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' => (
87 # because of issues with the Perl API
88 # to the typeglob in some versions, we
89 # need to just always grab a new
90 # reference to the hash here. Ideally
91 # we could just store a ref and it would
92 # Just Work, but oh well :\
95 \%{$_[0]->name . '::'}
99 # protect this from silliness
100 init_arg => '!............( DO NOT DO THIS )............!',
105 # use the metaclass to construct the meta-package
106 # which is a superclass of the metaclass itself :P
107 Class::MOP::Package->meta->add_method('initialize' => sub {
109 my $package_name = shift;
110 $class->meta->new_object(':package' => $package_name, @_);
113 ## --------------------------------------------------------
114 ## Class::MOP::Module
117 # yeah this is kind of stretching things a bit,
118 # but truthfully the version should be an attribute
119 # of the Module, the weirdness comes from having to
120 # stick to Perl 5 convention and store it in the
121 # $VERSION package variable. Basically if you just
122 # squint at it, it will look how you want it to look.
123 # Either as a package variable, or as a attribute of
124 # the metaclass, isn't abstraction great :)
126 Class::MOP::Module->meta->add_attribute(
127 Class::MOP::Attribute->new('$:version' => (
131 ${$self->get_package_symbol('$VERSION')};
135 # protect this from silliness
136 init_arg => '!............( DO NOT DO THIS )............!',
141 # By following the same conventions as version here,
142 # we are opening up the possibility that people can
143 # use the $AUTHORITY in non-Class::MOP modules as
146 Class::MOP::Module->meta->add_attribute(
147 Class::MOP::Attribute->new('$:authority' => (
151 ${$self->get_package_symbol('$AUTHORITY')};
155 # protect this from silliness
156 init_arg => '!............( DO NOT DO THIS )............!',
160 ## --------------------------------------------------------
163 Class::MOP::Class->meta->add_attribute(
164 Class::MOP::Attribute->new('%:attributes' => (
166 # NOTE: we need to do this in order
167 # for the instance meta-object to
168 # not fall into meta-circular death
169 'get_attribute_map' => sub { (shift)->{'%:attributes'} }
171 init_arg => ':attributes',
172 default => sub { {} }
176 Class::MOP::Class->meta->add_attribute(
177 Class::MOP::Attribute->new('$:attribute_metaclass' => (
178 reader => 'attribute_metaclass',
179 init_arg => ':attribute_metaclass',
180 default => 'Class::MOP::Attribute',
184 Class::MOP::Class->meta->add_attribute(
185 Class::MOP::Attribute->new('$:method_metaclass' => (
186 reader => 'method_metaclass',
187 init_arg => ':method_metaclass',
188 default => 'Class::MOP::Method',
192 Class::MOP::Class->meta->add_attribute(
193 Class::MOP::Attribute->new('$:instance_metaclass' => (
195 # NOTE: we need to do this in order
196 # for the instance meta-object to
197 # not fall into meta-circular death
198 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
200 init_arg => ':instance_metaclass',
201 default => 'Class::MOP::Instance',
206 # we don't actually need to tie the knot with
207 # Class::MOP::Class here, it is actually handled
208 # within Class::MOP::Class itself in the
209 # construct_class_instance method.
211 ## --------------------------------------------------------
212 ## Class::MOP::Attribute
214 Class::MOP::Attribute->meta->add_attribute(
215 Class::MOP::Attribute->new('name' => (
217 # NOTE: we need to do this in order
218 # for the instance meta-object to
219 # not fall into meta-circular death
220 'name' => sub { (shift)->{name} }
225 Class::MOP::Attribute->meta->add_attribute(
226 Class::MOP::Attribute->new('associated_class' => (
228 # NOTE: we need to do this in order
229 # for the instance meta-object to
230 # not fall into meta-circular death
231 'associated_class' => sub { (shift)->{associated_class} }
236 Class::MOP::Attribute->meta->add_attribute(
237 Class::MOP::Attribute->new('accessor' => (
238 reader => 'accessor',
239 predicate => 'has_accessor',
243 Class::MOP::Attribute->meta->add_attribute(
244 Class::MOP::Attribute->new('reader' => (
246 predicate => 'has_reader',
250 Class::MOP::Attribute->meta->add_attribute(
251 Class::MOP::Attribute->new('writer' => (
253 predicate => 'has_writer',
257 Class::MOP::Attribute->meta->add_attribute(
258 Class::MOP::Attribute->new('predicate' => (
259 reader => 'predicate',
260 predicate => 'has_predicate',
264 Class::MOP::Attribute->meta->add_attribute(
265 Class::MOP::Attribute->new('clearer' => (
267 predicate => 'has_clearer',
271 Class::MOP::Attribute->meta->add_attribute(
272 Class::MOP::Attribute->new('init_arg' => (
273 reader => 'init_arg',
274 predicate => 'has_init_arg',
278 Class::MOP::Attribute->meta->add_attribute(
279 Class::MOP::Attribute->new('default' => (
280 # default has a custom 'reader' method ...
281 predicate => 'has_default',
286 # NOTE: (meta-circularity)
287 # This should be one of the last things done
288 # it will "tie the knot" with Class::MOP::Attribute
289 # so that it uses the attributes meta-objects
290 # to construct itself.
291 Class::MOP::Attribute->meta->add_method('new' => sub {
296 (defined $name && $name)
297 || confess "You must provide a name for the attribute";
298 $options{init_arg} = $name
299 if not exists $options{init_arg};
301 (Class::MOP::Attribute::is_default_a_coderef(\%options))
302 || confess("References are not allowed as default values, you must ".
303 "wrap then in a CODE reference (ex: sub { [] } and not [])")
304 if exists $options{default} && ref $options{default};
306 # return the new object
307 $class->meta->new_object(name => $name, %options);
310 Class::MOP::Attribute->meta->add_method('clone' => sub {
312 $self->meta->clone_object($self, @_);
315 ## --------------------------------------------------------
316 ## Now close all the Class::MOP::* classes
318 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
319 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
320 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
321 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
322 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
323 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
324 Class::MOP::Object ->meta->make_immutable(inline_constructor => 0);
334 Class::MOP - A Meta Object Protocol for Perl 5
338 # ... This will come later, for now see
339 # the other SYNOPSIS for more information
343 This module is an attempt to create a meta object protocol for the
344 Perl 5 object system. It makes no attempt to change the behavior or
345 characteristics of the Perl 5 object system, only to create a
346 protocol for its manipulation and introspection.
348 That said, it does attempt to create the tools for building a rich
349 set of extensions to the Perl 5 object system. Every attempt has been
350 made for these tools to keep to the spirit of the Perl 5 object
351 system that we all know and love.
353 This documentation is admittedly sparse on details, as time permits
354 I will try to improve them. For now, I suggest looking at the items
355 listed in the L<SEE ALSO> section for more information. In particular
356 the book "The Art of the Meta Object Protocol" was very influential
357 in the development of this system.
359 =head2 What is a Meta Object Protocol?
361 A meta object protocol is an API to an object system.
363 To be more specific, it is a set of abstractions of the components of
364 an object system (typically things like; classes, object, methods,
365 object attributes, etc.). These abstractions can then be used to both
366 inspect and manipulate the object system which they describe.
368 It can be said that there are two MOPs for any object system; the
369 implicit MOP, and the explicit MOP. The implicit MOP handles things
370 like method dispatch or inheritance, which happen automatically as
371 part of how the object system works. The explicit MOP typically
372 handles the introspection/reflection features of the object system.
373 All object systems have implicit MOPs, without one, they would not
374 work. Explict MOPs however as less common, and depending on the
375 language can vary from restrictive (Reflection in Java or C#) to
376 wide open (CLOS is a perfect example).
378 =head2 Yet Another Class Builder!! Why?
380 This is B<not> a class builder so much as it is a I<class builder
381 B<builder>>. My intent is that an end user does not use this module
382 directly, but instead this module is used by module authors to
383 build extensions and features onto the Perl 5 object system.
385 =head2 Who is this module for?
387 This module is specifically for anyone who has ever created or
388 wanted to create a module for the Class:: namespace. The tools which
389 this module will provide will hopefully make it easier to do more
390 complex things with Perl 5 classes by removing such barriers as
391 the need to hack the symbol tables, or understand the fine details
394 =head2 What changes do I have to make to use this module?
396 This module was designed to be as unintrusive as possible. Many of
397 its features are accessible without B<any> change to your existsing
398 code at all. It is meant to be a compliment to your existing code and
399 not an intrusion on your code base. Unlike many other B<Class::>
400 modules, this module B<does not> require you subclass it, or even that
401 you C<use> it in within your module's package.
403 The only features which requires additions to your code are the
404 attribute handling and instance construction features, and these are
405 both completely optional features. The only reason for this is because
406 Perl 5's object system does not actually have these features built
407 in. More information about this feature can be found below.
409 =head2 A Note about Performance?
411 It is a common misconception that explict MOPs are performance drains.
412 But this is not a universal truth at all, it is an side-effect of
413 specific implementations. For instance, using Java reflection is much
414 slower because the JVM cannot take advantage of any compiler
415 optimizations, and the JVM has to deal with much more runtime type
416 information as well. Reflection in C# is marginally better as it was
417 designed into the language and runtime (the CLR). In contrast, CLOS
418 (the Common Lisp Object System) was built to support an explicit MOP,
419 and so performance is tuned for it.
421 This library in particular does it's absolute best to avoid putting
422 B<any> drain at all upon your code's performance. In fact, by itself
423 it does nothing to affect your existing code. So you only pay for
424 what you actually use.
426 =head2 About Metaclass compatibility
428 This module makes sure that all metaclasses created are both upwards
429 and downwards compatible. The topic of metaclass compatibility is
430 highly esoteric and is something only encountered when doing deep and
431 involved metaclass hacking. There are two basic kinds of metaclass
432 incompatibility; upwards and downwards.
434 Upwards metaclass compatibility means that the metaclass of a
435 given class is either the same as (or a subclass of) all of the
438 Downward metaclass compatibility means that the metaclasses of a
439 given class's anscestors are all either the same as (or a subclass
442 Here is a diagram showing a set of two classes (C<A> and C<B>) and
443 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
444 metaclass compatibility both upwards and downwards.
446 +---------+ +---------+
447 | Meta::A |<----| Meta::B | <....... (instance of )
448 +---------+ +---------+ <------- (inherits from)
451 +---------+ +---------+
453 +---------+ +---------+
455 As I said this is a highly esoteric topic and one you will only run
456 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
457 are interested in why this is an issue see the paper
458 I<Uniform and safe metaclass composition> linked to in the
459 L<SEE ALSO> section of this document.
461 =head2 Using custom metaclasses
463 Always use the metaclass pragma when using a custom metaclass, this
464 will ensure the proper initialization order and not accidentely
465 create an incorrect type of metaclass for you. This is a very rare
466 problem, and one which can only occur if you are doing deep metaclass
467 programming. So in other words, don't worry about it.
471 The protocol is divided into 3 main sub-protocols:
475 =item The Class protocol
477 This provides a means of manipulating and introspecting a Perl 5
478 class. It handles all of symbol table hacking for you, and provides
479 a rich set of methods that go beyond simple package introspection.
481 See L<Class::MOP::Class> for more details.
483 =item The Attribute protocol
485 This provides a consistent represenation for an attribute of a
486 Perl 5 class. Since there are so many ways to create and handle
487 atttributes in Perl 5 OO, this attempts to provide as much of a
488 unified approach as possible, while giving the freedom and
489 flexibility to subclass for specialization.
491 See L<Class::MOP::Attribute> for more details.
493 =item The Method protocol
495 This provides a means of manipulating and introspecting methods in
496 the Perl 5 object system. As with attributes, there are many ways to
497 approach this topic, so we try to keep it pretty basic, while still
498 making it possible to extend the system in many ways.
500 See L<Class::MOP::Method> for more details.
506 Class::MOP holds a cache of metaclasses, the following are functions
507 (B<not methods>) which can be used to access that cache. It is not
508 recommended that you mess with this, bad things could happen. But if
509 you are brave and willing to risk it, go for it.
513 =item B<get_all_metaclasses>
515 This will return an hash of all the metaclass instances that have
516 been cached by B<Class::MOP::Class> keyed by the package name.
518 =item B<get_all_metaclass_instances>
520 This will return an array of all the metaclass instances that have
521 been cached by B<Class::MOP::Class>.
523 =item B<get_all_metaclass_names>
525 This will return an array of all the metaclass names that have
526 been cached by B<Class::MOP::Class>.
528 =item B<get_metaclass_by_name ($name)>
530 =item B<store_metaclass_by_name ($name, $meta)>
532 =item B<weaken_metaclass ($name)>
534 =item B<does_metaclass_exist ($name)>
536 =item B<remove_metaclass_by_name ($name)>
544 There are very few books out on Meta Object Protocols and Metaclasses
545 because it is such an esoteric topic. The following books are really
546 the only ones I have found. If you know of any more, B<I<please>>
547 email me and let me know, I would love to hear about them.
551 =item "The Art of the Meta Object Protocol"
553 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
555 =item "Putting MetaClasses to Work"
557 =item "Smalltalk: The Language"
565 =item Uniform and safe metaclass composition
567 An excellent paper by the people who brought us the original Traits paper.
568 This paper is on how Traits can be used to do safe metaclass composition,
569 and offers an excellent introduction section which delves into the topic of
570 metaclass compatibility.
572 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
574 =item Safe Metaclass Programming
576 This paper seems to precede the above paper, and propose a mix-in based
577 approach as opposed to the Traits based approach. Both papers have similar
578 information on the metaclass compatibility problem space.
580 L<http://citeseer.ist.psu.edu/37617.html>
588 =item The Perl 6 MetaModel work in the Pugs project
592 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
594 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
600 =head1 SIMILAR MODULES
602 As I have said above, this module is a class-builder-builder, so it is
603 not the same thing as modules like L<Class::Accessor> and
604 L<Class::MethodMaker>. That being said there are very few modules on CPAN
605 with similar goals to this module. The one I have found which is most
606 like this module is L<Class::Meta>, although it's philosophy and the MOP it
607 creates are very different from this modules.
611 All complex software has bugs lurking in it, and this module is no
612 exception. If you find a bug please either email me, or add the bug
617 I use L<Devel::Cover> to test the code coverage of my tests, below is the
618 L<Devel::Cover> report on this module's test suite.
620 ---------------------------- ------ ------ ------ ------ ------ ------ ------
621 File stmt bran cond sub pod time total
622 ---------------------------- ------ ------ ------ ------ ------ ------ ------
623 Class/MOP.pm 78.0 87.5 55.6 71.4 100.0 12.4 76.8
624 Class/MOP/Attribute.pm 83.4 75.6 86.7 94.4 100.0 8.9 85.2
625 Class/MOP/Class.pm 96.9 75.8 43.2 98.0 100.0 55.3 83.6
626 Class/MOP/Class/Immutable.pm 88.5 53.8 n/a 95.8 100.0 1.1 84.7
627 Class/MOP/Instance.pm 87.9 75.0 33.3 89.7 100.0 10.1 89.1
628 Class/MOP/Method.pm 97.6 60.0 57.9 76.9 100.0 1.5 82.8
629 Class/MOP/Module.pm 87.5 n/a 11.1 83.3 100.0 0.3 66.7
630 Class/MOP/Object.pm 100.0 n/a 33.3 100.0 100.0 0.1 89.5
631 Class/MOP/Package.pm 95.1 69.0 33.3 100.0 100.0 9.9 85.5
632 metaclass.pm 100.0 100.0 83.3 100.0 n/a 0.5 97.7
633 ---------------------------- ------ ------ ------ ------ ------ ------ ------
634 Total 91.5 72.1 48.8 90.7 100.0 100.0 84.2
635 ---------------------------- ------ ------ ------ ------ ------ ------ ------
637 =head1 ACKNOWLEDGEMENTS
643 Thanks to Rob for actually getting the development of this module kick-started.
649 Stevan Little E<lt>stevan@iinteractive.comE<gt>
651 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
653 =head1 COPYRIGHT AND LICENSE
655 Copyright 2006 by Infinity Interactive, Inc.
657 L<http://www.iinteractive.com>
659 This library is free software; you can redistribute it and/or modify
660 it under the same terms as Perl itself.