10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 our $VERSION = '0.10';
16 ## ----------------------------------------------------------------------------
17 ## Setting up our environment ...
18 ## ----------------------------------------------------------------------------
19 ## Class::MOP needs to have a few things in the global perl environment so
20 ## that it can operate effectively. Those things are done here.
21 ## ----------------------------------------------------------------------------
23 # ... nothing yet actually ;)
25 ## ----------------------------------------------------------------------------
27 ## ----------------------------------------------------------------------------
28 ## The code below here is to bootstrap our MOP with itself. This is also
29 ## sometimes called "tying the knot". By doing this, we make it much easier
30 ## to extend the MOP through subclassing and such since now you can use the
31 ## MOP itself to extend itself.
33 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
34 ## ----------------------------------------------------------------------------
36 # We need to add in the meta-attributes here so that
37 # any subclass of Class::MOP::* will be able to
38 # inherit them using &construct_instance
42 Class::MOP::Class->meta->add_attribute(
43 Class::MOP::Attribute->new('$:package' => (
45 init_arg => ':package',
49 Class::MOP::Class->meta->add_attribute(
50 Class::MOP::Attribute->new('%:attributes' => (
51 reader => 'get_attribute_map',
52 init_arg => ':attributes',
57 Class::MOP::Class->meta->add_attribute(
58 Class::MOP::Attribute->new('$:attribute_metaclass' => (
59 reader => 'attribute_metaclass',
60 init_arg => ':attribute_metaclass',
61 default => 'Class::MOP::Attribute',
65 Class::MOP::Class->meta->add_attribute(
66 Class::MOP::Attribute->new('$:method_metaclass' => (
67 reader => 'method_metaclass',
68 init_arg => ':method_metaclass',
69 default => 'Class::MOP::Method',
73 ## Class::MOP::Attribute
75 Class::MOP::Attribute->meta->add_attribute(
76 Class::MOP::Attribute->new('name' => (
81 Class::MOP::Attribute->meta->add_attribute(
82 Class::MOP::Attribute->new('associated_class' => (
83 reader => 'associated_class'
87 Class::MOP::Attribute->meta->add_attribute(
88 Class::MOP::Attribute->new('accessor' => (
90 predicate => 'has_accessor',
94 Class::MOP::Attribute->meta->add_attribute(
95 Class::MOP::Attribute->new('reader' => (
97 predicate => 'has_reader',
101 Class::MOP::Attribute->meta->add_attribute(
102 Class::MOP::Attribute->new('writer' => (
104 predicate => 'has_writer',
108 Class::MOP::Attribute->meta->add_attribute(
109 Class::MOP::Attribute->new('predicate' => (
110 reader => 'predicate',
111 predicate => 'has_predicate',
115 Class::MOP::Attribute->meta->add_attribute(
116 Class::MOP::Attribute->new('init_arg' => (
117 reader => 'init_arg',
118 predicate => 'has_init_arg',
122 Class::MOP::Attribute->meta->add_attribute(
123 Class::MOP::Attribute->new('default' => (
124 # default has a custom 'reader' method ...
125 predicate => 'has_default',
130 # NOTE: (meta-circularity)
131 # This should be one of the last things done
132 # it will "tie the knot" with Class::MOP::Attribute
133 # so that it uses the attributes meta-objects
134 # to construct itself.
135 Class::MOP::Attribute->meta->add_method('new' => sub {
140 (defined $name && $name)
141 || confess "You must provide a name for the attribute";
142 $options{init_arg} = $name
143 if not exists $options{init_arg};
145 # return the new object
146 $class->meta->new_object(name => $name, %options);
149 Class::MOP::Attribute->meta->add_method('clone' => sub {
151 my $class = $self->associated_class;
152 $self->detach_from_class() if defined $class;
153 my $clone = $self->meta->clone_object($self, @_);
154 if (defined $class) {
155 $self->attach_to_class($class);
156 $clone->attach_to_class($class);
169 Class::MOP - A Meta Object Protocol for Perl 5
173 # ... This will come later, for now see
174 # the other SYNOPSIS for more information
178 This module is an attempt to create a meta object protocol for the
179 Perl 5 object system. It makes no attempt to change the behavior or
180 characteristics of the Perl 5 object system, only to create a
181 protocol for its manipulation and introspection.
183 That said, it does attempt to create the tools for building a rich
184 set of extensions to the Perl 5 object system. Every attempt has been
185 made for these tools to keep to the spirit of the Perl 5 object
186 system that we all know and love.
188 =head2 What is a Meta Object Protocol?
190 A meta object protocol is an API to an object system.
192 To be more specific, it is a set of abstractions of the components of
193 an object system (typically things like; classes, object, methods,
194 object attributes, etc.). These abstractions can then be used to both
195 inspect and manipulate the object system which they describe.
197 It can be said that there are two MOPs for any object system; the
198 implicit MOP, and the explicit MOP. The implicit MOP handles things
199 like method dispatch or inheritance, which happen automatically as
200 part of how the object system works. The explicit MOP typically
201 handles the introspection/reflection features of the object system.
202 All object systems have implicit MOPs, without one, they would not
203 work. Explict MOPs however as less common, and depending on the
204 language can vary from restrictive (Reflection in Java or C#) to
205 wide open (CLOS is a perfect example).
207 =head2 Yet Another Class Builder!! Why?
209 This is B<not> a class builder so much as it is a I<class builder
210 B<builder>>. My intent is that an end user does not use this module
211 directly, but instead this module is used by module authors to
212 build extensions and features onto the Perl 5 object system.
214 =head2 Who is this module for?
216 This module is specifically for anyone who has ever created or
217 wanted to create a module for the Class:: namespace. The tools which
218 this module will provide will hopefully make it easier to do more
219 complex things with Perl 5 classes by removing such barriers as
220 the need to hack the symbol tables, or understand the fine details
223 =head2 What changes do I have to make to use this module?
225 This module was designed to be as unintrusive as possible. Many of
226 its features are accessible without B<any> change to your existsing
227 code at all. It is meant to be a compliment to your existing code and
228 not an intrusion on your code base. Unlike many other B<Class::>
229 modules, this module B<does not> require you subclass it, or even that
230 you C<use> it in within your module's package.
232 The only features which requires additions to your code are the
233 attribute handling and instance construction features, and these are
234 both completely optional features. The only reason for this is because
235 Perl 5's object system does not actually have these features built
236 in. More information about this feature can be found below.
238 =head2 A Note about Performance?
240 It is a common misconception that explict MOPs are performance drains.
241 But this is not a universal truth at all, it is an side-effect of
242 specific implementations. For instance, using Java reflection is much
243 slower because the JVM cannot take advantage of any compiler
244 optimizations, and the JVM has to deal with much more runtime type
245 information as well. Reflection in C# is marginally better as it was
246 designed into the language and runtime (the CLR). In contrast, CLOS
247 (the Common Lisp Object System) was built to support an explicit MOP,
248 and so performance is tuned for it.
250 This library in particular does it's absolute best to avoid putting
251 B<any> drain at all upon your code's performance. In fact, by itself
252 it does nothing to affect your existing code. So you only pay for
253 what you actually use.
255 =head2 About Metaclass compatibility
257 This module makes sure that all metaclasses created are both upwards
258 and downwards compatible. The topic of metaclass compatibility is
259 highly esoteric and is something only encountered when doing deep and
260 involved metaclass hacking. There are two basic kinds of metaclass
261 incompatibility; upwards and downwards.
263 Upwards metaclass compatibility means that the metaclass of a
264 given class is either the same as (or a subclass of) all of the
267 Downward metaclass compatibility means that the metaclasses of a
268 given class's anscestors are all either the same as (or a subclass
271 Here is a diagram showing a set of two classes (C<A> and C<B>) and
272 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
273 metaclass compatibility both upwards and downwards.
275 +---------+ +---------+
276 | Meta::A |<----| Meta::B | <....... (instance of )
277 +---------+ +---------+ <------- (inherits from)
280 +---------+ +---------+
282 +---------+ +---------+
284 As I said this is a highly esoteric topic and one you will only run
285 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
286 are interested in why this is an issue see the paper
287 I<Uniform and safe metaclass composition> linked to in the
288 L<SEE ALSO> section of this document.
290 =head2 Using custom metaclasses
292 Always use the metaclass pragma when using a custom metaclass, this
293 will ensure the proper initialization order and not accidentely
294 create an incorrect type of metaclass for you. This is a very rare
295 problem, and one which can only occur if you are doing deep metaclass
296 programming. So in other words, don't worry about it.
300 The protocol is divided into 3 main sub-protocols:
304 =item The Class protocol
306 This provides a means of manipulating and introspecting a Perl 5
307 class. It handles all of symbol table hacking for you, and provides
308 a rich set of methods that go beyond simple package introspection.
310 See L<Class::MOP::Class> for more details.
312 =item The Attribute protocol
314 This provides a consistent represenation for an attribute of a
315 Perl 5 class. Since there are so many ways to create and handle
316 atttributes in Perl 5 OO, this attempts to provide as much of a
317 unified approach as possible, while giving the freedom and
318 flexibility to subclass for specialization.
320 See L<Class::MOP::Attribute> for more details.
322 =item The Method protocol
324 This provides a means of manipulating and introspecting methods in
325 the Perl 5 object system. As with attributes, there are many ways to
326 approach this topic, so we try to keep it pretty basic, while still
327 making it possible to extend the system in many ways.
329 See L<Class::MOP::Method> for more details.
337 There are very few books out on Meta Object Protocols and Metaclasses
338 because it is such an esoteric topic. The following books are really
339 the only ones I have found. If you know of any more, B<I<please>>
340 email me and let me know, I would love to hear about them.
344 =item "The Art of the Meta Object Protocol"
346 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
348 =item "Putting MetaClasses to Work"
350 =item "Smalltalk: The Language"
358 =item Uniform and safe metaclass composition
360 An excellent paper by the people who brought us the original Traits paper.
361 This paper is on how Traits can be used to do safe metaclass composition,
362 and offers an excellent introduction section which delves into the topic of
363 metaclass compatibility.
365 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
367 =item Safe Metaclass Programming
369 This paper seems to precede the above paper, and propose a mix-in based
370 approach as opposed to the Traits based approach. Both papers have similar
371 information on the metaclass compatibility problem space.
373 L<http://citeseer.ist.psu.edu/37617.html>
381 =item The Perl 6 MetaModel work in the Pugs project
385 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
387 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
393 =head1 SIMILAR MODULES
395 As I have said above, this module is a class-builder-builder, so it is
396 not the same thing as modules like L<Class::Accessor> and
397 L<Class::MethodMaker>. That being said there are very few modules on CPAN
398 with similar goals to this module. The one I have found which is most
399 like this module is L<Class::Meta>, although it's philosophy and the MOP it
400 creates are very different from this modules.
404 All complex software has bugs lurking in it, and this module is no
405 exception. If you find a bug please either email me, or add the bug
410 I use L<Devel::Cover> to test the code coverage of my tests, below is the
411 L<Devel::Cover> report on this module's test suite.
413 ---------------------------- ------ ------ ------ ------ ------ ------ ------
414 File stmt bran cond sub pod time total
415 ---------------------------- ------ ------ ------ ------ ------ ------ ------
416 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 21.4 100.0
417 Class/MOP/Attribute.pm 100.0 100.0 88.9 100.0 100.0 27.1 99.3
418 Class/MOP/Class.pm 100.0 100.0 93.7 100.0 100.0 44.8 99.1
419 Class/MOP/Method.pm 100.0 100.0 83.3 100.0 100.0 4.8 97.1
420 metaclass.pm 100.0 100.0 80.0 100.0 n/a 1.9 97.3
421 ---------------------------- ------ ------ ------ ------ ------ ------ ------
422 Total 100.0 100.0 92.2 100.0 100.0 100.0 99.0
423 ---------------------------- ------ ------ ------ ------ ------ ------ ------
425 =head1 ACKNOWLEDGEMENTS
429 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
431 Thanks to Rob for actually getting the development of this module kick-started.
437 Stevan Little E<lt>stevan@iinteractive.comE<gt>
439 =head1 COPYRIGHT AND LICENSE
441 Copyright 2006 by Infinity Interactive, Inc.
443 L<http://www.iinteractive.com>
445 This library is free software; you can redistribute it and/or modify
446 it under the same terms as Perl itself.