10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 use Class::MOP::Class::Immutable;
16 our $VERSION = '0.32';
17 our $AUTHORITY = 'cpan:STEVAN';
19 ## ----------------------------------------------------------------------------
20 ## Setting up our environment ...
21 ## ----------------------------------------------------------------------------
22 ## Class::MOP needs to have a few things in the global perl environment so
23 ## that it can operate effectively. Those things are done here.
24 ## ----------------------------------------------------------------------------
26 # ... nothing yet actually ;)
28 ## ----------------------------------------------------------------------------
30 ## ----------------------------------------------------------------------------
31 ## The code below here is to bootstrap our MOP with itself. This is also
32 ## sometimes called "tying the knot". By doing this, we make it much easier
33 ## to extend the MOP through subclassing and such since now you can use the
34 ## MOP itself to extend itself.
36 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
37 ## ----------------------------------------------------------------------------
39 # We need to add in the meta-attributes here so that
40 # any subclass of Class::MOP::* will be able to
41 # inherit them using &construct_instance
43 ## --------------------------------------------------------
44 ## Class::MOP::Package
46 Class::MOP::Package->meta->add_attribute(
47 Class::MOP::Attribute->new('$:package' => (
49 # NOTE: we need to do this in order
50 # for the instance meta-object to
51 # not fall into meta-circular death
52 'name' => sub { (shift)->{'$:package'} }
54 init_arg => ':package',
58 Class::MOP::Package->meta->add_attribute(
59 Class::MOP::Attribute->new('%:namespace' => (
61 'namespace' => sub { (shift)->{'%:namespace'} }
66 return \%{$class->name . '::'};
69 # protect this from silliness
70 init_arg => '!............( DO NOT DO THIS )............!',
75 # use the metaclass to construct the meta-package
76 # which is a superclass of the metaclass itself :P
77 Class::MOP::Package->meta->add_method('initialize' => sub {
79 my $package_name = shift;
80 $class->meta->new_object(':package' => $package_name, @_);
83 ## --------------------------------------------------------
87 # yeah this is kind of stretching things a bit,
88 # but truthfully the version should be an attribute
89 # of the Module, the weirdness comes from having to
90 # stick to Perl 5 convention and store it in the
91 # $VERSION package variable. Basically if you just
92 # squint at it, it will look how you want it to look.
93 # Either as a package variable, or as a attribute of
94 # the metaclass, isn't abstraction great :)
96 Class::MOP::Module->meta->add_attribute(
97 Class::MOP::Attribute->new('$:version' => (
101 ${$self->get_package_symbol('$VERSION')};
105 # protect this from silliness
106 init_arg => '!............( DO NOT DO THIS )............!',
111 # By following the same conventions as version here,
112 # we are opening up the possibility that people can
113 # use the $AUTHORITY in non-Class::MOP modules as
116 Class::MOP::Module->meta->add_attribute(
117 Class::MOP::Attribute->new('$:authority' => (
121 ${$self->get_package_symbol('$AUTHORITY')};
125 # protect this from silliness
126 init_arg => '!............( DO NOT DO THIS )............!',
130 ## --------------------------------------------------------
133 Class::MOP::Class->meta->add_attribute(
134 Class::MOP::Attribute->new('%:attributes' => (
136 # NOTE: we need to do this in order
137 # for the instance meta-object to
138 # not fall into meta-circular death
139 'get_attribute_map' => sub { (shift)->{'%:attributes'} }
141 init_arg => ':attributes',
142 default => sub { {} }
146 Class::MOP::Class->meta->add_attribute(
147 Class::MOP::Attribute->new('$:attribute_metaclass' => (
148 reader => 'attribute_metaclass',
149 init_arg => ':attribute_metaclass',
150 default => 'Class::MOP::Attribute',
154 Class::MOP::Class->meta->add_attribute(
155 Class::MOP::Attribute->new('$:method_metaclass' => (
156 reader => 'method_metaclass',
157 init_arg => ':method_metaclass',
158 default => 'Class::MOP::Method',
162 Class::MOP::Class->meta->add_attribute(
163 Class::MOP::Attribute->new('$:instance_metaclass' => (
165 # NOTE: we need to do this in order
166 # for the instance meta-object to
167 # not fall into meta-circular death
168 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
170 init_arg => ':instance_metaclass',
171 default => 'Class::MOP::Instance',
176 # we don't actually need to tie the knot with
177 # Class::MOP::Class here, it is actually handled
178 # within Class::MOP::Class itself in the
179 # construct_class_instance method.
181 ## --------------------------------------------------------
182 ## Class::MOP::Attribute
184 Class::MOP::Attribute->meta->add_attribute(
185 Class::MOP::Attribute->new('name' => (
187 # NOTE: we need to do this in order
188 # for the instance meta-object to
189 # not fall into meta-circular death
190 'name' => sub { (shift)->{name} }
195 Class::MOP::Attribute->meta->add_attribute(
196 Class::MOP::Attribute->new('associated_class' => (
198 # NOTE: we need to do this in order
199 # for the instance meta-object to
200 # not fall into meta-circular death
201 'associated_class' => sub { (shift)->{associated_class} }
206 Class::MOP::Attribute->meta->add_attribute(
207 Class::MOP::Attribute->new('accessor' => (
208 reader => 'accessor',
209 predicate => 'has_accessor',
213 Class::MOP::Attribute->meta->add_attribute(
214 Class::MOP::Attribute->new('reader' => (
216 predicate => 'has_reader',
220 Class::MOP::Attribute->meta->add_attribute(
221 Class::MOP::Attribute->new('writer' => (
223 predicate => 'has_writer',
227 Class::MOP::Attribute->meta->add_attribute(
228 Class::MOP::Attribute->new('predicate' => (
229 reader => 'predicate',
230 predicate => 'has_predicate',
234 Class::MOP::Attribute->meta->add_attribute(
235 Class::MOP::Attribute->new('clearer' => (
237 predicate => 'has_clearer',
241 Class::MOP::Attribute->meta->add_attribute(
242 Class::MOP::Attribute->new('init_arg' => (
243 reader => 'init_arg',
244 predicate => 'has_init_arg',
248 Class::MOP::Attribute->meta->add_attribute(
249 Class::MOP::Attribute->new('default' => (
250 # default has a custom 'reader' method ...
251 predicate => 'has_default',
256 # NOTE: (meta-circularity)
257 # This should be one of the last things done
258 # it will "tie the knot" with Class::MOP::Attribute
259 # so that it uses the attributes meta-objects
260 # to construct itself.
261 Class::MOP::Attribute->meta->add_method('new' => sub {
266 (defined $name && $name)
267 || confess "You must provide a name for the attribute";
268 $options{init_arg} = $name
269 if not exists $options{init_arg};
271 # return the new object
272 $class->meta->new_object(name => $name, %options);
275 Class::MOP::Attribute->meta->add_method('clone' => sub {
277 $self->meta->clone_object($self, @_);
280 ## --------------------------------------------------------
281 ## Now close all the Class::MOP::* classes
283 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
284 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
285 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
286 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
287 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
288 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
289 Class::MOP::Object ->meta->make_immutable(inline_constructor => 0);
299 Class::MOP - A Meta Object Protocol for Perl 5
303 # ... This will come later, for now see
304 # the other SYNOPSIS for more information
308 This module is an attempt to create a meta object protocol for the
309 Perl 5 object system. It makes no attempt to change the behavior or
310 characteristics of the Perl 5 object system, only to create a
311 protocol for its manipulation and introspection.
313 That said, it does attempt to create the tools for building a rich
314 set of extensions to the Perl 5 object system. Every attempt has been
315 made for these tools to keep to the spirit of the Perl 5 object
316 system that we all know and love.
318 This documentation is admittedly sparse on details, as time permits
319 I will try to improve them. For now, I suggest looking at the items
320 listed in the L<SEE ALSO> section for more information. In particular
321 the book "The Art of the Meta Object Protocol" was very influential
322 in the development of this system.
324 =head2 What is a Meta Object Protocol?
326 A meta object protocol is an API to an object system.
328 To be more specific, it is a set of abstractions of the components of
329 an object system (typically things like; classes, object, methods,
330 object attributes, etc.). These abstractions can then be used to both
331 inspect and manipulate the object system which they describe.
333 It can be said that there are two MOPs for any object system; the
334 implicit MOP, and the explicit MOP. The implicit MOP handles things
335 like method dispatch or inheritance, which happen automatically as
336 part of how the object system works. The explicit MOP typically
337 handles the introspection/reflection features of the object system.
338 All object systems have implicit MOPs, without one, they would not
339 work. Explict MOPs however as less common, and depending on the
340 language can vary from restrictive (Reflection in Java or C#) to
341 wide open (CLOS is a perfect example).
343 =head2 Yet Another Class Builder!! Why?
345 This is B<not> a class builder so much as it is a I<class builder
346 B<builder>>. My intent is that an end user does not use this module
347 directly, but instead this module is used by module authors to
348 build extensions and features onto the Perl 5 object system.
350 =head2 Who is this module for?
352 This module is specifically for anyone who has ever created or
353 wanted to create a module for the Class:: namespace. The tools which
354 this module will provide will hopefully make it easier to do more
355 complex things with Perl 5 classes by removing such barriers as
356 the need to hack the symbol tables, or understand the fine details
359 =head2 What changes do I have to make to use this module?
361 This module was designed to be as unintrusive as possible. Many of
362 its features are accessible without B<any> change to your existsing
363 code at all. It is meant to be a compliment to your existing code and
364 not an intrusion on your code base. Unlike many other B<Class::>
365 modules, this module B<does not> require you subclass it, or even that
366 you C<use> it in within your module's package.
368 The only features which requires additions to your code are the
369 attribute handling and instance construction features, and these are
370 both completely optional features. The only reason for this is because
371 Perl 5's object system does not actually have these features built
372 in. More information about this feature can be found below.
374 =head2 A Note about Performance?
376 It is a common misconception that explict MOPs are performance drains.
377 But this is not a universal truth at all, it is an side-effect of
378 specific implementations. For instance, using Java reflection is much
379 slower because the JVM cannot take advantage of any compiler
380 optimizations, and the JVM has to deal with much more runtime type
381 information as well. Reflection in C# is marginally better as it was
382 designed into the language and runtime (the CLR). In contrast, CLOS
383 (the Common Lisp Object System) was built to support an explicit MOP,
384 and so performance is tuned for it.
386 This library in particular does it's absolute best to avoid putting
387 B<any> drain at all upon your code's performance. In fact, by itself
388 it does nothing to affect your existing code. So you only pay for
389 what you actually use.
391 =head2 About Metaclass compatibility
393 This module makes sure that all metaclasses created are both upwards
394 and downwards compatible. The topic of metaclass compatibility is
395 highly esoteric and is something only encountered when doing deep and
396 involved metaclass hacking. There are two basic kinds of metaclass
397 incompatibility; upwards and downwards.
399 Upwards metaclass compatibility means that the metaclass of a
400 given class is either the same as (or a subclass of) all of the
403 Downward metaclass compatibility means that the metaclasses of a
404 given class's anscestors are all either the same as (or a subclass
407 Here is a diagram showing a set of two classes (C<A> and C<B>) and
408 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
409 metaclass compatibility both upwards and downwards.
411 +---------+ +---------+
412 | Meta::A |<----| Meta::B | <....... (instance of )
413 +---------+ +---------+ <------- (inherits from)
416 +---------+ +---------+
418 +---------+ +---------+
420 As I said this is a highly esoteric topic and one you will only run
421 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
422 are interested in why this is an issue see the paper
423 I<Uniform and safe metaclass composition> linked to in the
424 L<SEE ALSO> section of this document.
426 =head2 Using custom metaclasses
428 Always use the metaclass pragma when using a custom metaclass, this
429 will ensure the proper initialization order and not accidentely
430 create an incorrect type of metaclass for you. This is a very rare
431 problem, and one which can only occur if you are doing deep metaclass
432 programming. So in other words, don't worry about it.
436 The protocol is divided into 3 main sub-protocols:
440 =item The Class protocol
442 This provides a means of manipulating and introspecting a Perl 5
443 class. It handles all of symbol table hacking for you, and provides
444 a rich set of methods that go beyond simple package introspection.
446 See L<Class::MOP::Class> for more details.
448 =item The Attribute protocol
450 This provides a consistent represenation for an attribute of a
451 Perl 5 class. Since there are so many ways to create and handle
452 atttributes in Perl 5 OO, this attempts to provide as much of a
453 unified approach as possible, while giving the freedom and
454 flexibility to subclass for specialization.
456 See L<Class::MOP::Attribute> for more details.
458 =item The Method protocol
460 This provides a means of manipulating and introspecting methods in
461 the Perl 5 object system. As with attributes, there are many ways to
462 approach this topic, so we try to keep it pretty basic, while still
463 making it possible to extend the system in many ways.
465 See L<Class::MOP::Method> for more details.
473 There are very few books out on Meta Object Protocols and Metaclasses
474 because it is such an esoteric topic. The following books are really
475 the only ones I have found. If you know of any more, B<I<please>>
476 email me and let me know, I would love to hear about them.
480 =item "The Art of the Meta Object Protocol"
482 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
484 =item "Putting MetaClasses to Work"
486 =item "Smalltalk: The Language"
494 =item Uniform and safe metaclass composition
496 An excellent paper by the people who brought us the original Traits paper.
497 This paper is on how Traits can be used to do safe metaclass composition,
498 and offers an excellent introduction section which delves into the topic of
499 metaclass compatibility.
501 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
503 =item Safe Metaclass Programming
505 This paper seems to precede the above paper, and propose a mix-in based
506 approach as opposed to the Traits based approach. Both papers have similar
507 information on the metaclass compatibility problem space.
509 L<http://citeseer.ist.psu.edu/37617.html>
517 =item The Perl 6 MetaModel work in the Pugs project
521 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
523 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
529 =head1 SIMILAR MODULES
531 As I have said above, this module is a class-builder-builder, so it is
532 not the same thing as modules like L<Class::Accessor> and
533 L<Class::MethodMaker>. That being said there are very few modules on CPAN
534 with similar goals to this module. The one I have found which is most
535 like this module is L<Class::Meta>, although it's philosophy and the MOP it
536 creates are very different from this modules.
540 All complex software has bugs lurking in it, and this module is no
541 exception. If you find a bug please either email me, or add the bug
546 I use L<Devel::Cover> to test the code coverage of my tests, below is the
547 L<Devel::Cover> report on this module's test suite.
549 ---------------------------- ------ ------ ------ ------ ------ ------ ------
550 File stmt bran cond sub pod time total
551 ---------------------------- ------ ------ ------ ------ ------ ------ ------
552 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 19.8 100.0
553 Class/MOP/Attribute.pm 100.0 100.0 91.7 61.2 100.0 14.3 87.9
554 Class/MOP/Class.pm 97.6 91.3 77.3 98.4 100.0 56.4 93.2
555 Class/MOP/Instance.pm 91.1 75.0 33.3 91.7 100.0 6.8 90.7
556 Class/MOP/Method.pm 97.6 60.0 52.9 76.9 100.0 1.6 82.6
557 metaclass.pm 100.0 100.0 83.3 100.0 n/a 1.0 97.7
558 ---------------------------- ------ ------ ------ ------ ------ ------ ------
559 Total 97.5 88.5 75.5 82.8 100.0 100.0 91.2
560 ---------------------------- ------ ------ ------ ------ ------ ------ ------
562 =head1 ACKNOWLEDGEMENTS
566 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
568 Thanks to Rob for actually getting the development of this module kick-started.
574 Stevan Little E<lt>stevan@iinteractive.comE<gt>
576 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
578 =head1 COPYRIGHT AND LICENSE
580 Copyright 2006 by Infinity Interactive, Inc.
582 L<http://www.iinteractive.com>
584 This library is free software; you can redistribute it and/or modify
585 it under the same terms as Perl itself.