the type of your instances from the default hash reference to
874 some other type of reference. Several examples are provided in the
875 F<examples/> directory included in this distribution.
877 See L<Class::MOP::Instance> for more details.
881 Note that this module does not export any constants or functions.
887 =item I<Class::MOP::IS_RUNNING_ON_5_10>
889 We set this constant depending on what version perl we are on, this
890 allows us to take advantage of new 5.10 features and stay backwards
895 =head2 Utility functions
897 Note that these are all called as B<functions, not methods>.
901 =item B<Class::MOP::load_class($class_name)>
903 This will load the specified C<$class_name>, if it is not already
904 loaded (as reported by C<is_class_loaded>). This function can be used
905 in place of tricks like C<eval "use $module"> or using C<require>
908 If the module cannot be loaded, an exception is thrown.
910 For historical reasons, this function returns explicitly returns a true value.
912 =item B<Class::MOP::is_class_loaded($class_name)>
914 Returns a boolean indicating whether or not C<$class_name> has been
917 This does a basic check of the symbol table to try and determine as
918 best it can if the C<$class_name> is loaded, it is probably correct
919 about 99% of the time, but it can be fooled into reporting false
920 positives. In particular, loading any of the core L<IO> modules will
921 cause most of the rest of the core L<IO> modules to falsely report
922 having been loaded, due to the way the base L<IO> module works.
924 =item B<Class::MOP::get_code_info($code)>
926 This function returns two values, the name of the package the C<$code>
927 is from and the name of the C<$code> itself. This is used by several
928 elements of the MOP to determine where a given C<$code> reference is
931 =item B<Class::MOP::class_of($instance_or_class_name)>
933 This will return the metaclass of the given instance or class name. If the
934 class lacks a metaclass, no metaclass will be initialized, and C<undef> will be
937 =item B<Class::MOP::check_package_cache_flag($pkg)>
939 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
941 This will return an integer that is managed by L<Class::MOP::Class> to
942 determine if a module's symbol table has been altered.
944 In Perl 5.10 or greater, this flag is package specific. However in
945 versions prior to 5.10, this will use the C<PL_sub_generation>
946 variable which is not package specific.
948 =item B<Class::MOP::load_first_existing_class(@class_names)>
950 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
952 Given a list of class names, this function will attempt to load each
955 If it finds a class it can load, it will return that class' name. If
956 none of the classes can be loaded, it will throw an exception.
960 =head2 Metaclass cache functions
962 Class::MOP holds a cache of metaclasses. The following are functions
963 (B<not methods>) which can be used to access that cache. It is not
964 recommended that you mess with these. Bad things could happen, but if
965 you are brave and willing to risk it: go for it!
969 =item B<Class::MOP::get_all_metaclasses>
971 This will return a hash of all the metaclass instances that have
972 been cached by L<Class::MOP::Class>, keyed by the package name.
974 =item B<Class::MOP::get_all_metaclass_instances>
976 This will return a list of all the metaclass instances that have
977 been cached by L<Class::MOP::Class>.
979 =item B<Class::MOP::get_all_metaclass_names>
981 This will return a list of all the metaclass names that have
982 been cached by L<Class::MOP::Class>.
984 =item B<Class::MOP::get_metaclass_by_name($name)>
986 This will return a cached L<Class::MOP::Class> instance, or nothing
987 if no metaclass exists with that C<$name>.
989 =item B<Class::MOP::store_metaclass_by_name($name, $meta)>
991 This will store a metaclass in the cache at the supplied C<$key>.
993 =item B<Class::MOP::weaken_metaclass($name)>
995 In rare cases (e.g. anonymous metaclasses) it is desirable to
996 store a weakened reference in the metaclass cache. This
997 function will weaken the reference to the metaclass stored
1000 =item B<Class::MOP::does_metaclass_exist($name)>
1002 This will return true of there exists a metaclass stored in the
1003 C<$name> key, and return false otherwise.
1005 =item B<Class::MOP::remove_metaclass_by_name($name)>
1007 This will remove the metaclass stored in the C<$name> key.
1015 There are very few books out on Meta Object Protocols and Metaclasses
1016 because it is such an esoteric topic. The following books are really
1017 the only ones I have found. If you know of any more, B<I<please>>
1018 email me and let me know, I would love to hear about them.
1022 =item I<The Art of the Meta Object Protocol>
1024 =item I<Advances in Object-Oriented Metalevel Architecture and Reflection>
1026 =item I<Putting MetaClasses to Work>
1028 =item I<Smalltalk: The Language>
1036 =item "Uniform and safe metaclass composition"
1038 An excellent paper by the people who brought us the original Traits paper.
1039 This paper is on how Traits can be used to do safe metaclass composition,
1040 and offers an excellent introduction section which delves into the topic of
1041 metaclass compatibility.
1043 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1045 =item "Safe Metaclass Programming"
1047 This paper seems to precede the above paper, and propose a mix-in based
1048 approach as opposed to the Traits based approach. Both papers have similar
1049 information on the metaclass compatibility problem space.
1051 L<http://citeseer.ist.psu.edu/37617.html>
1059 =item The Perl 6 MetaModel work in the Pugs project
1063 =item L<http://svn.openfoundry.org/pugs/misc/Perl-MetaModel/>
1065 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1075 =item CPAN Module Review of Class::MOP
1077 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1081 =head1 SIMILAR MODULES
1083 As I have said above, this module is a class-builder-builder, so it is
1084 not the same thing as modules like L<Class::Accessor> and
1085 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1086 with similar goals to this module. The one I have found which is most
1087 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1088 creates are very different from this modules.
1092 All complex software has bugs lurking in it, and this module is no
1095 Please report any bugs to C<bug-class-mop@rt.cpan.org>, or through the
1096 web interface at L<http://rt.cpan.org>.
1098 You can also discuss feature requests or possible bugs on the Moose
1099 mailing list (moose@perl.org) or on IRC at
1100 L<irc://irc.perl.org/#moose>.
1102 =head1 ACKNOWLEDGEMENTS
1108 Thanks to Rob for actually getting the development of this module kick-started.
1114 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1116 B<with contributions from:>
1118 Brandon (blblack) Black
1120 Florian (rafl) Ragwitz
1122 Guillermo (groditi) Roditi
1124 Dave (autarch) Rolsky
1128 Rob (robkinyon) Kinyon
1130 Yuval (nothingmuch) Kogman
1132 Scott (konobi) McWhirter
1136 =head1 COPYRIGHT AND LICENSE
1138 Copyright 2006-2009 by Infinity Interactive, Inc.
1140 L<http://www.iinteractive.com>
1142 This library is free software; you can redistribute it and/or modify
1143 it under the same terms as Perl itself.