10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 use Class::MOP::Class::Immutable;
16 our $VERSION = '0.29_02';
18 ## ----------------------------------------------------------------------------
19 ## Setting up our environment ...
20 ## ----------------------------------------------------------------------------
21 ## Class::MOP needs to have a few things in the global perl environment so
22 ## that it can operate effectively. Those things are done here.
23 ## ----------------------------------------------------------------------------
25 # ... nothing yet actually ;)
27 ## ----------------------------------------------------------------------------
29 ## ----------------------------------------------------------------------------
30 ## The code below here is to bootstrap our MOP with itself. This is also
31 ## sometimes called "tying the knot". By doing this, we make it much easier
32 ## to extend the MOP through subclassing and such since now you can use the
33 ## MOP itself to extend itself.
35 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
36 ## ----------------------------------------------------------------------------
38 # We need to add in the meta-attributes here so that
39 # any subclass of Class::MOP::* will be able to
40 # inherit them using &construct_instance
42 ## Class::MOP::Package
44 Class::MOP::Package->meta->add_attribute(
45 Class::MOP::Attribute->new('$: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)->{'$:package'} }
52 init_arg => ':package',
58 Class::MOP::Class->meta->add_attribute(
59 Class::MOP::Attribute->new('%:attributes' => (
61 # NOTE: we need to do this in order
62 # for the instance meta-object to
63 # not fall into meta-circular death
64 'get_attribute_map' => sub { (shift)->{'%:attributes'} }
66 init_arg => ':attributes',
71 Class::MOP::Class->meta->add_attribute(
72 Class::MOP::Attribute->new('$:attribute_metaclass' => (
73 reader => 'attribute_metaclass',
74 init_arg => ':attribute_metaclass',
75 default => 'Class::MOP::Attribute',
79 Class::MOP::Class->meta->add_attribute(
80 Class::MOP::Attribute->new('$:method_metaclass' => (
81 reader => 'method_metaclass',
82 init_arg => ':method_metaclass',
83 default => 'Class::MOP::Method',
87 Class::MOP::Class->meta->add_attribute(
88 Class::MOP::Attribute->new('$:instance_metaclass' => (
90 # NOTE: we need to do this in order
91 # for the instance meta-object to
92 # not fall into meta-circular death
93 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
95 init_arg => ':instance_metaclass',
96 default => 'Class::MOP::Instance',
100 ## Class::MOP::Attribute
102 Class::MOP::Attribute->meta->add_attribute(
103 Class::MOP::Attribute->new('name' => (
105 # NOTE: we need to do this in order
106 # for the instance meta-object to
107 # not fall into meta-circular death
108 'name' => sub { (shift)->{name} }
113 Class::MOP::Attribute->meta->add_attribute(
114 Class::MOP::Attribute->new('associated_class' => (
116 # NOTE: we need to do this in order
117 # for the instance meta-object to
118 # not fall into meta-circular death
119 'associated_class' => sub { (shift)->{associated_class} }
124 Class::MOP::Attribute->meta->add_attribute(
125 Class::MOP::Attribute->new('accessor' => (
126 reader => 'accessor',
127 predicate => 'has_accessor',
131 Class::MOP::Attribute->meta->add_attribute(
132 Class::MOP::Attribute->new('reader' => (
134 predicate => 'has_reader',
138 Class::MOP::Attribute->meta->add_attribute(
139 Class::MOP::Attribute->new('writer' => (
141 predicate => 'has_writer',
145 Class::MOP::Attribute->meta->add_attribute(
146 Class::MOP::Attribute->new('predicate' => (
147 reader => 'predicate',
148 predicate => 'has_predicate',
152 Class::MOP::Attribute->meta->add_attribute(
153 Class::MOP::Attribute->new('init_arg' => (
154 reader => 'init_arg',
155 predicate => 'has_init_arg',
159 Class::MOP::Attribute->meta->add_attribute(
160 Class::MOP::Attribute->new('default' => (
161 # default has a custom 'reader' method ...
162 predicate => 'has_default',
167 # NOTE: (meta-circularity)
168 # This should be one of the last things done
169 # it will "tie the knot" with Class::MOP::Attribute
170 # so that it uses the attributes meta-objects
171 # to construct itself.
172 Class::MOP::Attribute->meta->add_method('new' => sub {
177 (defined $name && $name)
178 || confess "You must provide a name for the attribute";
179 $options{init_arg} = $name
180 if not exists $options{init_arg};
182 # return the new object
183 $class->meta->new_object(name => $name, %options);
186 Class::MOP::Attribute->meta->add_method('clone' => sub {
188 $self->meta->clone_object($self, @_);
199 Class::MOP - A Meta Object Protocol for Perl 5
203 # ... This will come later, for now see
204 # the other SYNOPSIS for more information
208 This module is an attempt to create a meta object protocol for the
209 Perl 5 object system. It makes no attempt to change the behavior or
210 characteristics of the Perl 5 object system, only to create a
211 protocol for its manipulation and introspection.
213 That said, it does attempt to create the tools for building a rich
214 set of extensions to the Perl 5 object system. Every attempt has been
215 made for these tools to keep to the spirit of the Perl 5 object
216 system that we all know and love.
218 This documentation is admittedly sparse on details, as time permits
219 I will try to improve them. For now, I suggest looking at the items
220 listed in the L<SEE ALSO> section for more information. In particular
221 the book "The Art of the Meta Object Protocol" was very influential
222 in the development of this system.
224 =head2 What is a Meta Object Protocol?
226 A meta object protocol is an API to an object system.
228 To be more specific, it is a set of abstractions of the components of
229 an object system (typically things like; classes, object, methods,
230 object attributes, etc.). These abstractions can then be used to both
231 inspect and manipulate the object system which they describe.
233 It can be said that there are two MOPs for any object system; the
234 implicit MOP, and the explicit MOP. The implicit MOP handles things
235 like method dispatch or inheritance, which happen automatically as
236 part of how the object system works. The explicit MOP typically
237 handles the introspection/reflection features of the object system.
238 All object systems have implicit MOPs, without one, they would not
239 work. Explict MOPs however as less common, and depending on the
240 language can vary from restrictive (Reflection in Java or C#) to
241 wide open (CLOS is a perfect example).
243 =head2 Yet Another Class Builder!! Why?
245 This is B<not> a class builder so much as it is a I<class builder
246 B<builder>>. My intent is that an end user does not use this module
247 directly, but instead this module is used by module authors to
248 build extensions and features onto the Perl 5 object system.
250 =head2 Who is this module for?
252 This module is specifically for anyone who has ever created or
253 wanted to create a module for the Class:: namespace. The tools which
254 this module will provide will hopefully make it easier to do more
255 complex things with Perl 5 classes by removing such barriers as
256 the need to hack the symbol tables, or understand the fine details
259 =head2 What changes do I have to make to use this module?
261 This module was designed to be as unintrusive as possible. Many of
262 its features are accessible without B<any> change to your existsing
263 code at all. It is meant to be a compliment to your existing code and
264 not an intrusion on your code base. Unlike many other B<Class::>
265 modules, this module B<does not> require you subclass it, or even that
266 you C<use> it in within your module's package.
268 The only features which requires additions to your code are the
269 attribute handling and instance construction features, and these are
270 both completely optional features. The only reason for this is because
271 Perl 5's object system does not actually have these features built
272 in. More information about this feature can be found below.
274 =head2 A Note about Performance?
276 It is a common misconception that explict MOPs are performance drains.
277 But this is not a universal truth at all, it is an side-effect of
278 specific implementations. For instance, using Java reflection is much
279 slower because the JVM cannot take advantage of any compiler
280 optimizations, and the JVM has to deal with much more runtime type
281 information as well. Reflection in C# is marginally better as it was
282 designed into the language and runtime (the CLR). In contrast, CLOS
283 (the Common Lisp Object System) was built to support an explicit MOP,
284 and so performance is tuned for it.
286 This library in particular does it's absolute best to avoid putting
287 B<any> drain at all upon your code's performance. In fact, by itself
288 it does nothing to affect your existing code. So you only pay for
289 what you actually use.
291 =head2 About Metaclass compatibility
293 This module makes sure that all metaclasses created are both upwards
294 and downwards compatible. The topic of metaclass compatibility is
295 highly esoteric and is something only encountered when doing deep and
296 involved metaclass hacking. There are two basic kinds of metaclass
297 incompatibility; upwards and downwards.
299 Upwards metaclass compatibility means that the metaclass of a
300 given class is either the same as (or a subclass of) all of the
303 Downward metaclass compatibility means that the metaclasses of a
304 given class's anscestors are all either the same as (or a subclass
307 Here is a diagram showing a set of two classes (C<A> and C<B>) and
308 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
309 metaclass compatibility both upwards and downwards.
311 +---------+ +---------+
312 | Meta::A |<----| Meta::B | <....... (instance of )
313 +---------+ +---------+ <------- (inherits from)
316 +---------+ +---------+
318 +---------+ +---------+
320 As I said this is a highly esoteric topic and one you will only run
321 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
322 are interested in why this is an issue see the paper
323 I<Uniform and safe metaclass composition> linked to in the
324 L<SEE ALSO> section of this document.
326 =head2 Using custom metaclasses
328 Always use the metaclass pragma when using a custom metaclass, this
329 will ensure the proper initialization order and not accidentely
330 create an incorrect type of metaclass for you. This is a very rare
331 problem, and one which can only occur if you are doing deep metaclass
332 programming. So in other words, don't worry about it.
336 The protocol is divided into 3 main sub-protocols:
340 =item The Class protocol
342 This provides a means of manipulating and introspecting a Perl 5
343 class. It handles all of symbol table hacking for you, and provides
344 a rich set of methods that go beyond simple package introspection.
346 See L<Class::MOP::Class> for more details.
348 =item The Attribute protocol
350 This provides a consistent represenation for an attribute of a
351 Perl 5 class. Since there are so many ways to create and handle
352 atttributes in Perl 5 OO, this attempts to provide as much of a
353 unified approach as possible, while giving the freedom and
354 flexibility to subclass for specialization.
356 See L<Class::MOP::Attribute> for more details.
358 =item The Method protocol
360 This provides a means of manipulating and introspecting methods in
361 the Perl 5 object system. As with attributes, there are many ways to
362 approach this topic, so we try to keep it pretty basic, while still
363 making it possible to extend the system in many ways.
365 See L<Class::MOP::Method> for more details.
373 There are very few books out on Meta Object Protocols and Metaclasses
374 because it is such an esoteric topic. The following books are really
375 the only ones I have found. If you know of any more, B<I<please>>
376 email me and let me know, I would love to hear about them.
380 =item "The Art of the Meta Object Protocol"
382 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
384 =item "Putting MetaClasses to Work"
386 =item "Smalltalk: The Language"
394 =item Uniform and safe metaclass composition
396 An excellent paper by the people who brought us the original Traits paper.
397 This paper is on how Traits can be used to do safe metaclass composition,
398 and offers an excellent introduction section which delves into the topic of
399 metaclass compatibility.
401 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
403 =item Safe Metaclass Programming
405 This paper seems to precede the above paper, and propose a mix-in based
406 approach as opposed to the Traits based approach. Both papers have similar
407 information on the metaclass compatibility problem space.
409 L<http://citeseer.ist.psu.edu/37617.html>
417 =item The Perl 6 MetaModel work in the Pugs project
421 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
423 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
429 =head1 SIMILAR MODULES
431 As I have said above, this module is a class-builder-builder, so it is
432 not the same thing as modules like L<Class::Accessor> and
433 L<Class::MethodMaker>. That being said there are very few modules on CPAN
434 with similar goals to this module. The one I have found which is most
435 like this module is L<Class::Meta>, although it's philosophy and the MOP it
436 creates are very different from this modules.
440 All complex software has bugs lurking in it, and this module is no
441 exception. If you find a bug please either email me, or add the bug
446 I use L<Devel::Cover> to test the code coverage of my tests, below is the
447 L<Devel::Cover> report on this module's test suite.
449 ---------------------------- ------ ------ ------ ------ ------ ------ ------
450 File stmt bran cond sub pod time total
451 ---------------------------- ------ ------ ------ ------ ------ ------ ------
452 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 19.8 100.0
453 Class/MOP/Attribute.pm 100.0 100.0 91.7 61.2 100.0 14.3 87.9
454 Class/MOP/Class.pm 97.6 91.3 77.3 98.4 100.0 56.4 93.2
455 Class/MOP/Instance.pm 91.1 75.0 33.3 91.7 100.0 6.8 90.7
456 Class/MOP/Method.pm 97.6 60.0 52.9 76.9 100.0 1.6 82.6
457 metaclass.pm 100.0 100.0 83.3 100.0 n/a 1.0 97.7
458 ---------------------------- ------ ------ ------ ------ ------ ------ ------
459 Total 97.5 88.5 75.5 82.8 100.0 100.0 91.2
460 ---------------------------- ------ ------ ------ ------ ------ ------ ------
462 =head1 ACKNOWLEDGEMENTS
466 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
468 Thanks to Rob for actually getting the development of this module kick-started.
474 Stevan Little E<lt>stevan@iinteractive.comE<gt>
476 =head1 COPYRIGHT AND LICENSE
478 Copyright 2006 by Infinity Interactive, Inc.
480 L<http://www.iinteractive.com>
482 This library is free software; you can redistribute it and/or modify
483 it under the same terms as Perl itself.