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)
27 *HAVE_ISAREV = defined(&mro::get_isarev)
32 # we may not use this yet, but once
33 # the get_code_info XS gets merged
34 # upstream to it, we will always use
35 # it. But for now it is just kinda
38 require Sub::Identify;
40 # stash these for a sec, and see how things go
41 my $_PP_subname = sub { $_[1] };
42 my $_PP_get_code_info = \&Sub::Identify::get_code_info;
44 if ($ENV{CLASS_MOP_NO_XS}) {
46 # this is if you really want things
47 # to be slow, then you can force the
48 # no-XS rule this way, otherwise we
49 # make an effort to load as much of
52 no warnings 'prototype', 'redefine';
54 # this is either part of core or set up appropriately by MRO::Compat
55 *check_package_cache_flag = \&mro::get_pkg_gen;
57 # our own version of Sub::Name
58 *subname = $_PP_subname;
59 # and the Sub::Identify version of the get_code_info
60 *get_code_info = $_PP_get_code_info;
63 # now try our best to get as much
64 # of the XS loaded as possible
69 XSLoader::load( 'Class::MOP', $VERSION );
71 die $@ if $@ && $@ !~ /object version|loadable object/;
73 # okay, so the XS failed to load, so
74 # use the pure perl one instead.
75 *get_code_info = $_PP_get_code_info if $@;
78 # get it from MRO::Compat
79 *check_package_cache_flag = \&mro::get_pkg_gen;
81 # now try and load the Sub::Name
82 # module and use that as a means
83 # for naming our CVs, if not, we
84 # use the workaround instead.
85 if ( eval { require Sub::Name } ) {
86 *subname = \&Sub::Name::subname;
89 *subname = $_PP_subname;
95 # Metaclasses are singletons, so we cache them here.
96 # there is no need to worry about destruction though
97 # because they should die only when the program dies.
98 # After all, do package definitions even get reaped?
101 # means of accessing all the metaclasses that have
102 # been initialized thus far (for mugwumps obj browser)
103 sub get_all_metaclasses { %METAS }
104 sub get_all_metaclass_instances { values %METAS }
105 sub get_all_metaclass_names { keys %METAS }
106 sub get_metaclass_by_name { $METAS{$_[0]} }
107 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
108 sub weaken_metaclass { weaken($METAS{$_[0]}) }
109 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
110 sub remove_metaclass_by_name { $METAS{$_[0]} = undef }
113 # We only cache metaclasses, meaning instances of
114 # Class::MOP::Class. We do not cache instance of
115 # Class::MOP::Package or Class::MOP::Module. Mostly
116 # because I don't yet see a good reason to do so.
122 if (ref($class) || !defined($class) || !length($class)) {
123 my $display = defined($class) ? $class : 'undef';
124 confess "Invalid class name ($display)";
127 # if the class is not already loaded in the symbol table..
128 unless (is_class_loaded($class)) {
130 my $file = $class . '.pm';
132 eval { CORE::require($file) };
133 confess "Could not load class ($class) because : $@" if $@;
136 # initialize a metaclass if necessary
137 unless (does_metaclass_exist($class)) {
138 eval { Class::MOP::Class->initialize($class) };
139 confess "Could not initialize class ($class) because : $@" if $@;
142 return get_metaclass_by_name($class);
145 sub is_class_loaded {
148 return 0 if ref($class) || !defined($class) || !length($class);
150 # walk the symbol table tree to avoid autovififying
151 # \*{${main::}{"Foo::"}} == \*main::Foo::
154 foreach my $part (split('::', $class)) {
155 return 0 unless exists ${$$pack}{"${part}::"};
156 $pack = \*{${$$pack}{"${part}::"}};
159 # check for $VERSION or @ISA
160 return 1 if exists ${$$pack}{VERSION}
161 && defined *{${$$pack}{VERSION}}{SCALAR};
162 return 1 if exists ${$$pack}{ISA}
163 && defined *{${$$pack}{ISA}}{ARRAY};
165 # check for any method
166 foreach ( keys %{$$pack} ) {
167 next if substr($_, -2, 2) eq '::';
169 my $glob = ${$$pack}{$_} || next;
172 if ( IS_RUNNING_ON_5_10 ) {
173 return 1 if ref $glob eq 'SCALAR';
176 return 1 if defined *{$glob}{CODE};
184 ## ----------------------------------------------------------------------------
185 ## Setting up our environment ...
186 ## ----------------------------------------------------------------------------
187 ## Class::MOP needs to have a few things in the global perl environment so
188 ## that it can operate effectively. Those things are done here.
189 ## ----------------------------------------------------------------------------
191 # ... nothing yet actually ;)
193 ## ----------------------------------------------------------------------------
195 ## ----------------------------------------------------------------------------
196 ## The code below here is to bootstrap our MOP with itself. This is also
197 ## sometimes called "tying the knot". By doing this, we make it much easier
198 ## to extend the MOP through subclassing and such since now you can use the
199 ## MOP itself to extend itself.
201 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
202 ## ----------------------------------------------------------------------------
204 # We need to add in the meta-attributes here so that
205 # any subclass of Class::MOP::* will be able to
206 # inherit them using &construct_instance
208 ## --------------------------------------------------------
209 ## Class::MOP::Package
211 Class::MOP::Package->meta->add_attribute(
212 Class::MOP::Attribute->new('package' => (
214 # NOTE: we need to do this in order
215 # for the instance meta-object to
216 # not fall into meta-circular death
218 # we just alias the original method
219 # rather than re-produce it here
220 'name' => \&Class::MOP::Package::name
222 init_arg => 'package',
226 Class::MOP::Package->meta->add_attribute(
227 Class::MOP::Attribute->new('namespace' => (
230 # we just alias the original method
231 # rather than re-produce it here
232 'namespace' => \&Class::MOP::Package::namespace
235 default => sub { \undef }
240 # use the metaclass to construct the meta-package
241 # which is a superclass of the metaclass itself :P
242 Class::MOP::Package->meta->add_method('initialize' => sub {
244 my $package_name = shift;
245 $class->meta->new_object('package' => $package_name, @_);
248 ## --------------------------------------------------------
249 ## Class::MOP::Module
252 # yeah this is kind of stretching things a bit,
253 # but truthfully the version should be an attribute
254 # of the Module, the weirdness comes from having to
255 # stick to Perl 5 convention and store it in the
256 # $VERSION package variable. Basically if you just
257 # squint at it, it will look how you want it to look.
258 # Either as a package variable, or as a attribute of
259 # the metaclass, isn't abstraction great :)
261 Class::MOP::Module->meta->add_attribute(
262 Class::MOP::Attribute->new('version' => (
265 # we just alias the original method
266 # rather than re-produce it here
267 'version' => \&Class::MOP::Module::version
270 default => sub { \undef }
275 # By following the same conventions as version here,
276 # we are opening up the possibility that people can
277 # use the $AUTHORITY in non-Class::MOP modules as
280 Class::MOP::Module->meta->add_attribute(
281 Class::MOP::Attribute->new('authority' => (
284 # we just alias the original method
285 # rather than re-produce it here
286 'authority' => \&Class::MOP::Module::authority
289 default => sub { \undef }
293 ## --------------------------------------------------------
296 Class::MOP::Class->meta->add_attribute(
297 Class::MOP::Attribute->new('attributes' => (
299 # NOTE: we need to do this in order
300 # for the instance meta-object to
301 # not fall into meta-circular death
303 # we just alias the original method
304 # rather than re-produce it here
305 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
307 init_arg => 'attributes',
308 default => sub { {} }
312 Class::MOP::Class->meta->add_attribute(
313 Class::MOP::Attribute->new('methods' => (
314 init_arg => 'methods',
317 # we just alias the original method
318 # rather than re-produce it here
319 'get_method_map' => \&Class::MOP::Class::get_method_map
321 default => sub { {} }
325 Class::MOP::Class->meta->add_attribute(
326 Class::MOP::Attribute->new('superclasses' => (
329 # we just alias the original method
330 # rather than re-produce it here
331 'superclasses' => \&Class::MOP::Class::superclasses
334 default => sub { \undef }
338 Class::MOP::Class->meta->add_attribute(
339 Class::MOP::Attribute->new('attribute_metaclass' => (
342 # we just alias the original method
343 # rather than re-produce it here
344 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
346 init_arg => 'attribute_metaclass',
347 default => 'Class::MOP::Attribute',
351 Class::MOP::Class->meta->add_attribute(
352 Class::MOP::Attribute->new('method_metaclass' => (
355 # we just alias the original method
356 # rather than re-produce it here
357 'method_metaclass' => \&Class::MOP::Class::method_metaclass
359 init_arg => 'method_metaclass',
360 default => 'Class::MOP::Method',
364 Class::MOP::Class->meta->add_attribute(
365 Class::MOP::Attribute->new('instance_metaclass' => (
367 # NOTE: we need to do this in order
368 # for the instance meta-object to
369 # not fall into meta-circular death
371 # we just alias the original method
372 # rather than re-produce it here
373 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
375 init_arg => 'instance_metaclass',
376 default => 'Class::MOP::Instance',
381 # we don't actually need to tie the knot with
382 # Class::MOP::Class here, it is actually handled
383 # within Class::MOP::Class itself in the
384 # construct_class_instance method.
386 ## --------------------------------------------------------
387 ## Class::MOP::Attribute
389 Class::MOP::Attribute->meta->add_attribute(
390 Class::MOP::Attribute->new('name' => (
393 # NOTE: we need to do this in order
394 # for the instance meta-object to
395 # not fall into meta-circular death
397 # we just alias the original method
398 # rather than re-produce it here
399 'name' => \&Class::MOP::Attribute::name
404 Class::MOP::Attribute->meta->add_attribute(
405 Class::MOP::Attribute->new('associated_class' => (
406 init_arg => 'associated_class',
408 # NOTE: we need to do this in order
409 # for the instance meta-object to
410 # not fall into meta-circular death
412 # we just alias the original method
413 # rather than re-produce it here
414 'associated_class' => \&Class::MOP::Attribute::associated_class
419 Class::MOP::Attribute->meta->add_attribute(
420 Class::MOP::Attribute->new('accessor' => (
421 init_arg => 'accessor',
422 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
423 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
427 Class::MOP::Attribute->meta->add_attribute(
428 Class::MOP::Attribute->new('reader' => (
429 init_arg => '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 init_arg => 'initializer',
438 reader => { 'initializer' => \&Class::MOP::Attribute::initializer },
439 predicate => { 'has_initializer' => \&Class::MOP::Attribute::has_initializer },
443 Class::MOP::Attribute->meta->add_attribute(
444 Class::MOP::Attribute->new('writer' => (
445 init_arg => 'writer',
446 reader => { 'writer' => \&Class::MOP::Attribute::writer },
447 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
451 Class::MOP::Attribute->meta->add_attribute(
452 Class::MOP::Attribute->new('predicate' => (
453 init_arg => 'predicate',
454 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
455 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
459 Class::MOP::Attribute->meta->add_attribute(
460 Class::MOP::Attribute->new('clearer' => (
461 init_arg => 'clearer',
462 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
463 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
467 Class::MOP::Attribute->meta->add_attribute(
468 Class::MOP::Attribute->new('builder' => (
469 init_arg => 'builder',
470 reader => { 'builder' => \&Class::MOP::Attribute::builder },
471 predicate => { 'has_builder' => \&Class::MOP::Attribute::has_builder },
475 Class::MOP::Attribute->meta->add_attribute(
476 Class::MOP::Attribute->new('init_arg' => (
477 init_arg => '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('default' => (
485 init_arg => 'default',
486 # default has a custom 'reader' method ...
487 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
491 Class::MOP::Attribute->meta->add_attribute(
492 Class::MOP::Attribute->new('associated_methods' => (
493 init_arg => 'associated_methods',
494 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
495 default => sub { [] }
499 # NOTE: (meta-circularity)
500 # This should be one of the last things done
501 # it will "tie the knot" with Class::MOP::Attribute
502 # so that it uses the attributes meta-objects
503 # to construct itself.
504 Class::MOP::Attribute->meta->add_method('new' => sub {
505 my ( $class, @args ) = @_;
507 unshift @args, "name" if @args % 2 == 1;
510 my $name = $options{name};
512 (defined $name && $name)
513 || confess "You must provide a name for the attribute";
514 $options{init_arg} = $name
515 if not exists $options{init_arg};
517 if(exists $options{builder}){
518 confess("builder must be a defined scalar value which is a method name")
519 if ref $options{builder} || !(defined $options{builder});
520 confess("Setting both default and builder is not allowed.")
521 if exists $options{default};
523 (Class::MOP::Attribute::is_default_a_coderef(\%options))
524 || confess("References are not allowed as default values, you must ".
525 "wrap the default of '$name' in a CODE reference (ex: sub { [] } and not [])")
526 if exists $options{default} && ref $options{default};
529 # return the new object
530 $class->meta->new_object(%options);
533 Class::MOP::Attribute->meta->add_method('clone' => sub {
535 $self->meta->clone_object($self, @_);
538 ## --------------------------------------------------------
539 ## Class::MOP::Method
540 Class::MOP::Method->meta->add_attribute(
541 Class::MOP::Attribute->new('body' => (
543 reader => { 'body' => \&Class::MOP::Method::body },
547 Class::MOP::Method->meta->add_attribute(
548 Class::MOP::Attribute->new('associated_metaclass' => (
549 init_arg => 'associated_metaclass',
550 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
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('attributes')
733 Class::MOP::Instance->meta->add_attribute(
734 Class::MOP::Attribute->new('slots')
737 Class::MOP::Instance->meta->add_attribute(
738 Class::MOP::Attribute->new('slot_hash')
742 # we need the meta instance of the meta instance to be created now, in order
743 # for the constructor to be able to use it
744 Class::MOP::Instance->meta->get_meta_instance;
746 Class::MOP::Instance->meta->add_method('new' => sub {
748 my $options = $class->BUILDARGS(@_);
750 my $self = $class->meta->new_object(%$options);
752 Scalar::Util::weaken($self->{'associated_metaclass'});
757 # pretend the add_method never happenned. it hasn't yet affected anything
758 undef Class::MOP::Instance->meta->{_package_cache_flag};
760 ## --------------------------------------------------------
761 ## Now close all the Class::MOP::* classes
764 # we don't need to inline the
765 # constructors or the accessors
766 # this only lengthens the compile
767 # time of the MOP, and gives us
768 # no actual benefits.
770 $_->meta->make_immutable(
771 inline_constructor => 0,
772 inline_accessors => 0,
778 Class::MOP::Attribute
784 Class::MOP::Method::Generated
786 Class::MOP::Method::Accessor
787 Class::MOP::Method::Constructor
788 Class::MOP::Method::Wrapped
799 Class::MOP - A Meta Object Protocol for Perl 5
803 This module is a fully functioning meta object protocol for the
804 Perl 5 object system. It makes no attempt to change the behavior or
805 characteristics of the Perl 5 object system, only to create a
806 protocol for its manipulation and introspection.
808 That said, it does attempt to create the tools for building a rich
809 set of extensions to the Perl 5 object system. Every attempt has been
810 made for these tools to keep to the spirit of the Perl 5 object
811 system that we all know and love.
813 This documentation is admittedly sparse on details, as time permits
814 I will try to improve them. For now, I suggest looking at the items
815 listed in the L<SEE ALSO> section for more information. In particular
816 the book "The Art of the Meta Object Protocol" was very influential
817 in the development of this system.
819 =head2 What is a Meta Object Protocol?
821 A meta object protocol is an API to an object system.
823 To be more specific, it is a set of abstractions of the components of
824 an object system (typically things like; classes, object, methods,
825 object attributes, etc.). These abstractions can then be used to both
826 inspect and manipulate the object system which they describe.
828 It can be said that there are two MOPs for any object system; the
829 implicit MOP, and the explicit MOP. The implicit MOP handles things
830 like method dispatch or inheritance, which happen automatically as
831 part of how the object system works. The explicit MOP typically
832 handles the introspection/reflection features of the object system.
833 All object systems have implicit MOPs, without one, they would not
834 work. Explict MOPs however as less common, and depending on the
835 language can vary from restrictive (Reflection in Java or C#) to
836 wide open (CLOS is a perfect example).
838 =head2 Yet Another Class Builder!! Why?
840 This is B<not> a class builder so much as it is a I<class builder
841 B<builder>>. My intent is that an end user does not use this module
842 directly, but instead this module is used by module authors to
843 build extensions and features onto the Perl 5 object system.
845 =head2 Who is this module for?
847 This module is specifically for anyone who has ever created or
848 wanted to create a module for the Class:: namespace. The tools which
849 this module will provide will hopefully make it easier to do more
850 complex things with Perl 5 classes by removing such barriers as
851 the need to hack the symbol tables, or understand the fine details
854 =head2 What changes do I have to make to use this module?
856 This module was designed to be as unintrusive as possible. Many of
857 its features are accessible without B<any> change to your existsing
858 code at all. It is meant to be a compliment to your existing code and
859 not an intrusion on your code base. Unlike many other B<Class::>
860 modules, this module B<does not> require you subclass it, or even that
861 you C<use> it in within your module's package.
863 The only features which requires additions to your code are the
864 attribute handling and instance construction features, and these are
865 both completely optional features. The only reason for this is because
866 Perl 5's object system does not actually have these features built
867 in. More information about this feature can be found below.
869 =head2 A Note about Performance?
871 It is a common misconception that explict MOPs are performance drains.
872 But this is not a universal truth at all, it is an side-effect of
873 specific implementations. For instance, using Java reflection is much
874 slower because the JVM cannot take advantage of any compiler
875 optimizations, and the JVM has to deal with much more runtime type
876 information as well. Reflection in C# is marginally better as it was
877 designed into the language and runtime (the CLR). In contrast, CLOS
878 (the Common Lisp Object System) was built to support an explicit MOP,
879 and so performance is tuned for it.
881 This library in particular does it's absolute best to avoid putting
882 B<any> drain at all upon your code's performance. In fact, by itself
883 it does nothing to affect your existing code. So you only pay for
884 what you actually use.
886 =head2 About Metaclass compatibility
888 This module makes sure that all metaclasses created are both upwards
889 and downwards compatible. The topic of metaclass compatibility is
890 highly esoteric and is something only encountered when doing deep and
891 involved metaclass hacking. There are two basic kinds of metaclass
892 incompatibility; upwards and downwards.
894 Upwards metaclass compatibility means that the metaclass of a
895 given class is either the same as (or a subclass of) all of the
898 Downward metaclass compatibility means that the metaclasses of a
899 given class's anscestors are all either the same as (or a subclass
902 Here is a diagram showing a set of two classes (C<A> and C<B>) and
903 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
904 metaclass compatibility both upwards and downwards.
906 +---------+ +---------+
907 | Meta::A |<----| Meta::B | <....... (instance of )
908 +---------+ +---------+ <------- (inherits from)
911 +---------+ +---------+
913 +---------+ +---------+
915 As I said this is a highly esoteric topic and one you will only run
916 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
917 are interested in why this is an issue see the paper
918 I<Uniform and safe metaclass composition> linked to in the
919 L<SEE ALSO> section of this document.
921 =head2 Using custom metaclasses
923 Always use the metaclass pragma when using a custom metaclass, this
924 will ensure the proper initialization order and not accidentely
925 create an incorrect type of metaclass for you. This is a very rare
926 problem, and one which can only occur if you are doing deep metaclass
927 programming. So in other words, don't worry about it.
931 The protocol is divided into 4 main sub-protocols:
935 =item The Class protocol
937 This provides a means of manipulating and introspecting a Perl 5
938 class. It handles all of symbol table hacking for you, and provides
939 a rich set of methods that go beyond simple package introspection.
941 See L<Class::MOP::Class> for more details.
943 =item The Attribute protocol
945 This provides a consistent represenation for an attribute of a
946 Perl 5 class. Since there are so many ways to create and handle
947 attributes in Perl 5 OO, this attempts to provide as much of a
948 unified approach as possible, while giving the freedom and
949 flexibility to subclass for specialization.
951 See L<Class::MOP::Attribute> for more details.
953 =item The Method protocol
955 This provides a means of manipulating and introspecting methods in
956 the Perl 5 object system. As with attributes, there are many ways to
957 approach this topic, so we try to keep it pretty basic, while still
958 making it possible to extend the system in many ways.
960 See L<Class::MOP::Method> for more details.
962 =item The Instance protocol
964 This provides a layer of abstraction for creating object instances.
965 Since the other layers use this protocol, it is relatively easy to
966 change the type of your instances from the default HASH ref to other
967 types of references. Several examples are provided in the F<examples/>
968 directory included in this distribution.
970 See L<Class::MOP::Instance> for more details.
980 =item I<IS_RUNNING_ON_5_10>
982 We set this constant depending on what version perl we are on, this
983 allows us to take advantage of new 5.10 features and stay backwards
988 Whether or not C<mro> provides C<get_isarev>, a much faster way to get all the
989 subclasses of a certain class.
993 =head2 Utility functions
997 =item B<load_class ($class_name)>
999 This will load a given C<$class_name> and if it does not have an
1000 already initialized metaclass, then it will intialize one for it.
1001 This function can be used in place of tricks like
1002 C<eval "use $module"> or using C<require>.
1004 =item B<is_class_loaded ($class_name)>
1006 This will return a boolean depending on if the C<$class_name> has
1009 NOTE: This does a basic check of the symbol table to try and
1010 determine as best it can if the C<$class_name> is loaded, it
1011 is probably correct about 99% of the time.
1013 =item B<check_package_cache_flag ($pkg)>
1015 This will return an integer that is managed by C<Class::MOP::Class>
1016 to determine if a module's symbol table has been altered.
1018 In Perl 5.10 or greater, this flag is package specific. However in
1019 versions prior to 5.10, this will use the C<PL_sub_generation> variable
1020 which is not package specific.
1022 =item B<get_code_info ($code)>
1024 This function returns two values, the name of the package the C<$code>
1025 is from and the name of the C<$code> itself. This is used by several
1026 elements of the MOP to detemine where a given C<$code> reference is from.
1028 =item B<subname ($name, $code)>
1030 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1032 If possible, we will load the L<Sub::Name> module and this will function
1033 as C<Sub::Name::subname> does, otherwise it will just return the C<$code>
1038 =head2 Metaclass cache functions
1040 Class::MOP holds a cache of metaclasses, the following are functions
1041 (B<not methods>) which can be used to access that cache. It is not
1042 recommended that you mess with this, bad things could happen. But if
1043 you are brave and willing to risk it, go for it.
1047 =item B<get_all_metaclasses>
1049 This will return an hash of all the metaclass instances that have
1050 been cached by B<Class::MOP::Class> keyed by the package name.
1052 =item B<get_all_metaclass_instances>
1054 This will return an array of all the metaclass instances that have
1055 been cached by B<Class::MOP::Class>.
1057 =item B<get_all_metaclass_names>
1059 This will return an array of all the metaclass names that have
1060 been cached by B<Class::MOP::Class>.
1062 =item B<get_metaclass_by_name ($name)>
1064 This will return a cached B<Class::MOP::Class> instance of nothing
1065 if no metaclass exist by that C<$name>.
1067 =item B<store_metaclass_by_name ($name, $meta)>
1069 This will store a metaclass in the cache at the supplied C<$key>.
1071 =item B<weaken_metaclass ($name)>
1073 In rare cases it is desireable to store a weakened reference in
1074 the metaclass cache. This function will weaken the reference to
1075 the metaclass stored in C<$name>.
1077 =item B<does_metaclass_exist ($name)>
1079 This will return true of there exists a metaclass stored in the
1080 C<$name> key and return false otherwise.
1082 =item B<remove_metaclass_by_name ($name)>
1084 This will remove a the metaclass stored in the C<$name> key.
1092 There are very few books out on Meta Object Protocols and Metaclasses
1093 because it is such an esoteric topic. The following books are really
1094 the only ones I have found. If you know of any more, B<I<please>>
1095 email me and let me know, I would love to hear about them.
1099 =item "The Art of the Meta Object Protocol"
1101 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
1103 =item "Putting MetaClasses to Work"
1105 =item "Smalltalk: The Language"
1113 =item Uniform and safe metaclass composition
1115 An excellent paper by the people who brought us the original Traits paper.
1116 This paper is on how Traits can be used to do safe metaclass composition,
1117 and offers an excellent introduction section which delves into the topic of
1118 metaclass compatibility.
1120 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1122 =item Safe Metaclass Programming
1124 This paper seems to precede the above paper, and propose a mix-in based
1125 approach as opposed to the Traits based approach. Both papers have similar
1126 information on the metaclass compatibility problem space.
1128 L<http://citeseer.ist.psu.edu/37617.html>
1136 =item The Perl 6 MetaModel work in the Pugs project
1140 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
1142 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1152 =item CPAN Module Review of Class::MOP
1154 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1158 =head1 SIMILAR MODULES
1160 As I have said above, this module is a class-builder-builder, so it is
1161 not the same thing as modules like L<Class::Accessor> and
1162 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1163 with similar goals to this module. The one I have found which is most
1164 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1165 creates are very different from this modules.
1169 All complex software has bugs lurking in it, and this module is no
1170 exception. If you find a bug please either email me, or add the bug
1173 =head1 ACKNOWLEDGEMENTS
1179 Thanks to Rob for actually getting the development of this module kick-started.
1185 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1187 B<with contributions from:>
1189 Brandon (blblack) Black
1191 Guillermo (groditi) Roditi
1195 Rob (robkinyon) Kinyon
1197 Yuval (nothingmuch) Kogman
1199 Scott (konobi) McWhirter
1201 =head1 COPYRIGHT AND LICENSE
1203 Copyright 2006-2008 by Infinity Interactive, Inc.
1205 L<http://www.iinteractive.com>
1207 This library is free software; you can redistribute it and/or modify
1208 it under the same terms as Perl itself.