8 use Scalar::Util 'weaken';
10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 use Class::MOP::Immutable;
16 our $VERSION = '0.36';
17 our $AUTHORITY = 'cpan:STEVAN';
20 # Metaclasses are singletons, so we cache them here.
21 # there is no need to worry about destruction though
22 # because they should die only when the program dies.
23 # After all, do package definitions even get reaped?
26 # means of accessing all the metaclasses that have
27 # been initialized thus far (for mugwumps obj browser)
28 sub get_all_metaclasses { %METAS }
29 sub get_all_metaclass_instances { values %METAS }
30 sub get_all_metaclass_names { keys %METAS }
31 sub get_metaclass_by_name { $METAS{$_[0]} }
32 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
33 sub weaken_metaclass { weaken($METAS{$_[0]}) }
34 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
35 sub remove_metaclass_by_name { $METAS{$_[0]} = undef }
38 # We only cache metaclasses, meaning instances of
39 # Class::MOP::Class. We do not cache instance of
40 # Class::MOP::Package or Class::MOP::Module. Mostly
41 # because I don't yet see a good reason to do so.
44 ## ----------------------------------------------------------------------------
45 ## Setting up our environment ...
46 ## ----------------------------------------------------------------------------
47 ## Class::MOP needs to have a few things in the global perl environment so
48 ## that it can operate effectively. Those things are done here.
49 ## ----------------------------------------------------------------------------
51 # ... nothing yet actually ;)
53 ## ----------------------------------------------------------------------------
55 ## ----------------------------------------------------------------------------
56 ## The code below here is to bootstrap our MOP with itself. This is also
57 ## sometimes called "tying the knot". By doing this, we make it much easier
58 ## to extend the MOP through subclassing and such since now you can use the
59 ## MOP itself to extend itself.
61 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
62 ## ----------------------------------------------------------------------------
64 # We need to add in the meta-attributes here so that
65 # any subclass of Class::MOP::* will be able to
66 # inherit them using &construct_instance
68 ## --------------------------------------------------------
69 ## Class::MOP::Package
71 Class::MOP::Package->meta->add_attribute(
72 Class::MOP::Attribute->new('$!package' => (
74 # NOTE: we need to do this in order
75 # for the instance meta-object to
76 # not fall into meta-circular death
78 # we just alias the original method
79 # rather than re-produce it here
80 'name' => \&Class::MOP::Package::name
82 init_arg => 'package',
86 Class::MOP::Package->meta->add_attribute(
87 Class::MOP::Attribute->new('%!namespace' => (
90 # we just alias the original method
91 # rather than re-produce it here
92 'namespace' => \&Class::MOP::Package::namespace
95 # protect this from silliness
96 init_arg => '!............( DO NOT DO THIS )............!',
97 default => sub { \undef }
102 # use the metaclass to construct the meta-package
103 # which is a superclass of the metaclass itself :P
104 Class::MOP::Package->meta->add_method('initialize' => sub {
106 my $package_name = shift;
107 $class->meta->new_object('package' => $package_name, @_);
110 ## --------------------------------------------------------
111 ## Class::MOP::Module
114 # yeah this is kind of stretching things a bit,
115 # but truthfully the version should be an attribute
116 # of the Module, the weirdness comes from having to
117 # stick to Perl 5 convention and store it in the
118 # $VERSION package variable. Basically if you just
119 # squint at it, it will look how you want it to look.
120 # Either as a package variable, or as a attribute of
121 # the metaclass, isn't abstraction great :)
123 Class::MOP::Module->meta->add_attribute(
124 Class::MOP::Attribute->new('$!version' => (
127 # we just alias the original method
128 # rather than re-produce it here
129 'version' => \&Class::MOP::Module::version
132 # protect this from silliness
133 init_arg => '!............( DO NOT DO THIS )............!',
134 default => sub { \undef }
139 # By following the same conventions as version here,
140 # we are opening up the possibility that people can
141 # use the $AUTHORITY in non-Class::MOP modules as
144 Class::MOP::Module->meta->add_attribute(
145 Class::MOP::Attribute->new('$!authority' => (
148 # we just alias the original method
149 # rather than re-produce it here
150 'authority' => \&Class::MOP::Module::authority
153 # protect this from silliness
154 init_arg => '!............( DO NOT DO THIS )............!',
155 default => sub { \undef }
159 ## --------------------------------------------------------
162 Class::MOP::Class->meta->add_attribute(
163 Class::MOP::Attribute->new('%!attributes' => (
165 # NOTE: we need to do this in order
166 # for the instance meta-object to
167 # not fall into meta-circular death
169 # we just alias the original method
170 # rather than re-produce it here
171 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
173 init_arg => 'attributes',
174 default => sub { {} }
178 Class::MOP::Class->meta->add_attribute(
179 Class::MOP::Attribute->new('%!methods' => (
180 init_arg => 'methods',
183 # we just alias the original method
184 # rather than re-produce it here
185 'get_method_map' => \&Class::MOP::Class::get_method_map
187 default => sub { {} }
191 Class::MOP::Class->meta->add_attribute(
192 Class::MOP::Attribute->new('@!superclasses' => (
195 # we just alias the original method
196 # rather than re-produce it here
197 'superclasses' => \&Class::MOP::Class::superclasses
200 # protect this from silliness
201 init_arg => '!............( DO NOT DO THIS )............!',
202 default => sub { \undef }
206 Class::MOP::Class->meta->add_attribute(
207 Class::MOP::Attribute->new('$!attribute_metaclass' => (
210 # we just alias the original method
211 # rather than re-produce it here
212 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
214 init_arg => 'attribute_metaclass',
215 default => 'Class::MOP::Attribute',
219 Class::MOP::Class->meta->add_attribute(
220 Class::MOP::Attribute->new('$!method_metaclass' => (
223 # we just alias the original method
224 # rather than re-produce it here
225 'method_metaclass' => \&Class::MOP::Class::method_metaclass
227 init_arg => 'method_metaclass',
228 default => 'Class::MOP::Method',
232 Class::MOP::Class->meta->add_attribute(
233 Class::MOP::Attribute->new('$!instance_metaclass' => (
235 # NOTE: we need to do this in order
236 # for the instance meta-object to
237 # not fall into meta-circular death
239 # we just alias the original method
240 # rather than re-produce it here
241 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
243 init_arg => 'instance_metaclass',
244 default => 'Class::MOP::Instance',
249 # we don't actually need to tie the knot with
250 # Class::MOP::Class here, it is actually handled
251 # within Class::MOP::Class itself in the
252 # construct_class_instance method.
254 ## --------------------------------------------------------
255 ## Class::MOP::Attribute
257 Class::MOP::Attribute->meta->add_attribute(
258 Class::MOP::Attribute->new('$!name' => (
261 # NOTE: we need to do this in order
262 # for the instance meta-object to
263 # not fall into meta-circular death
265 # we just alias the original method
266 # rather than re-produce it here
267 'name' => \&Class::MOP::Attribute::name
272 Class::MOP::Attribute->meta->add_attribute(
273 Class::MOP::Attribute->new('$!associated_class' => (
274 init_arg => 'associated_class',
276 # NOTE: we need to do this in order
277 # for the instance meta-object to
278 # not fall into meta-circular death
280 # we just alias the original method
281 # rather than re-produce it here
282 'associated_class' => \&Class::MOP::Attribute::associated_class
287 Class::MOP::Attribute->meta->add_attribute(
288 Class::MOP::Attribute->new('$!accessor' => (
289 init_arg => 'accessor',
290 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
291 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
295 Class::MOP::Attribute->meta->add_attribute(
296 Class::MOP::Attribute->new('$!reader' => (
297 init_arg => 'reader',
298 reader => { 'reader' => \&Class::MOP::Attribute::reader },
299 predicate => { 'has_reader' => \&Class::MOP::Attribute::has_reader },
303 Class::MOP::Attribute->meta->add_attribute(
304 Class::MOP::Attribute->new('$!writer' => (
305 init_arg => 'writer',
306 reader => { 'writer' => \&Class::MOP::Attribute::writer },
307 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
311 Class::MOP::Attribute->meta->add_attribute(
312 Class::MOP::Attribute->new('$!predicate' => (
313 init_arg => 'predicate',
314 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
315 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
319 Class::MOP::Attribute->meta->add_attribute(
320 Class::MOP::Attribute->new('$!clearer' => (
321 init_arg => 'clearer',
322 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
323 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
327 Class::MOP::Attribute->meta->add_attribute(
328 Class::MOP::Attribute->new('$!init_arg' => (
329 init_arg => 'init_arg',
330 reader => { 'init_arg' => \&Class::MOP::Attribute::init_arg },
331 predicate => { 'has_init_arg' => \&Class::MOP::Attribute::has_init_arg },
335 Class::MOP::Attribute->meta->add_attribute(
336 Class::MOP::Attribute->new('$!default' => (
337 init_arg => 'default',
338 # default has a custom 'reader' method ...
339 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
343 Class::MOP::Attribute->meta->add_attribute(
344 Class::MOP::Attribute->new('@!associated_methods' => (
345 init_arg => 'associated_methods',
346 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
347 default => sub { [] }
351 # NOTE: (meta-circularity)
352 # This should be one of the last things done
353 # it will "tie the knot" with Class::MOP::Attribute
354 # so that it uses the attributes meta-objects
355 # to construct itself.
356 Class::MOP::Attribute->meta->add_method('new' => sub {
361 (defined $name && $name)
362 || confess "You must provide a name for the attribute";
363 $options{init_arg} = $name
364 if not exists $options{init_arg};
366 (Class::MOP::Attribute::is_default_a_coderef(\%options))
367 || confess("References are not allowed as default values, you must ".
368 "wrap then in a CODE reference (ex: sub { [] } and not [])")
369 if exists $options{default} && ref $options{default};
371 # return the new object
372 $class->meta->new_object(name => $name, %options);
375 Class::MOP::Attribute->meta->add_method('clone' => sub {
377 $self->meta->clone_object($self, @_);
380 ## --------------------------------------------------------
381 ## Class::MOP::Method
383 Class::MOP::Method->meta->add_attribute(
384 Class::MOP::Attribute->new('&!body' => (
386 reader => { 'body' => \&Class::MOP::Method::body },
390 ## --------------------------------------------------------
391 ## Class::MOP::Method::Wrapped
394 # the way this item is initialized, this
395 # really does not follow the standard
396 # practices of attributes, but we put
397 # it here for completeness
398 Class::MOP::Method::Wrapped->meta->add_attribute(
399 Class::MOP::Attribute->new('%!modifier_table')
402 ## --------------------------------------------------------
403 ## Class::MOP::Method::Accessor
405 Class::MOP::Method::Accessor->meta->add_attribute(
406 Class::MOP::Attribute->new('$!attribute' => (
407 init_arg => 'attribute',
409 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
414 Class::MOP::Method::Accessor->meta->add_attribute(
415 Class::MOP::Attribute->new('$!accessor_type' => (
416 init_arg => 'accessor_type',
417 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
421 Class::MOP::Method::Accessor->meta->add_attribute(
422 Class::MOP::Attribute->new('$!is_inline' => (
423 init_arg => 'is_inline',
424 reader => { 'is_inline' => \&Class::MOP::Method::Accessor::is_inline },
428 ## --------------------------------------------------------
429 ## Class::MOP::Method::Constructor
431 Class::MOP::Method::Constructor->meta->add_attribute(
432 Class::MOP::Attribute->new('%!options' => (
433 init_arg => 'options',
435 'options' => \&Class::MOP::Method::Constructor::options
440 Class::MOP::Method::Constructor->meta->add_attribute(
441 Class::MOP::Attribute->new('$!metaclass' => (
442 init_arg => 'metaclass',
446 ## --------------------------------------------------------
447 ## Class::MOP::Instance
450 # these don't yet do much of anything, but are just
451 # included for completeness
453 Class::MOP::Instance->meta->add_attribute(
454 Class::MOP::Attribute->new('$!meta')
457 Class::MOP::Instance->meta->add_attribute(
458 Class::MOP::Attribute->new('@!slots')
461 ## --------------------------------------------------------
462 ## Now close all the Class::MOP::* classes
465 # we don't need to inline the
466 # constructors or the accessors
467 # this only lengthens the compile
468 # time of the MOP, and gives us
469 # no actual benefits.
471 $_->meta->make_immutable(
472 inline_constructor => 0,
473 inline_accessors => 0,
479 Class::MOP::Attribute
485 Class::MOP::Method::Accessor
486 Class::MOP::Method::Constructor
487 Class::MOP::Method::Wrapped
498 Class::MOP - A Meta Object Protocol for Perl 5
502 # ... This will come later, for now see
503 # the other SYNOPSIS for more information
507 This module is an attempt to create a meta object protocol for the
508 Perl 5 object system. It makes no attempt to change the behavior or
509 characteristics of the Perl 5 object system, only to create a
510 protocol for its manipulation and introspection.
512 That said, it does attempt to create the tools for building a rich
513 set of extensions to the Perl 5 object system. Every attempt has been
514 made for these tools to keep to the spirit of the Perl 5 object
515 system that we all know and love.
517 This documentation is admittedly sparse on details, as time permits
518 I will try to improve them. For now, I suggest looking at the items
519 listed in the L<SEE ALSO> section for more information. In particular
520 the book "The Art of the Meta Object Protocol" was very influential
521 in the development of this system.
523 =head2 What is a Meta Object Protocol?
525 A meta object protocol is an API to an object system.
527 To be more specific, it is a set of abstractions of the components of
528 an object system (typically things like; classes, object, methods,
529 object attributes, etc.). These abstractions can then be used to both
530 inspect and manipulate the object system which they describe.
532 It can be said that there are two MOPs for any object system; the
533 implicit MOP, and the explicit MOP. The implicit MOP handles things
534 like method dispatch or inheritance, which happen automatically as
535 part of how the object system works. The explicit MOP typically
536 handles the introspection/reflection features of the object system.
537 All object systems have implicit MOPs, without one, they would not
538 work. Explict MOPs however as less common, and depending on the
539 language can vary from restrictive (Reflection in Java or C#) to
540 wide open (CLOS is a perfect example).
542 =head2 Yet Another Class Builder!! Why?
544 This is B<not> a class builder so much as it is a I<class builder
545 B<builder>>. My intent is that an end user does not use this module
546 directly, but instead this module is used by module authors to
547 build extensions and features onto the Perl 5 object system.
549 =head2 Who is this module for?
551 This module is specifically for anyone who has ever created or
552 wanted to create a module for the Class:: namespace. The tools which
553 this module will provide will hopefully make it easier to do more
554 complex things with Perl 5 classes by removing such barriers as
555 the need to hack the symbol tables, or understand the fine details
558 =head2 What changes do I have to make to use this module?
560 This module was designed to be as unintrusive as possible. Many of
561 its features are accessible without B<any> change to your existsing
562 code at all. It is meant to be a compliment to your existing code and
563 not an intrusion on your code base. Unlike many other B<Class::>
564 modules, this module B<does not> require you subclass it, or even that
565 you C<use> it in within your module's package.
567 The only features which requires additions to your code are the
568 attribute handling and instance construction features, and these are
569 both completely optional features. The only reason for this is because
570 Perl 5's object system does not actually have these features built
571 in. More information about this feature can be found below.
573 =head2 A Note about Performance?
575 It is a common misconception that explict MOPs are performance drains.
576 But this is not a universal truth at all, it is an side-effect of
577 specific implementations. For instance, using Java reflection is much
578 slower because the JVM cannot take advantage of any compiler
579 optimizations, and the JVM has to deal with much more runtime type
580 information as well. Reflection in C# is marginally better as it was
581 designed into the language and runtime (the CLR). In contrast, CLOS
582 (the Common Lisp Object System) was built to support an explicit MOP,
583 and so performance is tuned for it.
585 This library in particular does it's absolute best to avoid putting
586 B<any> drain at all upon your code's performance. In fact, by itself
587 it does nothing to affect your existing code. So you only pay for
588 what you actually use.
590 =head2 About Metaclass compatibility
592 This module makes sure that all metaclasses created are both upwards
593 and downwards compatible. The topic of metaclass compatibility is
594 highly esoteric and is something only encountered when doing deep and
595 involved metaclass hacking. There are two basic kinds of metaclass
596 incompatibility; upwards and downwards.
598 Upwards metaclass compatibility means that the metaclass of a
599 given class is either the same as (or a subclass of) all of the
602 Downward metaclass compatibility means that the metaclasses of a
603 given class's anscestors are all either the same as (or a subclass
606 Here is a diagram showing a set of two classes (C<A> and C<B>) and
607 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
608 metaclass compatibility both upwards and downwards.
610 +---------+ +---------+
611 | Meta::A |<----| Meta::B | <....... (instance of )
612 +---------+ +---------+ <------- (inherits from)
615 +---------+ +---------+
617 +---------+ +---------+
619 As I said this is a highly esoteric topic and one you will only run
620 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
621 are interested in why this is an issue see the paper
622 I<Uniform and safe metaclass composition> linked to in the
623 L<SEE ALSO> section of this document.
625 =head2 Using custom metaclasses
627 Always use the metaclass pragma when using a custom metaclass, this
628 will ensure the proper initialization order and not accidentely
629 create an incorrect type of metaclass for you. This is a very rare
630 problem, and one which can only occur if you are doing deep metaclass
631 programming. So in other words, don't worry about it.
635 The protocol is divided into 3 main sub-protocols:
639 =item The Class protocol
641 This provides a means of manipulating and introspecting a Perl 5
642 class. It handles all of symbol table hacking for you, and provides
643 a rich set of methods that go beyond simple package introspection.
645 See L<Class::MOP::Class> for more details.
647 =item The Attribute protocol
649 This provides a consistent represenation for an attribute of a
650 Perl 5 class. Since there are so many ways to create and handle
651 atttributes in Perl 5 OO, this attempts to provide as much of a
652 unified approach as possible, while giving the freedom and
653 flexibility to subclass for specialization.
655 See L<Class::MOP::Attribute> for more details.
657 =item The Method protocol
659 This provides a means of manipulating and introspecting methods in
660 the Perl 5 object system. As with attributes, there are many ways to
661 approach this topic, so we try to keep it pretty basic, while still
662 making it possible to extend the system in many ways.
664 See L<Class::MOP::Method> for more details.
670 Class::MOP holds a cache of metaclasses, the following are functions
671 (B<not methods>) which can be used to access that cache. It is not
672 recommended that you mess with this, bad things could happen. But if
673 you are brave and willing to risk it, go for it.
677 =item B<get_all_metaclasses>
679 This will return an hash of all the metaclass instances that have
680 been cached by B<Class::MOP::Class> keyed by the package name.
682 =item B<get_all_metaclass_instances>
684 This will return an array of all the metaclass instances that have
685 been cached by B<Class::MOP::Class>.
687 =item B<get_all_metaclass_names>
689 This will return an array of all the metaclass names that have
690 been cached by B<Class::MOP::Class>.
692 =item B<get_metaclass_by_name ($name)>
694 =item B<store_metaclass_by_name ($name, $meta)>
696 =item B<weaken_metaclass ($name)>
698 =item B<does_metaclass_exist ($name)>
700 =item B<remove_metaclass_by_name ($name)>
708 There are very few books out on Meta Object Protocols and Metaclasses
709 because it is such an esoteric topic. The following books are really
710 the only ones I have found. If you know of any more, B<I<please>>
711 email me and let me know, I would love to hear about them.
715 =item "The Art of the Meta Object Protocol"
717 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
719 =item "Putting MetaClasses to Work"
721 =item "Smalltalk: The Language"
729 =item Uniform and safe metaclass composition
731 An excellent paper by the people who brought us the original Traits paper.
732 This paper is on how Traits can be used to do safe metaclass composition,
733 and offers an excellent introduction section which delves into the topic of
734 metaclass compatibility.
736 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
738 =item Safe Metaclass Programming
740 This paper seems to precede the above paper, and propose a mix-in based
741 approach as opposed to the Traits based approach. Both papers have similar
742 information on the metaclass compatibility problem space.
744 L<http://citeseer.ist.psu.edu/37617.html>
752 =item The Perl 6 MetaModel work in the Pugs project
756 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
758 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
768 =item CPAN Module Review of Class::MOP
770 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
774 =head1 SIMILAR MODULES
776 As I have said above, this module is a class-builder-builder, so it is
777 not the same thing as modules like L<Class::Accessor> and
778 L<Class::MethodMaker>. That being said there are very few modules on CPAN
779 with similar goals to this module. The one I have found which is most
780 like this module is L<Class::Meta>, although it's philosophy and the MOP it
781 creates are very different from this modules.
785 All complex software has bugs lurking in it, and this module is no
786 exception. If you find a bug please either email me, or add the bug
791 I use L<Devel::Cover> to test the code coverage of my tests, below is the
792 L<Devel::Cover> report on this module's test suite.
794 ---------------------------- ------ ------ ------ ------ ------ ------ ------
795 File stmt bran cond sub pod time total
796 ---------------------------- ------ ------ ------ ------ ------ ------ ------
797 Class/MOP.pm 97.7 100.0 88.9 94.7 100.0 3.2 96.6
798 Class/MOP/Attribute.pm 75.5 77.9 82.4 88.3 100.0 4.0 81.5
799 Class/MOP/Class.pm 96.9 88.8 72.1 98.2 100.0 35.8 91.4
800 Class/MOP/Class/Immutable.pm 88.2 60.0 n/a 95.5 100.0 0.5 84.6
801 Class/MOP/Instance.pm 86.4 75.0 33.3 86.2 100.0 1.2 87.5
802 Class/MOP/Method.pm 97.5 75.0 61.5 80.6 100.0 12.7 89.7
803 Class/MOP/Module.pm 100.0 n/a 55.6 100.0 100.0 0.1 90.7
804 Class/MOP/Object.pm 73.3 n/a 20.0 80.0 100.0 0.1 66.7
805 Class/MOP/Package.pm 94.6 71.7 33.3 100.0 100.0 42.2 87.0
806 metaclass.pm 100.0 100.0 83.3 100.0 n/a 0.2 97.7
807 ---------------------------- ------ ------ ------ ------ ------ ------ ------
808 Total 91.3 80.4 69.8 91.9 100.0 100.0 88.1
809 ---------------------------- ------ ------ ------ ------ ------ ------ ------
811 =head1 ACKNOWLEDGEMENTS
817 Thanks to Rob for actually getting the development of this module kick-started.
823 Stevan Little E<lt>stevan@iinteractive.comE<gt>
825 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
827 =head1 COPYRIGHT AND LICENSE
829 Copyright 2006 by Infinity Interactive, Inc.
831 L<http://www.iinteractive.com>
833 This library is free software; you can redistribute it and/or modify
834 it under the same terms as Perl itself.