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('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 =head2 Utility functions
992 =item B<load_class ($class_name)>
994 This will load a given C<$class_name> and if it does not have an
995 already initialized metaclass, then it will intialize one for it.
996 This function can be used in place of tricks like
997 C<eval "use $module"> or using C<require>.
999 =item B<is_class_loaded ($class_name)>
1001 This will return a boolean depending on if the C<$class_name> has
1004 NOTE: This does a basic check of the symbol table to try and
1005 determine as best it can if the C<$class_name> is loaded, it
1006 is probably correct about 99% of the time.
1008 =item B<check_package_cache_flag ($pkg)>
1010 This will return an integer that is managed by C<Class::MOP::Class>
1011 to determine if a module's symbol table has been altered.
1013 In Perl 5.10 or greater, this flag is package specific. However in
1014 versions prior to 5.10, this will use the C<PL_sub_generation> variable
1015 which is not package specific.
1017 =item B<get_code_info ($code)>
1019 This function returns two values, the name of the package the C<$code>
1020 is from and the name of the C<$code> itself. This is used by several
1021 elements of the MOP to detemine where a given C<$code> reference is from.
1023 =item B<subname ($name, $code)>
1025 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1027 If possible, we will load the L<Sub::Name> module and this will function
1028 as C<Sub::Name::subname> does, otherwise it will just return the C<$code>
1033 =head2 Metaclass cache functions
1035 Class::MOP holds a cache of metaclasses, the following are functions
1036 (B<not methods>) which can be used to access that cache. It is not
1037 recommended that you mess with this, bad things could happen. But if
1038 you are brave and willing to risk it, go for it.
1042 =item B<get_all_metaclasses>
1044 This will return an hash of all the metaclass instances that have
1045 been cached by B<Class::MOP::Class> keyed by the package name.
1047 =item B<get_all_metaclass_instances>
1049 This will return an array of all the metaclass instances that have
1050 been cached by B<Class::MOP::Class>.
1052 =item B<get_all_metaclass_names>
1054 This will return an array of all the metaclass names that have
1055 been cached by B<Class::MOP::Class>.
1057 =item B<get_metaclass_by_name ($name)>
1059 This will return a cached B<Class::MOP::Class> instance of nothing
1060 if no metaclass exist by that C<$name>.
1062 =item B<store_metaclass_by_name ($name, $meta)>
1064 This will store a metaclass in the cache at the supplied C<$key>.
1066 =item B<weaken_metaclass ($name)>
1068 In rare cases it is desireable to store a weakened reference in
1069 the metaclass cache. This function will weaken the reference to
1070 the metaclass stored in C<$name>.
1072 =item B<does_metaclass_exist ($name)>
1074 This will return true of there exists a metaclass stored in the
1075 C<$name> key and return false otherwise.
1077 =item B<remove_metaclass_by_name ($name)>
1079 This will remove a the metaclass stored in the C<$name> key.
1087 There are very few books out on Meta Object Protocols and Metaclasses
1088 because it is such an esoteric topic. The following books are really
1089 the only ones I have found. If you know of any more, B<I<please>>
1090 email me and let me know, I would love to hear about them.
1094 =item "The Art of the Meta Object Protocol"
1096 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
1098 =item "Putting MetaClasses to Work"
1100 =item "Smalltalk: The Language"
1108 =item Uniform and safe metaclass composition
1110 An excellent paper by the people who brought us the original Traits paper.
1111 This paper is on how Traits can be used to do safe metaclass composition,
1112 and offers an excellent introduction section which delves into the topic of
1113 metaclass compatibility.
1115 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1117 =item Safe Metaclass Programming
1119 This paper seems to precede the above paper, and propose a mix-in based
1120 approach as opposed to the Traits based approach. Both papers have similar
1121 information on the metaclass compatibility problem space.
1123 L<http://citeseer.ist.psu.edu/37617.html>
1131 =item The Perl 6 MetaModel work in the Pugs project
1135 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
1137 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1147 =item CPAN Module Review of Class::MOP
1149 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1153 =head1 SIMILAR MODULES
1155 As I have said above, this module is a class-builder-builder, so it is
1156 not the same thing as modules like L<Class::Accessor> and
1157 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1158 with similar goals to this module. The one I have found which is most
1159 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1160 creates are very different from this modules.
1164 All complex software has bugs lurking in it, and this module is no
1165 exception. If you find a bug please either email me, or add the bug
1168 =head1 ACKNOWLEDGEMENTS
1174 Thanks to Rob for actually getting the development of this module kick-started.
1180 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1182 B<with contributions from:>
1184 Brandon (blblack) Black
1186 Guillermo (groditi) Roditi
1190 Rob (robkinyon) Kinyon
1192 Yuval (nothingmuch) Kogman
1194 Scott (konobi) McWhirter
1196 =head1 COPYRIGHT AND LICENSE
1198 Copyright 2006-2008 by Infinity Interactive, Inc.
1200 L<http://www.iinteractive.com>
1202 This library is free software; you can redistribute it and/or modify
1203 it under the same terms as Perl itself.