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.
86 if ( eval { require Sub::Name } ) {
87 *subname = \&Sub::Name::subname;
90 *subname = $_PP_subname;
96 # Metaclasses are singletons, so we cache them here.
97 # there is no need to worry about destruction though
98 # because they should die only when the program dies.
99 # After all, do package definitions even get reaped?
102 # means of accessing all the metaclasses that have
103 # been initialized thus far (for mugwumps obj browser)
104 sub get_all_metaclasses { %METAS }
105 sub get_all_metaclass_instances { values %METAS }
106 sub get_all_metaclass_names { keys %METAS }
107 sub get_metaclass_by_name { $METAS{$_[0]} }
108 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
109 sub weaken_metaclass { weaken($METAS{$_[0]}) }
110 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
111 sub remove_metaclass_by_name { $METAS{$_[0]} = undef }
114 # We only cache metaclasses, meaning instances of
115 # Class::MOP::Class. We do not cache instance of
116 # Class::MOP::Package or Class::MOP::Module. Mostly
117 # because I don't yet see a good reason to do so.
123 if (ref($class) || !defined($class) || !length($class)) {
124 my $display = defined($class) ? $class : 'undef';
125 confess "Invalid class name ($display)";
128 # if the class is not already loaded in the symbol table..
129 unless (is_class_loaded($class)) {
131 my $file = $class . '.pm';
133 my $e = do { local $@; eval { require($file) }; $@ };
134 confess "Could not load class ($class) because : $e" if $e;
137 # initialize a metaclass if necessary
138 unless (does_metaclass_exist($class)) {
139 my $e = do { local $@; eval { Class::MOP::Class->initialize($class) }; $@ };
140 confess "Could not initialize class ($class) because : $e" if $e;
143 return get_metaclass_by_name($class) if defined wantarray;
146 sub is_class_loaded {
149 return 0 if ref($class) || !defined($class) || !length($class);
151 # walk the symbol table tree to avoid autovififying
152 # \*{${main::}{"Foo::"}} == \*main::Foo::
155 foreach my $part (split('::', $class)) {
156 return 0 unless exists ${$$pack}{"${part}::"};
157 $pack = \*{${$$pack}{"${part}::"}};
160 # check for $VERSION or @ISA
161 return 1 if exists ${$$pack}{VERSION}
162 && defined *{${$$pack}{VERSION}}{SCALAR};
163 return 1 if exists ${$$pack}{ISA}
164 && defined *{${$$pack}{ISA}}{ARRAY};
166 # check for any method
167 foreach ( keys %{$$pack} ) {
168 next if substr($_, -2, 2) eq '::';
170 my $glob = ${$$pack}{$_} || next;
173 if ( IS_RUNNING_ON_5_10 ) {
174 return 1 if ref $glob eq 'SCALAR';
177 return 1 if defined *{$glob}{CODE};
185 ## ----------------------------------------------------------------------------
186 ## Setting up our environment ...
187 ## ----------------------------------------------------------------------------
188 ## Class::MOP needs to have a few things in the global perl environment so
189 ## that it can operate effectively. Those things are done here.
190 ## ----------------------------------------------------------------------------
192 # ... nothing yet actually ;)
194 ## ----------------------------------------------------------------------------
196 ## ----------------------------------------------------------------------------
197 ## The code below here is to bootstrap our MOP with itself. This is also
198 ## sometimes called "tying the knot". By doing this, we make it much easier
199 ## to extend the MOP through subclassing and such since now you can use the
200 ## MOP itself to extend itself.
202 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
203 ## ----------------------------------------------------------------------------
205 # We need to add in the meta-attributes here so that
206 # any subclass of Class::MOP::* will be able to
207 # inherit them using &construct_instance
209 ## --------------------------------------------------------
210 ## Class::MOP::Package
212 Class::MOP::Package->meta->add_attribute(
213 Class::MOP::Attribute->new('package' => (
215 # NOTE: we need to do this in order
216 # for the instance meta-object to
217 # not fall into meta-circular death
219 # we just alias the original method
220 # rather than re-produce it here
221 'name' => \&Class::MOP::Package::name
223 init_arg => 'package',
227 Class::MOP::Package->meta->add_attribute(
228 Class::MOP::Attribute->new('namespace' => (
231 # we just alias the original method
232 # rather than re-produce it here
233 'namespace' => \&Class::MOP::Package::namespace
236 default => sub { \undef }
241 # use the metaclass to construct the meta-package
242 # which is a superclass of the metaclass itself :P
243 Class::MOP::Package->meta->add_method('initialize' => sub {
245 my $package_name = shift;
246 $class->meta->new_object('package' => $package_name, @_);
249 ## --------------------------------------------------------
250 ## Class::MOP::Module
253 # yeah this is kind of stretching things a bit,
254 # but truthfully the version should be an attribute
255 # of the Module, the weirdness comes from having to
256 # stick to Perl 5 convention and store it in the
257 # $VERSION package variable. Basically if you just
258 # squint at it, it will look how you want it to look.
259 # Either as a package variable, or as a attribute of
260 # the metaclass, isn't abstraction great :)
262 Class::MOP::Module->meta->add_attribute(
263 Class::MOP::Attribute->new('version' => (
266 # we just alias the original method
267 # rather than re-produce it here
268 'version' => \&Class::MOP::Module::version
271 default => sub { \undef }
276 # By following the same conventions as version here,
277 # we are opening up the possibility that people can
278 # use the $AUTHORITY in non-Class::MOP modules as
281 Class::MOP::Module->meta->add_attribute(
282 Class::MOP::Attribute->new('authority' => (
285 # we just alias the original method
286 # rather than re-produce it here
287 'authority' => \&Class::MOP::Module::authority
290 default => sub { \undef }
294 ## --------------------------------------------------------
297 Class::MOP::Class->meta->add_attribute(
298 Class::MOP::Attribute->new('attributes' => (
300 # NOTE: we need to do this in order
301 # for the instance meta-object to
302 # not fall into meta-circular death
304 # we just alias the original method
305 # rather than re-produce it here
306 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
308 init_arg => 'attributes',
309 default => sub { {} }
313 Class::MOP::Class->meta->add_attribute(
314 Class::MOP::Attribute->new('methods' => (
315 init_arg => 'methods',
318 # we just alias the original method
319 # rather than re-produce it here
320 'get_method_map' => \&Class::MOP::Class::get_method_map
322 default => sub { {} }
326 Class::MOP::Class->meta->add_attribute(
327 Class::MOP::Attribute->new('superclasses' => (
330 # we just alias the original method
331 # rather than re-produce it here
332 'superclasses' => \&Class::MOP::Class::superclasses
335 default => sub { \undef }
339 Class::MOP::Class->meta->add_attribute(
340 Class::MOP::Attribute->new('attribute_metaclass' => (
343 # we just alias the original method
344 # rather than re-produce it here
345 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
347 init_arg => 'attribute_metaclass',
348 default => 'Class::MOP::Attribute',
352 Class::MOP::Class->meta->add_attribute(
353 Class::MOP::Attribute->new('method_metaclass' => (
356 # we just alias the original method
357 # rather than re-produce it here
358 'method_metaclass' => \&Class::MOP::Class::method_metaclass
360 init_arg => 'method_metaclass',
361 default => 'Class::MOP::Method',
365 Class::MOP::Class->meta->add_attribute(
366 Class::MOP::Attribute->new('instance_metaclass' => (
368 # NOTE: we need to do this in order
369 # for the instance meta-object to
370 # not fall into meta-circular death
372 # we just alias the original method
373 # rather than re-produce it here
374 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
376 init_arg => 'instance_metaclass',
377 default => 'Class::MOP::Instance',
382 # we don't actually need to tie the knot with
383 # Class::MOP::Class here, it is actually handled
384 # within Class::MOP::Class itself in the
385 # construct_class_instance method.
387 ## --------------------------------------------------------
388 ## Class::MOP::Attribute
390 Class::MOP::Attribute->meta->add_attribute(
391 Class::MOP::Attribute->new('name' => (
394 # NOTE: we need to do this in order
395 # for the instance meta-object to
396 # not fall into meta-circular death
398 # we just alias the original method
399 # rather than re-produce it here
400 'name' => \&Class::MOP::Attribute::name
405 Class::MOP::Attribute->meta->add_attribute(
406 Class::MOP::Attribute->new('associated_class' => (
407 init_arg => 'associated_class',
409 # NOTE: we need to do this in order
410 # for the instance meta-object to
411 # not fall into meta-circular death
413 # we just alias the original method
414 # rather than re-produce it here
415 'associated_class' => \&Class::MOP::Attribute::associated_class
420 Class::MOP::Attribute->meta->add_attribute(
421 Class::MOP::Attribute->new('accessor' => (
422 init_arg => 'accessor',
423 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
424 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
428 Class::MOP::Attribute->meta->add_attribute(
429 Class::MOP::Attribute->new('reader' => (
430 init_arg => 'reader',
431 reader => { 'reader' => \&Class::MOP::Attribute::reader },
432 predicate => { 'has_reader' => \&Class::MOP::Attribute::has_reader },
436 Class::MOP::Attribute->meta->add_attribute(
437 Class::MOP::Attribute->new('initializer' => (
438 init_arg => 'initializer',
439 reader => { 'initializer' => \&Class::MOP::Attribute::initializer },
440 predicate => { 'has_initializer' => \&Class::MOP::Attribute::has_initializer },
444 Class::MOP::Attribute->meta->add_attribute(
445 Class::MOP::Attribute->new('writer' => (
446 init_arg => 'writer',
447 reader => { 'writer' => \&Class::MOP::Attribute::writer },
448 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
452 Class::MOP::Attribute->meta->add_attribute(
453 Class::MOP::Attribute->new('predicate' => (
454 init_arg => 'predicate',
455 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
456 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
460 Class::MOP::Attribute->meta->add_attribute(
461 Class::MOP::Attribute->new('clearer' => (
462 init_arg => 'clearer',
463 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
464 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
468 Class::MOP::Attribute->meta->add_attribute(
469 Class::MOP::Attribute->new('builder' => (
470 init_arg => 'builder',
471 reader => { 'builder' => \&Class::MOP::Attribute::builder },
472 predicate => { 'has_builder' => \&Class::MOP::Attribute::has_builder },
476 Class::MOP::Attribute->meta->add_attribute(
477 Class::MOP::Attribute->new('init_arg' => (
478 init_arg => 'init_arg',
479 reader => { 'init_arg' => \&Class::MOP::Attribute::init_arg },
480 predicate => { 'has_init_arg' => \&Class::MOP::Attribute::has_init_arg },
484 Class::MOP::Attribute->meta->add_attribute(
485 Class::MOP::Attribute->new('default' => (
486 init_arg => 'default',
487 # default has a custom 'reader' method ...
488 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
492 Class::MOP::Attribute->meta->add_attribute(
493 Class::MOP::Attribute->new('associated_methods' => (
494 init_arg => 'associated_methods',
495 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
496 default => sub { [] }
500 # NOTE: (meta-circularity)
501 # This should be one of the last things done
502 # it will "tie the knot" with Class::MOP::Attribute
503 # so that it uses the attributes meta-objects
504 # to construct itself.
505 Class::MOP::Attribute->meta->add_method('new' => sub {
506 my ( $class, @args ) = @_;
508 unshift @args, "name" if @args % 2 == 1;
511 my $name = $options{name};
513 (defined $name && $name)
514 || confess "You must provide a name for the attribute";
515 $options{init_arg} = $name
516 if not exists $options{init_arg};
518 if(exists $options{builder}){
519 confess("builder must be a defined scalar value which is a method name")
520 if ref $options{builder} || !(defined $options{builder});
521 confess("Setting both default and builder is not allowed.")
522 if exists $options{default};
524 (Class::MOP::Attribute::is_default_a_coderef(\%options))
525 || confess("References are not allowed as default values, you must ".
526 "wrap the default of '$name' in a CODE reference (ex: sub { [] } and not [])")
527 if exists $options{default} && ref $options{default};
530 # return the new object
531 $class->meta->new_object(%options);
534 Class::MOP::Attribute->meta->add_method('clone' => sub {
536 $self->meta->clone_object($self, @_);
539 ## --------------------------------------------------------
540 ## Class::MOP::Method
541 Class::MOP::Method->meta->add_attribute(
542 Class::MOP::Attribute->new('body' => (
544 reader => { 'body' => \&Class::MOP::Method::body },
548 Class::MOP::Method->meta->add_attribute(
549 Class::MOP::Attribute->new('associated_metaclass' => (
550 init_arg => 'associated_metaclass',
551 reader => { 'associated_metaclass' => \&Class::MOP::Method::associated_metaclass },
555 Class::MOP::Method->meta->add_attribute(
556 Class::MOP::Attribute->new('package_name' => (
557 init_arg => 'package_name',
558 reader => { 'package_name' => \&Class::MOP::Method::package_name },
562 Class::MOP::Method->meta->add_attribute(
563 Class::MOP::Attribute->new('name' => (
565 reader => { 'name' => \&Class::MOP::Method::name },
569 Class::MOP::Method->meta->add_method('wrap' => sub {
570 my ( $class, @args ) = @_;
572 unshift @args, 'body' if @args % 2 == 1;
575 my $code = $options{body};
577 ('CODE' eq ref($code))
578 || confess "You must supply a CODE reference to bless, not (" . ($code || 'undef') . ")";
580 ($options{package_name} && $options{name})
581 || confess "You must supply the package_name and name parameters";
583 # return the new object
584 $class->meta->new_object(%options);
587 Class::MOP::Method->meta->add_method('clone' => sub {
589 $self->meta->clone_object($self, @_);
592 ## --------------------------------------------------------
593 ## Class::MOP::Method::Wrapped
596 # the way this item is initialized, this
597 # really does not follow the standard
598 # practices of attributes, but we put
599 # it here for completeness
600 Class::MOP::Method::Wrapped->meta->add_attribute(
601 Class::MOP::Attribute->new('modifier_table')
604 ## --------------------------------------------------------
605 ## Class::MOP::Method::Generated
607 Class::MOP::Method::Generated->meta->add_attribute(
608 Class::MOP::Attribute->new('is_inline' => (
609 init_arg => 'is_inline',
610 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
615 Class::MOP::Method::Generated->meta->add_method('new' => sub {
616 my ($class, %options) = @_;
617 ($options{package_name} && $options{name})
618 || confess "You must supply the package_name and name parameters";
619 my $self = $class->meta->new_object(%options);
620 $self->initialize_body;
624 ## --------------------------------------------------------
625 ## Class::MOP::Method::Accessor
627 Class::MOP::Method::Accessor->meta->add_attribute(
628 Class::MOP::Attribute->new('attribute' => (
629 init_arg => 'attribute',
631 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
636 Class::MOP::Method::Accessor->meta->add_attribute(
637 Class::MOP::Attribute->new('accessor_type' => (
638 init_arg => 'accessor_type',
639 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
643 Class::MOP::Method::Accessor->meta->add_method('new' => sub {
647 (exists $options{attribute})
648 || confess "You must supply an attribute to construct with";
650 (exists $options{accessor_type})
651 || confess "You must supply an accessor_type to construct with";
653 (Scalar::Util::blessed($options{attribute}) && $options{attribute}->isa('Class::MOP::Attribute'))
654 || confess "You must supply an attribute which is a 'Class::MOP::Attribute' instance";
656 ($options{package_name} && $options{name})
657 || confess "You must supply the package_name and name parameters";
659 # return the new object
660 my $self = $class->meta->new_object(%options);
662 # we don't want this creating
663 # a cycle in the code, if not
665 Scalar::Util::weaken($self->{'attribute'});
667 $self->initialize_body;
673 ## --------------------------------------------------------
674 ## Class::MOP::Method::Constructor
676 Class::MOP::Method::Constructor->meta->add_attribute(
677 Class::MOP::Attribute->new('options' => (
678 init_arg => 'options',
680 'options' => \&Class::MOP::Method::Constructor::options
682 default => sub { +{} }
686 Class::MOP::Method::Constructor->meta->add_attribute(
687 Class::MOP::Attribute->new('associated_metaclass' => (
688 init_arg => 'metaclass',
690 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
695 Class::MOP::Method::Constructor->meta->add_method('new' => sub {
699 (Scalar::Util::blessed $options{metaclass} && $options{metaclass}->isa('Class::MOP::Class'))
700 || confess "You must pass a metaclass instance if you want to inline"
701 if $options{is_inline};
703 ($options{package_name} && $options{name})
704 || confess "You must supply the package_name and name parameters";
706 # return the new object
707 my $self = $class->meta->new_object(%options);
709 # we don't want this creating
710 # a cycle in the code, if not
712 Scalar::Util::weaken($self->{'associated_metaclass'});
714 $self->initialize_body;
719 ## --------------------------------------------------------
720 ## Class::MOP::Instance
723 # these don't yet do much of anything, but are just
724 # included for completeness
726 Class::MOP::Instance->meta->add_attribute(
727 Class::MOP::Attribute->new('associated_metaclass')
730 Class::MOP::Instance->meta->add_attribute(
731 Class::MOP::Attribute->new('attributes')
734 Class::MOP::Instance->meta->add_attribute(
735 Class::MOP::Attribute->new('slots')
738 Class::MOP::Instance->meta->add_attribute(
739 Class::MOP::Attribute->new('slot_hash')
743 # we need the meta instance of the meta instance to be created now, in order
744 # for the constructor to be able to use it
745 Class::MOP::Instance->meta->get_meta_instance;
747 Class::MOP::Instance->meta->add_method('new' => sub {
749 my $options = $class->BUILDARGS(@_);
751 my $self = $class->meta->new_object(%$options);
753 Scalar::Util::weaken($self->{'associated_metaclass'});
758 # pretend the add_method never happenned. it hasn't yet affected anything
759 undef Class::MOP::Instance->meta->{_package_cache_flag};
761 ## --------------------------------------------------------
762 ## Now close all the Class::MOP::* classes
765 # we don't need to inline the
766 # constructors or the accessors
767 # this only lengthens the compile
768 # time of the MOP, and gives us
769 # no actual benefits.
771 $_->meta->make_immutable(
772 inline_constructor => 0,
773 inline_accessors => 0,
779 Class::MOP::Attribute
785 Class::MOP::Method::Generated
787 Class::MOP::Method::Accessor
788 Class::MOP::Method::Constructor
789 Class::MOP::Method::Wrapped
800 Class::MOP - A Meta Object Protocol for Perl 5
804 This module is a fully functioning meta object protocol for the
805 Perl 5 object system. It makes no attempt to change the behavior or
806 characteristics of the Perl 5 object system, only to create a
807 protocol for its manipulation and introspection.
809 That said, it does attempt to create the tools for building a rich
810 set of extensions to the Perl 5 object system. Every attempt has been
811 made for these tools to keep to the spirit of the Perl 5 object
812 system that we all know and love.
814 This documentation is admittedly sparse on details, as time permits
815 I will try to improve them. For now, I suggest looking at the items
816 listed in the L<SEE ALSO> section for more information. In particular
817 the book "The Art of the Meta Object Protocol" was very influential
818 in the development of this system.
820 =head2 What is a Meta Object Protocol?
822 A meta object protocol is an API to an object system.
824 To be more specific, it is a set of abstractions of the components of
825 an object system (typically things like; classes, object, methods,
826 object attributes, etc.). These abstractions can then be used to both
827 inspect and manipulate the object system which they describe.
829 It can be said that there are two MOPs for any object system; the
830 implicit MOP, and the explicit MOP. The implicit MOP handles things
831 like method dispatch or inheritance, which happen automatically as
832 part of how the object system works. The explicit MOP typically
833 handles the introspection/reflection features of the object system.
834 All object systems have implicit MOPs, without one, they would not
835 work. Explict MOPs however as less common, and depending on the
836 language can vary from restrictive (Reflection in Java or C#) to
837 wide open (CLOS is a perfect example).
839 =head2 Yet Another Class Builder!! Why?
841 This is B<not> a class builder so much as it is a I<class builder
842 B<builder>>. My intent is that an end user does not use this module
843 directly, but instead this module is used by module authors to
844 build extensions and features onto the Perl 5 object system.
846 =head2 Who is this module for?
848 This module is specifically for anyone who has ever created or
849 wanted to create a module for the Class:: namespace. The tools which
850 this module will provide will hopefully make it easier to do more
851 complex things with Perl 5 classes by removing such barriers as
852 the need to hack the symbol tables, or understand the fine details
855 =head2 What changes do I have to make to use this module?
857 This module was designed to be as unintrusive as possible. Many of
858 its features are accessible without B<any> change to your existsing
859 code at all. It is meant to be a compliment to your existing code and
860 not an intrusion on your code base. Unlike many other B<Class::>
861 modules, this module B<does not> require you subclass it, or even that
862 you C<use> it in within your module's package.
864 The only features which requires additions to your code are the
865 attribute handling and instance construction features, and these are
866 both completely optional features. The only reason for this is because
867 Perl 5's object system does not actually have these features built
868 in. More information about this feature can be found below.
870 =head2 A Note about Performance?
872 It is a common misconception that explict MOPs are performance drains.
873 But this is not a universal truth at all, it is an side-effect of
874 specific implementations. For instance, using Java reflection is much
875 slower because the JVM cannot take advantage of any compiler
876 optimizations, and the JVM has to deal with much more runtime type
877 information as well. Reflection in C# is marginally better as it was
878 designed into the language and runtime (the CLR). In contrast, CLOS
879 (the Common Lisp Object System) was built to support an explicit MOP,
880 and so performance is tuned for it.
882 This library in particular does it's absolute best to avoid putting
883 B<any> drain at all upon your code's performance. In fact, by itself
884 it does nothing to affect your existing code. So you only pay for
885 what you actually use.
887 =head2 About Metaclass compatibility
889 This module makes sure that all metaclasses created are both upwards
890 and downwards compatible. The topic of metaclass compatibility is
891 highly esoteric and is something only encountered when doing deep and
892 involved metaclass hacking. There are two basic kinds of metaclass
893 incompatibility; upwards and downwards.
895 Upwards metaclass compatibility means that the metaclass of a
896 given class is either the same as (or a subclass of) all of the
899 Downward metaclass compatibility means that the metaclasses of a
900 given class's anscestors are all either the same as (or a subclass
903 Here is a diagram showing a set of two classes (C<A> and C<B>) and
904 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
905 metaclass compatibility both upwards and downwards.
907 +---------+ +---------+
908 | Meta::A |<----| Meta::B | <....... (instance of )
909 +---------+ +---------+ <------- (inherits from)
912 +---------+ +---------+
914 +---------+ +---------+
916 As I said this is a highly esoteric topic and one you will only run
917 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
918 are interested in why this is an issue see the paper
919 I<Uniform and safe metaclass composition> linked to in the
920 L<SEE ALSO> section of this document.
922 =head2 Using custom metaclasses
924 Always use the metaclass pragma when using a custom metaclass, this
925 will ensure the proper initialization order and not accidentely
926 create an incorrect type of metaclass for you. This is a very rare
927 problem, and one which can only occur if you are doing deep metaclass
928 programming. So in other words, don't worry about it.
932 The protocol is divided into 4 main sub-protocols:
936 =item The Class protocol
938 This provides a means of manipulating and introspecting a Perl 5
939 class. It handles all of symbol table hacking for you, and provides
940 a rich set of methods that go beyond simple package introspection.
942 See L<Class::MOP::Class> for more details.
944 =item The Attribute protocol
946 This provides a consistent represenation for an attribute of a
947 Perl 5 class. Since there are so many ways to create and handle
948 attributes in Perl 5 OO, this attempts to provide as much of a
949 unified approach as possible, while giving the freedom and
950 flexibility to subclass for specialization.
952 See L<Class::MOP::Attribute> for more details.
954 =item The Method protocol
956 This provides a means of manipulating and introspecting methods in
957 the Perl 5 object system. As with attributes, there are many ways to
958 approach this topic, so we try to keep it pretty basic, while still
959 making it possible to extend the system in many ways.
961 See L<Class::MOP::Method> for more details.
963 =item The Instance protocol
965 This provides a layer of abstraction for creating object instances.
966 Since the other layers use this protocol, it is relatively easy to
967 change the type of your instances from the default HASH ref to other
968 types of references. Several examples are provided in the F<examples/>
969 directory included in this distribution.
971 See L<Class::MOP::Instance> for more details.
981 =item I<IS_RUNNING_ON_5_10>
983 We set this constant depending on what version perl we are on, this
984 allows us to take advantage of new 5.10 features and stay backwards
989 Whether or not C<mro> provides C<get_isarev>, a much faster way to get all the
990 subclasses of a certain class.
994 =head2 Utility functions
998 =item B<load_class ($class_name)>
1000 This will load a given C<$class_name> and if it does not have an
1001 already initialized metaclass, then it will intialize one for it.
1002 This function can be used in place of tricks like
1003 C<eval "use $module"> or using C<require>.
1005 =item B<is_class_loaded ($class_name)>
1007 This will return a boolean depending on if the C<$class_name> has
1010 NOTE: This does a basic check of the symbol table to try and
1011 determine as best it can if the C<$class_name> is loaded, it
1012 is probably correct about 99% of the time.
1014 =item B<check_package_cache_flag ($pkg)>
1016 This will return an integer that is managed by C<Class::MOP::Class>
1017 to determine if a module's symbol table has been altered.
1019 In Perl 5.10 or greater, this flag is package specific. However in
1020 versions prior to 5.10, this will use the C<PL_sub_generation> variable
1021 which is not package specific.
1023 =item B<get_code_info ($code)>
1025 This function returns two values, the name of the package the C<$code>
1026 is from and the name of the C<$code> itself. This is used by several
1027 elements of the MOP to detemine where a given C<$code> reference is from.
1029 =item B<subname ($name, $code)>
1031 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
1033 If possible, we will load the L<Sub::Name> module and this will function
1034 as C<Sub::Name::subname> does, otherwise it will just return the C<$code>
1039 =head2 Metaclass cache functions
1041 Class::MOP holds a cache of metaclasses, the following are functions
1042 (B<not methods>) which can be used to access that cache. It is not
1043 recommended that you mess with this, bad things could happen. But if
1044 you are brave and willing to risk it, go for it.
1048 =item B<get_all_metaclasses>
1050 This will return an hash of all the metaclass instances that have
1051 been cached by B<Class::MOP::Class> keyed by the package name.
1053 =item B<get_all_metaclass_instances>
1055 This will return an array of all the metaclass instances that have
1056 been cached by B<Class::MOP::Class>.
1058 =item B<get_all_metaclass_names>
1060 This will return an array of all the metaclass names that have
1061 been cached by B<Class::MOP::Class>.
1063 =item B<get_metaclass_by_name ($name)>
1065 This will return a cached B<Class::MOP::Class> instance of nothing
1066 if no metaclass exist by that C<$name>.
1068 =item B<store_metaclass_by_name ($name, $meta)>
1070 This will store a metaclass in the cache at the supplied C<$key>.
1072 =item B<weaken_metaclass ($name)>
1074 In rare cases it is desireable to store a weakened reference in
1075 the metaclass cache. This function will weaken the reference to
1076 the metaclass stored in C<$name>.
1078 =item B<does_metaclass_exist ($name)>
1080 This will return true of there exists a metaclass stored in the
1081 C<$name> key and return false otherwise.
1083 =item B<remove_metaclass_by_name ($name)>
1085 This will remove a the metaclass stored in the C<$name> key.
1093 There are very few books out on Meta Object Protocols and Metaclasses
1094 because it is such an esoteric topic. The following books are really
1095 the only ones I have found. If you know of any more, B<I<please>>
1096 email me and let me know, I would love to hear about them.
1100 =item "The Art of the Meta Object Protocol"
1102 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
1104 =item "Putting MetaClasses to Work"
1106 =item "Smalltalk: The Language"
1114 =item Uniform and safe metaclass composition
1116 An excellent paper by the people who brought us the original Traits paper.
1117 This paper is on how Traits can be used to do safe metaclass composition,
1118 and offers an excellent introduction section which delves into the topic of
1119 metaclass compatibility.
1121 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1123 =item Safe Metaclass Programming
1125 This paper seems to precede the above paper, and propose a mix-in based
1126 approach as opposed to the Traits based approach. Both papers have similar
1127 information on the metaclass compatibility problem space.
1129 L<http://citeseer.ist.psu.edu/37617.html>
1137 =item The Perl 6 MetaModel work in the Pugs project
1141 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
1143 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1153 =item CPAN Module Review of Class::MOP
1155 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1159 =head1 SIMILAR MODULES
1161 As I have said above, this module is a class-builder-builder, so it is
1162 not the same thing as modules like L<Class::Accessor> and
1163 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1164 with similar goals to this module. The one I have found which is most
1165 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1166 creates are very different from this modules.
1170 All complex software has bugs lurking in it, and this module is no
1171 exception. If you find a bug please either email me, or add the bug
1174 =head1 ACKNOWLEDGEMENTS
1180 Thanks to Rob for actually getting the development of this module kick-started.
1186 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1188 B<with contributions from:>
1190 Brandon (blblack) Black
1192 Guillermo (groditi) Roditi
1196 Rob (robkinyon) Kinyon
1198 Yuval (nothingmuch) Kogman
1200 Scott (konobi) McWhirter
1202 =head1 COPYRIGHT AND LICENSE
1204 Copyright 2006-2008 by Infinity Interactive, Inc.
1206 L<http://www.iinteractive.com>
1208 This library is free software; you can redistribute it and/or modify
1209 it under the same terms as Perl itself.