8 use Scalar::Util 'weaken';
10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 use Class::MOP::Immutable;
17 our $VERSION = '0.53';
18 our $AUTHORITY = 'cpan:STEVAN';
21 XSLoader::load( 'Class::MOP', $VERSION );
23 unless ($] < 5.009_005) {
25 no warnings 'redefine', 'prototype';
26 *check_package_cache_flag = \&mro::get_pkg_gen;
27 *IS_RUNNING_ON_5_10 = sub () { 1 };
30 *IS_RUNNING_ON_5_10 = sub () { 0 };
35 # Metaclasses are singletons, so we cache them here.
36 # there is no need to worry about destruction though
37 # because they should die only when the program dies.
38 # After all, do package definitions even get reaped?
41 # means of accessing all the metaclasses that have
42 # been initialized thus far (for mugwumps obj browser)
43 sub get_all_metaclasses { %METAS }
44 sub get_all_metaclass_instances { values %METAS }
45 sub get_all_metaclass_names { keys %METAS }
46 sub get_metaclass_by_name { $METAS{$_[0]} }
47 sub store_metaclass_by_name { $METAS{$_[0]} = $_[1] }
48 sub weaken_metaclass { weaken($METAS{$_[0]}) }
49 sub does_metaclass_exist { exists $METAS{$_[0]} && defined $METAS{$_[0]} }
50 sub remove_metaclass_by_name { $METAS{$_[0]} = undef }
53 # We only cache metaclasses, meaning instances of
54 # Class::MOP::Class. We do not cache instance of
55 # Class::MOP::Package or Class::MOP::Module. Mostly
56 # because I don't yet see a good reason to do so.
61 # see if this is already
62 # loaded in the symbol table
63 return 1 if is_class_loaded($class);
64 # otherwise require it ...
65 my $file = $class . '.pm';
67 eval { CORE::require($file) };
68 confess "Could not load class ($class) because : $@" if $@;
69 unless (does_metaclass_exist($class)) {
70 eval { Class::MOP::Class->initialize($class) };
71 confess "Could not initialize class ($class) because : $@" if $@;
73 1; # return true if it worked
79 return 1 if defined ${"${class}::VERSION"} || defined @{"${class}::ISA"};
80 foreach (keys %{"${class}::"}) {
81 next if substr($_, -2, 2) eq '::';
82 return 1 if defined &{"${class}::$_"};
88 ## ----------------------------------------------------------------------------
89 ## Setting up our environment ...
90 ## ----------------------------------------------------------------------------
91 ## Class::MOP needs to have a few things in the global perl environment so
92 ## that it can operate effectively. Those things are done here.
93 ## ----------------------------------------------------------------------------
95 # ... nothing yet actually ;)
97 ## ----------------------------------------------------------------------------
99 ## ----------------------------------------------------------------------------
100 ## The code below here is to bootstrap our MOP with itself. This is also
101 ## sometimes called "tying the knot". By doing this, we make it much easier
102 ## to extend the MOP through subclassing and such since now you can use the
103 ## MOP itself to extend itself.
105 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
106 ## ----------------------------------------------------------------------------
108 # We need to add in the meta-attributes here so that
109 # any subclass of Class::MOP::* will be able to
110 # inherit them using &construct_instance
112 ## --------------------------------------------------------
113 ## Class::MOP::Package
115 Class::MOP::Package->meta->add_attribute(
116 Class::MOP::Attribute->new('$!package' => (
118 # NOTE: we need to do this in order
119 # for the instance meta-object to
120 # not fall into meta-circular death
122 # we just alias the original method
123 # rather than re-produce it here
124 'name' => \&Class::MOP::Package::name
126 init_arg => 'package',
130 Class::MOP::Package->meta->add_attribute(
131 Class::MOP::Attribute->new('%!namespace' => (
134 # we just alias the original method
135 # rather than re-produce it here
136 'namespace' => \&Class::MOP::Package::namespace
139 default => sub { \undef }
144 # use the metaclass to construct the meta-package
145 # which is a superclass of the metaclass itself :P
146 Class::MOP::Package->meta->add_method('initialize' => sub {
148 my $package_name = shift;
149 $class->meta->new_object('package' => $package_name, @_);
152 ## --------------------------------------------------------
153 ## Class::MOP::Module
156 # yeah this is kind of stretching things a bit,
157 # but truthfully the version should be an attribute
158 # of the Module, the weirdness comes from having to
159 # stick to Perl 5 convention and store it in the
160 # $VERSION package variable. Basically if you just
161 # squint at it, it will look how you want it to look.
162 # Either as a package variable, or as a attribute of
163 # the metaclass, isn't abstraction great :)
165 Class::MOP::Module->meta->add_attribute(
166 Class::MOP::Attribute->new('$!version' => (
169 # we just alias the original method
170 # rather than re-produce it here
171 'version' => \&Class::MOP::Module::version
174 default => sub { \undef }
179 # By following the same conventions as version here,
180 # we are opening up the possibility that people can
181 # use the $AUTHORITY in non-Class::MOP modules as
184 Class::MOP::Module->meta->add_attribute(
185 Class::MOP::Attribute->new('$!authority' => (
188 # we just alias the original method
189 # rather than re-produce it here
190 'authority' => \&Class::MOP::Module::authority
193 default => sub { \undef }
197 ## --------------------------------------------------------
200 Class::MOP::Class->meta->add_attribute(
201 Class::MOP::Attribute->new('%!attributes' => (
203 # NOTE: we need to do this in order
204 # for the instance meta-object to
205 # not fall into meta-circular death
207 # we just alias the original method
208 # rather than re-produce it here
209 'get_attribute_map' => \&Class::MOP::Class::get_attribute_map
211 init_arg => 'attributes',
212 default => sub { {} }
216 Class::MOP::Class->meta->add_attribute(
217 Class::MOP::Attribute->new('%!methods' => (
218 init_arg => 'methods',
221 # we just alias the original method
222 # rather than re-produce it here
223 'get_method_map' => \&Class::MOP::Class::get_method_map
225 default => sub { {} }
229 Class::MOP::Class->meta->add_attribute(
230 Class::MOP::Attribute->new('@!superclasses' => (
233 # we just alias the original method
234 # rather than re-produce it here
235 'superclasses' => \&Class::MOP::Class::superclasses
238 default => sub { \undef }
242 Class::MOP::Class->meta->add_attribute(
243 Class::MOP::Attribute->new('$!attribute_metaclass' => (
246 # we just alias the original method
247 # rather than re-produce it here
248 'attribute_metaclass' => \&Class::MOP::Class::attribute_metaclass
250 init_arg => 'attribute_metaclass',
251 default => 'Class::MOP::Attribute',
255 Class::MOP::Class->meta->add_attribute(
256 Class::MOP::Attribute->new('$!method_metaclass' => (
259 # we just alias the original method
260 # rather than re-produce it here
261 'method_metaclass' => \&Class::MOP::Class::method_metaclass
263 init_arg => 'method_metaclass',
264 default => 'Class::MOP::Method',
268 Class::MOP::Class->meta->add_attribute(
269 Class::MOP::Attribute->new('$!instance_metaclass' => (
271 # NOTE: we need to do this in order
272 # for the instance meta-object to
273 # not fall into meta-circular death
275 # we just alias the original method
276 # rather than re-produce it here
277 'instance_metaclass' => \&Class::MOP::Class::instance_metaclass
279 init_arg => 'instance_metaclass',
280 default => 'Class::MOP::Instance',
285 # we don't actually need to tie the knot with
286 # Class::MOP::Class here, it is actually handled
287 # within Class::MOP::Class itself in the
288 # construct_class_instance method.
290 ## --------------------------------------------------------
291 ## Class::MOP::Attribute
293 Class::MOP::Attribute->meta->add_attribute(
294 Class::MOP::Attribute->new('$!name' => (
297 # NOTE: we need to do this in order
298 # for the instance meta-object to
299 # not fall into meta-circular death
301 # we just alias the original method
302 # rather than re-produce it here
303 'name' => \&Class::MOP::Attribute::name
308 Class::MOP::Attribute->meta->add_attribute(
309 Class::MOP::Attribute->new('$!associated_class' => (
310 init_arg => 'associated_class',
312 # NOTE: we need to do this in order
313 # for the instance meta-object to
314 # not fall into meta-circular death
316 # we just alias the original method
317 # rather than re-produce it here
318 'associated_class' => \&Class::MOP::Attribute::associated_class
323 Class::MOP::Attribute->meta->add_attribute(
324 Class::MOP::Attribute->new('$!accessor' => (
325 init_arg => 'accessor',
326 reader => { 'accessor' => \&Class::MOP::Attribute::accessor },
327 predicate => { 'has_accessor' => \&Class::MOP::Attribute::has_accessor },
331 Class::MOP::Attribute->meta->add_attribute(
332 Class::MOP::Attribute->new('$!reader' => (
333 init_arg => 'reader',
334 reader => { 'reader' => \&Class::MOP::Attribute::reader },
335 predicate => { 'has_reader' => \&Class::MOP::Attribute::has_reader },
339 Class::MOP::Attribute->meta->add_attribute(
340 Class::MOP::Attribute->new('$!initializer' => (
341 init_arg => 'initializer',
342 reader => { 'initializer' => \&Class::MOP::Attribute::initializer },
343 predicate => { 'has_initializer' => \&Class::MOP::Attribute::has_initializer },
347 Class::MOP::Attribute->meta->add_attribute(
348 Class::MOP::Attribute->new('$!writer' => (
349 init_arg => 'writer',
350 reader => { 'writer' => \&Class::MOP::Attribute::writer },
351 predicate => { 'has_writer' => \&Class::MOP::Attribute::has_writer },
355 Class::MOP::Attribute->meta->add_attribute(
356 Class::MOP::Attribute->new('$!predicate' => (
357 init_arg => 'predicate',
358 reader => { 'predicate' => \&Class::MOP::Attribute::predicate },
359 predicate => { 'has_predicate' => \&Class::MOP::Attribute::has_predicate },
363 Class::MOP::Attribute->meta->add_attribute(
364 Class::MOP::Attribute->new('$!clearer' => (
365 init_arg => 'clearer',
366 reader => { 'clearer' => \&Class::MOP::Attribute::clearer },
367 predicate => { 'has_clearer' => \&Class::MOP::Attribute::has_clearer },
371 Class::MOP::Attribute->meta->add_attribute(
372 Class::MOP::Attribute->new('$!builder' => (
373 init_arg => 'builder',
374 reader => { 'builder' => \&Class::MOP::Attribute::builder },
375 predicate => { 'has_builder' => \&Class::MOP::Attribute::has_builder },
379 Class::MOP::Attribute->meta->add_attribute(
380 Class::MOP::Attribute->new('$!init_arg' => (
381 init_arg => 'init_arg',
382 reader => { 'init_arg' => \&Class::MOP::Attribute::init_arg },
383 predicate => { 'has_init_arg' => \&Class::MOP::Attribute::has_init_arg },
387 Class::MOP::Attribute->meta->add_attribute(
388 Class::MOP::Attribute->new('$!default' => (
389 init_arg => 'default',
390 # default has a custom 'reader' method ...
391 predicate => { 'has_default' => \&Class::MOP::Attribute::has_default },
395 Class::MOP::Attribute->meta->add_attribute(
396 Class::MOP::Attribute->new('@!associated_methods' => (
397 init_arg => 'associated_methods',
398 reader => { 'associated_methods' => \&Class::MOP::Attribute::associated_methods },
399 default => sub { [] }
403 # NOTE: (meta-circularity)
404 # This should be one of the last things done
405 # it will "tie the knot" with Class::MOP::Attribute
406 # so that it uses the attributes meta-objects
407 # to construct itself.
408 Class::MOP::Attribute->meta->add_method('new' => sub {
413 (defined $name && $name)
414 || confess "You must provide a name for the attribute";
415 $options{init_arg} = $name
416 if not exists $options{init_arg};
418 if(exists $options{builder}){
419 confess("builder must be a defined scalar value which is a method name")
420 if ref $options{builder} || !(defined $options{builder});
421 confess("Setting both default and builder is not allowed.")
422 if exists $options{default};
424 (Class::MOP::Attribute::is_default_a_coderef(\%options))
425 || confess("References are not allowed as default values, you must ".
426 "wrap then in a CODE reference (ex: sub { [] } and not [])")
427 if exists $options{default} && ref $options{default};
429 # return the new object
430 $class->meta->new_object(name => $name, %options);
433 Class::MOP::Attribute->meta->add_method('clone' => sub {
435 $self->meta->clone_object($self, @_);
438 ## --------------------------------------------------------
439 ## Class::MOP::Method
441 Class::MOP::Method->meta->add_attribute(
442 Class::MOP::Attribute->new('&!body' => (
444 reader => { 'body' => \&Class::MOP::Method::body },
448 ## --------------------------------------------------------
449 ## Class::MOP::Method::Wrapped
452 # the way this item is initialized, this
453 # really does not follow the standard
454 # practices of attributes, but we put
455 # it here for completeness
456 Class::MOP::Method::Wrapped->meta->add_attribute(
457 Class::MOP::Attribute->new('%!modifier_table')
460 ## --------------------------------------------------------
461 ## Class::MOP::Method::Generated
463 Class::MOP::Method::Generated->meta->add_attribute(
464 Class::MOP::Attribute->new('$!is_inline' => (
465 init_arg => 'is_inline',
466 reader => { 'is_inline' => \&Class::MOP::Method::Generated::is_inline },
470 ## --------------------------------------------------------
471 ## Class::MOP::Method::Accessor
473 Class::MOP::Method::Accessor->meta->add_attribute(
474 Class::MOP::Attribute->new('$!attribute' => (
475 init_arg => 'attribute',
477 'associated_attribute' => \&Class::MOP::Method::Accessor::associated_attribute
482 Class::MOP::Method::Accessor->meta->add_attribute(
483 Class::MOP::Attribute->new('$!accessor_type' => (
484 init_arg => 'accessor_type',
485 reader => { 'accessor_type' => \&Class::MOP::Method::Accessor::accessor_type },
490 ## --------------------------------------------------------
491 ## Class::MOP::Method::Constructor
493 Class::MOP::Method::Constructor->meta->add_attribute(
494 Class::MOP::Attribute->new('%!options' => (
495 init_arg => 'options',
497 'options' => \&Class::MOP::Method::Constructor::options
502 Class::MOP::Method::Constructor->meta->add_attribute(
503 Class::MOP::Attribute->new('$!associated_metaclass' => (
504 init_arg => 'metaclass',
506 'associated_metaclass' => \&Class::MOP::Method::Constructor::associated_metaclass
511 ## --------------------------------------------------------
512 ## Class::MOP::Instance
515 # these don't yet do much of anything, but are just
516 # included for completeness
518 Class::MOP::Instance->meta->add_attribute(
519 Class::MOP::Attribute->new('$!meta')
522 Class::MOP::Instance->meta->add_attribute(
523 Class::MOP::Attribute->new('@!slots')
526 ## --------------------------------------------------------
527 ## Now close all the Class::MOP::* classes
530 # we don't need to inline the
531 # constructors or the accessors
532 # this only lengthens the compile
533 # time of the MOP, and gives us
534 # no actual benefits.
536 $_->meta->make_immutable(
537 inline_constructor => 0,
538 inline_accessors => 0,
544 Class::MOP::Attribute
550 Class::MOP::Method::Generated
552 Class::MOP::Method::Accessor
553 Class::MOP::Method::Constructor
554 Class::MOP::Method::Wrapped
565 Class::MOP - A Meta Object Protocol for Perl 5
569 This module is a fully functioning meta object protocol for the
570 Perl 5 object system. It makes no attempt to change the behavior or
571 characteristics of the Perl 5 object system, only to create a
572 protocol for its manipulation and introspection.
574 That said, it does attempt to create the tools for building a rich
575 set of extensions to the Perl 5 object system. Every attempt has been
576 made for these tools to keep to the spirit of the Perl 5 object
577 system that we all know and love.
579 This documentation is admittedly sparse on details, as time permits
580 I will try to improve them. For now, I suggest looking at the items
581 listed in the L<SEE ALSO> section for more information. In particular
582 the book "The Art of the Meta Object Protocol" was very influential
583 in the development of this system.
585 =head2 What is a Meta Object Protocol?
587 A meta object protocol is an API to an object system.
589 To be more specific, it is a set of abstractions of the components of
590 an object system (typically things like; classes, object, methods,
591 object attributes, etc.). These abstractions can then be used to both
592 inspect and manipulate the object system which they describe.
594 It can be said that there are two MOPs for any object system; the
595 implicit MOP, and the explicit MOP. The implicit MOP handles things
596 like method dispatch or inheritance, which happen automatically as
597 part of how the object system works. The explicit MOP typically
598 handles the introspection/reflection features of the object system.
599 All object systems have implicit MOPs, without one, they would not
600 work. Explict MOPs however as less common, and depending on the
601 language can vary from restrictive (Reflection in Java or C#) to
602 wide open (CLOS is a perfect example).
604 =head2 Yet Another Class Builder!! Why?
606 This is B<not> a class builder so much as it is a I<class builder
607 B<builder>>. My intent is that an end user does not use this module
608 directly, but instead this module is used by module authors to
609 build extensions and features onto the Perl 5 object system.
611 =head2 Who is this module for?
613 This module is specifically for anyone who has ever created or
614 wanted to create a module for the Class:: namespace. The tools which
615 this module will provide will hopefully make it easier to do more
616 complex things with Perl 5 classes by removing such barriers as
617 the need to hack the symbol tables, or understand the fine details
620 =head2 What changes do I have to make to use this module?
622 This module was designed to be as unintrusive as possible. Many of
623 its features are accessible without B<any> change to your existsing
624 code at all. It is meant to be a compliment to your existing code and
625 not an intrusion on your code base. Unlike many other B<Class::>
626 modules, this module B<does not> require you subclass it, or even that
627 you C<use> it in within your module's package.
629 The only features which requires additions to your code are the
630 attribute handling and instance construction features, and these are
631 both completely optional features. The only reason for this is because
632 Perl 5's object system does not actually have these features built
633 in. More information about this feature can be found below.
635 =head2 A Note about Performance?
637 It is a common misconception that explict MOPs are performance drains.
638 But this is not a universal truth at all, it is an side-effect of
639 specific implementations. For instance, using Java reflection is much
640 slower because the JVM cannot take advantage of any compiler
641 optimizations, and the JVM has to deal with much more runtime type
642 information as well. Reflection in C# is marginally better as it was
643 designed into the language and runtime (the CLR). In contrast, CLOS
644 (the Common Lisp Object System) was built to support an explicit MOP,
645 and so performance is tuned for it.
647 This library in particular does it's absolute best to avoid putting
648 B<any> drain at all upon your code's performance. In fact, by itself
649 it does nothing to affect your existing code. So you only pay for
650 what you actually use.
652 =head2 About Metaclass compatibility
654 This module makes sure that all metaclasses created are both upwards
655 and downwards compatible. The topic of metaclass compatibility is
656 highly esoteric and is something only encountered when doing deep and
657 involved metaclass hacking. There are two basic kinds of metaclass
658 incompatibility; upwards and downwards.
660 Upwards metaclass compatibility means that the metaclass of a
661 given class is either the same as (or a subclass of) all of the
664 Downward metaclass compatibility means that the metaclasses of a
665 given class's anscestors are all either the same as (or a subclass
668 Here is a diagram showing a set of two classes (C<A> and C<B>) and
669 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
670 metaclass compatibility both upwards and downwards.
672 +---------+ +---------+
673 | Meta::A |<----| Meta::B | <....... (instance of )
674 +---------+ +---------+ <------- (inherits from)
677 +---------+ +---------+
679 +---------+ +---------+
681 As I said this is a highly esoteric topic and one you will only run
682 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
683 are interested in why this is an issue see the paper
684 I<Uniform and safe metaclass composition> linked to in the
685 L<SEE ALSO> section of this document.
687 =head2 Using custom metaclasses
689 Always use the metaclass pragma when using a custom metaclass, this
690 will ensure the proper initialization order and not accidentely
691 create an incorrect type of metaclass for you. This is a very rare
692 problem, and one which can only occur if you are doing deep metaclass
693 programming. So in other words, don't worry about it.
697 The protocol is divided into 4 main sub-protocols:
701 =item The Class protocol
703 This provides a means of manipulating and introspecting a Perl 5
704 class. It handles all of symbol table hacking for you, and provides
705 a rich set of methods that go beyond simple package introspection.
707 See L<Class::MOP::Class> for more details.
709 =item The Attribute protocol
711 This provides a consistent represenation for an attribute of a
712 Perl 5 class. Since there are so many ways to create and handle
713 attributes in Perl 5 OO, this attempts to provide as much of a
714 unified approach as possible, while giving the freedom and
715 flexibility to subclass for specialization.
717 See L<Class::MOP::Attribute> for more details.
719 =item The Method protocol
721 This provides a means of manipulating and introspecting methods in
722 the Perl 5 object system. As with attributes, there are many ways to
723 approach this topic, so we try to keep it pretty basic, while still
724 making it possible to extend the system in many ways.
726 See L<Class::MOP::Method> for more details.
728 =item The Instance protocol
730 This provides a layer of abstraction for creating object instances.
731 Since the other layers use this protocol, it is relatively easy to
732 change the type of your instances from the default HASH ref to other
733 types of references. Several examples are provided in the F<examples/>
734 directory included in this distribution.
736 See L<Class::MOP::Instance> for more details.
746 =item I<IS_RUNNING_ON_5_10>
748 We set this constant depending on what version perl we are on, this
749 allows us to take advantage of new 5.10 features and stay backwards
754 =head2 Utility functions
758 =item B<load_class ($class_name)>
760 This will load a given C<$class_name> and if it does not have an
761 already initialized metaclass, then it will intialize one for it.
762 This function can be used in place of tricks like
763 C<eval "use $module"> or using C<require>.
765 =item B<is_class_loaded ($class_name)>
767 This will return a boolean depending on if the C<$class_name> has
770 NOTE: This does a basic check of the symbol table to try and
771 determine as best it can if the C<$class_name> is loaded, it
772 is probably correct about 99% of the time.
774 =item B<check_package_cache_flag ($pkg)>
776 This will return an integer that is managed by C<Class::MOP::Class>
777 to determine if a module's symbol table has been altered.
779 In Perl 5.10 or greater, this flag is package specific. However in
780 versions prior to 5.10, this will use the C<PL_sub_generation> variable
781 which is not package specific.
783 =item B<get_code_info ($code)>
785 This function returns two values, the name of the package the C<$code>
786 is from and the name of the C<$code> itself. This is used by several
787 elements of the MOP to detemine where a given C<$code> reference is from.
791 =head2 Metaclass cache functions
793 Class::MOP holds a cache of metaclasses, the following are functions
794 (B<not methods>) which can be used to access that cache. It is not
795 recommended that you mess with this, bad things could happen. But if
796 you are brave and willing to risk it, go for it.
800 =item B<get_all_metaclasses>
802 This will return an hash of all the metaclass instances that have
803 been cached by B<Class::MOP::Class> keyed by the package name.
805 =item B<get_all_metaclass_instances>
807 This will return an array of all the metaclass instances that have
808 been cached by B<Class::MOP::Class>.
810 =item B<get_all_metaclass_names>
812 This will return an array of all the metaclass names that have
813 been cached by B<Class::MOP::Class>.
815 =item B<get_metaclass_by_name ($name)>
817 This will return a cached B<Class::MOP::Class> instance of nothing
818 if no metaclass exist by that C<$name>.
820 =item B<store_metaclass_by_name ($name, $meta)>
822 This will store a metaclass in the cache at the supplied C<$key>.
824 =item B<weaken_metaclass ($name)>
826 In rare cases it is desireable to store a weakened reference in
827 the metaclass cache. This function will weaken the reference to
828 the metaclass stored in C<$name>.
830 =item B<does_metaclass_exist ($name)>
832 This will return true of there exists a metaclass stored in the
833 C<$name> key and return false otherwise.
835 =item B<remove_metaclass_by_name ($name)>
837 This will remove a the metaclass stored in the C<$name> key.
845 There are very few books out on Meta Object Protocols and Metaclasses
846 because it is such an esoteric topic. The following books are really
847 the only ones I have found. If you know of any more, B<I<please>>
848 email me and let me know, I would love to hear about them.
852 =item "The Art of the Meta Object Protocol"
854 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
856 =item "Putting MetaClasses to Work"
858 =item "Smalltalk: The Language"
866 =item Uniform and safe metaclass composition
868 An excellent paper by the people who brought us the original Traits paper.
869 This paper is on how Traits can be used to do safe metaclass composition,
870 and offers an excellent introduction section which delves into the topic of
871 metaclass compatibility.
873 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
875 =item Safe Metaclass Programming
877 This paper seems to precede the above paper, and propose a mix-in based
878 approach as opposed to the Traits based approach. Both papers have similar
879 information on the metaclass compatibility problem space.
881 L<http://citeseer.ist.psu.edu/37617.html>
889 =item The Perl 6 MetaModel work in the Pugs project
893 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
895 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
905 =item CPAN Module Review of Class::MOP
907 L<http://www.oreillynet.com/onlamp/blog/2006/06/cpan_module_review_classmop.html>
911 =head1 SIMILAR MODULES
913 As I have said above, this module is a class-builder-builder, so it is
914 not the same thing as modules like L<Class::Accessor> and
915 L<Class::MethodMaker>. That being said there are very few modules on CPAN
916 with similar goals to this module. The one I have found which is most
917 like this module is L<Class::Meta>, although it's philosophy and the MOP it
918 creates are very different from this modules.
922 All complex software has bugs lurking in it, and this module is no
923 exception. If you find a bug please either email me, or add the bug
926 =head1 ACKNOWLEDGEMENTS
932 Thanks to Rob for actually getting the development of this module kick-started.
938 Stevan Little E<lt>stevan@iinteractive.comE<gt>
940 B<with contributions from:>
942 Brandon (blblack) Black
944 Guillermo (groditi) Roditi
948 Rob (robkinyon) Kinyon
950 Yuval (nothingmuch) Kogman
952 Scott (konobi) McWhirter
954 =head1 COPYRIGHT AND LICENSE
956 Copyright 2006-2008 by Infinity Interactive, Inc.
958 L<http://www.iinteractive.com>
960 This library is free software; you can redistribute it and/or modify
961 it under the same terms as Perl itself.