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 = '0.34';
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' => (
182 # we just alias the original method
183 # rather than re-produce it here
184 'get_method_map' => \&Class::MOP::Class::get_method_map
186 default => sub { {} }
190 Class::MOP::Class->meta->add_attribute(
191 Class::MOP::Attribute->new('$:attribute_metaclass' => (
192 reader => 'attribute_metaclass',
193 init_arg => ':attribute_metaclass',
194 default => 'Class::MOP::Attribute',
198 Class::MOP::Class->meta->add_attribute(
199 Class::MOP::Attribute->new('$:method_metaclass' => (
200 reader => 'method_metaclass',
201 init_arg => ':method_metaclass',
202 default => 'Class::MOP::Method',
206 Class::MOP::Class->meta->add_attribute(
207 Class::MOP::Attribute->new('$:instance_metaclass' => (
209 # NOTE: we need to do this in order
210 # for the instance meta-object to
211 # not fall into meta-circular death
213 # we just alias the original method
214 # rather than re-produce it here
215 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
217 init_arg => ':instance_metaclass',
218 default => 'Class::MOP::Instance',
223 # we don't actually need to tie the knot with
224 # Class::MOP::Class here, it is actually handled
225 # within Class::MOP::Class itself in the
226 # construct_class_instance method.
228 ## --------------------------------------------------------
229 ## Class::MOP::Attribute
231 Class::MOP::Attribute->meta->add_attribute(
232 Class::MOP::Attribute->new('name' => (
234 # NOTE: we need to do this in order
235 # for the instance meta-object to
236 # not fall into meta-circular death
238 # we just alias the original method
239 # rather than re-produce it here
240 'name' => \&Class::MOP::Attribute::name
245 Class::MOP::Attribute->meta->add_attribute(
246 Class::MOP::Attribute->new('associated_class' => (
248 # NOTE: we need to do this in order
249 # for the instance meta-object to
250 # not fall into meta-circular death
252 # we just alias the original method
253 # rather than re-produce it here
254 'associated_class' => \&Class::MOP::Attribute::associated_class
259 Class::MOP::Attribute->meta->add_attribute(
260 Class::MOP::Attribute->new('accessor' => (
261 reader => 'accessor',
262 predicate => 'has_accessor',
266 Class::MOP::Attribute->meta->add_attribute(
267 Class::MOP::Attribute->new('reader' => (
269 predicate => 'has_reader',
273 Class::MOP::Attribute->meta->add_attribute(
274 Class::MOP::Attribute->new('writer' => (
276 predicate => 'has_writer',
280 Class::MOP::Attribute->meta->add_attribute(
281 Class::MOP::Attribute->new('predicate' => (
282 reader => 'predicate',
283 predicate => 'has_predicate',
287 Class::MOP::Attribute->meta->add_attribute(
288 Class::MOP::Attribute->new('clearer' => (
290 predicate => 'has_clearer',
294 Class::MOP::Attribute->meta->add_attribute(
295 Class::MOP::Attribute->new('init_arg' => (
296 reader => 'init_arg',
297 predicate => 'has_init_arg',
301 Class::MOP::Attribute->meta->add_attribute(
302 Class::MOP::Attribute->new('default' => (
303 # default has a custom 'reader' method ...
304 predicate => 'has_default',
309 # NOTE: (meta-circularity)
310 # This should be one of the last things done
311 # it will "tie the knot" with Class::MOP::Attribute
312 # so that it uses the attributes meta-objects
313 # to construct itself.
314 Class::MOP::Attribute->meta->add_method('new' => sub {
319 (defined $name && $name)
320 || confess "You must provide a name for the attribute";
321 $options{init_arg} = $name
322 if not exists $options{init_arg};
324 (Class::MOP::Attribute::is_default_a_coderef(\%options))
325 || confess("References are not allowed as default values, you must ".
326 "wrap then in a CODE reference (ex: sub { [] } and not [])")
327 if exists $options{default} && ref $options{default};
329 # return the new object
330 $class->meta->new_object(name => $name, %options);
333 Class::MOP::Attribute->meta->add_method('clone' => sub {
335 $self->meta->clone_object($self, @_);
338 ## --------------------------------------------------------
339 ## Class::MOP::Method
341 Class::MOP::Method->meta->add_attribute(
342 Class::MOP::Attribute->new('body' => (
347 ## --------------------------------------------------------
348 ## Class::MOP::Method::Wrapped
351 # the way this item is initialized, this
352 # really does not follow the standard
353 # practices of attributes, but we put
354 # it here for completeness
355 Class::MOP::Method::Wrapped->meta->add_attribute(
356 Class::MOP::Attribute->new('modifier_table')
359 ## --------------------------------------------------------
360 ## Now close all the Class::MOP::* classes
362 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
363 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
364 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
365 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
366 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
367 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
368 Class::MOP::Object ->meta->make_immutable(inline_constructor => 0);
370 # Class::MOP::Method subclasses
371 Class::MOP::Attribute::Accessor->meta->make_immutable(inline_constructor => 0);
372 Class::MOP::Method::Wrapped ->meta->make_immutable(inline_constructor => 0);
382 Class::MOP - A Meta Object Protocol for Perl 5
386 # ... This will come later, for now see
387 # the other SYNOPSIS for more information
391 This module is an attempt to create a meta object protocol for the
392 Perl 5 object system. It makes no attempt to change the behavior or
393 characteristics of the Perl 5 object system, only to create a
394 protocol for its manipulation and introspection.
396 That said, it does attempt to create the tools for building a rich
397 set of extensions to the Perl 5 object system. Every attempt has been
398 made for these tools to keep to the spirit of the Perl 5 object
399 system that we all know and love.
401 This documentation is admittedly sparse on details, as time permits
402 I will try to improve them. For now, I suggest looking at the items
403 listed in the L<SEE ALSO> section for more information. In particular
404 the book "The Art of the Meta Object Protocol" was very influential
405 in the development of this system.
407 =head2 What is a Meta Object Protocol?
409 A meta object protocol is an API to an object system.
411 To be more specific, it is a set of abstractions of the components of
412 an object system (typically things like; classes, object, methods,
413 object attributes, etc.). These abstractions can then be used to both
414 inspect and manipulate the object system which they describe.
416 It can be said that there are two MOPs for any object system; the
417 implicit MOP, and the explicit MOP. The implicit MOP handles things
418 like method dispatch or inheritance, which happen automatically as
419 part of how the object system works. The explicit MOP typically
420 handles the introspection/reflection features of the object system.
421 All object systems have implicit MOPs, without one, they would not
422 work. Explict MOPs however as less common, and depending on the
423 language can vary from restrictive (Reflection in Java or C#) to
424 wide open (CLOS is a perfect example).
426 =head2 Yet Another Class Builder!! Why?
428 This is B<not> a class builder so much as it is a I<class builder
429 B<builder>>. My intent is that an end user does not use this module
430 directly, but instead this module is used by module authors to
431 build extensions and features onto the Perl 5 object system.
433 =head2 Who is this module for?
435 This module is specifically for anyone who has ever created or
436 wanted to create a module for the Class:: namespace. The tools which
437 this module will provide will hopefully make it easier to do more
438 complex things with Perl 5 classes by removing such barriers as
439 the need to hack the symbol tables, or understand the fine details
442 =head2 What changes do I have to make to use this module?
444 This module was designed to be as unintrusive as possible. Many of
445 its features are accessible without B<any> change to your existsing
446 code at all. It is meant to be a compliment to your existing code and
447 not an intrusion on your code base. Unlike many other B<Class::>
448 modules, this module B<does not> require you subclass it, or even that
449 you C<use> it in within your module's package.
451 The only features which requires additions to your code are the
452 attribute handling and instance construction features, and these are
453 both completely optional features. The only reason for this is because
454 Perl 5's object system does not actually have these features built
455 in. More information about this feature can be found below.
457 =head2 A Note about Performance?
459 It is a common misconception that explict MOPs are performance drains.
460 But this is not a universal truth at all, it is an side-effect of
461 specific implementations. For instance, using Java reflection is much
462 slower because the JVM cannot take advantage of any compiler
463 optimizations, and the JVM has to deal with much more runtime type
464 information as well. Reflection in C# is marginally better as it was
465 designed into the language and runtime (the CLR). In contrast, CLOS
466 (the Common Lisp Object System) was built to support an explicit MOP,
467 and so performance is tuned for it.
469 This library in particular does it's absolute best to avoid putting
470 B<any> drain at all upon your code's performance. In fact, by itself
471 it does nothing to affect your existing code. So you only pay for
472 what you actually use.
474 =head2 About Metaclass compatibility
476 This module makes sure that all metaclasses created are both upwards
477 and downwards compatible. The topic of metaclass compatibility is
478 highly esoteric and is something only encountered when doing deep and
479 involved metaclass hacking. There are two basic kinds of metaclass
480 incompatibility; upwards and downwards.
482 Upwards metaclass compatibility means that the metaclass of a
483 given class is either the same as (or a subclass of) all of the
486 Downward metaclass compatibility means that the metaclasses of a
487 given class's anscestors are all either the same as (or a subclass
490 Here is a diagram showing a set of two classes (C<A> and C<B>) and
491 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
492 metaclass compatibility both upwards and downwards.
494 +---------+ +---------+
495 | Meta::A |<----| Meta::B | <....... (instance of )
496 +---------+ +---------+ <------- (inherits from)
499 +---------+ +---------+
501 +---------+ +---------+
503 As I said this is a highly esoteric topic and one you will only run
504 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
505 are interested in why this is an issue see the paper
506 I<Uniform and safe metaclass composition> linked to in the
507 L<SEE ALSO> section of this document.
509 =head2 Using custom metaclasses
511 Always use the metaclass pragma when using a custom metaclass, this
512 will ensure the proper initialization order and not accidentely
513 create an incorrect type of metaclass for you. This is a very rare
514 problem, and one which can only occur if you are doing deep metaclass
515 programming. So in other words, don't worry about it.
519 The protocol is divided into 3 main sub-protocols:
523 =item The Class protocol
525 This provides a means of manipulating and introspecting a Perl 5
526 class. It handles all of symbol table hacking for you, and provides
527 a rich set of methods that go beyond simple package introspection.
529 See L<Class::MOP::Class> for more details.
531 =item The Attribute protocol
533 This provides a consistent represenation for an attribute of a
534 Perl 5 class. Since there are so many ways to create and handle
535 atttributes in Perl 5 OO, this attempts to provide as much of a
536 unified approach as possible, while giving the freedom and
537 flexibility to subclass for specialization.
539 See L<Class::MOP::Attribute> for more details.
541 =item The Method protocol
543 This provides a means of manipulating and introspecting methods in
544 the Perl 5 object system. As with attributes, there are many ways to
545 approach this topic, so we try to keep it pretty basic, while still
546 making it possible to extend the system in many ways.
548 See L<Class::MOP::Method> for more details.
554 Class::MOP holds a cache of metaclasses, the following are functions
555 (B<not methods>) which can be used to access that cache. It is not
556 recommended that you mess with this, bad things could happen. But if
557 you are brave and willing to risk it, go for it.
561 =item B<get_all_metaclasses>
563 This will return an hash of all the metaclass instances that have
564 been cached by B<Class::MOP::Class> keyed by the package name.
566 =item B<get_all_metaclass_instances>
568 This will return an array of all the metaclass instances that have
569 been cached by B<Class::MOP::Class>.
571 =item B<get_all_metaclass_names>
573 This will return an array of all the metaclass names that have
574 been cached by B<Class::MOP::Class>.
576 =item B<get_metaclass_by_name ($name)>
578 =item B<store_metaclass_by_name ($name, $meta)>
580 =item B<weaken_metaclass ($name)>
582 =item B<does_metaclass_exist ($name)>
584 =item B<remove_metaclass_by_name ($name)>
592 There are very few books out on Meta Object Protocols and Metaclasses
593 because it is such an esoteric topic. The following books are really
594 the only ones I have found. If you know of any more, B<I<please>>
595 email me and let me know, I would love to hear about them.
599 =item "The Art of the Meta Object Protocol"
601 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
603 =item "Putting MetaClasses to Work"
605 =item "Smalltalk: The Language"
613 =item Uniform and safe metaclass composition
615 An excellent paper by the people who brought us the original Traits paper.
616 This paper is on how Traits can be used to do safe metaclass composition,
617 and offers an excellent introduction section which delves into the topic of
618 metaclass compatibility.
620 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
622 =item Safe Metaclass Programming
624 This paper seems to precede the above paper, and propose a mix-in based
625 approach as opposed to the Traits based approach. Both papers have similar
626 information on the metaclass compatibility problem space.
628 L<http://citeseer.ist.psu.edu/37617.html>
636 =item The Perl 6 MetaModel work in the Pugs project
640 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
642 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
648 =head1 SIMILAR MODULES
650 As I have said above, this module is a class-builder-builder, so it is
651 not the same thing as modules like L<Class::Accessor> and
652 L<Class::MethodMaker>. That being said there are very few modules on CPAN
653 with similar goals to this module. The one I have found which is most
654 like this module is L<Class::Meta>, although it's philosophy and the MOP it
655 creates are very different from this modules.
659 All complex software has bugs lurking in it, and this module is no
660 exception. If you find a bug please either email me, or add the bug
665 I use L<Devel::Cover> to test the code coverage of my tests, below is the
666 L<Devel::Cover> report on this module's test suite.
668 ---------------------------- ------ ------ ------ ------ ------ ------ ------
669 File stmt bran cond sub pod time total
670 ---------------------------- ------ ------ ------ ------ ------ ------ ------
671 Class/MOP.pm 78.0 87.5 55.6 71.4 100.0 12.4 76.8
672 Class/MOP/Attribute.pm 83.4 75.6 86.7 94.4 100.0 8.9 85.2
673 Class/MOP/Class.pm 96.9 75.8 43.2 98.0 100.0 55.3 83.6
674 Class/MOP/Class/Immutable.pm 88.5 53.8 n/a 95.8 100.0 1.1 84.7
675 Class/MOP/Instance.pm 87.9 75.0 33.3 89.7 100.0 10.1 89.1
676 Class/MOP/Method.pm 97.6 60.0 57.9 76.9 100.0 1.5 82.8
677 Class/MOP/Module.pm 87.5 n/a 11.1 83.3 100.0 0.3 66.7
678 Class/MOP/Object.pm 100.0 n/a 33.3 100.0 100.0 0.1 89.5
679 Class/MOP/Package.pm 95.1 69.0 33.3 100.0 100.0 9.9 85.5
680 metaclass.pm 100.0 100.0 83.3 100.0 n/a 0.5 97.7
681 ---------------------------- ------ ------ ------ ------ ------ ------ ------
682 Total 91.5 72.1 48.8 90.7 100.0 100.0 84.2
683 ---------------------------- ------ ------ ------ ------ ------ ------ ------
685 =head1 ACKNOWLEDGEMENTS
691 Thanks to Rob for actually getting the development of this module kick-started.
697 Stevan Little E<lt>stevan@iinteractive.comE<gt>
699 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
701 =head1 COPYRIGHT AND LICENSE
703 Copyright 2006 by Infinity Interactive, Inc.
705 L<http://www.iinteractive.com>
707 This library is free software; you can redistribute it and/or modify
708 it under the same terms as Perl itself.