10 use Class::MOP::Class;
11 use Class::MOP::Attribute;
12 use Class::MOP::Method;
14 use Class::MOP::Class::Immutable;
16 our $VERSION = '0.33';
17 our $AUTHORITY = 'cpan:STEVAN';
19 ## ----------------------------------------------------------------------------
20 ## Setting up our environment ...
21 ## ----------------------------------------------------------------------------
22 ## Class::MOP needs to have a few things in the global perl environment so
23 ## that it can operate effectively. Those things are done here.
24 ## ----------------------------------------------------------------------------
26 # ... nothing yet actually ;)
28 ## ----------------------------------------------------------------------------
30 ## ----------------------------------------------------------------------------
31 ## The code below here is to bootstrap our MOP with itself. This is also
32 ## sometimes called "tying the knot". By doing this, we make it much easier
33 ## to extend the MOP through subclassing and such since now you can use the
34 ## MOP itself to extend itself.
36 ## Yes, I know, thats weird and insane, but it's a good thing, trust me :)
37 ## ----------------------------------------------------------------------------
39 # We need to add in the meta-attributes here so that
40 # any subclass of Class::MOP::* will be able to
41 # inherit them using &construct_instance
43 ## --------------------------------------------------------
44 ## Class::MOP::Package
46 Class::MOP::Package->meta->add_attribute(
47 Class::MOP::Attribute->new('$:package' => (
49 # NOTE: we need to do this in order
50 # for the instance meta-object to
51 # not fall into meta-circular death
52 'name' => sub { (shift)->{'$:package'} }
54 init_arg => ':package',
58 Class::MOP::Package->meta->add_attribute(
59 Class::MOP::Attribute->new('%:namespace' => (
61 'namespace' => sub { (shift)->{'%:namespace'} }
66 return \%{$class->name . '::'};
69 # protect this from silliness
70 init_arg => '!............( DO NOT DO THIS )............!',
75 # use the metaclass to construct the meta-package
76 # which is a superclass of the metaclass itself :P
77 Class::MOP::Package->meta->add_method('initialize' => sub {
79 my $package_name = shift;
80 $class->meta->new_object(':package' => $package_name, @_);
83 ## --------------------------------------------------------
87 # yeah this is kind of stretching things a bit,
88 # but truthfully the version should be an attribute
89 # of the Module, the weirdness comes from having to
90 # stick to Perl 5 convention and store it in the
91 # $VERSION package variable. Basically if you just
92 # squint at it, it will look how you want it to look.
93 # Either as a package variable, or as a attribute of
94 # the metaclass, isn't abstraction great :)
96 Class::MOP::Module->meta->add_attribute(
97 Class::MOP::Attribute->new('$:version' => (
101 ${$self->get_package_symbol('$VERSION')};
105 # protect this from silliness
106 init_arg => '!............( DO NOT DO THIS )............!',
111 # By following the same conventions as version here,
112 # we are opening up the possibility that people can
113 # use the $AUTHORITY in non-Class::MOP modules as
116 Class::MOP::Module->meta->add_attribute(
117 Class::MOP::Attribute->new('$:authority' => (
121 ${$self->get_package_symbol('$AUTHORITY')};
125 # protect this from silliness
126 init_arg => '!............( DO NOT DO THIS )............!',
130 ## --------------------------------------------------------
133 Class::MOP::Class->meta->add_attribute(
134 Class::MOP::Attribute->new('%:attributes' => (
136 # NOTE: we need to do this in order
137 # for the instance meta-object to
138 # not fall into meta-circular death
139 'get_attribute_map' => sub { (shift)->{'%:attributes'} }
141 init_arg => ':attributes',
142 default => sub { {} }
146 Class::MOP::Class->meta->add_attribute(
147 Class::MOP::Attribute->new('$:attribute_metaclass' => (
148 reader => 'attribute_metaclass',
149 init_arg => ':attribute_metaclass',
150 default => 'Class::MOP::Attribute',
154 Class::MOP::Class->meta->add_attribute(
155 Class::MOP::Attribute->new('$:method_metaclass' => (
156 reader => 'method_metaclass',
157 init_arg => ':method_metaclass',
158 default => 'Class::MOP::Method',
162 Class::MOP::Class->meta->add_attribute(
163 Class::MOP::Attribute->new('$:instance_metaclass' => (
165 # NOTE: we need to do this in order
166 # for the instance meta-object to
167 # not fall into meta-circular death
168 'instance_metaclass' => sub { (shift)->{'$:instance_metaclass'} }
170 init_arg => ':instance_metaclass',
171 default => 'Class::MOP::Instance',
176 # we don't actually need to tie the knot with
177 # Class::MOP::Class here, it is actually handled
178 # within Class::MOP::Class itself in the
179 # construct_class_instance method.
181 ## --------------------------------------------------------
182 ## Class::MOP::Attribute
184 Class::MOP::Attribute->meta->add_attribute(
185 Class::MOP::Attribute->new('name' => (
187 # NOTE: we need to do this in order
188 # for the instance meta-object to
189 # not fall into meta-circular death
190 'name' => sub { (shift)->{name} }
195 Class::MOP::Attribute->meta->add_attribute(
196 Class::MOP::Attribute->new('associated_class' => (
198 # NOTE: we need to do this in order
199 # for the instance meta-object to
200 # not fall into meta-circular death
201 'associated_class' => sub { (shift)->{associated_class} }
206 Class::MOP::Attribute->meta->add_attribute(
207 Class::MOP::Attribute->new('accessor' => (
208 reader => 'accessor',
209 predicate => 'has_accessor',
213 Class::MOP::Attribute->meta->add_attribute(
214 Class::MOP::Attribute->new('reader' => (
216 predicate => 'has_reader',
220 Class::MOP::Attribute->meta->add_attribute(
221 Class::MOP::Attribute->new('writer' => (
223 predicate => 'has_writer',
227 Class::MOP::Attribute->meta->add_attribute(
228 Class::MOP::Attribute->new('predicate' => (
229 reader => 'predicate',
230 predicate => 'has_predicate',
234 Class::MOP::Attribute->meta->add_attribute(
235 Class::MOP::Attribute->new('clearer' => (
237 predicate => 'has_clearer',
241 Class::MOP::Attribute->meta->add_attribute(
242 Class::MOP::Attribute->new('init_arg' => (
243 reader => 'init_arg',
244 predicate => 'has_init_arg',
248 Class::MOP::Attribute->meta->add_attribute(
249 Class::MOP::Attribute->new('default' => (
250 # default has a custom 'reader' method ...
251 predicate => 'has_default',
256 # NOTE: (meta-circularity)
257 # This should be one of the last things done
258 # it will "tie the knot" with Class::MOP::Attribute
259 # so that it uses the attributes meta-objects
260 # to construct itself.
261 Class::MOP::Attribute->meta->add_method('new' => sub {
266 (defined $name && $name)
267 || confess "You must provide a name for the attribute";
268 $options{init_arg} = $name
269 if not exists $options{init_arg};
271 (Class::MOP::Attribute::is_default_a_coderef(\%options))
272 || confess("References are not allowed as default values, you must ".
273 "wrap then in a CODE reference (ex: sub { [] } and not [])")
274 if exists $options{default} && ref $options{default};
276 # return the new object
277 $class->meta->new_object(name => $name, %options);
280 Class::MOP::Attribute->meta->add_method('clone' => sub {
282 $self->meta->clone_object($self, @_);
285 ## --------------------------------------------------------
286 ## Now close all the Class::MOP::* classes
288 Class::MOP::Package ->meta->make_immutable(inline_constructor => 0);
289 Class::MOP::Module ->meta->make_immutable(inline_constructor => 0);
290 Class::MOP::Class ->meta->make_immutable(inline_constructor => 0);
291 Class::MOP::Attribute->meta->make_immutable(inline_constructor => 0);
292 Class::MOP::Method ->meta->make_immutable(inline_constructor => 0);
293 Class::MOP::Instance ->meta->make_immutable(inline_constructor => 0);
294 Class::MOP::Object ->meta->make_immutable(inline_constructor => 0);
304 Class::MOP - A Meta Object Protocol for Perl 5
308 # ... This will come later, for now see
309 # the other SYNOPSIS for more information
313 This module is an attempt to create a meta object protocol for the
314 Perl 5 object system. It makes no attempt to change the behavior or
315 characteristics of the Perl 5 object system, only to create a
316 protocol for its manipulation and introspection.
318 That said, it does attempt to create the tools for building a rich
319 set of extensions to the Perl 5 object system. Every attempt has been
320 made for these tools to keep to the spirit of the Perl 5 object
321 system that we all know and love.
323 This documentation is admittedly sparse on details, as time permits
324 I will try to improve them. For now, I suggest looking at the items
325 listed in the L<SEE ALSO> section for more information. In particular
326 the book "The Art of the Meta Object Protocol" was very influential
327 in the development of this system.
329 =head2 What is a Meta Object Protocol?
331 A meta object protocol is an API to an object system.
333 To be more specific, it is a set of abstractions of the components of
334 an object system (typically things like; classes, object, methods,
335 object attributes, etc.). These abstractions can then be used to both
336 inspect and manipulate the object system which they describe.
338 It can be said that there are two MOPs for any object system; the
339 implicit MOP, and the explicit MOP. The implicit MOP handles things
340 like method dispatch or inheritance, which happen automatically as
341 part of how the object system works. The explicit MOP typically
342 handles the introspection/reflection features of the object system.
343 All object systems have implicit MOPs, without one, they would not
344 work. Explict MOPs however as less common, and depending on the
345 language can vary from restrictive (Reflection in Java or C#) to
346 wide open (CLOS is a perfect example).
348 =head2 Yet Another Class Builder!! Why?
350 This is B<not> a class builder so much as it is a I<class builder
351 B<builder>>. My intent is that an end user does not use this module
352 directly, but instead this module is used by module authors to
353 build extensions and features onto the Perl 5 object system.
355 =head2 Who is this module for?
357 This module is specifically for anyone who has ever created or
358 wanted to create a module for the Class:: namespace. The tools which
359 this module will provide will hopefully make it easier to do more
360 complex things with Perl 5 classes by removing such barriers as
361 the need to hack the symbol tables, or understand the fine details
364 =head2 What changes do I have to make to use this module?
366 This module was designed to be as unintrusive as possible. Many of
367 its features are accessible without B<any> change to your existsing
368 code at all. It is meant to be a compliment to your existing code and
369 not an intrusion on your code base. Unlike many other B<Class::>
370 modules, this module B<does not> require you subclass it, or even that
371 you C<use> it in within your module's package.
373 The only features which requires additions to your code are the
374 attribute handling and instance construction features, and these are
375 both completely optional features. The only reason for this is because
376 Perl 5's object system does not actually have these features built
377 in. More information about this feature can be found below.
379 =head2 A Note about Performance?
381 It is a common misconception that explict MOPs are performance drains.
382 But this is not a universal truth at all, it is an side-effect of
383 specific implementations. For instance, using Java reflection is much
384 slower because the JVM cannot take advantage of any compiler
385 optimizations, and the JVM has to deal with much more runtime type
386 information as well. Reflection in C# is marginally better as it was
387 designed into the language and runtime (the CLR). In contrast, CLOS
388 (the Common Lisp Object System) was built to support an explicit MOP,
389 and so performance is tuned for it.
391 This library in particular does it's absolute best to avoid putting
392 B<any> drain at all upon your code's performance. In fact, by itself
393 it does nothing to affect your existing code. So you only pay for
394 what you actually use.
396 =head2 About Metaclass compatibility
398 This module makes sure that all metaclasses created are both upwards
399 and downwards compatible. The topic of metaclass compatibility is
400 highly esoteric and is something only encountered when doing deep and
401 involved metaclass hacking. There are two basic kinds of metaclass
402 incompatibility; upwards and downwards.
404 Upwards metaclass compatibility means that the metaclass of a
405 given class is either the same as (or a subclass of) all of the
408 Downward metaclass compatibility means that the metaclasses of a
409 given class's anscestors are all either the same as (or a subclass
412 Here is a diagram showing a set of two classes (C<A> and C<B>) and
413 two metaclasses (C<Meta::A> and C<Meta::B>) which have correct
414 metaclass compatibility both upwards and downwards.
416 +---------+ +---------+
417 | Meta::A |<----| Meta::B | <....... (instance of )
418 +---------+ +---------+ <------- (inherits from)
421 +---------+ +---------+
423 +---------+ +---------+
425 As I said this is a highly esoteric topic and one you will only run
426 into if you do a lot of subclassing of B<Class::MOP::Class>. If you
427 are interested in why this is an issue see the paper
428 I<Uniform and safe metaclass composition> linked to in the
429 L<SEE ALSO> section of this document.
431 =head2 Using custom metaclasses
433 Always use the metaclass pragma when using a custom metaclass, this
434 will ensure the proper initialization order and not accidentely
435 create an incorrect type of metaclass for you. This is a very rare
436 problem, and one which can only occur if you are doing deep metaclass
437 programming. So in other words, don't worry about it.
441 The protocol is divided into 3 main sub-protocols:
445 =item The Class protocol
447 This provides a means of manipulating and introspecting a Perl 5
448 class. It handles all of symbol table hacking for you, and provides
449 a rich set of methods that go beyond simple package introspection.
451 See L<Class::MOP::Class> for more details.
453 =item The Attribute protocol
455 This provides a consistent represenation for an attribute of a
456 Perl 5 class. Since there are so many ways to create and handle
457 atttributes in Perl 5 OO, this attempts to provide as much of a
458 unified approach as possible, while giving the freedom and
459 flexibility to subclass for specialization.
461 See L<Class::MOP::Attribute> for more details.
463 =item The Method protocol
465 This provides a means of manipulating and introspecting methods in
466 the Perl 5 object system. As with attributes, there are many ways to
467 approach this topic, so we try to keep it pretty basic, while still
468 making it possible to extend the system in many ways.
470 See L<Class::MOP::Method> for more details.
478 There are very few books out on Meta Object Protocols and Metaclasses
479 because it is such an esoteric topic. The following books are really
480 the only ones I have found. If you know of any more, B<I<please>>
481 email me and let me know, I would love to hear about them.
485 =item "The Art of the Meta Object Protocol"
487 =item "Advances in Object-Oriented Metalevel Architecture and Reflection"
489 =item "Putting MetaClasses to Work"
491 =item "Smalltalk: The Language"
499 =item Uniform and safe metaclass composition
501 An excellent paper by the people who brought us the original Traits paper.
502 This paper is on how Traits can be used to do safe metaclass composition,
503 and offers an excellent introduction section which delves into the topic of
504 metaclass compatibility.
506 L<http://www.iam.unibe.ch/~scg/Archive/Papers/Duca05ySafeMetaclassTrait.pdf>
508 =item Safe Metaclass Programming
510 This paper seems to precede the above paper, and propose a mix-in based
511 approach as opposed to the Traits based approach. Both papers have similar
512 information on the metaclass compatibility problem space.
514 L<http://citeseer.ist.psu.edu/37617.html>
522 =item The Perl 6 MetaModel work in the Pugs project
526 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel>
528 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-ObjectSpace>
534 =head1 SIMILAR MODULES
536 As I have said above, this module is a class-builder-builder, so it is
537 not the same thing as modules like L<Class::Accessor> and
538 L<Class::MethodMaker>. That being said there are very few modules on CPAN
539 with similar goals to this module. The one I have found which is most
540 like this module is L<Class::Meta>, although it's philosophy and the MOP it
541 creates are very different from this modules.
545 All complex software has bugs lurking in it, and this module is no
546 exception. If you find a bug please either email me, or add the bug
551 I use L<Devel::Cover> to test the code coverage of my tests, below is the
552 L<Devel::Cover> report on this module's test suite.
554 ---------------------------- ------ ------ ------ ------ ------ ------ ------
555 File stmt bran cond sub pod time total
556 ---------------------------- ------ ------ ------ ------ ------ ------ ------
557 Class/MOP.pm 100.0 100.0 100.0 100.0 n/a 19.8 100.0
558 Class/MOP/Attribute.pm 100.0 100.0 91.7 61.2 100.0 14.3 87.9
559 Class/MOP/Class.pm 97.6 91.3 77.3 98.4 100.0 56.4 93.2
560 Class/MOP/Instance.pm 91.1 75.0 33.3 91.7 100.0 6.8 90.7
561 Class/MOP/Method.pm 97.6 60.0 52.9 76.9 100.0 1.6 82.6
562 metaclass.pm 100.0 100.0 83.3 100.0 n/a 1.0 97.7
563 ---------------------------- ------ ------ ------ ------ ------ ------ ------
564 Total 97.5 88.5 75.5 82.8 100.0 100.0 91.2
565 ---------------------------- ------ ------ ------ ------ ------ ------ ------
567 =head1 ACKNOWLEDGEMENTS
571 =item Rob Kinyon E<lt>rob@iinteractive.comE<gt>
573 Thanks to Rob for actually getting the development of this module kick-started.
579 Stevan Little E<lt>stevan@iinteractive.comE<gt>
581 Yuval Kogman E<lt>nothingmuch@woobling.comE<gt>
583 =head1 COPYRIGHT AND LICENSE
585 Copyright 2006 by Infinity Interactive, Inc.
587 L<http://www.iinteractive.com>
589 This library is free software; you can redistribute it and/or modify
590 it under the same terms as Perl itself.