7 use Scalar::Util 'blessed';
11 use Class::MOP::Class;
12 use Class::MOP::Attribute;
13 use Class::MOP::Method;
15 our $VERSION = '0.07';
20 if ($_[0] eq ':universal') {
21 *UNIVERSAL::meta = sub {
22 Class::MOP::Class->initialize(blessed($_[0]) || $_[0])
28 *{$pkg . '::' . $_[0]} = sub {
29 Class::MOP::Class->initialize(blessed($_[0]) || $_[0])
34 ## ----------------------------------------------------------------------------
36 ## ----------------------------------------------------------------------------
37 ## The code below here is to bootstrap our MOP with itself. This is also
38 ## sometimes called "tying the knot". By doing this, we make it much easier
39 ## to extend the MOP through subclassing and such since now you can use the
40 ## MOP itself to extend itself.
42 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
43 ## ----------------------------------------------------------------------------
45 # We need to add in the meta-attributes here so that
46 # any subclass of Class::MOP::* will be able to
47 # inherit them using &construct_instance
51 Class::MOP::Class->meta->add_attribute(
52 Class::MOP::Attribute->new('$:package' => (
54 init_arg => ':package',
58 Class::MOP::Class->meta->add_attribute(
59 Class::MOP::Attribute->new('%:attributes' => (
60 reader => 'get_attribute_map',
61 init_arg => ':attributes',
66 Class::MOP::Class->meta->add_attribute(
67 Class::MOP::Attribute->new('$:attribute_metaclass' => (
68 reader => 'attribute_metaclass',
69 init_arg => ':attribute_metaclass',
70 default => 'Class::MOP::Attribute',
74 Class::MOP::Class->meta->add_attribute(
75 Class::MOP::Attribute->new('$:method_metaclass' => (
76 reader => 'method_metaclass',
77 init_arg => ':method_metaclass',
78 default => 'Class::MOP::Method',
82 ## Class::MOP::Attribute
84 Class::MOP::Attribute->meta->add_attribute(
85 Class::MOP::Attribute->new('name' => (
90 Class::MOP::Attribute->meta->add_attribute(
91 Class::MOP::Attribute->new('associated_class' => (
92 reader => 'associated_class'
96 Class::MOP::Attribute->meta->add_attribute(
97 Class::MOP::Attribute->new('accessor' => (
99 predicate => 'has_accessor',
103 Class::MOP::Attribute->meta->add_attribute(
104 Class::MOP::Attribute->new('reader' => (
106 predicate => 'has_reader',
110 Class::MOP::Attribute->meta->add_attribute(
111 Class::MOP::Attribute->new('writer' => (
113 predicate => 'has_writer',
117 Class::MOP::Attribute->meta->add_attribute(
118 Class::MOP::Attribute->new('predicate' => (
119 reader => 'predicate',
120 predicate => 'has_predicate',
124 Class::MOP::Attribute->meta->add_attribute(
125 Class::MOP::Attribute->new('init_arg' => (
126 reader => 'init_arg',
127 predicate => 'has_init_arg',
131 Class::MOP::Attribute->meta->add_attribute(
132 Class::MOP::Attribute->new('default' => (
133 # default has a custom 'reader' method ...
134 predicate => 'has_default',
139 # NOTE: (meta-circularity)
140 # This should be one of the last things done
141 # it will "tie the knot" with Class::MOP::Attribute
142 # so that it uses the attributes meta-objects
143 # to construct itself.
144 Class::MOP::Attribute->meta->add_method('new' => sub {
149 (defined $name && $name)
150 || confess "You must provide a name for the attribute";
151 $options{init_arg} = $name
152 if not exists $options{init_arg};
154 # return the new object
155 $class->meta->new_object(name => $name, %options);
158 Class::MOP::Attribute->meta->add_method('clone' => sub {
160 my $class = $self->associated_class;
161 $self->detach_from_class() if defined $class;
162 my $clone = $self->meta->clone_object($self, @_);
163 if (defined $class) {
164 $self->attach_to_class($class);
165 $clone->attach_to_class($class);
178 Class::MOP - A Meta Object Protocol for Perl 5
182 # ... This will come later, for now see
183 # the other SYNOPSIS for more information
187 This module is an attempt to create a meta object protocol for the
188 Perl 5 object system. It makes no attempt to change the behavior or
189 characteristics of the Perl 5 object system, only to create a
190 protocol for its manipulation and introspection.
192 That said, it does attempt to create the tools for building a rich
193 set of extensions to the Perl 5 object system. Every attempt has been
194 made for these tools to keep to the spirit of the Perl 5 object
195 system that we all know and love.
197 =head2 What is a Meta Object Protocol?
199 A meta object protocol is an API to an object system.
201 To be more specific, it is a set of abstractions of the components of
202 an object system (typically things like; classes, object, methods,
203 object attributes, etc.). These abstractions can then be used to both
204 inspect and manipulate the object system which they describe.
206 It can be said that there are two MOPs for any object system; the
207 implicit MOP, and the explicit MOP. The implicit MOP handles things
208 like method dispatch or inheritance, which happen automatically as
209 part of how the object system works. The explicit MOP typically
210 handles the introspection/reflection features of the object system.
211 All object systems have implicit MOPs, without one, they would not
212 work. Explict MOPs however as less common, and depending on the
213 language can vary from restrictive (Reflection in Java or C#) to
214 wide open (CLOS is a perfect example).
216 =head2 Yet Another Class Builder!! Why?
218 This is B<not> a class builder so much as it is a I<class builder
219 B<builder>>. My intent is that an end user does not use this module
220 directly, but instead this module is used by module authors to
221 build extensions and features onto the Perl 5 object system.
223 =head2 Who is this module for?
225 This module is specifically for anyone who has ever created or
226 wanted to create a module for the Class:: namespace. The tools which
227 this module will provide will hopefully make it easier to do more
228 complex things with Perl 5 classes by removing such barriers as
229 the need to hack the symbol tables, or understand the fine details
232 =head2 What changes do I have to make to use this module?
234 This module was designed to be as unintrusive as possible. Many of
235 its features are accessible without B<any> change to your existsing
236 code at all. It is meant to be a compliment to your existing code and
237 not an intrusion on your code base. Unlike many other B<Class::>
238 modules, this module B<does not> require you subclass it, or even that
239 you C<use> it in within your module's package.
241 The only features which requires additions to your code are the
242 attribute handling and instance construction features, and these are
243 both completely optional features. The only reason for this is because
244 Perl 5's object system does not actually have these features built
245 in. More information about this feature can be found below.
247 =head2 A Note about Performance?
249 It is a common misconception that explict MOPs are performance drains.
250 But this is not a universal truth at all, it is an side-effect of
251 specific implementations. For instance, using Java reflection is much
252 slower because the JVM cannot take advantage of any compiler
253 optimizations, and the JVM has to deal with much more runtime type
254 information as well. Reflection in C# is marginally better as it was
255 designed into the language and runtime (the CLR). In contrast, CLOS
256 (the Common Lisp Object System) was built to support an explicit MOP,
257 and so performance is tuned for it.
259 This library in particular does it's absolute best to avoid putting
260 B<any> drain at all upon your code's performance. In fact, by itself
261 it does nothing to affect your existing code. So you only pay for
262 what you actually use.
264 =head2 About Metaclass compatibility
266 This module makes sure that all metaclasses created are both upwards
267 and downwards compatible. The topic of metaclass compatibility is
268 highly esoteric and is something only encountered when doing deep and
269 involved metaclass hacking. There are two basic kinds of metaclass
270 incompatibility; upwards and downwards.
272 Upwards metaclass compatibility means that the metaclass of a
273 given class is either the same as (or a subclass of) all of the
276 Downward metaclass compatibility means that the metaclasses of a
277 given class's anscestors are all either the same as (or a subclass
280 Here is a diagram showing a set of two classes (C<A> and C<B>) and
281 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
282 metaclass compatibility both upwards and downwards.
284 +---------+ +---------+
285 | Meta::A |<----| Meta::B | <....... (instance of )
286 +---------+ +---------+ <------- (inherits from)
289 +---------+ +---------+
291 +---------+ +---------+
293 As I said this is a highly esoteric topic and one you will only run
294 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
295 are interested in why this is an issue see the paper
296 I<Uniform and safe metaclass composition> linked to in the
297 L<SEE ALSO> section of this document.
301 The protocol is divided into 3 main sub-protocols:
305 =item The Class protocol
307 This provides a means of manipulating and introspecting a Perl 5
308 class. It handles all of symbol table hacking for you, and provides
309 a rich set of methods that go beyond simple package introspection.
311 See L<Class::MOP::Class> for more details.
313 =item The Attribute protocol
315 This provides a consistent represenation for an attribute of a
316 Perl 5 class. Since there are so many ways to create and handle
317 atttributes in Perl 5 OO, this attempts to provide as much of a
318 unified approach as possible, while giving the freedom and
319 flexibility to subclass for specialization.
321 See L<Class::MOP::Attribute> for more details.
323 =item The Method protocol
325 This provides a means of manipulating and introspecting methods in
326 the Perl 5 object system. As with attributes, there are many ways to
327 approach this topic, so we try to keep it pretty basic, while still
328 making it possible to extend the system in many ways.
330 See L<Class::MOP::Method> for more details.
338 There are very few books out on Meta Object Protocols and Metaclasses
339 because it is such an esoteric topic. The following books are really
340 the only ones I have found. If you know of any more, B<I<please>>
341 email me and let me know, I would love to hear about them.
345 =item "The Art of the Meta Object Protocol"
347 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
349 =item "Putting MetaClasses to Work"
351 =item "Smalltalk: The Language"
359 =item Uniform and safe metaclass composition
361 An excellent paper by the people who brought us the original Traits paper.
362 This paper is on how Traits can be used to do safe metaclass composition,
363 and offers an excellent introduction section which delves into the topic of
364 metaclass compatibility.
366 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
368 =item Safe Metaclass Programming
370 This paper seems to precede the above paper, and propose a mix-in based
371 approach as opposed to the Traits based approach. Both papers have similar
372 information on the metaclass compatibility problem space.
374 L<http://citeseer.ist.psu.edu/37617.html>
382 =item The Perl 6 MetaModel work in the Pugs project
386 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
388 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
394 =head1 SIMILAR MODULES
396 As I have said above, this module is a class-builder-builder, so it is
397 not the same thing as modules like L<Class::Accessor> and
398 L<Class::MethodMaker>. That being said there are very few modules on CPAN
399 with similar goals to this module. The one I have found which is most
400 like this module is L<Class::Meta>, although it's philosophy and the MOP it
401 creates are very different from this modules.
405 All complex software has bugs lurking in it, and this module is no
406 exception. If you find a bug please either email me, or add the bug
409 =head1 ACKNOWLEDGEMENTS
413 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
415 Thanks to Rob for actually getting the development of this module kick-started.
421 Stevan Little E<lt>stevan@iinteractive.comE<gt>
423 =head1 COPYRIGHT AND LICENSE
425 Copyright 2006 by Infinity Interactive, Inc.
427 L<http://www.iinteractive.com>
429 This library is free software; you can redistribute it and/or modify
430 it under the same terms as Perl itself.