8 use Scalar::Util 'weaken';
10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 use Class::MOP::Class::Immutable;
16 our $VERSION = '0.33';
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('$:package' => (
74 # NOTE: we need to do this in order
75 # for the instance meta-object to
76 # not fall into meta-circular death
77 'name' => sub { (shift)->{'$:package'} }
79 init_arg => ':package',
83 Class::MOP::Package->meta->add_attribute(
84 Class::MOP::Attribute->new('%:namespace' => (
86 'namespace' => sub { (shift)->{'%:namespace'} }
91 return \%{$class->name . '::'};
94 # protect this from silliness
95 init_arg => '!............( DO NOT DO THIS )............!',
100 # use the metaclass to construct the meta-package
101 # which is a superclass of the metaclass itself :P
102 Class::MOP::Package->meta->add_method('initialize' => sub {
104 my $package_name = shift;
105 $class->meta->new_object(':package' => $package_name, @_);
108 ## --------------------------------------------------------
109 ## Class::MOP::Module
112 # yeah this is kind of stretching things a bit,
113 # but truthfully the version should be an attribute
114 # of the Module, the weirdness comes from having to
115 # stick to Perl 5 convention and store it in the
116 # $VERSION package variable. Basically if you just
117 # squint at it, it will look how you want it to look.
118 # Either as a package variable, or as a attribute of
119 # the metaclass, isn't abstraction great :)
121 Class::MOP::Module->meta->add_attribute(
122 Class::MOP::Attribute->new('$:version' => (
126 ${$self->get_package_symbol('$VERSION')};
130 # protect this from silliness
131 init_arg => '!............( DO NOT DO THIS )............!',
136 # By following the same conventions as version here,
137 # we are opening up the possibility that people can
138 # use the $AUTHORITY in non-Class::MOP modules as
141 Class::MOP::Module->meta->add_attribute(
142 Class::MOP::Attribute->new('$:authority' => (
146 ${$self->get_package_symbol('$AUTHORITY')};
150 # protect this from silliness
151 init_arg => '!............( DO NOT DO THIS )............!',
155 ## --------------------------------------------------------
158 Class::MOP::Class->meta->add_attribute(
159 Class::MOP::Attribute->new('%:attributes' => (
161 # NOTE: we need to do this in order
162 # for the instance meta-object to
163 # not fall into meta-circular death
164 'get_attribute_map' => sub { (shift)->{'%:attributes'} }
166 init_arg => ':attributes',
167 default => sub { {} }
171 Class::MOP::Class->meta->add_attribute(
172 Class::MOP::Attribute->new('$:attribute_metaclass' => (
173 reader => 'attribute_metaclass',
174 init_arg => ':attribute_metaclass',
175 default => 'Class::MOP::Attribute',
179 Class::MOP::Class->meta->add_attribute(
180 Class::MOP::Attribute->new('$:method_metaclass' => (
181 reader => 'method_metaclass',
182 init_arg => ':method_metaclass',
183 default => 'Class::MOP::Method',
187 Class::MOP::Class->meta->add_attribute(
188 Class::MOP::Attribute->new('$:instance_metaclass' => (
190 # NOTE: we need to do this in order
191 # for the instance meta-object to
192 # not fall into meta-circular death
193 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
195 init_arg => ':instance_metaclass',
196 default => 'Class::MOP::Instance',
201 # we don't actually need to tie the knot with
202 # Class::MOP::Class here, it is actually handled
203 # within Class::MOP::Class itself in the
204 # construct_class_instance method.
206 ## --------------------------------------------------------
207 ## Class::MOP::Attribute
209 Class::MOP::Attribute->meta->add_attribute(
210 Class::MOP::Attribute->new('name' => (
212 # NOTE: we need to do this in order
213 # for the instance meta-object to
214 # not fall into meta-circular death
215 'name' => sub { (shift)->{name} }
220 Class::MOP::Attribute->meta->add_attribute(
221 Class::MOP::Attribute->new('associated_class' => (
223 # NOTE: we need to do this in order
224 # for the instance meta-object to
225 # not fall into meta-circular death
226 'associated_class' => sub { (shift)->{associated_class} }
231 Class::MOP::Attribute->meta->add_attribute(
232 Class::MOP::Attribute->new('accessor' => (
233 reader => 'accessor',
234 predicate => 'has_accessor',
238 Class::MOP::Attribute->meta->add_attribute(
239 Class::MOP::Attribute->new('reader' => (
241 predicate => 'has_reader',
245 Class::MOP::Attribute->meta->add_attribute(
246 Class::MOP::Attribute->new('writer' => (
248 predicate => 'has_writer',
252 Class::MOP::Attribute->meta->add_attribute(
253 Class::MOP::Attribute->new('predicate' => (
254 reader => 'predicate',
255 predicate => 'has_predicate',
259 Class::MOP::Attribute->meta->add_attribute(
260 Class::MOP::Attribute->new('clearer' => (
262 predicate => 'has_clearer',
266 Class::MOP::Attribute->meta->add_attribute(
267 Class::MOP::Attribute->new('init_arg' => (
268 reader => 'init_arg',
269 predicate => 'has_init_arg',
273 Class::MOP::Attribute->meta->add_attribute(
274 Class::MOP::Attribute->new('default' => (
275 # default has a custom 'reader' method ...
276 predicate => 'has_default',
281 # NOTE: (meta-circularity)
282 # This should be one of the last things done
283 # it will "tie the knot" with Class::MOP::Attribute
284 # so that it uses the attributes meta-objects
285 # to construct itself.
286 Class::MOP::Attribute->meta->add_method('new' => sub {
291 (defined $name && $name)
292 || confess "You must provide a name for the attribute";
293 $options{init_arg} = $name
294 if not exists $options{init_arg};
296 (Class::MOP::Attribute::is_default_a_coderef(\%options))
297 || confess("References are not allowed as default values, you must ".
298 "wrap then in a CODE reference (ex: sub { [] } and not [])")
299 if exists $options{default} && ref $options{default};
301 # return the new object
302 $class->meta->new_object(name => $name, %options);
305 Class::MOP::Attribute->meta->add_method('clone' => sub {
307 $self->meta->clone_object($self, @_);
310 ## --------------------------------------------------------
311 ## Now close all the Class::MOP::* classes
313 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
314 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
315 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
316 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
317 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
318 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
319 Class::MOP::Object ->meta->make_immutable(inline_constructor => 0);
329 Class::MOP - A Meta Object Protocol for Perl 5
333 # ... This will come later, for now see
334 # the other SYNOPSIS for more information
338 This module is an attempt to create a meta object protocol for the
339 Perl 5 object system. It makes no attempt to change the behavior or
340 characteristics of the Perl 5 object system, only to create a
341 protocol for its manipulation and introspection.
343 That said, it does attempt to create the tools for building a rich
344 set of extensions to the Perl 5 object system. Every attempt has been
345 made for these tools to keep to the spirit of the Perl 5 object
346 system that we all know and love.
348 This documentation is admittedly sparse on details, as time permits
349 I will try to improve them. For now, I suggest looking at the items
350 listed in the L<SEE ALSO> section for more information. In particular
351 the book "The Art of the Meta Object Protocol" was very influential
352 in the development of this system.
354 =head2 What is a Meta Object Protocol?
356 A meta object protocol is an API to an object system.
358 To be more specific, it is a set of abstractions of the components of
359 an object system (typically things like; classes, object, methods,
360 object attributes, etc.). These abstractions can then be used to both
361 inspect and manipulate the object system which they describe.
363 It can be said that there are two MOPs for any object system; the
364 implicit MOP, and the explicit MOP. The implicit MOP handles things
365 like method dispatch or inheritance, which happen automatically as
366 part of how the object system works. The explicit MOP typically
367 handles the introspection/reflection features of the object system.
368 All object systems have implicit MOPs, without one, they would not
369 work. Explict MOPs however as less common, and depending on the
370 language can vary from restrictive (Reflection in Java or C#) to
371 wide open (CLOS is a perfect example).
373 =head2 Yet Another Class Builder!! Why?
375 This is B<not> a class builder so much as it is a I<class builder
376 B<builder>>. My intent is that an end user does not use this module
377 directly, but instead this module is used by module authors to
378 build extensions and features onto the Perl 5 object system.
380 =head2 Who is this module for?
382 This module is specifically for anyone who has ever created or
383 wanted to create a module for the Class:: namespace. The tools which
384 this module will provide will hopefully make it easier to do more
385 complex things with Perl 5 classes by removing such barriers as
386 the need to hack the symbol tables, or understand the fine details
389 =head2 What changes do I have to make to use this module?
391 This module was designed to be as unintrusive as possible. Many of
392 its features are accessible without B<any> change to your existsing
393 code at all. It is meant to be a compliment to your existing code and
394 not an intrusion on your code base. Unlike many other B<Class::>
395 modules, this module B<does not> require you subclass it, or even that
396 you C<use> it in within your module's package.
398 The only features which requires additions to your code are the
399 attribute handling and instance construction features, and these are
400 both completely optional features. The only reason for this is because
401 Perl 5's object system does not actually have these features built
402 in. More information about this feature can be found below.
404 =head2 A Note about Performance?
406 It is a common misconception that explict MOPs are performance drains.
407 But this is not a universal truth at all, it is an side-effect of
408 specific implementations. For instance, using Java reflection is much
409 slower because the JVM cannot take advantage of any compiler
410 optimizations, and the JVM has to deal with much more runtime type
411 information as well. Reflection in C# is marginally better as it was
412 designed into the language and runtime (the CLR). In contrast, CLOS
413 (the Common Lisp Object System) was built to support an explicit MOP,
414 and so performance is tuned for it.
416 This library in particular does it's absolute best to avoid putting
417 B<any> drain at all upon your code's performance. In fact, by itself
418 it does nothing to affect your existing code. So you only pay for
419 what you actually use.
421 =head2 About Metaclass compatibility
423 This module makes sure that all metaclasses created are both upwards
424 and downwards compatible. The topic of metaclass compatibility is
425 highly esoteric and is something only encountered when doing deep and
426 involved metaclass hacking. There are two basic kinds of metaclass
427 incompatibility; upwards and downwards.
429 Upwards metaclass compatibility means that the metaclass of a
430 given class is either the same as (or a subclass of) all of the
433 Downward metaclass compatibility means that the metaclasses of a
434 given class's anscestors are all either the same as (or a subclass
437 Here is a diagram showing a set of two classes (C<A> and C<B>) and
438 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
439 metaclass compatibility both upwards and downwards.
441 +---------+ +---------+
442 | Meta::A |<----| Meta::B | <....... (instance of )
443 +---------+ +---------+ <------- (inherits from)
446 +---------+ +---------+
448 +---------+ +---------+
450 As I said this is a highly esoteric topic and one you will only run
451 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
452 are interested in why this is an issue see the paper
453 I<Uniform and safe metaclass composition> linked to in the
454 L<SEE ALSO> section of this document.
456 =head2 Using custom metaclasses
458 Always use the metaclass pragma when using a custom metaclass, this
459 will ensure the proper initialization order and not accidentely
460 create an incorrect type of metaclass for you. This is a very rare
461 problem, and one which can only occur if you are doing deep metaclass
462 programming. So in other words, don't worry about it.
466 The protocol is divided into 3 main sub-protocols:
470 =item The Class protocol
472 This provides a means of manipulating and introspecting a Perl 5
473 class. It handles all of symbol table hacking for you, and provides
474 a rich set of methods that go beyond simple package introspection.
476 See L<Class::MOP::Class> for more details.
478 =item The Attribute protocol
480 This provides a consistent represenation for an attribute of a
481 Perl 5 class. Since there are so many ways to create and handle
482 atttributes in Perl 5 OO, this attempts to provide as much of a
483 unified approach as possible, while giving the freedom and
484 flexibility to subclass for specialization.
486 See L<Class::MOP::Attribute> for more details.
488 =item The Method protocol
490 This provides a means of manipulating and introspecting methods in
491 the Perl 5 object system. As with attributes, there are many ways to
492 approach this topic, so we try to keep it pretty basic, while still
493 making it possible to extend the system in many ways.
495 See L<Class::MOP::Method> for more details.
501 Class::MOP holds a cache of metaclasses, the following are functions
502 (B<not methods>) which can be used to access that cache. It is not
503 recommended that you mess with this, bad things could happen. But if
504 you are brave and willing to risk it, go for it.
508 =item B<get_all_metaclasses>
510 =item B<get_all_metaclass_instances>
512 =item B<get_all_metaclass_names>
514 =item B<get_metaclass_by_name ($name)>
516 =item B<store_metaclass_by_name ($name, $meta)>
518 =item B<weaken_metaclass ($name)>
520 =item B<does_metaclass_exist ($name)>
522 =item B<remove_metaclass_by_name ($name)>
530 There are very few books out on Meta Object Protocols and Metaclasses
531 because it is such an esoteric topic. The following books are really
532 the only ones I have found. If you know of any more, B<I<please>>
533 email me and let me know, I would love to hear about them.
537 =item "The Art of the Meta Object Protocol"
539 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
541 =item "Putting MetaClasses to Work"
543 =item "Smalltalk: The Language"
551 =item Uniform and safe metaclass composition
553 An excellent paper by the people who brought us the original Traits paper.
554 This paper is on how Traits can be used to do safe metaclass composition,
555 and offers an excellent introduction section which delves into the topic of
556 metaclass compatibility.
558 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
560 =item Safe Metaclass Programming
562 This paper seems to precede the above paper, and propose a mix-in based
563 approach as opposed to the Traits based approach. Both papers have similar
564 information on the metaclass compatibility problem space.
566 L<http://citeseer.ist.psu.edu/37617.html>
574 =item The Perl 6 MetaModel work in the Pugs project
578 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
580 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
586 =head1 SIMILAR MODULES
588 As I have said above, this module is a class-builder-builder, so it is
589 not the same thing as modules like L<Class::Accessor> and
590 L<Class::MethodMaker>. That being said there are very few modules on CPAN
591 with similar goals to this module. The one I have found which is most
592 like this module is L<Class::Meta>, although it's philosophy and the MOP it
593 creates are very different from this modules.
597 All complex software has bugs lurking in it, and this module is no
598 exception. If you find a bug please either email me, or add the bug
603 I use L<Devel::Cover> to test the code coverage of my tests, below is the
604 L<Devel::Cover> report on this module's test suite.
606 ---------------------------- ------ ------ ------ ------ ------ ------ ------
607 File stmt bran cond sub pod time total
608 ---------------------------- ------ ------ ------ ------ ------ ------ ------
609 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 19.8 100.0
610 Class/MOP/Attribute.pm 100.0 100.0 91.7 61.2 100.0 14.3 87.9
611 Class/MOP/Class.pm 97.6 91.3 77.3 98.4 100.0 56.4 93.2
612 Class/MOP/Instance.pm 91.1 75.0 33.3 91.7 100.0 6.8 90.7
613 Class/MOP/Method.pm 97.6 60.0 52.9 76.9 100.0 1.6 82.6
614 metaclass.pm 100.0 100.0 83.3 100.0 n/a 1.0 97.7
615 ---------------------------- ------ ------ ------ ------ ------ ------ ------
616 Total 97.5 88.5 75.5 82.8 100.0 100.0 91.2
617 ---------------------------- ------ ------ ------ ------ ------ ------ ------
619 =head1 ACKNOWLEDGEMENTS
623 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
625 Thanks to Rob for actually getting the development of this module kick-started.
631 Stevan Little E<lt>stevan@iinteractive.comE<gt>
633 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
635 =head1 COPYRIGHT AND LICENSE
637 Copyright 2006 by Infinity Interactive, Inc.
639 L<http://www.iinteractive.com>
641 This library is free software; you can redistribute it and/or modify
642 it under the same terms as Perl itself.