10 use Scalar::Util 'weaken';
12 use Class::MOP::Class;
13 use Class::MOP::Attribute;
14 use Class::MOP::Method;
16 use Class::MOP::Immutable;
20 our $VERSION = '0.65';
21 our $AUTHORITY = 'cpan:STEVAN';
23 *IS_RUNNING_ON_5_10 = ($] < 5.009_005)
28 # we may not use this yet, but once
29 # the get_code_info XS gets merged
30 # upstream to it, we will always use
31 # it. But for now it is just kinda
34 require Sub::Identify;
36 # stash these for a sec, and see how things go
37 my $_PP_subname = sub { $_[1] };
38 my $_PP_get_code_info = \&Sub::Identify::get_code_info;
40 if ($ENV{CLASS_MOP_NO_XS}) {
42 # this is if you really want things
43 # to be slow, then you can force the
44 # no-XS rule this way, otherwise we
45 # make an effort to load as much of
48 no warnings 'prototype', 'redefine';
50 unless (IS_RUNNING_ON_5_10()) {
51 # get this from MRO::Compat ...
52 *check_package_cache_flag = \&MRO::Compat::__get_pkg_gen_pp;
56 # but if we are running 5.10
57 # there is no need to use the
58 # Pure Perl version since we
59 # can use the built in mro
62 *check_package_cache_flag = \&mro::get_pkg_gen;
64 # our own version of Sub::Name
65 *subname = $_PP_subname;
66 # and the Sub::Identify version of the get_code_info
67 *get_code_info = $_PP_get_code_info;
70 # now try our best to get as much
71 # of the XS loaded as possible
76 XSLoader::load( 'Class::MOP', $VERSION );
78 die $@ if $@ && $@ !~ /object version|loadable object/;
80 # okay, so the XS failed to load, so
81 # use the pure perl one instead.
82 *get_code_info = $_PP_get_code_info if $@;
85 # get it from MRO::Compat
86 *check_package_cache_flag = \&mro::get_pkg_gen;
88 # now try and load the Sub::Name
89 # module and use that as a means
90 # for naming our CVs, if not, we
91 # use the workaround instead.
92 if ( eval { require Sub::Name } ) {
93 *subname = \&Sub::Name::subname;
96 *subname = $_PP_subname;
102 # Metaclasses are singletons, so we cache them here.
103 # there is no need to worry about destruction though
104 # because they should die only when the program dies.
105 # After all, do package definitions even get reaped?
108 # means of accessing all the metaclasses that have
109 # been initialized thus far (for mugwumps obj browser)
110 sub get_all_metaclasses { %METAS }
111 sub get_all_metaclass_instances { values %METAS }
112 sub get_all_metaclass_names { keys %METAS }
113 sub get_metaclass_by_name { $METAS{$_[0]} }
114 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
115 sub weaken_metaclass { weaken($METAS{$_[0]}) }
116 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
117 sub remove_metaclass_by_name { $METAS{$_[0]} = undef }
120 # We only cache metaclasses, meaning instances of
121 # Class::MOP::Class. We do not cache instance of
122 # Class::MOP::Package or Class::MOP::Module. Mostly
123 # because I don't yet see a good reason to do so.
129 if (ref($class) || !defined($class) || !length($class)) {
130 my $display = defined($class) ? $class : 'undef';
131 confess "Invalid class name ($display)";
134 # if the class is not already loaded in the symbol table..
135 unless (is_class_loaded($class)) {
137 my $file = $class . '.pm';
139 eval { CORE::require($file) };
140 confess "Could not load class ($class) because : $@" if $@;
143 # initialize a metaclass if necessary
144 unless (does_metaclass_exist($class)) {
145 eval { Class::MOP::Class->initialize($class) };
146 confess "Could not initialize class ($class) because : $@" if $@;
149 return get_metaclass_by_name($class);
152 sub is_class_loaded {
155 return 0 if ref($class) || !defined($class) || !length($class);
157 # walk the symbol table tree to avoid autovififying
158 # \*{${main::}{"Foo::"}} == \*main::Foo::
161 foreach my $part (split('::', $class)) {
162 return 0 unless exists ${$$pack}{"${part}::"};
163 $pack = \*{${$$pack}{"${part}::"}};
166 # check for $VERSION or @ISA
167 return 1 if exists ${$$pack}{VERSION}
168 && defined *{${$$pack}{VERSION}}{SCALAR};
169 return 1 if exists ${$$pack}{ISA}
170 && defined *{${$$pack}{ISA}}{ARRAY};
172 # check for any method
173 foreach ( keys %{$$pack} ) {
174 next if substr($_, -2, 2) eq '::';
176 my $glob = ${$$pack}{$_} || next;
179 if ( IS_RUNNING_ON_5_10 ) {
180 return 1 if ref $glob eq 'SCALAR';
183 return 1 if defined *{$glob}{CODE};
191 ## ----------------------------------------------------------------------------
192 ## Setting up our environment ...
193 ## ----------------------------------------------------------------------------
194 ## Class::MOP needs to have a few things in the global perl environment so
195 ## that it can operate effectively. Those things are done here.
196 ## ----------------------------------------------------------------------------
198 # ... nothing yet actually ;)
200 ## ----------------------------------------------------------------------------
202 ## ----------------------------------------------------------------------------
203 ## The code below here is to bootstrap our MOP with itself. This is also
204 ## sometimes called "tying the knot". By doing this, we make it much easier
205 ## to extend the MOP through subclassing and such since now you can use the
206 ## MOP itself to extend itself.
208 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
209 ## ----------------------------------------------------------------------------
211 # We need to add in the meta-attributes here so that
212 # any subclass of Class::MOP::* will be able to
213 # inherit them using &construct_instance
215 ## --------------------------------------------------------
216 ## Class::MOP::Package
218 Class::MOP::Package->meta->add_attribute(
219 Class::MOP::Attribute->new('package' => (
221 # NOTE: we need to do this in order
222 # for the instance meta-object to
223 # not fall into meta-circular death
225 # we just alias the original method
226 # rather than re-produce it here
227 'name' => \&Class::MOP::Package::name
229 init_arg => 'package',
233 Class::MOP::Package->meta->add_attribute(
234 Class::MOP::Attribute->new('namespace' => (
237 # we just alias the original method
238 # rather than re-produce it here
239 'namespace' => \&Class::MOP::Package::namespace
242 default => sub { \undef }
247 # use the metaclass to construct the meta-package
248 # which is a superclass of the metaclass itself :P
249 Class::MOP::Package->meta->add_method('initialize' => sub {
251 my $package_name = shift;
252 $class->meta->new_object('package' => $package_name, @_);
255 ## --------------------------------------------------------
256 ## Class::MOP::Module
259 # yeah this is kind of stretching things a bit,
260 # but truthfully the version should be an attribute
261 # of the Module, the weirdness comes from having to
262 # stick to Perl 5 convention and store it in the
263 # $VERSION package variable. Basically if you just
264 # squint at it, it will look how you want it to look.
265 # Either as a package variable, or as a attribute of
266 # the metaclass, isn't abstraction great :)
268 Class::MOP::Module->meta->add_attribute(
269 Class::MOP::Attribute->new('version' => (
272 # we just alias the original method
273 # rather than re-produce it here
274 'version' => \&Class::MOP::Module::version
277 default => sub { \undef }
282 # By following the same conventions as version here,
283 # we are opening up the possibility that people can
284 # use the $AUTHORITY in non-Class::MOP modules as
287 Class::MOP::Module->meta->add_attribute(
288 Class::MOP::Attribute->new('authority' => (
291 # we just alias the original method
292 # rather than re-produce it here
293 'authority' => \&Class::MOP::Module::authority
296 default => sub { \undef }
300 ## --------------------------------------------------------
303 Class::MOP::Class->meta->add_attribute(
304 Class::MOP::Attribute->new('attributes' => (
306 # NOTE: we need to do this in order
307 # for the instance meta-object to
308 # not fall into meta-circular death
310 # we just alias the original method
311 # rather than re-produce it here
312 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
314 init_arg => 'attributes',
315 default => sub { {} }
319 Class::MOP::Class->meta->add_attribute(
320 Class::MOP::Attribute->new('methods' => (
321 init_arg => 'methods',
324 # we just alias the original method
325 # rather than re-produce it here
326 'get_method_map' => \&Class::MOP::Class::get_method_map
328 default => sub { {} }
332 Class::MOP::Class->meta->add_attribute(
333 Class::MOP::Attribute->new('superclasses' => (
336 # we just alias the original method
337 # rather than re-produce it here
338 'superclasses' => \&Class::MOP::Class::superclasses
341 default => sub { \undef }
345 Class::MOP::Class->meta->add_attribute(
346 Class::MOP::Attribute->new('attribute_metaclass' => (
349 # we just alias the original method
350 # rather than re-produce it here
351 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
353 init_arg => 'attribute_metaclass',
354 default => 'Class::MOP::Attribute',
358 Class::MOP::Class->meta->add_attribute(
359 Class::MOP::Attribute->new('method_metaclass' => (
362 # we just alias the original method
363 # rather than re-produce it here
364 'method_metaclass' => \&Class::MOP::Class::method_metaclass
366 init_arg => 'method_metaclass',
367 default => 'Class::MOP::Method',
371 Class::MOP::Class->meta->add_attribute(
372 Class::MOP::Attribute->new('instance_metaclass' => (
374 # NOTE: we need to do this in order
375 # for the instance meta-object to
376 # not fall into meta-circular death
378 # we just alias the original method
379 # rather than re-produce it here
380 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
382 init_arg => 'instance_metaclass',
383 default => 'Class::MOP::Instance',
388 # we don't actually need to tie the knot with
389 # Class::MOP::Class here, it is actually handled
390 # within Class::MOP::Class itself in the
391 # construct_class_instance method.
393 ## --------------------------------------------------------
394 ## Class::MOP::Attribute
396 Class::MOP::Attribute->meta->add_attribute(
397 Class::MOP::Attribute->new('name' => (
400 # NOTE: we need to do this in order
401 # for the instance meta-object to
402 # not fall into meta-circular death
404 # we just alias the original method
405 # rather than re-produce it here
406 'name' => \&Class::MOP::Attribute::name
411 Class::MOP::Attribute->meta->add_attribute(
412 Class::MOP::Attribute->new('associated_class' => (
413 init_arg => 'associated_class',
415 # NOTE: we need to do this in order
416 # for the instance meta-object to
417 # not fall into meta-circular death
419 # we just alias the original method
420 # rather than re-produce it here
421 'associated_class' => \&Class::MOP::Attribute::associated_class
426 Class::MOP::Attribute->meta->add_attribute(
427 Class::MOP::Attribute->new('accessor' => (
428 init_arg => 'accessor',
429 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
430 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
434 Class::MOP::Attribute->meta->add_attribute(
435 Class::MOP::Attribute->new('reader' => (
436 init_arg => 'reader',
437 reader => { 'reader' => \&Class::MOP::Attribute::reader },
438 predicate => { 'has_reader' => \&Class::MOP::Attribute::has_reader },
442 Class::MOP::Attribute->meta->add_attribute(
443 Class::MOP::Attribute->new('initializer' => (
444 init_arg => 'initializer',
445 reader => { 'initializer' => \&Class::MOP::Attribute::initializer },
446 predicate => { 'has_initializer' => \&Class::MOP::Attribute::has_initializer },
450 Class::MOP::Attribute->meta->add_attribute(
451 Class::MOP::Attribute->new('writer' => (
452 init_arg => 'writer',
453 reader => { 'writer' => \&Class::MOP::Attribute::writer },
454 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
458 Class::MOP::Attribute->meta->add_attribute(
459 Class::MOP::Attribute->new('predicate' => (
460 init_arg => 'predicate',
461 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
462 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
466 Class::MOP::Attribute->meta->add_attribute(
467 Class::MOP::Attribute->new('clearer' => (
468 init_arg => 'clearer',
469 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
470 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
474 Class::MOP::Attribute->meta->add_attribute(
475 Class::MOP::Attribute->new('builder' => (
476 init_arg => 'builder',
477 reader => { 'builder' => \&Class::MOP::Attribute::builder },
478 predicate => { 'has_builder' => \&Class::MOP::Attribute::has_builder },
482 Class::MOP::Attribute->meta->add_attribute(
483 Class::MOP::Attribute->new('init_arg' => (
484 init_arg => 'init_arg',
485 reader => { 'init_arg' => \&Class::MOP::Attribute::init_arg },
486 predicate => { 'has_init_arg' => \&Class::MOP::Attribute::has_init_arg },
490 Class::MOP::Attribute->meta->add_attribute(
491 Class::MOP::Attribute->new('default' => (
492 init_arg => 'default',
493 # default has a custom 'reader' method ...
494 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
498 Class::MOP::Attribute->meta->add_attribute(
499 Class::MOP::Attribute->new('associated_methods' => (
500 init_arg => 'associated_methods',
501 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
502 default => sub { [] }
506 # NOTE: (meta-circularity)
507 # This should be one of the last things done
508 # it will "tie the knot" with Class::MOP::Attribute
509 # so that it uses the attributes meta-objects
510 # to construct itself.
511 Class::MOP::Attribute->meta->add_method('new' => sub {
512 my ( $class, @args ) = @_;
514 unshift @args, "name" if @args % 2 == 1;
517 my $name = $options{name};
519 (defined $name && $name)
520 || confess "You must provide a name for the attribute";
521 $options{init_arg} = $name
522 if not exists $options{init_arg};
524 if(exists $options{builder}){
525 confess("builder must be a defined scalar value which is a method name")
526 if ref $options{builder} || !(defined $options{builder});
527 confess("Setting both default and builder is not allowed.")
528 if exists $options{default};
530 (Class::MOP::Attribute::is_default_a_coderef(\%options))
531 || confess("References are not allowed as default values, you must ".
532 "wrap the default of '$name' in a CODE reference (ex: sub { [] } and not [])")
533 if exists $options{default} && ref $options{default};
536 # return the new object
537 $class->meta->new_object(%options);
540 Class::MOP::Attribute->meta->add_method('clone' => sub {
542 $self->meta->clone_object($self, @_);
545 ## --------------------------------------------------------
546 ## Class::MOP::Method
547 Class::MOP::Method->meta->add_attribute(
548 Class::MOP::Attribute->new('body' => (
550 reader => { 'body' => \&Class::MOP::Method::body },
554 Class::MOP::Method->meta->add_attribute(
555 Class::MOP::Attribute->new('package_name' => (
556 init_arg => 'package_name',
557 reader => { 'package_name' => \&Class::MOP::Method::package_name },
561 Class::MOP::Method->meta->add_attribute(
562 Class::MOP::Attribute->new('name' => (
564 reader => { 'name' => \&Class::MOP::Method::name },
568 Class::MOP::Method->meta->add_method('wrap' => sub {
569 my ( $class, @args ) = @_;
571 unshift @args, 'body' if @args % 2 == 1;
574 my $code = $options{body};
576 ('CODE' eq ref($code))
577 || confess "You must supply a CODE reference to bless, not (" . ($code || 'undef') . ")";
579 ($options{package_name} && $options{name})
580 || confess "You must supply the package_name and name parameters";
582 # return the new object
583 $class->meta->new_object(%options);
586 Class::MOP::Method->meta->add_method('clone' => sub {
588 $self->meta->clone_object($self, @_);
591 ## --------------------------------------------------------
592 ## Class::MOP::Method::Wrapped
595 # the way this item is initialized, this
596 # really does not follow the standard
597 # practices of attributes, but we put
598 # it here for completeness
599 Class::MOP::Method::Wrapped->meta->add_attribute(
600 Class::MOP::Attribute->new('modifier_table')
603 ## --------------------------------------------------------
604 ## Class::MOP::Method::Generated
606 Class::MOP::Method::Generated->meta->add_attribute(
607 Class::MOP::Attribute->new('is_inline' => (
608 init_arg => 'is_inline',
609 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
614 Class::MOP::Method::Generated->meta->add_method('new' => sub {
615 my ($class, %options) = @_;
616 ($options{package_name} && $options{name})
617 || confess "You must supply the package_name and name parameters";
618 my $self = $class->meta->new_object(%options);
619 $self->initialize_body;
623 ## --------------------------------------------------------
624 ## Class::MOP::Method::Accessor
626 Class::MOP::Method::Accessor->meta->add_attribute(
627 Class::MOP::Attribute->new('attribute' => (
628 init_arg => 'attribute',
630 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
635 Class::MOP::Method::Accessor->meta->add_attribute(
636 Class::MOP::Attribute->new('accessor_type' => (
637 init_arg => 'accessor_type',
638 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
642 Class::MOP::Method::Accessor->meta->add_method('new' => sub {
646 (exists $options{attribute})
647 || confess "You must supply an attribute to construct with";
649 (exists $options{accessor_type})
650 || confess "You must supply an accessor_type to construct with";
652 (Scalar::Util::blessed($options{attribute}) && $options{attribute}->isa('Class::MOP::Attribute'))
653 || confess "You must supply an attribute which is a 'Class::MOP::Attribute' instance";
655 ($options{package_name} && $options{name})
656 || confess "You must supply the package_name and name parameters";
658 # return the new object
659 my $self = $class->meta->new_object(%options);
661 # we don't want this creating
662 # a cycle in the code, if not
664 Scalar::Util::weaken($self->{'attribute'});
666 $self->initialize_body;
672 ## --------------------------------------------------------
673 ## Class::MOP::Method::Constructor
675 Class::MOP::Method::Constructor->meta->add_attribute(
676 Class::MOP::Attribute->new('options' => (
677 init_arg => 'options',
679 'options' => \&Class::MOP::Method::Constructor::options
681 default => sub { +{} }
685 Class::MOP::Method::Constructor->meta->add_attribute(
686 Class::MOP::Attribute->new('associated_metaclass' => (
687 init_arg => 'metaclass',
689 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
694 Class::MOP::Method::Constructor->meta->add_method('new' => sub {
698 (Scalar::Util::blessed $options{metaclass} && $options{metaclass}->isa('Class::MOP::Class'))
699 || confess "You must pass a metaclass instance if you want to inline"
700 if $options{is_inline};
702 ($options{package_name} && $options{name})
703 || confess "You must supply the package_name and name parameters";
705 # return the new object
706 my $self = $class->meta->new_object(%options);
708 # we don't want this creating
709 # a cycle in the code, if not
711 Scalar::Util::weaken($self->{'associated_metaclass'});
713 $self->initialize_body;
718 ## --------------------------------------------------------
719 ## Class::MOP::Instance
722 # these don't yet do much of anything, but are just
723 # included for completeness
725 Class::MOP::Instance->meta->add_attribute(
726 Class::MOP::Attribute->new('associated_metaclass')
729 Class::MOP::Instance->meta->add_attribute(
730 Class::MOP::Attribute->new('slots')
733 Class::MOP::Instance->meta->add_attribute(
734 Class::MOP::Attribute->new('slot_hash')
738 # we need the meta instance of the meta instance to be created now, in order
739 # for the constructor to be able to use it
740 Class::MOP::Instance->meta->get_meta_instance;
742 Class::MOP::Instance->meta->add_method('new' => sub {
744 my $options = $class->BUILDARGS(@_);
746 my $self = $class->meta->new_object(%$options);
748 Scalar::Util::weaken($self->{'associated_metaclass'});
753 # pretend the add_method never happenned. it hasn't yet affected anything
754 undef Class::MOP::Instance->meta->{_package_cache_flag};
756 ## --------------------------------------------------------
757 ## Now close all the Class::MOP::* classes
760 # we don't need to inline the
761 # constructors or the accessors
762 # this only lengthens the compile
763 # time of the MOP, and gives us
764 # no actual benefits.
766 $_->meta->make_immutable(
767 inline_constructor => 0,
768 inline_accessors => 0,
774 Class::MOP::Attribute
780 Class::MOP::Method::Generated
782 Class::MOP::Method::Accessor
783 Class::MOP::Method::Constructor
784 Class::MOP::Method::Wrapped
795 Class::MOP - A Meta Object Protocol for Perl 5
799 This module is a fully functioning meta object protocol for the
800 Perl 5 object system. It makes no attempt to change the behavior or
801 characteristics of the Perl 5 object system, only to create a
802 protocol for its manipulation and introspection.
804 That said, it does attempt to create the tools for building a rich
805 set of extensions to the Perl 5 object system. Every attempt has been
806 made for these tools to keep to the spirit of the Perl 5 object
807 system that we all know and love.
809 This documentation is admittedly sparse on details, as time permits
810 I will try to improve them. For now, I suggest looking at the items
811 listed in the L<SEE ALSO> section for more information. In particular
812 the book "The Art of the Meta Object Protocol" was very influential
813 in the development of this system.
815 =head2 What is a Meta Object Protocol?
817 A meta object protocol is an API to an object system.
819 To be more specific, it is a set of abstractions of the components of
820 an object system (typically things like; classes, object, methods,
821 object attributes, etc.). These abstractions can then be used to both
822 inspect and manipulate the object system which they describe.
824 It can be said that there are two MOPs for any object system; the
825 implicit MOP, and the explicit MOP. The implicit MOP handles things
826 like method dispatch or inheritance, which happen automatically as
827 part of how the object system works. The explicit MOP typically
828 handles the introspection/reflection features of the object system.
829 All object systems have implicit MOPs, without one, they would not
830 work. Explict MOPs however as less common, and depending on the
831 language can vary from restrictive (Reflection in Java or C#) to
832 wide open (CLOS is a perfect example).
834 =head2 Yet Another Class Builder!! Why?
836 This is B<not> a class builder so much as it is a I<class builder
837 B<builder>>. My intent is that an end user does not use this module
838 directly, but instead this module is used by module authors to
839 build extensions and features onto the Perl 5 object system.
841 =head2 Who is this module for?
843 This module is specifically for anyone who has ever created or
844 wanted to create a module for the Class:: namespace. The tools which
845 this module will provide will hopefully make it easier to do more
846 complex things with Perl 5 classes by removing such barriers as
847 the need to hack the symbol tables, or understand the fine details
850 =head2 What changes do I have to make to use this module?
852 This module was designed to be as unintrusive as possible. Many of
853 its features are accessible without B<any> change to your existsing
854 code at all. It is meant to be a compliment to your existing code and
855 not an intrusion on your code base. Unlike many other B<Class::>
856 modules, this module B<does not> require you subclass it, or even that
857 you C<use> it in within your module's package.
859 The only features which requires additions to your code are the
860 attribute handling and instance construction features, and these are
861 both completely optional features. The only reason for this is because
862 Perl 5's object system does not actually have these features built
863 in. More information about this feature can be found below.
865 =head2 A Note about Performance?
867 It is a common misconception that explict MOPs are performance drains.
868 But this is not a universal truth at all, it is an side-effect of
869 specific implementations. For instance, using Java reflection is much
870 slower because the JVM cannot take advantage of any compiler
871 optimizations, and the JVM has to deal with much more runtime type
872 information as well. Reflection in C# is marginally better as it was
873 designed into the language and runtime (the CLR). In contrast, CLOS
874 (the Common Lisp Object System) was built to support an explicit MOP,
875 and so performance is tuned for it.
877 This library in particular does it's absolute best to avoid putting
878 B<any> drain at all upon your code's performance. In fact, by itself
879 it does nothing to affect your existing code. So you only pay for
880 what you actually use.
882 =head2 About Metaclass compatibility
884 This module makes sure that all metaclasses created are both upwards
885 and downwards compatible. The topic of metaclass compatibility is
886 highly esoteric and is something only encountered when doing deep and
887 involved metaclass hacking. There are two basic kinds of metaclass
888 incompatibility; upwards and downwards.
890 Upwards metaclass compatibility means that the metaclass of a
891 given class is either the same as (or a subclass of) all of the
894 Downward metaclass compatibility means that the metaclasses of a
895 given class's anscestors are all either the same as (or a subclass
898 Here is a diagram showing a set of two classes (C<A> and C<B>) and
899 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
900 metaclass compatibility both upwards and downwards.
902 +---------+ +---------+
903 | Meta::A |<----| Meta::B | <....... (instance of )
904 +---------+ +---------+ <------- (inherits from)
907 +---------+ +---------+
909 +---------+ +---------+
911 As I said this is a highly esoteric topic and one you will only run
912 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
913 are interested in why this is an issue see the paper
914 I<Uniform and safe metaclass composition> linked to in the
915 L<SEE ALSO> section of this document.
917 =head2 Using custom metaclasses
919 Always use the metaclass pragma when using a custom metaclass, this
920 will ensure the proper initialization order and not accidentely
921 create an incorrect type of metaclass for you. This is a very rare
922 problem, and one which can only occur if you are doing deep metaclass
923 programming. So in other words, don't worry about it.
927 The protocol is divided into 4 main sub-protocols:
931 =item The Class protocol
933 This provides a means of manipulating and introspecting a Perl 5
934 class. It handles all of symbol table hacking for you, and provides
935 a rich set of methods that go beyond simple package introspection.
937 See L<Class::MOP::Class> for more details.
939 =item The Attribute protocol
941 This provides a consistent represenation for an attribute of a
942 Perl 5 class. Since there are so many ways to create and handle
943 attributes in Perl 5 OO, this attempts to provide as much of a
944 unified approach as possible, while giving the freedom and
945 flexibility to subclass for specialization.
947 See L<Class::MOP::Attribute> for more details.
949 =item The Method protocol
951 This provides a means of manipulating and introspecting methods in
952 the Perl 5 object system. As with attributes, there are many ways to
953 approach this topic, so we try to keep it pretty basic, while still
954 making it possible to extend the system in many ways.
956 See L<Class::MOP::Method> for more details.
958 =item The Instance protocol
960 This provides a layer of abstraction for creating object instances.
961 Since the other layers use this protocol, it is relatively easy to
962 change the type of your instances from the default HASH ref to other
963 types of references. Several examples are provided in the F<examples/>
964 directory included in this distribution.
966 See L<Class::MOP::Instance> for more details.
976 =item I<IS_RUNNING_ON_5_10>
978 We set this constant depending on what version perl we are on, this
979 allows us to take advantage of new 5.10 features and stay backwards
984 =head2 Utility functions
988 =item B<load_class ($class_name)>
990 This will load a given C<$class_name> and if it does not have an
991 already initialized metaclass, then it will intialize one for it.
992 This function can be used in place of tricks like
993 C<eval "use $module"> or using C<require>.
995 =item B<is_class_loaded ($class_name)>
997 This will return a boolean depending on if the C<$class_name> has
1000 NOTE: This does a basic check of the symbol table to try and
1001 determine as best it can if the C<$class_name> is loaded, it
1002 is probably correct about 99% of the time.
1004 =item B<check_package_cache_flag ($pkg)>
1006 This will return an integer that is managed by C<Class::MOP::Class>
1007 to determine if a module's symbol table has been altered.
1009 In Perl 5.10 or greater, this flag is package specific. However in
1010 versions prior to 5.10, this will use the C<PL_sub_generation> variable
1011 which is not package specific.
1013 =item B<get_code_info ($code)>
1015 This function returns two values, the name of the package the C<$code>
1016 is from and the name of the C<$code> itself. This is used by several
1017 elements of the MOP to detemine where a given C<$code> reference is from.
1019 =item B<subname ($name, $code)>
1021 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1023 If possible, we will load the L<Sub::Name> module and this will function
1024 as C<Sub::Name::subname> does, otherwise it will just return the C<$code>
1029 =head2 Metaclass cache functions
1031 Class::MOP holds a cache of metaclasses, the following are functions
1032 (B<not methods>) which can be used to access that cache. It is not
1033 recommended that you mess with this, bad things could happen. But if
1034 you are brave and willing to risk it, go for it.
1038 =item B<get_all_metaclasses>
1040 This will return an hash of all the metaclass instances that have
1041 been cached by B<Class::MOP::Class> keyed by the package name.
1043 =item B<get_all_metaclass_instances>
1045 This will return an array of all the metaclass instances that have
1046 been cached by B<Class::MOP::Class>.
1048 =item B<get_all_metaclass_names>
1050 This will return an array of all the metaclass names that have
1051 been cached by B<Class::MOP::Class>.
1053 =item B<get_metaclass_by_name ($name)>
1055 This will return a cached B<Class::MOP::Class> instance of nothing
1056 if no metaclass exist by that C<$name>.
1058 =item B<store_metaclass_by_name ($name, $meta)>
1060 This will store a metaclass in the cache at the supplied C<$key>.
1062 =item B<weaken_metaclass ($name)>
1064 In rare cases it is desireable to store a weakened reference in
1065 the metaclass cache. This function will weaken the reference to
1066 the metaclass stored in C<$name>.
1068 =item B<does_metaclass_exist ($name)>
1070 This will return true of there exists a metaclass stored in the
1071 C<$name> key and return false otherwise.
1073 =item B<remove_metaclass_by_name ($name)>
1075 This will remove a the metaclass stored in the C<$name> key.
1083 There are very few books out on Meta Object Protocols and Metaclasses
1084 because it is such an esoteric topic. The following books are really
1085 the only ones I have found. If you know of any more, B<I<please>>
1086 email me and let me know, I would love to hear about them.
1090 =item "The Art of the Meta Object Protocol"
1092 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
1094 =item "Putting MetaClasses to Work"
1096 =item "Smalltalk: The Language"
1104 =item Uniform and safe metaclass composition
1106 An excellent paper by the people who brought us the original Traits paper.
1107 This paper is on how Traits can be used to do safe metaclass composition,
1108 and offers an excellent introduction section which delves into the topic of
1109 metaclass compatibility.
1111 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1113 =item Safe Metaclass Programming
1115 This paper seems to precede the above paper, and propose a mix-in based
1116 approach as opposed to the Traits based approach. Both papers have similar
1117 information on the metaclass compatibility problem space.
1119 L<http://citeseer.ist.psu.edu/37617.html>
1127 =item The Perl 6 MetaModel work in the Pugs project
1131 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
1133 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1143 =item CPAN Module Review of Class::MOP
1145 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1149 =head1 SIMILAR MODULES
1151 As I have said above, this module is a class-builder-builder, so it is
1152 not the same thing as modules like L<Class::Accessor> and
1153 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1154 with similar goals to this module. The one I have found which is most
1155 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1156 creates are very different from this modules.
1160 All complex software has bugs lurking in it, and this module is no
1161 exception. If you find a bug please either email me, or add the bug
1164 =head1 ACKNOWLEDGEMENTS
1170 Thanks to Rob for actually getting the development of this module kick-started.
1176 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1178 B<with contributions from:>
1180 Brandon (blblack) Black
1182 Guillermo (groditi) Roditi
1186 Rob (robkinyon) Kinyon
1188 Yuval (nothingmuch) Kogman
1190 Scott (konobi) McWhirter
1192 =head1 COPYRIGHT AND LICENSE
1194 Copyright 2006-2008 by Infinity Interactive, Inc.
1196 L<http://www.iinteractive.com>
1198 This library is free software; you can redistribute it and/or modify
1199 it under the same terms as Perl itself.