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 = '0.61';
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 # see if this is already
135 # loaded in the symbol table
136 return 1 if is_class_loaded($class);
137 # otherwise require it ...
138 my $file = $class . '.pm';
140 eval { CORE::require($file) };
141 confess "Could not load class ($class) because : $@" if $@;
142 unless (does_metaclass_exist($class)) {
143 eval { Class::MOP::Class->initialize($class) };
144 confess "Could not initialize class ($class) because : $@" if $@;
146 1; # return true if it worked
149 sub is_class_loaded {
152 return 0 if ref($class) || !defined($class) || !length($class);
154 # walk the symbol table tree to avoid autovififying
155 # \*{${main::}{"Foo::"}} == \*main::Foo::
158 foreach my $part (split('::', $class)) {
159 return 0 unless exists ${$$pack}{"${part}::"};
160 $pack = \*{${$$pack}{"${part}::"}};
163 # check for $VERSION or @ISA
164 return 1 if exists ${$$pack}{VERSION}
165 && defined *{${$$pack}{VERSION}}{SCALAR};
166 return 1 if exists ${$$pack}{ISA}
167 && defined *{${$$pack}{ISA}}{ARRAY};
169 # check for any method
170 foreach ( keys %{$$pack} ) {
171 next if substr($_, -2, 2) eq '::';
172 return 1 if defined *{${$$pack}{$_}}{CODE};
180 ## ----------------------------------------------------------------------------
181 ## Setting up our environment ...
182 ## ----------------------------------------------------------------------------
183 ## Class::MOP needs to have a few things in the global perl environment so
184 ## that it can operate effectively. Those things are done here.
185 ## ----------------------------------------------------------------------------
187 # ... nothing yet actually ;)
189 ## ----------------------------------------------------------------------------
191 ## ----------------------------------------------------------------------------
192 ## The code below here is to bootstrap our MOP with itself. This is also
193 ## sometimes called "tying the knot". By doing this, we make it much easier
194 ## to extend the MOP through subclassing and such since now you can use the
195 ## MOP itself to extend itself.
197 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
198 ## ----------------------------------------------------------------------------
200 # We need to add in the meta-attributes here so that
201 # any subclass of Class::MOP::* will be able to
202 # inherit them using &construct_instance
204 ## --------------------------------------------------------
205 ## Class::MOP::Package
207 Class::MOP::Package->meta->add_attribute(
208 Class::MOP::Attribute->new('$!package' => (
210 # NOTE: we need to do this in order
211 # for the instance meta-object to
212 # not fall into meta-circular death
214 # we just alias the original method
215 # rather than re-produce it here
216 'name' => \&Class::MOP::Package::name
218 init_arg => 'package',
222 Class::MOP::Package->meta->add_attribute(
223 Class::MOP::Attribute->new('%!namespace' => (
226 # we just alias the original method
227 # rather than re-produce it here
228 'namespace' => \&Class::MOP::Package::namespace
231 default => sub { \undef }
236 # use the metaclass to construct the meta-package
237 # which is a superclass of the metaclass itself :P
238 Class::MOP::Package->meta->add_method('initialize' => sub {
240 my $package_name = shift;
241 $class->meta->new_object('package' => $package_name, @_);
244 ## --------------------------------------------------------
245 ## Class::MOP::Module
248 # yeah this is kind of stretching things a bit,
249 # but truthfully the version should be an attribute
250 # of the Module, the weirdness comes from having to
251 # stick to Perl 5 convention and store it in the
252 # $VERSION package variable. Basically if you just
253 # squint at it, it will look how you want it to look.
254 # Either as a package variable, or as a attribute of
255 # the metaclass, isn't abstraction great :)
257 Class::MOP::Module->meta->add_attribute(
258 Class::MOP::Attribute->new('$!version' => (
261 # we just alias the original method
262 # rather than re-produce it here
263 'version' => \&Class::MOP::Module::version
266 default => sub { \undef }
271 # By following the same conventions as version here,
272 # we are opening up the possibility that people can
273 # use the $AUTHORITY in non-Class::MOP modules as
276 Class::MOP::Module->meta->add_attribute(
277 Class::MOP::Attribute->new('$!authority' => (
280 # we just alias the original method
281 # rather than re-produce it here
282 'authority' => \&Class::MOP::Module::authority
285 default => sub { \undef }
289 ## --------------------------------------------------------
292 Class::MOP::Class->meta->add_attribute(
293 Class::MOP::Attribute->new('%!attributes' => (
295 # NOTE: we need to do this in order
296 # for the instance meta-object to
297 # not fall into meta-circular death
299 # we just alias the original method
300 # rather than re-produce it here
301 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
303 init_arg => 'attributes',
304 default => sub { {} }
308 Class::MOP::Class->meta->add_attribute(
309 Class::MOP::Attribute->new('%!methods' => (
310 init_arg => 'methods',
313 # we just alias the original method
314 # rather than re-produce it here
315 'get_method_map' => \&Class::MOP::Class::get_method_map
317 default => sub { {} }
321 Class::MOP::Class->meta->add_attribute(
322 Class::MOP::Attribute->new('@!superclasses' => (
325 # we just alias the original method
326 # rather than re-produce it here
327 'superclasses' => \&Class::MOP::Class::superclasses
330 default => sub { \undef }
334 Class::MOP::Class->meta->add_attribute(
335 Class::MOP::Attribute->new('$!attribute_metaclass' => (
338 # we just alias the original method
339 # rather than re-produce it here
340 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
342 init_arg => 'attribute_metaclass',
343 default => 'Class::MOP::Attribute',
347 Class::MOP::Class->meta->add_attribute(
348 Class::MOP::Attribute->new('$!method_metaclass' => (
351 # we just alias the original method
352 # rather than re-produce it here
353 'method_metaclass' => \&Class::MOP::Class::method_metaclass
355 init_arg => 'method_metaclass',
356 default => 'Class::MOP::Method',
360 Class::MOP::Class->meta->add_attribute(
361 Class::MOP::Attribute->new('$!instance_metaclass' => (
363 # NOTE: we need to do this in order
364 # for the instance meta-object to
365 # not fall into meta-circular death
367 # we just alias the original method
368 # rather than re-produce it here
369 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
371 init_arg => 'instance_metaclass',
372 default => 'Class::MOP::Instance',
377 # we don't actually need to tie the knot with
378 # Class::MOP::Class here, it is actually handled
379 # within Class::MOP::Class itself in the
380 # construct_class_instance method.
382 ## --------------------------------------------------------
383 ## Class::MOP::Attribute
385 Class::MOP::Attribute->meta->add_attribute(
386 Class::MOP::Attribute->new('$!name' => (
389 # NOTE: we need to do this in order
390 # for the instance meta-object to
391 # not fall into meta-circular death
393 # we just alias the original method
394 # rather than re-produce it here
395 'name' => \&Class::MOP::Attribute::name
400 Class::MOP::Attribute->meta->add_attribute(
401 Class::MOP::Attribute->new('$!associated_class' => (
402 init_arg => 'associated_class',
404 # NOTE: we need to do this in order
405 # for the instance meta-object to
406 # not fall into meta-circular death
408 # we just alias the original method
409 # rather than re-produce it here
410 'associated_class' => \&Class::MOP::Attribute::associated_class
415 Class::MOP::Attribute->meta->add_attribute(
416 Class::MOP::Attribute->new('$!accessor' => (
417 init_arg => 'accessor',
418 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
419 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
423 Class::MOP::Attribute->meta->add_attribute(
424 Class::MOP::Attribute->new('$!reader' => (
425 init_arg => 'reader',
426 reader => { 'reader' => \&Class::MOP::Attribute::reader },
427 predicate => { 'has_reader' => \&Class::MOP::Attribute::has_reader },
431 Class::MOP::Attribute->meta->add_attribute(
432 Class::MOP::Attribute->new('$!initializer' => (
433 init_arg => 'initializer',
434 reader => { 'initializer' => \&Class::MOP::Attribute::initializer },
435 predicate => { 'has_initializer' => \&Class::MOP::Attribute::has_initializer },
439 Class::MOP::Attribute->meta->add_attribute(
440 Class::MOP::Attribute->new('$!writer' => (
441 init_arg => 'writer',
442 reader => { 'writer' => \&Class::MOP::Attribute::writer },
443 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
447 Class::MOP::Attribute->meta->add_attribute(
448 Class::MOP::Attribute->new('$!predicate' => (
449 init_arg => 'predicate',
450 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
451 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
455 Class::MOP::Attribute->meta->add_attribute(
456 Class::MOP::Attribute->new('$!clearer' => (
457 init_arg => 'clearer',
458 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
459 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
463 Class::MOP::Attribute->meta->add_attribute(
464 Class::MOP::Attribute->new('$!builder' => (
465 init_arg => 'builder',
466 reader => { 'builder' => \&Class::MOP::Attribute::builder },
467 predicate => { 'has_builder' => \&Class::MOP::Attribute::has_builder },
471 Class::MOP::Attribute->meta->add_attribute(
472 Class::MOP::Attribute->new('$!init_arg' => (
473 init_arg => 'init_arg',
474 reader => { 'init_arg' => \&Class::MOP::Attribute::init_arg },
475 predicate => { 'has_init_arg' => \&Class::MOP::Attribute::has_init_arg },
479 Class::MOP::Attribute->meta->add_attribute(
480 Class::MOP::Attribute->new('$!default' => (
481 init_arg => 'default',
482 # default has a custom 'reader' method ...
483 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
487 Class::MOP::Attribute->meta->add_attribute(
488 Class::MOP::Attribute->new('@!associated_methods' => (
489 init_arg => 'associated_methods',
490 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
491 default => sub { [] }
495 # NOTE: (meta-circularity)
496 # This should be one of the last things done
497 # it will "tie the knot" with Class::MOP::Attribute
498 # so that it uses the attributes meta-objects
499 # to construct itself.
500 Class::MOP::Attribute->meta->add_method('new' => sub {
505 (defined $name && $name)
506 || confess "You must provide a name for the attribute";
507 $options{init_arg} = $name
508 if not exists $options{init_arg};
510 if(exists $options{builder}){
511 confess("builder must be a defined scalar value which is a method name")
512 if ref $options{builder} || !(defined $options{builder});
513 confess("Setting both default and builder is not allowed.")
514 if exists $options{default};
516 (Class::MOP::Attribute::is_default_a_coderef(\%options))
517 || confess("References are not allowed as default values, you must ".
518 "wrap the default of '$name' in a CODE reference (ex: sub { [] } and not [])")
519 if exists $options{default} && ref $options{default};
521 # return the new object
522 $class->meta->new_object(name => $name, %options);
525 Class::MOP::Attribute->meta->add_method('clone' => sub {
527 $self->meta->clone_object($self, @_);
530 ## --------------------------------------------------------
531 ## Class::MOP::Method
533 Class::MOP::Method->meta->add_attribute(
534 Class::MOP::Attribute->new('&!body' => (
536 reader => { 'body' => \&Class::MOP::Method::body },
540 Class::MOP::Method->meta->add_attribute(
541 Class::MOP::Attribute->new('$!package_name' => (
542 init_arg => 'package_name',
543 reader => { 'package_name' => \&Class::MOP::Method::package_name },
547 Class::MOP::Method->meta->add_attribute(
548 Class::MOP::Attribute->new('$!name' => (
550 reader => { 'name' => \&Class::MOP::Method::name },
554 Class::MOP::Method->meta->add_method('wrap' => sub {
559 ('CODE' eq ref($code))
560 || confess "You must supply a CODE reference to bless, not (" . ($code || 'undef') . ")";
562 ($options{package_name} && $options{name})
563 || confess "You must supply the package_name and name parameters";
565 # return the new object
566 $class->meta->new_object(body => $code, %options);
569 Class::MOP::Method->meta->add_method('clone' => sub {
571 $self->meta->clone_object($self, @_);
574 ## --------------------------------------------------------
575 ## Class::MOP::Method::Wrapped
578 # the way this item is initialized, this
579 # really does not follow the standard
580 # practices of attributes, but we put
581 # it here for completeness
582 Class::MOP::Method::Wrapped->meta->add_attribute(
583 Class::MOP::Attribute->new('%!modifier_table')
586 ## --------------------------------------------------------
587 ## Class::MOP::Method::Generated
589 Class::MOP::Method::Generated->meta->add_attribute(
590 Class::MOP::Attribute->new('$!is_inline' => (
591 init_arg => 'is_inline',
592 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
597 Class::MOP::Method::Generated->meta->add_method('new' => sub {
598 my ($class, %options) = @_;
599 ($options{package_name} && $options{name})
600 || confess "You must supply the package_name and name parameters";
601 my $self = $class->meta->new_object(%options);
602 $self->initialize_body;
606 ## --------------------------------------------------------
607 ## Class::MOP::Method::Accessor
609 Class::MOP::Method::Accessor->meta->add_attribute(
610 Class::MOP::Attribute->new('$!attribute' => (
611 init_arg => 'attribute',
613 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
618 Class::MOP::Method::Accessor->meta->add_attribute(
619 Class::MOP::Attribute->new('$!accessor_type' => (
620 init_arg => 'accessor_type',
621 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
625 Class::MOP::Method::Accessor->meta->add_method('new' => sub {
629 (exists $options{attribute})
630 || confess "You must supply an attribute to construct with";
632 (exists $options{accessor_type})
633 || confess "You must supply an accessor_type to construct with";
635 (Scalar::Util::blessed($options{attribute}) && $options{attribute}->isa('Class::MOP::Attribute'))
636 || confess "You must supply an attribute which is a 'Class::MOP::Attribute' instance";
638 ($options{package_name} && $options{name})
639 || confess "You must supply the package_name and name parameters";
641 # return the new object
642 my $self = $class->meta->new_object(%options);
644 # we don't want this creating
645 # a cycle in the code, if not
647 Scalar::Util::weaken($self->{'$!attribute'});
649 $self->initialize_body;
655 ## --------------------------------------------------------
656 ## Class::MOP::Method::Constructor
658 Class::MOP::Method::Constructor->meta->add_attribute(
659 Class::MOP::Attribute->new('%!options' => (
660 init_arg => 'options',
662 'options' => \&Class::MOP::Method::Constructor::options
664 default => sub { +{} }
668 Class::MOP::Method::Constructor->meta->add_attribute(
669 Class::MOP::Attribute->new('$!associated_metaclass' => (
670 init_arg => 'metaclass',
672 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
677 Class::MOP::Method::Constructor->meta->add_method('new' => sub {
681 (Scalar::Util::blessed $options{metaclass} && $options{metaclass}->isa('Class::MOP::Class'))
682 || confess "You must pass a metaclass instance if you want to inline"
683 if $options{is_inline};
685 ($options{package_name} && $options{name})
686 || confess "You must supply the package_name and name parameters";
688 # return the new object
689 my $self = $class->meta->new_object(%options);
691 # we don't want this creating
692 # a cycle in the code, if not
694 Scalar::Util::weaken($self->{'$!associated_metaclass'});
696 $self->initialize_body;
701 ## --------------------------------------------------------
702 ## Class::MOP::Instance
705 # these don't yet do much of anything, but are just
706 # included for completeness
708 Class::MOP::Instance->meta->add_attribute(
709 Class::MOP::Attribute->new('$!meta')
712 Class::MOP::Instance->meta->add_attribute(
713 Class::MOP::Attribute->new('@!slots')
716 ## --------------------------------------------------------
717 ## Now close all the Class::MOP::* classes
720 # we don't need to inline the
721 # constructors or the accessors
722 # this only lengthens the compile
723 # time of the MOP, and gives us
724 # no actual benefits.
726 $_->meta->make_immutable(
727 inline_constructor => 0,
728 inline_accessors => 0,
734 Class::MOP::Attribute
740 Class::MOP::Method::Generated
742 Class::MOP::Method::Accessor
743 Class::MOP::Method::Constructor
744 Class::MOP::Method::Wrapped
755 Class::MOP - A Meta Object Protocol for Perl 5
759 This module is a fully functioning meta object protocol for the
760 Perl 5 object system. It makes no attempt to change the behavior or
761 characteristics of the Perl 5 object system, only to create a
762 protocol for its manipulation and introspection.
764 That said, it does attempt to create the tools for building a rich
765 set of extensions to the Perl 5 object system. Every attempt has been
766 made for these tools to keep to the spirit of the Perl 5 object
767 system that we all know and love.
769 This documentation is admittedly sparse on details, as time permits
770 I will try to improve them. For now, I suggest looking at the items
771 listed in the L<SEE ALSO> section for more information. In particular
772 the book "The Art of the Meta Object Protocol" was very influential
773 in the development of this system.
775 =head2 What is a Meta Object Protocol?
777 A meta object protocol is an API to an object system.
779 To be more specific, it is a set of abstractions of the components of
780 an object system (typically things like; classes, object, methods,
781 object attributes, etc.). These abstractions can then be used to both
782 inspect and manipulate the object system which they describe.
784 It can be said that there are two MOPs for any object system; the
785 implicit MOP, and the explicit MOP. The implicit MOP handles things
786 like method dispatch or inheritance, which happen automatically as
787 part of how the object system works. The explicit MOP typically
788 handles the introspection/reflection features of the object system.
789 All object systems have implicit MOPs, without one, they would not
790 work. Explict MOPs however as less common, and depending on the
791 language can vary from restrictive (Reflection in Java or C#) to
792 wide open (CLOS is a perfect example).
794 =head2 Yet Another Class Builder!! Why?
796 This is B<not> a class builder so much as it is a I<class builder
797 B<builder>>. My intent is that an end user does not use this module
798 directly, but instead this module is used by module authors to
799 build extensions and features onto the Perl 5 object system.
801 =head2 Who is this module for?
803 This module is specifically for anyone who has ever created or
804 wanted to create a module for the Class:: namespace. The tools which
805 this module will provide will hopefully make it easier to do more
806 complex things with Perl 5 classes by removing such barriers as
807 the need to hack the symbol tables, or understand the fine details
810 =head2 What changes do I have to make to use this module?
812 This module was designed to be as unintrusive as possible. Many of
813 its features are accessible without B<any> change to your existsing
814 code at all. It is meant to be a compliment to your existing code and
815 not an intrusion on your code base. Unlike many other B<Class::>
816 modules, this module B<does not> require you subclass it, or even that
817 you C<use> it in within your module's package.
819 The only features which requires additions to your code are the
820 attribute handling and instance construction features, and these are
821 both completely optional features. The only reason for this is because
822 Perl 5's object system does not actually have these features built
823 in. More information about this feature can be found below.
825 =head2 A Note about Performance?
827 It is a common misconception that explict MOPs are performance drains.
828 But this is not a universal truth at all, it is an side-effect of
829 specific implementations. For instance, using Java reflection is much
830 slower because the JVM cannot take advantage of any compiler
831 optimizations, and the JVM has to deal with much more runtime type
832 information as well. Reflection in C# is marginally better as it was
833 designed into the language and runtime (the CLR). In contrast, CLOS
834 (the Common Lisp Object System) was built to support an explicit MOP,
835 and so performance is tuned for it.
837 This library in particular does it's absolute best to avoid putting
838 B<any> drain at all upon your code's performance. In fact, by itself
839 it does nothing to affect your existing code. So you only pay for
840 what you actually use.
842 =head2 About Metaclass compatibility
844 This module makes sure that all metaclasses created are both upwards
845 and downwards compatible. The topic of metaclass compatibility is
846 highly esoteric and is something only encountered when doing deep and
847 involved metaclass hacking. There are two basic kinds of metaclass
848 incompatibility; upwards and downwards.
850 Upwards metaclass compatibility means that the metaclass of a
851 given class is either the same as (or a subclass of) all of the
854 Downward metaclass compatibility means that the metaclasses of a
855 given class's anscestors are all either the same as (or a subclass
858 Here is a diagram showing a set of two classes (C<A> and C<B>) and
859 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
860 metaclass compatibility both upwards and downwards.
862 +---------+ +---------+
863 | Meta::A |<----| Meta::B | <....... (instance of )
864 +---------+ +---------+ <------- (inherits from)
867 +---------+ +---------+
869 +---------+ +---------+
871 As I said this is a highly esoteric topic and one you will only run
872 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
873 are interested in why this is an issue see the paper
874 I<Uniform and safe metaclass composition> linked to in the
875 L<SEE ALSO> section of this document.
877 =head2 Using custom metaclasses
879 Always use the metaclass pragma when using a custom metaclass, this
880 will ensure the proper initialization order and not accidentely
881 create an incorrect type of metaclass for you. This is a very rare
882 problem, and one which can only occur if you are doing deep metaclass
883 programming. So in other words, don't worry about it.
887 The protocol is divided into 4 main sub-protocols:
891 =item The Class protocol
893 This provides a means of manipulating and introspecting a Perl 5
894 class. It handles all of symbol table hacking for you, and provides
895 a rich set of methods that go beyond simple package introspection.
897 See L<Class::MOP::Class> for more details.
899 =item The Attribute protocol
901 This provides a consistent represenation for an attribute of a
902 Perl 5 class. Since there are so many ways to create and handle
903 attributes in Perl 5 OO, this attempts to provide as much of a
904 unified approach as possible, while giving the freedom and
905 flexibility to subclass for specialization.
907 See L<Class::MOP::Attribute> for more details.
909 =item The Method protocol
911 This provides a means of manipulating and introspecting methods in
912 the Perl 5 object system. As with attributes, there are many ways to
913 approach this topic, so we try to keep it pretty basic, while still
914 making it possible to extend the system in many ways.
916 See L<Class::MOP::Method> for more details.
918 =item The Instance protocol
920 This provides a layer of abstraction for creating object instances.
921 Since the other layers use this protocol, it is relatively easy to
922 change the type of your instances from the default HASH ref to other
923 types of references. Several examples are provided in the F<examples/>
924 directory included in this distribution.
926 See L<Class::MOP::Instance> for more details.
936 =item I<IS_RUNNING_ON_5_10>
938 We set this constant depending on what version perl we are on, this
939 allows us to take advantage of new 5.10 features and stay backwards
944 =head2 Utility functions
948 =item B<load_class ($class_name)>
950 This will load a given C<$class_name> and if it does not have an
951 already initialized metaclass, then it will intialize one for it.
952 This function can be used in place of tricks like
953 C<eval "use $module"> or using C<require>.
955 =item B<is_class_loaded ($class_name)>
957 This will return a boolean depending on if the C<$class_name> has
960 NOTE: This does a basic check of the symbol table to try and
961 determine as best it can if the C<$class_name> is loaded, it
962 is probably correct about 99% of the time.
964 =item B<check_package_cache_flag ($pkg)>
966 This will return an integer that is managed by C<Class::MOP::Class>
967 to determine if a module's symbol table has been altered.
969 In Perl 5.10 or greater, this flag is package specific. However in
970 versions prior to 5.10, this will use the C<PL_sub_generation> variable
971 which is not package specific.
973 =item B<get_code_info ($code)>
975 This function returns two values, the name of the package the C<$code>
976 is from and the name of the C<$code> itself. This is used by several
977 elements of the MOP to detemine where a given C<$code> reference is from.
979 =item B<subname ($name, $code)>
981 B<NOTE: DO NOT USE THIS FUNCTION, IT IS FOR INTERNAL USE ONLY!>
983 If possible, we will load the L<Sub::Name> module and this will function
984 as C<Sub::Name::subname> does, otherwise it will just return the C<$code>
989 =head2 Metaclass cache functions
991 Class::MOP holds a cache of metaclasses, the following are functions
992 (B<not methods>) which can be used to access that cache. It is not
993 recommended that you mess with this, bad things could happen. But if
994 you are brave and willing to risk it, go for it.
998 =item B<get_all_metaclasses>
1000 This will return an hash of all the metaclass instances that have
1001 been cached by B<Class::MOP::Class> keyed by the package name.
1003 =item B<get_all_metaclass_instances>
1005 This will return an array of all the metaclass instances that have
1006 been cached by B<Class::MOP::Class>.
1008 =item B<get_all_metaclass_names>
1010 This will return an array of all the metaclass names that have
1011 been cached by B<Class::MOP::Class>.
1013 =item B<get_metaclass_by_name ($name)>
1015 This will return a cached B<Class::MOP::Class> instance of nothing
1016 if no metaclass exist by that C<$name>.
1018 =item B<store_metaclass_by_name ($name, $meta)>
1020 This will store a metaclass in the cache at the supplied C<$key>.
1022 =item B<weaken_metaclass ($name)>
1024 In rare cases it is desireable to store a weakened reference in
1025 the metaclass cache. This function will weaken the reference to
1026 the metaclass stored in C<$name>.
1028 =item B<does_metaclass_exist ($name)>
1030 This will return true of there exists a metaclass stored in the
1031 C<$name> key and return false otherwise.
1033 =item B<remove_metaclass_by_name ($name)>
1035 This will remove a the metaclass stored in the C<$name> key.
1043 There are very few books out on Meta Object Protocols and Metaclasses
1044 because it is such an esoteric topic. The following books are really
1045 the only ones I have found. If you know of any more, B<I<please>>
1046 email me and let me know, I would love to hear about them.
1050 =item "The Art of the Meta Object Protocol"
1052 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
1054 =item "Putting MetaClasses to Work"
1056 =item "Smalltalk: The Language"
1064 =item Uniform and safe metaclass composition
1066 An excellent paper by the people who brought us the original Traits paper.
1067 This paper is on how Traits can be used to do safe metaclass composition,
1068 and offers an excellent introduction section which delves into the topic of
1069 metaclass compatibility.
1071 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
1073 =item Safe Metaclass Programming
1075 This paper seems to precede the above paper, and propose a mix-in based
1076 approach as opposed to the Traits based approach. Both papers have similar
1077 information on the metaclass compatibility problem space.
1079 L<http://citeseer.ist.psu.edu/37617.html>
1087 =item The Perl 6 MetaModel work in the Pugs project
1091 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
1093 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
1103 =item CPAN Module Review of Class::MOP
1105 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
1109 =head1 SIMILAR MODULES
1111 As I have said above, this module is a class-builder-builder, so it is
1112 not the same thing as modules like L<Class::Accessor> and
1113 L<Class::MethodMaker>. That being said there are very few modules on CPAN
1114 with similar goals to this module. The one I have found which is most
1115 like this module is L<Class::Meta>, although it's philosophy and the MOP it
1116 creates are very different from this modules.
1120 All complex software has bugs lurking in it, and this module is no
1121 exception. If you find a bug please either email me, or add the bug
1124 =head1 ACKNOWLEDGEMENTS
1130 Thanks to Rob for actually getting the development of this module kick-started.
1136 Stevan Little E<lt>stevan@iinteractive.comE<gt>
1138 B<with contributions from:>
1140 Brandon (blblack) Black
1142 Guillermo (groditi) Roditi
1146 Rob (robkinyon) Kinyon
1148 Yuval (nothingmuch) Kogman
1150 Scott (konobi) McWhirter
1152 =head1 COPYRIGHT AND LICENSE
1154 Copyright 2006-2008 by Infinity Interactive, Inc.
1156 L<http://www.iinteractive.com>
1158 This library is free software; you can redistribute it and/or modify
1159 it under the same terms as Perl itself.