10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 use Class::MOP::Class::Immutable;
16 our $VERSION = '0.32';
18 ## ----------------------------------------------------------------------------
19 ## Setting up our environment ...
20 ## ----------------------------------------------------------------------------
21 ## Class::MOP needs to have a few things in the global perl environment so
22 ## that it can operate effectively. Those things are done here.
23 ## ----------------------------------------------------------------------------
25 # ... nothing yet actually ;)
27 ## ----------------------------------------------------------------------------
29 ## ----------------------------------------------------------------------------
30 ## The code below here is to bootstrap our MOP with itself. This is also
31 ## sometimes called "tying the knot". By doing this, we make it much easier
32 ## to extend the MOP through subclassing and such since now you can use the
33 ## MOP itself to extend itself.
35 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
36 ## ----------------------------------------------------------------------------
38 # We need to add in the meta-attributes here so that
39 # any subclass of Class::MOP::* will be able to
40 # inherit them using &construct_instance
42 ## Class::MOP::Package
44 Class::MOP::Package->meta->add_attribute(
45 Class::MOP::Attribute->new('$:package' => (
47 # NOTE: we need to do this in order
48 # for the instance meta-object to
49 # not fall into meta-circular death
50 'name' => sub { (shift)->{'$:package'} }
52 init_arg => ':package',
56 Class::MOP::Package->meta->add_attribute(
57 Class::MOP::Attribute->new('%:namespace' => (
59 'namespace' => sub { (shift)->{'%:namespace'} }
64 return \%{$class->name . '::'};
67 # protect this from silliness
68 init_arg => '!............( DO NOT DO THIS )............!',
73 # use the metaclass to construct the meta-package
74 # which is a superclass of the metaclass itself :P
75 Class::MOP::Package->meta->add_method('initialize' => sub {
77 my $package_name = shift;
78 $class->meta->new_object(':package' => $package_name, @_);
83 Class::MOP::Class->meta->add_attribute(
84 Class::MOP::Attribute->new('%:attributes' => (
86 # NOTE: we need to do this in order
87 # for the instance meta-object to
88 # not fall into meta-circular death
89 'get_attribute_map' => sub { (shift)->{'%:attributes'} }
91 init_arg => ':attributes',
96 Class::MOP::Class->meta->add_attribute(
97 Class::MOP::Attribute->new('$:attribute_metaclass' => (
98 reader => 'attribute_metaclass',
99 init_arg => ':attribute_metaclass',
100 default => 'Class::MOP::Attribute',
104 Class::MOP::Class->meta->add_attribute(
105 Class::MOP::Attribute->new('$:method_metaclass' => (
106 reader => 'method_metaclass',
107 init_arg => ':method_metaclass',
108 default => 'Class::MOP::Method',
112 Class::MOP::Class->meta->add_attribute(
113 Class::MOP::Attribute->new('$:instance_metaclass' => (
115 # NOTE: we need to do this in order
116 # for the instance meta-object to
117 # not fall into meta-circular death
118 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
120 init_arg => ':instance_metaclass',
121 default => 'Class::MOP::Instance',
126 # we don't actually need to tie the knot with
127 # Class::MOP::Class here, it is actually handled
128 # within Class::MOP::Class itself in the
129 # construct_class_instance method.
131 ## Class::MOP::Attribute
133 Class::MOP::Attribute->meta->add_attribute(
134 Class::MOP::Attribute->new('name' => (
136 # NOTE: we need to do this in order
137 # for the instance meta-object to
138 # not fall into meta-circular death
139 'name' => sub { (shift)->{name} }
144 Class::MOP::Attribute->meta->add_attribute(
145 Class::MOP::Attribute->new('associated_class' => (
147 # NOTE: we need to do this in order
148 # for the instance meta-object to
149 # not fall into meta-circular death
150 'associated_class' => sub { (shift)->{associated_class} }
155 Class::MOP::Attribute->meta->add_attribute(
156 Class::MOP::Attribute->new('accessor' => (
157 reader => 'accessor',
158 predicate => 'has_accessor',
162 Class::MOP::Attribute->meta->add_attribute(
163 Class::MOP::Attribute->new('reader' => (
165 predicate => 'has_reader',
169 Class::MOP::Attribute->meta->add_attribute(
170 Class::MOP::Attribute->new('writer' => (
172 predicate => 'has_writer',
176 Class::MOP::Attribute->meta->add_attribute(
177 Class::MOP::Attribute->new('predicate' => (
178 reader => 'predicate',
179 predicate => 'has_predicate',
183 Class::MOP::Attribute->meta->add_attribute(
184 Class::MOP::Attribute->new('clearer' => (
186 predicate => 'has_clearer',
190 Class::MOP::Attribute->meta->add_attribute(
191 Class::MOP::Attribute->new('init_arg' => (
192 reader => 'init_arg',
193 predicate => 'has_init_arg',
197 Class::MOP::Attribute->meta->add_attribute(
198 Class::MOP::Attribute->new('default' => (
199 # default has a custom 'reader' method ...
200 predicate => 'has_default',
205 # NOTE: (meta-circularity)
206 # This should be one of the last things done
207 # it will "tie the knot" with Class::MOP::Attribute
208 # so that it uses the attributes meta-objects
209 # to construct itself.
210 Class::MOP::Attribute->meta->add_method('new' => sub {
215 (defined $name && $name)
216 || confess "You must provide a name for the attribute";
217 $options{init_arg} = $name
218 if not exists $options{init_arg};
220 # return the new object
221 $class->meta->new_object(name => $name, %options);
224 Class::MOP::Attribute->meta->add_method('clone' => sub {
226 $self->meta->clone_object($self, @_);
229 ## Try and close Class::MOP::*
231 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
232 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
233 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
234 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
235 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
236 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
247 Class::MOP - A Meta Object Protocol for Perl 5
251 # ... This will come later, for now see
252 # the other SYNOPSIS for more information
256 This module is an attempt to create a meta object protocol for the
257 Perl 5 object system. It makes no attempt to change the behavior or
258 characteristics of the Perl 5 object system, only to create a
259 protocol for its manipulation and introspection.
261 That said, it does attempt to create the tools for building a rich
262 set of extensions to the Perl 5 object system. Every attempt has been
263 made for these tools to keep to the spirit of the Perl 5 object
264 system that we all know and love.
266 This documentation is admittedly sparse on details, as time permits
267 I will try to improve them. For now, I suggest looking at the items
268 listed in the L<SEE ALSO> section for more information. In particular
269 the book "The Art of the Meta Object Protocol" was very influential
270 in the development of this system.
272 =head2 What is a Meta Object Protocol?
274 A meta object protocol is an API to an object system.
276 To be more specific, it is a set of abstractions of the components of
277 an object system (typically things like; classes, object, methods,
278 object attributes, etc.). These abstractions can then be used to both
279 inspect and manipulate the object system which they describe.
281 It can be said that there are two MOPs for any object system; the
282 implicit MOP, and the explicit MOP. The implicit MOP handles things
283 like method dispatch or inheritance, which happen automatically as
284 part of how the object system works. The explicit MOP typically
285 handles the introspection/reflection features of the object system.
286 All object systems have implicit MOPs, without one, they would not
287 work. Explict MOPs however as less common, and depending on the
288 language can vary from restrictive (Reflection in Java or C#) to
289 wide open (CLOS is a perfect example).
291 =head2 Yet Another Class Builder!! Why?
293 This is B<not> a class builder so much as it is a I<class builder
294 B<builder>>. My intent is that an end user does not use this module
295 directly, but instead this module is used by module authors to
296 build extensions and features onto the Perl 5 object system.
298 =head2 Who is this module for?
300 This module is specifically for anyone who has ever created or
301 wanted to create a module for the Class:: namespace. The tools which
302 this module will provide will hopefully make it easier to do more
303 complex things with Perl 5 classes by removing such barriers as
304 the need to hack the symbol tables, or understand the fine details
307 =head2 What changes do I have to make to use this module?
309 This module was designed to be as unintrusive as possible. Many of
310 its features are accessible without B<any> change to your existsing
311 code at all. It is meant to be a compliment to your existing code and
312 not an intrusion on your code base. Unlike many other B<Class::>
313 modules, this module B<does not> require you subclass it, or even that
314 you C<use> it in within your module's package.
316 The only features which requires additions to your code are the
317 attribute handling and instance construction features, and these are
318 both completely optional features. The only reason for this is because
319 Perl 5's object system does not actually have these features built
320 in. More information about this feature can be found below.
322 =head2 A Note about Performance?
324 It is a common misconception that explict MOPs are performance drains.
325 But this is not a universal truth at all, it is an side-effect of
326 specific implementations. For instance, using Java reflection is much
327 slower because the JVM cannot take advantage of any compiler
328 optimizations, and the JVM has to deal with much more runtime type
329 information as well. Reflection in C# is marginally better as it was
330 designed into the language and runtime (the CLR). In contrast, CLOS
331 (the Common Lisp Object System) was built to support an explicit MOP,
332 and so performance is tuned for it.
334 This library in particular does it's absolute best to avoid putting
335 B<any> drain at all upon your code's performance. In fact, by itself
336 it does nothing to affect your existing code. So you only pay for
337 what you actually use.
339 =head2 About Metaclass compatibility
341 This module makes sure that all metaclasses created are both upwards
342 and downwards compatible. The topic of metaclass compatibility is
343 highly esoteric and is something only encountered when doing deep and
344 involved metaclass hacking. There are two basic kinds of metaclass
345 incompatibility; upwards and downwards.
347 Upwards metaclass compatibility means that the metaclass of a
348 given class is either the same as (or a subclass of) all of the
351 Downward metaclass compatibility means that the metaclasses of a
352 given class's anscestors are all either the same as (or a subclass
355 Here is a diagram showing a set of two classes (C<A> and C<B>) and
356 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
357 metaclass compatibility both upwards and downwards.
359 +---------+ +---------+
360 | Meta::A |<----| Meta::B | <....... (instance of )
361 +---------+ +---------+ <------- (inherits from)
364 +---------+ +---------+
366 +---------+ +---------+
368 As I said this is a highly esoteric topic and one you will only run
369 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
370 are interested in why this is an issue see the paper
371 I<Uniform and safe metaclass composition> linked to in the
372 L<SEE ALSO> section of this document.
374 =head2 Using custom metaclasses
376 Always use the metaclass pragma when using a custom metaclass, this
377 will ensure the proper initialization order and not accidentely
378 create an incorrect type of metaclass for you. This is a very rare
379 problem, and one which can only occur if you are doing deep metaclass
380 programming. So in other words, don't worry about it.
384 The protocol is divided into 3 main sub-protocols:
388 =item The Class protocol
390 This provides a means of manipulating and introspecting a Perl 5
391 class. It handles all of symbol table hacking for you, and provides
392 a rich set of methods that go beyond simple package introspection.
394 See L<Class::MOP::Class> for more details.
396 =item The Attribute protocol
398 This provides a consistent represenation for an attribute of a
399 Perl 5 class. Since there are so many ways to create and handle
400 atttributes in Perl 5 OO, this attempts to provide as much of a
401 unified approach as possible, while giving the freedom and
402 flexibility to subclass for specialization.
404 See L<Class::MOP::Attribute> for more details.
406 =item The Method protocol
408 This provides a means of manipulating and introspecting methods in
409 the Perl 5 object system. As with attributes, there are many ways to
410 approach this topic, so we try to keep it pretty basic, while still
411 making it possible to extend the system in many ways.
413 See L<Class::MOP::Method> for more details.
421 There are very few books out on Meta Object Protocols and Metaclasses
422 because it is such an esoteric topic. The following books are really
423 the only ones I have found. If you know of any more, B<I<please>>
424 email me and let me know, I would love to hear about them.
428 =item "The Art of the Meta Object Protocol"
430 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
432 =item "Putting MetaClasses to Work"
434 =item "Smalltalk: The Language"
442 =item Uniform and safe metaclass composition
444 An excellent paper by the people who brought us the original Traits paper.
445 This paper is on how Traits can be used to do safe metaclass composition,
446 and offers an excellent introduction section which delves into the topic of
447 metaclass compatibility.
449 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
451 =item Safe Metaclass Programming
453 This paper seems to precede the above paper, and propose a mix-in based
454 approach as opposed to the Traits based approach. Both papers have similar
455 information on the metaclass compatibility problem space.
457 L<http://citeseer.ist.psu.edu/37617.html>
465 =item The Perl 6 MetaModel work in the Pugs project
469 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
471 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
477 =head1 SIMILAR MODULES
479 As I have said above, this module is a class-builder-builder, so it is
480 not the same thing as modules like L<Class::Accessor> and
481 L<Class::MethodMaker>. That being said there are very few modules on CPAN
482 with similar goals to this module. The one I have found which is most
483 like this module is L<Class::Meta>, although it's philosophy and the MOP it
484 creates are very different from this modules.
488 All complex software has bugs lurking in it, and this module is no
489 exception. If you find a bug please either email me, or add the bug
494 I use L<Devel::Cover> to test the code coverage of my tests, below is the
495 L<Devel::Cover> report on this module's test suite.
497 ---------------------------- ------ ------ ------ ------ ------ ------ ------
498 File stmt bran cond sub pod time total
499 ---------------------------- ------ ------ ------ ------ ------ ------ ------
500 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 19.8 100.0
501 Class/MOP/Attribute.pm 100.0 100.0 91.7 61.2 100.0 14.3 87.9
502 Class/MOP/Class.pm 97.6 91.3 77.3 98.4 100.0 56.4 93.2
503 Class/MOP/Instance.pm 91.1 75.0 33.3 91.7 100.0 6.8 90.7
504 Class/MOP/Method.pm 97.6 60.0 52.9 76.9 100.0 1.6 82.6
505 metaclass.pm 100.0 100.0 83.3 100.0 n/a 1.0 97.7
506 ---------------------------- ------ ------ ------ ------ ------ ------ ------
507 Total 97.5 88.5 75.5 82.8 100.0 100.0 91.2
508 ---------------------------- ------ ------ ------ ------ ------ ------ ------
510 =head1 ACKNOWLEDGEMENTS
514 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
516 Thanks to Rob for actually getting the development of this module kick-started.
522 Stevan Little E<lt>stevan@iinteractive.comE<gt>
524 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
526 =head1 COPYRIGHT AND LICENSE
528 Copyright 2006 by Infinity Interactive, Inc.
530 L<http://www.iinteractive.com>
532 This library is free software; you can redistribute it and/or modify
533 it under the same terms as Perl itself.