10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 use Class::MOP::Class::Immutable;
16 our $VERSION = '0.31';
18 ## ----------------------------------------------------------------------------
19 ## Setting up our environment ...
20 ## ----------------------------------------------------------------------------
21 ## Class::MOP needs to have a few things in the global perl environment so
22 ## that it can operate effectively. Those things are done here.
23 ## ----------------------------------------------------------------------------
25 # ... nothing yet actually ;)
27 ## ----------------------------------------------------------------------------
29 ## ----------------------------------------------------------------------------
30 ## The code below here is to bootstrap our MOP with itself. This is also
31 ## sometimes called "tying the knot". By doing this, we make it much easier
32 ## to extend the MOP through subclassing and such since now you can use the
33 ## MOP itself to extend itself.
35 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
36 ## ----------------------------------------------------------------------------
38 # We need to add in the meta-attributes here so that
39 # any subclass of Class::MOP::* will be able to
40 # inherit them using &construct_instance
42 ## Class::MOP::Package
44 Class::MOP::Package->meta->add_attribute(
45 Class::MOP::Attribute->new('$: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('init_arg' => (
185 reader => 'init_arg',
186 predicate => 'has_init_arg',
190 Class::MOP::Attribute->meta->add_attribute(
191 Class::MOP::Attribute->new('default' => (
192 # default has a custom 'reader' method ...
193 predicate => 'has_default',
198 # NOTE: (meta-circularity)
199 # This should be one of the last things done
200 # it will "tie the knot" with Class::MOP::Attribute
201 # so that it uses the attributes meta-objects
202 # to construct itself.
203 Class::MOP::Attribute->meta->add_method('new' => sub {
208 (defined $name && $name)
209 || confess "You must provide a name for the attribute";
210 $options{init_arg} = $name
211 if not exists $options{init_arg};
213 # return the new object
214 $class->meta->new_object(name => $name, %options);
217 Class::MOP::Attribute->meta->add_method('clone' => sub {
219 $self->meta->clone_object($self, @_);
222 ## Try and close Class::MOP::*
224 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
225 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
226 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
227 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
228 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
229 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
240 Class::MOP - A Meta Object Protocol for Perl 5
244 # ... This will come later, for now see
245 # the other SYNOPSIS for more information
249 This module is an attempt to create a meta object protocol for the
250 Perl 5 object system. It makes no attempt to change the behavior or
251 characteristics of the Perl 5 object system, only to create a
252 protocol for its manipulation and introspection.
254 That said, it does attempt to create the tools for building a rich
255 set of extensions to the Perl 5 object system. Every attempt has been
256 made for these tools to keep to the spirit of the Perl 5 object
257 system that we all know and love.
259 This documentation is admittedly sparse on details, as time permits
260 I will try to improve them. For now, I suggest looking at the items
261 listed in the L<SEE ALSO> section for more information. In particular
262 the book "The Art of the Meta Object Protocol" was very influential
263 in the development of this system.
265 =head2 What is a Meta Object Protocol?
267 A meta object protocol is an API to an object system.
269 To be more specific, it is a set of abstractions of the components of
270 an object system (typically things like; classes, object, methods,
271 object attributes, etc.). These abstractions can then be used to both
272 inspect and manipulate the object system which they describe.
274 It can be said that there are two MOPs for any object system; the
275 implicit MOP, and the explicit MOP. The implicit MOP handles things
276 like method dispatch or inheritance, which happen automatically as
277 part of how the object system works. The explicit MOP typically
278 handles the introspection/reflection features of the object system.
279 All object systems have implicit MOPs, without one, they would not
280 work. Explict MOPs however as less common, and depending on the
281 language can vary from restrictive (Reflection in Java or C#) to
282 wide open (CLOS is a perfect example).
284 =head2 Yet Another Class Builder!! Why?
286 This is B<not> a class builder so much as it is a I<class builder
287 B<builder>>. My intent is that an end user does not use this module
288 directly, but instead this module is used by module authors to
289 build extensions and features onto the Perl 5 object system.
291 =head2 Who is this module for?
293 This module is specifically for anyone who has ever created or
294 wanted to create a module for the Class:: namespace. The tools which
295 this module will provide will hopefully make it easier to do more
296 complex things with Perl 5 classes by removing such barriers as
297 the need to hack the symbol tables, or understand the fine details
300 =head2 What changes do I have to make to use this module?
302 This module was designed to be as unintrusive as possible. Many of
303 its features are accessible without B<any> change to your existsing
304 code at all. It is meant to be a compliment to your existing code and
305 not an intrusion on your code base. Unlike many other B<Class::>
306 modules, this module B<does not> require you subclass it, or even that
307 you C<use> it in within your module's package.
309 The only features which requires additions to your code are the
310 attribute handling and instance construction features, and these are
311 both completely optional features. The only reason for this is because
312 Perl 5's object system does not actually have these features built
313 in. More information about this feature can be found below.
315 =head2 A Note about Performance?
317 It is a common misconception that explict MOPs are performance drains.
318 But this is not a universal truth at all, it is an side-effect of
319 specific implementations. For instance, using Java reflection is much
320 slower because the JVM cannot take advantage of any compiler
321 optimizations, and the JVM has to deal with much more runtime type
322 information as well. Reflection in C# is marginally better as it was
323 designed into the language and runtime (the CLR). In contrast, CLOS
324 (the Common Lisp Object System) was built to support an explicit MOP,
325 and so performance is tuned for it.
327 This library in particular does it's absolute best to avoid putting
328 B<any> drain at all upon your code's performance. In fact, by itself
329 it does nothing to affect your existing code. So you only pay for
330 what you actually use.
332 =head2 About Metaclass compatibility
334 This module makes sure that all metaclasses created are both upwards
335 and downwards compatible. The topic of metaclass compatibility is
336 highly esoteric and is something only encountered when doing deep and
337 involved metaclass hacking. There are two basic kinds of metaclass
338 incompatibility; upwards and downwards.
340 Upwards metaclass compatibility means that the metaclass of a
341 given class is either the same as (or a subclass of) all of the
344 Downward metaclass compatibility means that the metaclasses of a
345 given class's anscestors are all either the same as (or a subclass
348 Here is a diagram showing a set of two classes (C<A> and C<B>) and
349 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
350 metaclass compatibility both upwards and downwards.
352 +---------+ +---------+
353 | Meta::A |<----| Meta::B | <....... (instance of )
354 +---------+ +---------+ <------- (inherits from)
357 +---------+ +---------+
359 +---------+ +---------+
361 As I said this is a highly esoteric topic and one you will only run
362 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
363 are interested in why this is an issue see the paper
364 I<Uniform and safe metaclass composition> linked to in the
365 L<SEE ALSO> section of this document.
367 =head2 Using custom metaclasses
369 Always use the metaclass pragma when using a custom metaclass, this
370 will ensure the proper initialization order and not accidentely
371 create an incorrect type of metaclass for you. This is a very rare
372 problem, and one which can only occur if you are doing deep metaclass
373 programming. So in other words, don't worry about it.
377 The protocol is divided into 3 main sub-protocols:
381 =item The Class protocol
383 This provides a means of manipulating and introspecting a Perl 5
384 class. It handles all of symbol table hacking for you, and provides
385 a rich set of methods that go beyond simple package introspection.
387 See L<Class::MOP::Class> for more details.
389 =item The Attribute protocol
391 This provides a consistent represenation for an attribute of a
392 Perl 5 class. Since there are so many ways to create and handle
393 atttributes in Perl 5 OO, this attempts to provide as much of a
394 unified approach as possible, while giving the freedom and
395 flexibility to subclass for specialization.
397 See L<Class::MOP::Attribute> for more details.
399 =item The Method protocol
401 This provides a means of manipulating and introspecting methods in
402 the Perl 5 object system. As with attributes, there are many ways to
403 approach this topic, so we try to keep it pretty basic, while still
404 making it possible to extend the system in many ways.
406 See L<Class::MOP::Method> for more details.
414 There are very few books out on Meta Object Protocols and Metaclasses
415 because it is such an esoteric topic. The following books are really
416 the only ones I have found. If you know of any more, B<I<please>>
417 email me and let me know, I would love to hear about them.
421 =item "The Art of the Meta Object Protocol"
423 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
425 =item "Putting MetaClasses to Work"
427 =item "Smalltalk: The Language"
435 =item Uniform and safe metaclass composition
437 An excellent paper by the people who brought us the original Traits paper.
438 This paper is on how Traits can be used to do safe metaclass composition,
439 and offers an excellent introduction section which delves into the topic of
440 metaclass compatibility.
442 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
444 =item Safe Metaclass Programming
446 This paper seems to precede the above paper, and propose a mix-in based
447 approach as opposed to the Traits based approach. Both papers have similar
448 information on the metaclass compatibility problem space.
450 L<http://citeseer.ist.psu.edu/37617.html>
458 =item The Perl 6 MetaModel work in the Pugs project
462 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
464 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
470 =head1 SIMILAR MODULES
472 As I have said above, this module is a class-builder-builder, so it is
473 not the same thing as modules like L<Class::Accessor> and
474 L<Class::MethodMaker>. That being said there are very few modules on CPAN
475 with similar goals to this module. The one I have found which is most
476 like this module is L<Class::Meta>, although it's philosophy and the MOP it
477 creates are very different from this modules.
481 All complex software has bugs lurking in it, and this module is no
482 exception. If you find a bug please either email me, or add the bug
487 I use L<Devel::Cover> to test the code coverage of my tests, below is the
488 L<Devel::Cover> report on this module's test suite.
490 ---------------------------- ------ ------ ------ ------ ------ ------ ------
491 File stmt bran cond sub pod time total
492 ---------------------------- ------ ------ ------ ------ ------ ------ ------
493 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 19.8 100.0
494 Class/MOP/Attribute.pm 100.0 100.0 91.7 61.2 100.0 14.3 87.9
495 Class/MOP/Class.pm 97.6 91.3 77.3 98.4 100.0 56.4 93.2
496 Class/MOP/Instance.pm 91.1 75.0 33.3 91.7 100.0 6.8 90.7
497 Class/MOP/Method.pm 97.6 60.0 52.9 76.9 100.0 1.6 82.6
498 metaclass.pm 100.0 100.0 83.3 100.0 n/a 1.0 97.7
499 ---------------------------- ------ ------ ------ ------ ------ ------ ------
500 Total 97.5 88.5 75.5 82.8 100.0 100.0 91.2
501 ---------------------------- ------ ------ ------ ------ ------ ------ ------
503 =head1 ACKNOWLEDGEMENTS
507 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
509 Thanks to Rob for actually getting the development of this module kick-started.
515 Stevan Little E<lt>stevan@iinteractive.comE<gt>
517 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
519 =head1 COPYRIGHT AND LICENSE
521 Copyright 2006 by Infinity Interactive, Inc.
523 L<http://www.iinteractive.com>
525 This library is free software; you can redistribute it and/or modify
526 it under the same terms as Perl itself.