10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 our $VERSION = '0.30';
16 ## ----------------------------------------------------------------------------
17 ## Setting up our environment ...
18 ## ----------------------------------------------------------------------------
19 ## Class::MOP needs to have a few things in the global perl environment so
20 ## that it can operate effectively. Those things are done here.
21 ## ----------------------------------------------------------------------------
23 # ... nothing yet actually ;)
25 ## ----------------------------------------------------------------------------
27 ## ----------------------------------------------------------------------------
28 ## The code below here is to bootstrap our MOP with itself. This is also
29 ## sometimes called "tying the knot". By doing this, we make it much easier
30 ## to extend the MOP through subclassing and such since now you can use the
31 ## MOP itself to extend itself.
33 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
34 ## ----------------------------------------------------------------------------
36 # We need to add in the meta-attributes here so that
37 # any subclass of Class::MOP::* will be able to
38 # inherit them using &construct_instance
42 Class::MOP::Class->meta->add_attribute(
43 Class::MOP::Attribute->new('$:package' => (
45 # NOTE: we need to do this in order
46 # for the instance meta-object to
47 # not fall into meta-circular death
48 'name' => sub { (shift)->{'$:package'} }
50 init_arg => ':package',
54 Class::MOP::Class->meta->add_attribute(
55 Class::MOP::Attribute->new('%:attributes' => (
56 reader => 'get_attribute_map',
57 init_arg => ':attributes',
62 Class::MOP::Class->meta->add_attribute(
63 Class::MOP::Attribute->new('$:attribute_metaclass' => (
64 reader => 'attribute_metaclass',
65 init_arg => ':attribute_metaclass',
66 default => 'Class::MOP::Attribute',
70 Class::MOP::Class->meta->add_attribute(
71 Class::MOP::Attribute->new('$:method_metaclass' => (
72 reader => 'method_metaclass',
73 init_arg => ':method_metaclass',
74 default => 'Class::MOP::Method',
78 Class::MOP::Class->meta->add_attribute(
79 Class::MOP::Attribute->new('$:instance_metaclass' => (
81 # NOTE: we need to do this in order
82 # for the instance meta-object to
83 # not fall into meta-circular death
84 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
86 init_arg => ':instance_metaclass',
87 default => 'Class::MOP::Instance',
91 ## Class::MOP::Attribute
93 Class::MOP::Attribute->meta->add_attribute(
94 Class::MOP::Attribute->new('name' => (
96 # NOTE: we need to do this in order
97 # for the instance meta-object to
98 # not fall into meta-circular death
99 'name' => sub { (shift)->{name} }
104 Class::MOP::Attribute->meta->add_attribute(
105 Class::MOP::Attribute->new('associated_class' => (
107 # NOTE: we need to do this in order
108 # for the instance meta-object to
109 # not fall into meta-circular death
110 'associated_class' => sub { (shift)->{associated_class} }
115 Class::MOP::Attribute->meta->add_attribute(
116 Class::MOP::Attribute->new('accessor' => (
117 reader => 'accessor',
118 predicate => 'has_accessor',
122 Class::MOP::Attribute->meta->add_attribute(
123 Class::MOP::Attribute->new('reader' => (
125 predicate => 'has_reader',
129 Class::MOP::Attribute->meta->add_attribute(
130 Class::MOP::Attribute->new('writer' => (
132 predicate => 'has_writer',
136 Class::MOP::Attribute->meta->add_attribute(
137 Class::MOP::Attribute->new('predicate' => (
138 reader => 'predicate',
139 predicate => 'has_predicate',
143 Class::MOP::Attribute->meta->add_attribute(
144 Class::MOP::Attribute->new('init_arg' => (
145 reader => 'init_arg',
146 predicate => 'has_init_arg',
150 Class::MOP::Attribute->meta->add_attribute(
151 Class::MOP::Attribute->new('default' => (
152 # default has a custom 'reader' method ...
153 predicate => 'has_default',
158 # NOTE: (meta-circularity)
159 # This should be one of the last things done
160 # it will "tie the knot" with Class::MOP::Attribute
161 # so that it uses the attributes meta-objects
162 # to construct itself.
163 Class::MOP::Attribute->meta->add_method('new' => sub {
168 (defined $name && $name)
169 || confess "You must provide a name for the attribute";
170 $options{init_arg} = $name
171 if not exists $options{init_arg};
173 # return the new object
174 $class->meta->new_object(name => $name, %options);
177 Class::MOP::Attribute->meta->add_method('clone' => sub {
179 $self->meta->clone_object($self, @_);
190 Class::MOP - A Meta Object Protocol for Perl 5
194 # ... This will come later, for now see
195 # the other SYNOPSIS for more information
199 This module is an attempt to create a meta object protocol for the
200 Perl 5 object system. It makes no attempt to change the behavior or
201 characteristics of the Perl 5 object system, only to create a
202 protocol for its manipulation and introspection.
204 That said, it does attempt to create the tools for building a rich
205 set of extensions to the Perl 5 object system. Every attempt has been
206 made for these tools to keep to the spirit of the Perl 5 object
207 system that we all know and love.
209 =head2 What is a Meta Object Protocol?
211 A meta object protocol is an API to an object system.
213 To be more specific, it is a set of abstractions of the components of
214 an object system (typically things like; classes, object, methods,
215 object attributes, etc.). These abstractions can then be used to both
216 inspect and manipulate the object system which they describe.
218 It can be said that there are two MOPs for any object system; the
219 implicit MOP, and the explicit MOP. The implicit MOP handles things
220 like method dispatch or inheritance, which happen automatically as
221 part of how the object system works. The explicit MOP typically
222 handles the introspection/reflection features of the object system.
223 All object systems have implicit MOPs, without one, they would not
224 work. Explict MOPs however as less common, and depending on the
225 language can vary from restrictive (Reflection in Java or C#) to
226 wide open (CLOS is a perfect example).
228 =head2 Yet Another Class Builder!! Why?
230 This is B<not> a class builder so much as it is a I<class builder
231 B<builder>>. My intent is that an end user does not use this module
232 directly, but instead this module is used by module authors to
233 build extensions and features onto the Perl 5 object system.
235 =head2 Who is this module for?
237 This module is specifically for anyone who has ever created or
238 wanted to create a module for the Class:: namespace. The tools which
239 this module will provide will hopefully make it easier to do more
240 complex things with Perl 5 classes by removing such barriers as
241 the need to hack the symbol tables, or understand the fine details
244 =head2 What changes do I have to make to use this module?
246 This module was designed to be as unintrusive as possible. Many of
247 its features are accessible without B<any> change to your existsing
248 code at all. It is meant to be a compliment to your existing code and
249 not an intrusion on your code base. Unlike many other B<Class::>
250 modules, this module B<does not> require you subclass it, or even that
251 you C<use> it in within your module's package.
253 The only features which requires additions to your code are the
254 attribute handling and instance construction features, and these are
255 both completely optional features. The only reason for this is because
256 Perl 5's object system does not actually have these features built
257 in. More information about this feature can be found below.
259 =head2 A Note about Performance?
261 It is a common misconception that explict MOPs are performance drains.
262 But this is not a universal truth at all, it is an side-effect of
263 specific implementations. For instance, using Java reflection is much
264 slower because the JVM cannot take advantage of any compiler
265 optimizations, and the JVM has to deal with much more runtime type
266 information as well. Reflection in C# is marginally better as it was
267 designed into the language and runtime (the CLR). In contrast, CLOS
268 (the Common Lisp Object System) was built to support an explicit MOP,
269 and so performance is tuned for it.
271 This library in particular does it's absolute best to avoid putting
272 B<any> drain at all upon your code's performance. In fact, by itself
273 it does nothing to affect your existing code. So you only pay for
274 what you actually use.
276 =head2 About Metaclass compatibility
278 This module makes sure that all metaclasses created are both upwards
279 and downwards compatible. The topic of metaclass compatibility is
280 highly esoteric and is something only encountered when doing deep and
281 involved metaclass hacking. There are two basic kinds of metaclass
282 incompatibility; upwards and downwards.
284 Upwards metaclass compatibility means that the metaclass of a
285 given class is either the same as (or a subclass of) all of the
288 Downward metaclass compatibility means that the metaclasses of a
289 given class's anscestors are all either the same as (or a subclass
292 Here is a diagram showing a set of two classes (C<A> and C<B>) and
293 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
294 metaclass compatibility both upwards and downwards.
296 +---------+ +---------+
297 | Meta::A |<----| Meta::B | <....... (instance of )
298 +---------+ +---------+ <------- (inherits from)
301 +---------+ +---------+
303 +---------+ +---------+
305 As I said this is a highly esoteric topic and one you will only run
306 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
307 are interested in why this is an issue see the paper
308 I<Uniform and safe metaclass composition> linked to in the
309 L<SEE ALSO> section of this document.
311 =head2 Using custom metaclasses
313 Always use the metaclass pragma when using a custom metaclass, this
314 will ensure the proper initialization order and not accidentely
315 create an incorrect type of metaclass for you. This is a very rare
316 problem, and one which can only occur if you are doing deep metaclass
317 programming. So in other words, don't worry about it.
321 The protocol is divided into 3 main sub-protocols:
325 =item The Class protocol
327 This provides a means of manipulating and introspecting a Perl 5
328 class. It handles all of symbol table hacking for you, and provides
329 a rich set of methods that go beyond simple package introspection.
331 See L<Class::MOP::Class> for more details.
333 =item The Attribute protocol
335 This provides a consistent represenation for an attribute of a
336 Perl 5 class. Since there are so many ways to create and handle
337 atttributes in Perl 5 OO, this attempts to provide as much of a
338 unified approach as possible, while giving the freedom and
339 flexibility to subclass for specialization.
341 See L<Class::MOP::Attribute> for more details.
343 =item The Method protocol
345 This provides a means of manipulating and introspecting methods in
346 the Perl 5 object system. As with attributes, there are many ways to
347 approach this topic, so we try to keep it pretty basic, while still
348 making it possible to extend the system in many ways.
350 See L<Class::MOP::Method> for more details.
358 There are very few books out on Meta Object Protocols and Metaclasses
359 because it is such an esoteric topic. The following books are really
360 the only ones I have found. If you know of any more, B<I<please>>
361 email me and let me know, I would love to hear about them.
365 =item "The Art of the Meta Object Protocol"
367 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
369 =item "Putting MetaClasses to Work"
371 =item "Smalltalk: The Language"
379 =item Uniform and safe metaclass composition
381 An excellent paper by the people who brought us the original Traits paper.
382 This paper is on how Traits can be used to do safe metaclass composition,
383 and offers an excellent introduction section which delves into the topic of
384 metaclass compatibility.
386 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
388 =item Safe Metaclass Programming
390 This paper seems to precede the above paper, and propose a mix-in based
391 approach as opposed to the Traits based approach. Both papers have similar
392 information on the metaclass compatibility problem space.
394 L<http://citeseer.ist.psu.edu/37617.html>
402 =item The Perl 6 MetaModel work in the Pugs project
406 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
408 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
414 =head1 SIMILAR MODULES
416 As I have said above, this module is a class-builder-builder, so it is
417 not the same thing as modules like L<Class::Accessor> and
418 L<Class::MethodMaker>. That being said there are very few modules on CPAN
419 with similar goals to this module. The one I have found which is most
420 like this module is L<Class::Meta>, although it's philosophy and the MOP it
421 creates are very different from this modules.
425 All complex software has bugs lurking in it, and this module is no
426 exception. If you find a bug please either email me, or add the bug
431 I use L<Devel::Cover> to test the code coverage of my tests, below is the
432 L<Devel::Cover> report on this module's test suite.
434 ---------------------------- ------ ------ ------ ------ ------ ------ ------
435 File stmt bran cond sub pod time total
436 ---------------------------- ------ ------ ------ ------ ------ ------ ------
437 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 9.6 100.0
438 Class/MOP/Attribute.pm 100.0 100.0 91.7 73.8 100.0 28.4 92.1
439 Class/MOP/Class.pm 100.0 93.5 82.3 98.2 100.0 56.6 95.7
440 Class/MOP/Method.pm 100.0 64.3 52.9 80.0 100.0 3.5 85.3
441 metaclass.pm 100.0 100.0 80.0 100.0 n/a 1.9 97.4
442 ---------------------------- ------ ------ ------ ------ ------ ------ ------
443 Total 100.0 90.8 79.7 86.2 100.0 100.0 93.6
444 ---------------------------- ------ ------ ------ ------ ------ ------ ------
446 =head1 ACKNOWLEDGEMENTS
450 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
452 Thanks to Rob for actually getting the development of this module kick-started.
458 Stevan Little E<lt>stevan@iinteractive.comE<gt>
460 =head1 COPYRIGHT AND LICENSE
462 Copyright 2006 by Infinity Interactive, Inc.
464 L<http://www.iinteractive.com>
466 This library is free software; you can redistribute it and/or modify
467 it under the same terms as Perl itself.