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('$:package' => (
74 # NOTE: we need to do this in order
75 # for the instance meta-object to
76 # not fall into meta-circular death
77 'name' => sub { (shift)->{'$: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 )............!',
101 default => sub { \undef }
106 # use the metaclass to construct the meta-package
107 # which is a superclass of the metaclass itself :P
108 Class::MOP::Package->meta->add_method('initialize' => sub {
110 my $package_name = shift;
111 $class->meta->new_object(':package' => $package_name, @_);
114 ## --------------------------------------------------------
115 ## Class::MOP::Module
118 # yeah this is kind of stretching things a bit,
119 # but truthfully the version should be an attribute
120 # of the Module, the weirdness comes from having to
121 # stick to Perl 5 convention and store it in the
122 # $VERSION package variable. Basically if you just
123 # squint at it, it will look how you want it to look.
124 # Either as a package variable, or as a attribute of
125 # the metaclass, isn't abstraction great :)
127 Class::MOP::Module->meta->add_attribute(
128 Class::MOP::Attribute->new('$:version' => (
132 ${$self->get_package_symbol('$VERSION')};
136 # protect this from silliness
137 init_arg => '!............( DO NOT DO THIS )............!',
138 default => sub { \undef }
143 # By following the same conventions as version here,
144 # we are opening up the possibility that people can
145 # use the $AUTHORITY in non-Class::MOP modules as
148 Class::MOP::Module->meta->add_attribute(
149 Class::MOP::Attribute->new('$:authority' => (
153 ${$self->get_package_symbol('$AUTHORITY')};
157 # protect this from silliness
158 init_arg => '!............( DO NOT DO THIS )............!',
159 default => sub { \undef }
163 ## --------------------------------------------------------
166 Class::MOP::Class->meta->add_attribute(
167 Class::MOP::Attribute->new('%:attributes' => (
169 # NOTE: we need to do this in order
170 # for the instance meta-object to
171 # not fall into meta-circular death
172 'get_attribute_map' => sub { (shift)->{'%:attributes'} }
174 init_arg => ':attributes',
175 default => sub { {} }
179 Class::MOP::Class->meta->add_attribute(
180 Class::MOP::Attribute->new('%:methods' => (
183 # as with the $VERSION and $AUTHORITY above
184 # sometimes we don't/can't store directly
185 # inside the instance, so we need the accessor
187 'get_method_map' => sub {
190 # there is a faster/better way
191 # to do this, I am sure :)
194 $_ => $self->get_method($_)
196 $self->has_method($_)
197 } $self->list_all_package_symbols
201 init_arg => '!............( DO NOT DO THIS )............!',
202 default => sub { \undef }
206 Class::MOP::Class->meta->add_attribute(
207 Class::MOP::Attribute->new('$:attribute_metaclass' => (
208 reader => 'attribute_metaclass',
209 init_arg => ':attribute_metaclass',
210 default => 'Class::MOP::Attribute',
214 Class::MOP::Class->meta->add_attribute(
215 Class::MOP::Attribute->new('$:method_metaclass' => (
216 reader => 'method_metaclass',
217 init_arg => ':method_metaclass',
218 default => 'Class::MOP::Method',
222 Class::MOP::Class->meta->add_attribute(
223 Class::MOP::Attribute->new('$:instance_metaclass' => (
225 # NOTE: we need to do this in order
226 # for the instance meta-object to
227 # not fall into meta-circular death
228 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
230 init_arg => ':instance_metaclass',
231 default => 'Class::MOP::Instance',
236 # we don't actually need to tie the knot with
237 # Class::MOP::Class here, it is actually handled
238 # within Class::MOP::Class itself in the
239 # construct_class_instance method.
241 ## --------------------------------------------------------
242 ## Class::MOP::Attribute
244 Class::MOP::Attribute->meta->add_attribute(
245 Class::MOP::Attribute->new('name' => (
247 # NOTE: we need to do this in order
248 # for the instance meta-object to
249 # not fall into meta-circular death
250 'name' => sub { (shift)->{name} }
255 Class::MOP::Attribute->meta->add_attribute(
256 Class::MOP::Attribute->new('associated_class' => (
258 # NOTE: we need to do this in order
259 # for the instance meta-object to
260 # not fall into meta-circular death
261 'associated_class' => sub { (shift)->{associated_class} }
266 Class::MOP::Attribute->meta->add_attribute(
267 Class::MOP::Attribute->new('accessor' => (
268 reader => 'accessor',
269 predicate => 'has_accessor',
273 Class::MOP::Attribute->meta->add_attribute(
274 Class::MOP::Attribute->new('reader' => (
276 predicate => 'has_reader',
280 Class::MOP::Attribute->meta->add_attribute(
281 Class::MOP::Attribute->new('writer' => (
283 predicate => 'has_writer',
287 Class::MOP::Attribute->meta->add_attribute(
288 Class::MOP::Attribute->new('predicate' => (
289 reader => 'predicate',
290 predicate => 'has_predicate',
294 Class::MOP::Attribute->meta->add_attribute(
295 Class::MOP::Attribute->new('clearer' => (
297 predicate => 'has_clearer',
301 Class::MOP::Attribute->meta->add_attribute(
302 Class::MOP::Attribute->new('init_arg' => (
303 reader => 'init_arg',
304 predicate => 'has_init_arg',
308 Class::MOP::Attribute->meta->add_attribute(
309 Class::MOP::Attribute->new('default' => (
310 # default has a custom 'reader' method ...
311 predicate => 'has_default',
316 # NOTE: (meta-circularity)
317 # This should be one of the last things done
318 # it will "tie the knot" with Class::MOP::Attribute
319 # so that it uses the attributes meta-objects
320 # to construct itself.
321 Class::MOP::Attribute->meta->add_method('new' => sub {
326 (defined $name && $name)
327 || confess "You must provide a name for the attribute";
328 $options{init_arg} = $name
329 if not exists $options{init_arg};
331 (Class::MOP::Attribute::is_default_a_coderef(\%options))
332 || confess("References are not allowed as default values, you must ".
333 "wrap then in a CODE reference (ex: sub { [] } and not [])")
334 if exists $options{default} && ref $options{default};
336 # return the new object
337 $class->meta->new_object(name => $name, %options);
340 Class::MOP::Attribute->meta->add_method('clone' => sub {
342 $self->meta->clone_object($self, @_);
345 ## --------------------------------------------------------
346 ## Now close all the Class::MOP::* classes
348 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
349 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
350 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
351 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
352 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
353 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
354 Class::MOP::Object ->meta->make_immutable(inline_constructor => 0);
364 Class::MOP - A Meta Object Protocol for Perl 5
368 # ... This will come later, for now see
369 # the other SYNOPSIS for more information
373 This module is an attempt to create a meta object protocol for the
374 Perl 5 object system. It makes no attempt to change the behavior or
375 characteristics of the Perl 5 object system, only to create a
376 protocol for its manipulation and introspection.
378 That said, it does attempt to create the tools for building a rich
379 set of extensions to the Perl 5 object system. Every attempt has been
380 made for these tools to keep to the spirit of the Perl 5 object
381 system that we all know and love.
383 This documentation is admittedly sparse on details, as time permits
384 I will try to improve them. For now, I suggest looking at the items
385 listed in the L<SEE ALSO> section for more information. In particular
386 the book "The Art of the Meta Object Protocol" was very influential
387 in the development of this system.
389 =head2 What is a Meta Object Protocol?
391 A meta object protocol is an API to an object system.
393 To be more specific, it is a set of abstractions of the components of
394 an object system (typically things like; classes, object, methods,
395 object attributes, etc.). These abstractions can then be used to both
396 inspect and manipulate the object system which they describe.
398 It can be said that there are two MOPs for any object system; the
399 implicit MOP, and the explicit MOP. The implicit MOP handles things
400 like method dispatch or inheritance, which happen automatically as
401 part of how the object system works. The explicit MOP typically
402 handles the introspection/reflection features of the object system.
403 All object systems have implicit MOPs, without one, they would not
404 work. Explict MOPs however as less common, and depending on the
405 language can vary from restrictive (Reflection in Java or C#) to
406 wide open (CLOS is a perfect example).
408 =head2 Yet Another Class Builder!! Why?
410 This is B<not> a class builder so much as it is a I<class builder
411 B<builder>>. My intent is that an end user does not use this module
412 directly, but instead this module is used by module authors to
413 build extensions and features onto the Perl 5 object system.
415 =head2 Who is this module for?
417 This module is specifically for anyone who has ever created or
418 wanted to create a module for the Class:: namespace. The tools which
419 this module will provide will hopefully make it easier to do more
420 complex things with Perl 5 classes by removing such barriers as
421 the need to hack the symbol tables, or understand the fine details
424 =head2 What changes do I have to make to use this module?
426 This module was designed to be as unintrusive as possible. Many of
427 its features are accessible without B<any> change to your existsing
428 code at all. It is meant to be a compliment to your existing code and
429 not an intrusion on your code base. Unlike many other B<Class::>
430 modules, this module B<does not> require you subclass it, or even that
431 you C<use> it in within your module's package.
433 The only features which requires additions to your code are the
434 attribute handling and instance construction features, and these are
435 both completely optional features. The only reason for this is because
436 Perl 5's object system does not actually have these features built
437 in. More information about this feature can be found below.
439 =head2 A Note about Performance?
441 It is a common misconception that explict MOPs are performance drains.
442 But this is not a universal truth at all, it is an side-effect of
443 specific implementations. For instance, using Java reflection is much
444 slower because the JVM cannot take advantage of any compiler
445 optimizations, and the JVM has to deal with much more runtime type
446 information as well. Reflection in C# is marginally better as it was
447 designed into the language and runtime (the CLR). In contrast, CLOS
448 (the Common Lisp Object System) was built to support an explicit MOP,
449 and so performance is tuned for it.
451 This library in particular does it's absolute best to avoid putting
452 B<any> drain at all upon your code's performance. In fact, by itself
453 it does nothing to affect your existing code. So you only pay for
454 what you actually use.
456 =head2 About Metaclass compatibility
458 This module makes sure that all metaclasses created are both upwards
459 and downwards compatible. The topic of metaclass compatibility is
460 highly esoteric and is something only encountered when doing deep and
461 involved metaclass hacking. There are two basic kinds of metaclass
462 incompatibility; upwards and downwards.
464 Upwards metaclass compatibility means that the metaclass of a
465 given class is either the same as (or a subclass of) all of the
468 Downward metaclass compatibility means that the metaclasses of a
469 given class's anscestors are all either the same as (or a subclass
472 Here is a diagram showing a set of two classes (C<A> and C<B>) and
473 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
474 metaclass compatibility both upwards and downwards.
476 +---------+ +---------+
477 | Meta::A |<----| Meta::B | <....... (instance of )
478 +---------+ +---------+ <------- (inherits from)
481 +---------+ +---------+
483 +---------+ +---------+
485 As I said this is a highly esoteric topic and one you will only run
486 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
487 are interested in why this is an issue see the paper
488 I<Uniform and safe metaclass composition> linked to in the
489 L<SEE ALSO> section of this document.
491 =head2 Using custom metaclasses
493 Always use the metaclass pragma when using a custom metaclass, this
494 will ensure the proper initialization order and not accidentely
495 create an incorrect type of metaclass for you. This is a very rare
496 problem, and one which can only occur if you are doing deep metaclass
497 programming. So in other words, don't worry about it.
501 The protocol is divided into 3 main sub-protocols:
505 =item The Class protocol
507 This provides a means of manipulating and introspecting a Perl 5
508 class. It handles all of symbol table hacking for you, and provides
509 a rich set of methods that go beyond simple package introspection.
511 See L<Class::MOP::Class> for more details.
513 =item The Attribute protocol
515 This provides a consistent represenation for an attribute of a
516 Perl 5 class. Since there are so many ways to create and handle
517 atttributes in Perl 5 OO, this attempts to provide as much of a
518 unified approach as possible, while giving the freedom and
519 flexibility to subclass for specialization.
521 See L<Class::MOP::Attribute> for more details.
523 =item The Method protocol
525 This provides a means of manipulating and introspecting methods in
526 the Perl 5 object system. As with attributes, there are many ways to
527 approach this topic, so we try to keep it pretty basic, while still
528 making it possible to extend the system in many ways.
530 See L<Class::MOP::Method> for more details.
536 Class::MOP holds a cache of metaclasses, the following are functions
537 (B<not methods>) which can be used to access that cache. It is not
538 recommended that you mess with this, bad things could happen. But if
539 you are brave and willing to risk it, go for it.
543 =item B<get_all_metaclasses>
545 This will return an hash of all the metaclass instances that have
546 been cached by B<Class::MOP::Class> keyed by the package name.
548 =item B<get_all_metaclass_instances>
550 This will return an array of all the metaclass instances that have
551 been cached by B<Class::MOP::Class>.
553 =item B<get_all_metaclass_names>
555 This will return an array of all the metaclass names that have
556 been cached by B<Class::MOP::Class>.
558 =item B<get_metaclass_by_name ($name)>
560 =item B<store_metaclass_by_name ($name, $meta)>
562 =item B<weaken_metaclass ($name)>
564 =item B<does_metaclass_exist ($name)>
566 =item B<remove_metaclass_by_name ($name)>
574 There are very few books out on Meta Object Protocols and Metaclasses
575 because it is such an esoteric topic. The following books are really
576 the only ones I have found. If you know of any more, B<I<please>>
577 email me and let me know, I would love to hear about them.
581 =item "The Art of the Meta Object Protocol"
583 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
585 =item "Putting MetaClasses to Work"
587 =item "Smalltalk: The Language"
595 =item Uniform and safe metaclass composition
597 An excellent paper by the people who brought us the original Traits paper.
598 This paper is on how Traits can be used to do safe metaclass composition,
599 and offers an excellent introduction section which delves into the topic of
600 metaclass compatibility.
602 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
604 =item Safe Metaclass Programming
606 This paper seems to precede the above paper, and propose a mix-in based
607 approach as opposed to the Traits based approach. Both papers have similar
608 information on the metaclass compatibility problem space.
610 L<http://citeseer.ist.psu.edu/37617.html>
618 =item The Perl 6 MetaModel work in the Pugs project
622 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
624 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
630 =head1 SIMILAR MODULES
632 As I have said above, this module is a class-builder-builder, so it is
633 not the same thing as modules like L<Class::Accessor> and
634 L<Class::MethodMaker>. That being said there are very few modules on CPAN
635 with similar goals to this module. The one I have found which is most
636 like this module is L<Class::Meta>, although it's philosophy and the MOP it
637 creates are very different from this modules.
641 All complex software has bugs lurking in it, and this module is no
642 exception. If you find a bug please either email me, or add the bug
647 I use L<Devel::Cover> to test the code coverage of my tests, below is the
648 L<Devel::Cover> report on this module's test suite.
650 ---------------------------- ------ ------ ------ ------ ------ ------ ------
651 File stmt bran cond sub pod time total
652 ---------------------------- ------ ------ ------ ------ ------ ------ ------
653 Class/MOP.pm 78.0 87.5 55.6 71.4 100.0 12.4 76.8
654 Class/MOP/Attribute.pm 83.4 75.6 86.7 94.4 100.0 8.9 85.2
655 Class/MOP/Class.pm 96.9 75.8 43.2 98.0 100.0 55.3 83.6
656 Class/MOP/Class/Immutable.pm 88.5 53.8 n/a 95.8 100.0 1.1 84.7
657 Class/MOP/Instance.pm 87.9 75.0 33.3 89.7 100.0 10.1 89.1
658 Class/MOP/Method.pm 97.6 60.0 57.9 76.9 100.0 1.5 82.8
659 Class/MOP/Module.pm 87.5 n/a 11.1 83.3 100.0 0.3 66.7
660 Class/MOP/Object.pm 100.0 n/a 33.3 100.0 100.0 0.1 89.5
661 Class/MOP/Package.pm 95.1 69.0 33.3 100.0 100.0 9.9 85.5
662 metaclass.pm 100.0 100.0 83.3 100.0 n/a 0.5 97.7
663 ---------------------------- ------ ------ ------ ------ ------ ------ ------
664 Total 91.5 72.1 48.8 90.7 100.0 100.0 84.2
665 ---------------------------- ------ ------ ------ ------ ------ ------ ------
667 =head1 ACKNOWLEDGEMENTS
673 Thanks to Rob for actually getting the development of this module kick-started.
679 Stevan Little E<lt>stevan@iinteractive.comE<gt>
681 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
683 =head1 COPYRIGHT AND LICENSE
685 Copyright 2006 by Infinity Interactive, Inc.
687 L<http://www.iinteractive.com>
689 This library is free software; you can redistribute it and/or modify
690 it under the same terms as Perl itself.