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' => (
182 'get_method_map' => sub {
184 my $map = $self->{'%:methods'};
185 foreach my $symbol ($self->list_all_package_symbols('CODE')) {
186 my $code = $self->get_package_symbol('&' . $symbol);
187 next if exists $map->{$symbol} && $map->{$symbol}->body == $code;
188 $map->{$symbol} = $self->method_metaclass->wrap($code);
193 default => sub { {} }
197 Class::MOP::Class->meta->add_attribute(
198 Class::MOP::Attribute->new('$:attribute_metaclass' => (
199 reader => 'attribute_metaclass',
200 init_arg => ':attribute_metaclass',
201 default => 'Class::MOP::Attribute',
205 Class::MOP::Class->meta->add_attribute(
206 Class::MOP::Attribute->new('$:method_metaclass' => (
207 reader => 'method_metaclass',
208 init_arg => ':method_metaclass',
209 default => 'Class::MOP::Method',
213 Class::MOP::Class->meta->add_attribute(
214 Class::MOP::Attribute->new('$:instance_metaclass' => (
216 # NOTE: we need to do this in order
217 # for the instance meta-object to
218 # not fall into meta-circular death
219 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
221 init_arg => ':instance_metaclass',
222 default => 'Class::MOP::Instance',
227 # we don't actually need to tie the knot with
228 # Class::MOP::Class here, it is actually handled
229 # within Class::MOP::Class itself in the
230 # construct_class_instance method.
232 ## --------------------------------------------------------
233 ## Class::MOP::Attribute
235 Class::MOP::Attribute->meta->add_attribute(
236 Class::MOP::Attribute->new('name' => (
238 # NOTE: we need to do this in order
239 # for the instance meta-object to
240 # not fall into meta-circular death
241 'name' => sub { (shift)->{name} }
246 Class::MOP::Attribute->meta->add_attribute(
247 Class::MOP::Attribute->new('associated_class' => (
249 # NOTE: we need to do this in order
250 # for the instance meta-object to
251 # not fall into meta-circular death
252 'associated_class' => sub { (shift)->{associated_class} }
257 Class::MOP::Attribute->meta->add_attribute(
258 Class::MOP::Attribute->new('accessor' => (
259 reader => 'accessor',
260 predicate => 'has_accessor',
264 Class::MOP::Attribute->meta->add_attribute(
265 Class::MOP::Attribute->new('reader' => (
267 predicate => 'has_reader',
271 Class::MOP::Attribute->meta->add_attribute(
272 Class::MOP::Attribute->new('writer' => (
274 predicate => 'has_writer',
278 Class::MOP::Attribute->meta->add_attribute(
279 Class::MOP::Attribute->new('predicate' => (
280 reader => 'predicate',
281 predicate => 'has_predicate',
285 Class::MOP::Attribute->meta->add_attribute(
286 Class::MOP::Attribute->new('clearer' => (
288 predicate => 'has_clearer',
292 Class::MOP::Attribute->meta->add_attribute(
293 Class::MOP::Attribute->new('init_arg' => (
294 reader => 'init_arg',
295 predicate => 'has_init_arg',
299 Class::MOP::Attribute->meta->add_attribute(
300 Class::MOP::Attribute->new('default' => (
301 # default has a custom 'reader' method ...
302 predicate => 'has_default',
307 # NOTE: (meta-circularity)
308 # This should be one of the last things done
309 # it will "tie the knot" with Class::MOP::Attribute
310 # so that it uses the attributes meta-objects
311 # to construct itself.
312 Class::MOP::Attribute->meta->add_method('new' => sub {
317 (defined $name && $name)
318 || confess "You must provide a name for the attribute";
319 $options{init_arg} = $name
320 if not exists $options{init_arg};
322 (Class::MOP::Attribute::is_default_a_coderef(\%options))
323 || confess("References are not allowed as default values, you must ".
324 "wrap then in a CODE reference (ex: sub { [] } and not [])")
325 if exists $options{default} && ref $options{default};
327 # return the new object
328 $class->meta->new_object(name => $name, %options);
331 Class::MOP::Attribute->meta->add_method('clone' => sub {
333 $self->meta->clone_object($self, @_);
336 ## --------------------------------------------------------
337 ## Now close all the Class::MOP::* classes
339 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
340 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
341 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
342 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
343 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
344 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
345 Class::MOP::Object ->meta->make_immutable(inline_constructor => 0);
355 Class::MOP - A Meta Object Protocol for Perl 5
359 # ... This will come later, for now see
360 # the other SYNOPSIS for more information
364 This module is an attempt to create a meta object protocol for the
365 Perl 5 object system. It makes no attempt to change the behavior or
366 characteristics of the Perl 5 object system, only to create a
367 protocol for its manipulation and introspection.
369 That said, it does attempt to create the tools for building a rich
370 set of extensions to the Perl 5 object system. Every attempt has been
371 made for these tools to keep to the spirit of the Perl 5 object
372 system that we all know and love.
374 This documentation is admittedly sparse on details, as time permits
375 I will try to improve them. For now, I suggest looking at the items
376 listed in the L<SEE ALSO> section for more information. In particular
377 the book "The Art of the Meta Object Protocol" was very influential
378 in the development of this system.
380 =head2 What is a Meta Object Protocol?
382 A meta object protocol is an API to an object system.
384 To be more specific, it is a set of abstractions of the components of
385 an object system (typically things like; classes, object, methods,
386 object attributes, etc.). These abstractions can then be used to both
387 inspect and manipulate the object system which they describe.
389 It can be said that there are two MOPs for any object system; the
390 implicit MOP, and the explicit MOP. The implicit MOP handles things
391 like method dispatch or inheritance, which happen automatically as
392 part of how the object system works. The explicit MOP typically
393 handles the introspection/reflection features of the object system.
394 All object systems have implicit MOPs, without one, they would not
395 work. Explict MOPs however as less common, and depending on the
396 language can vary from restrictive (Reflection in Java or C#) to
397 wide open (CLOS is a perfect example).
399 =head2 Yet Another Class Builder!! Why?
401 This is B<not> a class builder so much as it is a I<class builder
402 B<builder>>. My intent is that an end user does not use this module
403 directly, but instead this module is used by module authors to
404 build extensions and features onto the Perl 5 object system.
406 =head2 Who is this module for?
408 This module is specifically for anyone who has ever created or
409 wanted to create a module for the Class:: namespace. The tools which
410 this module will provide will hopefully make it easier to do more
411 complex things with Perl 5 classes by removing such barriers as
412 the need to hack the symbol tables, or understand the fine details
415 =head2 What changes do I have to make to use this module?
417 This module was designed to be as unintrusive as possible. Many of
418 its features are accessible without B<any> change to your existsing
419 code at all. It is meant to be a compliment to your existing code and
420 not an intrusion on your code base. Unlike many other B<Class::>
421 modules, this module B<does not> require you subclass it, or even that
422 you C<use> it in within your module's package.
424 The only features which requires additions to your code are the
425 attribute handling and instance construction features, and these are
426 both completely optional features. The only reason for this is because
427 Perl 5's object system does not actually have these features built
428 in. More information about this feature can be found below.
430 =head2 A Note about Performance?
432 It is a common misconception that explict MOPs are performance drains.
433 But this is not a universal truth at all, it is an side-effect of
434 specific implementations. For instance, using Java reflection is much
435 slower because the JVM cannot take advantage of any compiler
436 optimizations, and the JVM has to deal with much more runtime type
437 information as well. Reflection in C# is marginally better as it was
438 designed into the language and runtime (the CLR). In contrast, CLOS
439 (the Common Lisp Object System) was built to support an explicit MOP,
440 and so performance is tuned for it.
442 This library in particular does it's absolute best to avoid putting
443 B<any> drain at all upon your code's performance. In fact, by itself
444 it does nothing to affect your existing code. So you only pay for
445 what you actually use.
447 =head2 About Metaclass compatibility
449 This module makes sure that all metaclasses created are both upwards
450 and downwards compatible. The topic of metaclass compatibility is
451 highly esoteric and is something only encountered when doing deep and
452 involved metaclass hacking. There are two basic kinds of metaclass
453 incompatibility; upwards and downwards.
455 Upwards metaclass compatibility means that the metaclass of a
456 given class is either the same as (or a subclass of) all of the
459 Downward metaclass compatibility means that the metaclasses of a
460 given class's anscestors are all either the same as (or a subclass
463 Here is a diagram showing a set of two classes (C<A> and C<B>) and
464 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
465 metaclass compatibility both upwards and downwards.
467 +---------+ +---------+
468 | Meta::A |<----| Meta::B | <....... (instance of )
469 +---------+ +---------+ <------- (inherits from)
472 +---------+ +---------+
474 +---------+ +---------+
476 As I said this is a highly esoteric topic and one you will only run
477 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
478 are interested in why this is an issue see the paper
479 I<Uniform and safe metaclass composition> linked to in the
480 L<SEE ALSO> section of this document.
482 =head2 Using custom metaclasses
484 Always use the metaclass pragma when using a custom metaclass, this
485 will ensure the proper initialization order and not accidentely
486 create an incorrect type of metaclass for you. This is a very rare
487 problem, and one which can only occur if you are doing deep metaclass
488 programming. So in other words, don't worry about it.
492 The protocol is divided into 3 main sub-protocols:
496 =item The Class protocol
498 This provides a means of manipulating and introspecting a Perl 5
499 class. It handles all of symbol table hacking for you, and provides
500 a rich set of methods that go beyond simple package introspection.
502 See L<Class::MOP::Class> for more details.
504 =item The Attribute protocol
506 This provides a consistent represenation for an attribute of a
507 Perl 5 class. Since there are so many ways to create and handle
508 atttributes in Perl 5 OO, this attempts to provide as much of a
509 unified approach as possible, while giving the freedom and
510 flexibility to subclass for specialization.
512 See L<Class::MOP::Attribute> for more details.
514 =item The Method protocol
516 This provides a means of manipulating and introspecting methods in
517 the Perl 5 object system. As with attributes, there are many ways to
518 approach this topic, so we try to keep it pretty basic, while still
519 making it possible to extend the system in many ways.
521 See L<Class::MOP::Method> for more details.
527 Class::MOP holds a cache of metaclasses, the following are functions
528 (B<not methods>) which can be used to access that cache. It is not
529 recommended that you mess with this, bad things could happen. But if
530 you are brave and willing to risk it, go for it.
534 =item B<get_all_metaclasses>
536 This will return an hash of all the metaclass instances that have
537 been cached by B<Class::MOP::Class> keyed by the package name.
539 =item B<get_all_metaclass_instances>
541 This will return an array of all the metaclass instances that have
542 been cached by B<Class::MOP::Class>.
544 =item B<get_all_metaclass_names>
546 This will return an array of all the metaclass names that have
547 been cached by B<Class::MOP::Class>.
549 =item B<get_metaclass_by_name ($name)>
551 =item B<store_metaclass_by_name ($name, $meta)>
553 =item B<weaken_metaclass ($name)>
555 =item B<does_metaclass_exist ($name)>
557 =item B<remove_metaclass_by_name ($name)>
565 There are very few books out on Meta Object Protocols and Metaclasses
566 because it is such an esoteric topic. The following books are really
567 the only ones I have found. If you know of any more, B<I<please>>
568 email me and let me know, I would love to hear about them.
572 =item "The Art of the Meta Object Protocol"
574 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
576 =item "Putting MetaClasses to Work"
578 =item "Smalltalk: The Language"
586 =item Uniform and safe metaclass composition
588 An excellent paper by the people who brought us the original Traits paper.
589 This paper is on how Traits can be used to do safe metaclass composition,
590 and offers an excellent introduction section which delves into the topic of
591 metaclass compatibility.
593 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
595 =item Safe Metaclass Programming
597 This paper seems to precede the above paper, and propose a mix-in based
598 approach as opposed to the Traits based approach. Both papers have similar
599 information on the metaclass compatibility problem space.
601 L<http://citeseer.ist.psu.edu/37617.html>
609 =item The Perl 6 MetaModel work in the Pugs project
613 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
615 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
621 =head1 SIMILAR MODULES
623 As I have said above, this module is a class-builder-builder, so it is
624 not the same thing as modules like L<Class::Accessor> and
625 L<Class::MethodMaker>. That being said there are very few modules on CPAN
626 with similar goals to this module. The one I have found which is most
627 like this module is L<Class::Meta>, although it's philosophy and the MOP it
628 creates are very different from this modules.
632 All complex software has bugs lurking in it, and this module is no
633 exception. If you find a bug please either email me, or add the bug
638 I use L<Devel::Cover> to test the code coverage of my tests, below is the
639 L<Devel::Cover> report on this module's test suite.
641 ---------------------------- ------ ------ ------ ------ ------ ------ ------
642 File stmt bran cond sub pod time total
643 ---------------------------- ------ ------ ------ ------ ------ ------ ------
644 Class/MOP.pm 78.0 87.5 55.6 71.4 100.0 12.4 76.8
645 Class/MOP/Attribute.pm 83.4 75.6 86.7 94.4 100.0 8.9 85.2
646 Class/MOP/Class.pm 96.9 75.8 43.2 98.0 100.0 55.3 83.6
647 Class/MOP/Class/Immutable.pm 88.5 53.8 n/a 95.8 100.0 1.1 84.7
648 Class/MOP/Instance.pm 87.9 75.0 33.3 89.7 100.0 10.1 89.1
649 Class/MOP/Method.pm 97.6 60.0 57.9 76.9 100.0 1.5 82.8
650 Class/MOP/Module.pm 87.5 n/a 11.1 83.3 100.0 0.3 66.7
651 Class/MOP/Object.pm 100.0 n/a 33.3 100.0 100.0 0.1 89.5
652 Class/MOP/Package.pm 95.1 69.0 33.3 100.0 100.0 9.9 85.5
653 metaclass.pm 100.0 100.0 83.3 100.0 n/a 0.5 97.7
654 ---------------------------- ------ ------ ------ ------ ------ ------ ------
655 Total 91.5 72.1 48.8 90.7 100.0 100.0 84.2
656 ---------------------------- ------ ------ ------ ------ ------ ------ ------
658 =head1 ACKNOWLEDGEMENTS
664 Thanks to Rob for actually getting the development of this module kick-started.
670 Stevan Little E<lt>stevan@iinteractive.comE<gt>
672 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
674 =head1 COPYRIGHT AND LICENSE
676 Copyright 2006 by Infinity Interactive, Inc.
678 L<http://www.iinteractive.com>
680 This library is free software; you can redistribute it and/or modify
681 it under the same terms as Perl itself.