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' => (
181 #reader => 'get_method_map',
184 # # as with the $VERSION and $AUTHORITY above
185 # # sometimes we don't/can't store directly
186 # # inside the instance, so we need the accessor
188 # 'get_method_map' => sub {
191 # # there is a faster/better way
192 # # to do this, I am sure :)
195 # $_ => $self->method_metaclass->wrap($self->get_package_symbol('&' . $_))
197 # $self->has_package_symbol('&' . $_)
198 # } $self->list_all_package_symbols
202 #init_arg => '!............( DO NOT DO THIS )............!',
203 #default => sub { \undef }
204 default => sub { {} }
208 Class::MOP::Class->meta->add_attribute(
209 Class::MOP::Attribute->new('$:attribute_metaclass' => (
210 reader => 'attribute_metaclass',
211 init_arg => ':attribute_metaclass',
212 default => 'Class::MOP::Attribute',
216 Class::MOP::Class->meta->add_attribute(
217 Class::MOP::Attribute->new('$:method_metaclass' => (
218 reader => 'method_metaclass',
219 init_arg => ':method_metaclass',
220 default => 'Class::MOP::Method',
224 Class::MOP::Class->meta->add_attribute(
225 Class::MOP::Attribute->new('$:instance_metaclass' => (
227 # NOTE: we need to do this in order
228 # for the instance meta-object to
229 # not fall into meta-circular death
230 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
232 init_arg => ':instance_metaclass',
233 default => 'Class::MOP::Instance',
238 # we don't actually need to tie the knot with
239 # Class::MOP::Class here, it is actually handled
240 # within Class::MOP::Class itself in the
241 # construct_class_instance method.
243 ## --------------------------------------------------------
244 ## Class::MOP::Attribute
246 Class::MOP::Attribute->meta->add_attribute(
247 Class::MOP::Attribute->new('name' => (
249 # NOTE: we need to do this in order
250 # for the instance meta-object to
251 # not fall into meta-circular death
252 'name' => sub { (shift)->{name} }
257 Class::MOP::Attribute->meta->add_attribute(
258 Class::MOP::Attribute->new('associated_class' => (
260 # NOTE: we need to do this in order
261 # for the instance meta-object to
262 # not fall into meta-circular death
263 'associated_class' => sub { (shift)->{associated_class} }
268 Class::MOP::Attribute->meta->add_attribute(
269 Class::MOP::Attribute->new('accessor' => (
270 reader => 'accessor',
271 predicate => 'has_accessor',
275 Class::MOP::Attribute->meta->add_attribute(
276 Class::MOP::Attribute->new('reader' => (
278 predicate => 'has_reader',
282 Class::MOP::Attribute->meta->add_attribute(
283 Class::MOP::Attribute->new('writer' => (
285 predicate => 'has_writer',
289 Class::MOP::Attribute->meta->add_attribute(
290 Class::MOP::Attribute->new('predicate' => (
291 reader => 'predicate',
292 predicate => 'has_predicate',
296 Class::MOP::Attribute->meta->add_attribute(
297 Class::MOP::Attribute->new('clearer' => (
299 predicate => 'has_clearer',
303 Class::MOP::Attribute->meta->add_attribute(
304 Class::MOP::Attribute->new('init_arg' => (
305 reader => 'init_arg',
306 predicate => 'has_init_arg',
310 Class::MOP::Attribute->meta->add_attribute(
311 Class::MOP::Attribute->new('default' => (
312 # default has a custom 'reader' method ...
313 predicate => 'has_default',
318 # NOTE: (meta-circularity)
319 # This should be one of the last things done
320 # it will "tie the knot" with Class::MOP::Attribute
321 # so that it uses the attributes meta-objects
322 # to construct itself.
323 Class::MOP::Attribute->meta->add_method('new' => sub {
328 (defined $name && $name)
329 || confess "You must provide a name for the attribute";
330 $options{init_arg} = $name
331 if not exists $options{init_arg};
333 (Class::MOP::Attribute::is_default_a_coderef(\%options))
334 || confess("References are not allowed as default values, you must ".
335 "wrap then in a CODE reference (ex: sub { [] } and not [])")
336 if exists $options{default} && ref $options{default};
338 # return the new object
339 $class->meta->new_object(name => $name, %options);
342 Class::MOP::Attribute->meta->add_method('clone' => sub {
344 $self->meta->clone_object($self, @_);
347 ## --------------------------------------------------------
348 ## Now close all the Class::MOP::* classes
350 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
351 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
352 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
353 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
354 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
355 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
356 Class::MOP::Object ->meta->make_immutable(inline_constructor => 0);
366 Class::MOP - A Meta Object Protocol for Perl 5
370 # ... This will come later, for now see
371 # the other SYNOPSIS for more information
375 This module is an attempt to create a meta object protocol for the
376 Perl 5 object system. It makes no attempt to change the behavior or
377 characteristics of the Perl 5 object system, only to create a
378 protocol for its manipulation and introspection.
380 That said, it does attempt to create the tools for building a rich
381 set of extensions to the Perl 5 object system. Every attempt has been
382 made for these tools to keep to the spirit of the Perl 5 object
383 system that we all know and love.
385 This documentation is admittedly sparse on details, as time permits
386 I will try to improve them. For now, I suggest looking at the items
387 listed in the L<SEE ALSO> section for more information. In particular
388 the book "The Art of the Meta Object Protocol" was very influential
389 in the development of this system.
391 =head2 What is a Meta Object Protocol?
393 A meta object protocol is an API to an object system.
395 To be more specific, it is a set of abstractions of the components of
396 an object system (typically things like; classes, object, methods,
397 object attributes, etc.). These abstractions can then be used to both
398 inspect and manipulate the object system which they describe.
400 It can be said that there are two MOPs for any object system; the
401 implicit MOP, and the explicit MOP. The implicit MOP handles things
402 like method dispatch or inheritance, which happen automatically as
403 part of how the object system works. The explicit MOP typically
404 handles the introspection/reflection features of the object system.
405 All object systems have implicit MOPs, without one, they would not
406 work. Explict MOPs however as less common, and depending on the
407 language can vary from restrictive (Reflection in Java or C#) to
408 wide open (CLOS is a perfect example).
410 =head2 Yet Another Class Builder!! Why?
412 This is B<not> a class builder so much as it is a I<class builder
413 B<builder>>. My intent is that an end user does not use this module
414 directly, but instead this module is used by module authors to
415 build extensions and features onto the Perl 5 object system.
417 =head2 Who is this module for?
419 This module is specifically for anyone who has ever created or
420 wanted to create a module for the Class:: namespace. The tools which
421 this module will provide will hopefully make it easier to do more
422 complex things with Perl 5 classes by removing such barriers as
423 the need to hack the symbol tables, or understand the fine details
426 =head2 What changes do I have to make to use this module?
428 This module was designed to be as unintrusive as possible. Many of
429 its features are accessible without B<any> change to your existsing
430 code at all. It is meant to be a compliment to your existing code and
431 not an intrusion on your code base. Unlike many other B<Class::>
432 modules, this module B<does not> require you subclass it, or even that
433 you C<use> it in within your module's package.
435 The only features which requires additions to your code are the
436 attribute handling and instance construction features, and these are
437 both completely optional features. The only reason for this is because
438 Perl 5's object system does not actually have these features built
439 in. More information about this feature can be found below.
441 =head2 A Note about Performance?
443 It is a common misconception that explict MOPs are performance drains.
444 But this is not a universal truth at all, it is an side-effect of
445 specific implementations. For instance, using Java reflection is much
446 slower because the JVM cannot take advantage of any compiler
447 optimizations, and the JVM has to deal with much more runtime type
448 information as well. Reflection in C# is marginally better as it was
449 designed into the language and runtime (the CLR). In contrast, CLOS
450 (the Common Lisp Object System) was built to support an explicit MOP,
451 and so performance is tuned for it.
453 This library in particular does it's absolute best to avoid putting
454 B<any> drain at all upon your code's performance. In fact, by itself
455 it does nothing to affect your existing code. So you only pay for
456 what you actually use.
458 =head2 About Metaclass compatibility
460 This module makes sure that all metaclasses created are both upwards
461 and downwards compatible. The topic of metaclass compatibility is
462 highly esoteric and is something only encountered when doing deep and
463 involved metaclass hacking. There are two basic kinds of metaclass
464 incompatibility; upwards and downwards.
466 Upwards metaclass compatibility means that the metaclass of a
467 given class is either the same as (or a subclass of) all of the
470 Downward metaclass compatibility means that the metaclasses of a
471 given class's anscestors are all either the same as (or a subclass
474 Here is a diagram showing a set of two classes (C<A> and C<B>) and
475 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
476 metaclass compatibility both upwards and downwards.
478 +---------+ +---------+
479 | Meta::A |<----| Meta::B | <....... (instance of )
480 +---------+ +---------+ <------- (inherits from)
483 +---------+ +---------+
485 +---------+ +---------+
487 As I said this is a highly esoteric topic and one you will only run
488 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
489 are interested in why this is an issue see the paper
490 I<Uniform and safe metaclass composition> linked to in the
491 L<SEE ALSO> section of this document.
493 =head2 Using custom metaclasses
495 Always use the metaclass pragma when using a custom metaclass, this
496 will ensure the proper initialization order and not accidentely
497 create an incorrect type of metaclass for you. This is a very rare
498 problem, and one which can only occur if you are doing deep metaclass
499 programming. So in other words, don't worry about it.
503 The protocol is divided into 3 main sub-protocols:
507 =item The Class protocol
509 This provides a means of manipulating and introspecting a Perl 5
510 class. It handles all of symbol table hacking for you, and provides
511 a rich set of methods that go beyond simple package introspection.
513 See L<Class::MOP::Class> for more details.
515 =item The Attribute protocol
517 This provides a consistent represenation for an attribute of a
518 Perl 5 class. Since there are so many ways to create and handle
519 atttributes in Perl 5 OO, this attempts to provide as much of a
520 unified approach as possible, while giving the freedom and
521 flexibility to subclass for specialization.
523 See L<Class::MOP::Attribute> for more details.
525 =item The Method protocol
527 This provides a means of manipulating and introspecting methods in
528 the Perl 5 object system. As with attributes, there are many ways to
529 approach this topic, so we try to keep it pretty basic, while still
530 making it possible to extend the system in many ways.
532 See L<Class::MOP::Method> for more details.
538 Class::MOP holds a cache of metaclasses, the following are functions
539 (B<not methods>) which can be used to access that cache. It is not
540 recommended that you mess with this, bad things could happen. But if
541 you are brave and willing to risk it, go for it.
545 =item B<get_all_metaclasses>
547 This will return an hash of all the metaclass instances that have
548 been cached by B<Class::MOP::Class> keyed by the package name.
550 =item B<get_all_metaclass_instances>
552 This will return an array of all the metaclass instances that have
553 been cached by B<Class::MOP::Class>.
555 =item B<get_all_metaclass_names>
557 This will return an array of all the metaclass names that have
558 been cached by B<Class::MOP::Class>.
560 =item B<get_metaclass_by_name ($name)>
562 =item B<store_metaclass_by_name ($name, $meta)>
564 =item B<weaken_metaclass ($name)>
566 =item B<does_metaclass_exist ($name)>
568 =item B<remove_metaclass_by_name ($name)>
576 There are very few books out on Meta Object Protocols and Metaclasses
577 because it is such an esoteric topic. The following books are really
578 the only ones I have found. If you know of any more, B<I<please>>
579 email me and let me know, I would love to hear about them.
583 =item "The Art of the Meta Object Protocol"
585 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
587 =item "Putting MetaClasses to Work"
589 =item "Smalltalk: The Language"
597 =item Uniform and safe metaclass composition
599 An excellent paper by the people who brought us the original Traits paper.
600 This paper is on how Traits can be used to do safe metaclass composition,
601 and offers an excellent introduction section which delves into the topic of
602 metaclass compatibility.
604 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
606 =item Safe Metaclass Programming
608 This paper seems to precede the above paper, and propose a mix-in based
609 approach as opposed to the Traits based approach. Both papers have similar
610 information on the metaclass compatibility problem space.
612 L<http://citeseer.ist.psu.edu/37617.html>
620 =item The Perl 6 MetaModel work in the Pugs project
624 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
626 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
632 =head1 SIMILAR MODULES
634 As I have said above, this module is a class-builder-builder, so it is
635 not the same thing as modules like L<Class::Accessor> and
636 L<Class::MethodMaker>. That being said there are very few modules on CPAN
637 with similar goals to this module. The one I have found which is most
638 like this module is L<Class::Meta>, although it's philosophy and the MOP it
639 creates are very different from this modules.
643 All complex software has bugs lurking in it, and this module is no
644 exception. If you find a bug please either email me, or add the bug
649 I use L<Devel::Cover> to test the code coverage of my tests, below is the
650 L<Devel::Cover> report on this module's test suite.
652 ---------------------------- ------ ------ ------ ------ ------ ------ ------
653 File stmt bran cond sub pod time total
654 ---------------------------- ------ ------ ------ ------ ------ ------ ------
655 Class/MOP.pm 78.0 87.5 55.6 71.4 100.0 12.4 76.8
656 Class/MOP/Attribute.pm 83.4 75.6 86.7 94.4 100.0 8.9 85.2
657 Class/MOP/Class.pm 96.9 75.8 43.2 98.0 100.0 55.3 83.6
658 Class/MOP/Class/Immutable.pm 88.5 53.8 n/a 95.8 100.0 1.1 84.7
659 Class/MOP/Instance.pm 87.9 75.0 33.3 89.7 100.0 10.1 89.1
660 Class/MOP/Method.pm 97.6 60.0 57.9 76.9 100.0 1.5 82.8
661 Class/MOP/Module.pm 87.5 n/a 11.1 83.3 100.0 0.3 66.7
662 Class/MOP/Object.pm 100.0 n/a 33.3 100.0 100.0 0.1 89.5
663 Class/MOP/Package.pm 95.1 69.0 33.3 100.0 100.0 9.9 85.5
664 metaclass.pm 100.0 100.0 83.3 100.0 n/a 0.5 97.7
665 ---------------------------- ------ ------ ------ ------ ------ ------ ------
666 Total 91.5 72.1 48.8 90.7 100.0 100.0 84.2
667 ---------------------------- ------ ------ ------ ------ ------ ------ ------
669 =head1 ACKNOWLEDGEMENTS
675 Thanks to Rob for actually getting the development of this module kick-started.
681 Stevan Little E<lt>stevan@iinteractive.comE<gt>
683 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
685 =head1 COPYRIGHT AND LICENSE
687 Copyright 2006 by Infinity Interactive, Inc.
689 L<http://www.iinteractive.com>
691 This library is free software; you can redistribute it and/or modify
692 it under the same terms as Perl itself.