18 eval "require Class::C3::XS";
24 die $error if $error !~ /\blocate\b/;
25 require Algorithm::C3;
26 require Class::C3::next;
31 # this is our global stash of both
32 # MRO's and method dispatch tables
33 # the structure basically looks like
37 # MRO => [ <class precendence list> ],
39 # orig => <original location of method>,
40 # code => \&<ref to original method>
42 # has_overload_fallback => (1 | 0)
47 # use these for debugging ...
48 sub _dump_MRO_table { %MRO }
51 # state tracking for initialize()/uninitialize()
52 our $_initialized = 0;
56 # skip if the caller is main::
57 # since that is clearly not relevant
58 return if $class eq 'main';
60 return if $TURN_OFF_C3;
61 mro::set_mro($class, 'c3') if $C3_IN_CORE;
63 # make a note to calculate $class
65 $MRO{$class} = undef unless exists $MRO{$class};
71 %next::METHOD_CACHE = ();
72 # why bother if we don't have anything ...
73 return unless keys %MRO;
75 mro::set_mro($_, 'c3') for keys %MRO;
80 $MRO{$_} = undef foreach keys %MRO;
82 _calculate_method_dispatch_tables();
83 _apply_method_dispatch_tables();
89 # why bother if we don't have anything ...
90 %next::METHOD_CACHE = ();
91 return unless keys %MRO;
93 mro::set_mro($_, 'dfs') for keys %MRO;
96 _remove_method_dispatch_tables();
101 sub reinitialize { goto &initialize }
103 ## functions for applying C3 to classes
105 sub _calculate_method_dispatch_tables {
106 return if $C3_IN_CORE;
108 foreach my $class (keys %MRO) {
109 _calculate_method_dispatch_table($class, \%merge_cache);
113 sub _calculate_method_dispatch_table {
114 return if $C3_IN_CORE;
115 my ($class, $merge_cache) = @_;
117 my @MRO = calculateMRO($class, $merge_cache);
118 $MRO{$class} = { MRO => \@MRO };
119 my $has_overload_fallback;
122 # we do @MRO[1 .. $#MRO] here because it
123 # makes no sense to interogate the class
124 # which you are calculating for.
125 foreach my $local (@MRO[1 .. $#MRO]) {
126 # if overload has tagged this module to
127 # have use "fallback", then we want to
129 $has_overload_fallback = ${"${local}::()"}
130 if !defined $has_overload_fallback && defined ${"${local}::()"};
131 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
132 # skip if already overriden in local class
133 next unless !defined *{"${class}::$method"}{CODE};
134 $methods{$method} = {
135 orig => "${local}::$method",
136 code => \&{"${local}::$method"}
137 } unless exists $methods{$method};
140 # now stash them in our %MRO table
141 $MRO{$class}->{methods} = \%methods;
142 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
145 sub _apply_method_dispatch_tables {
146 return if $C3_IN_CORE;
147 foreach my $class (keys %MRO) {
148 _apply_method_dispatch_table($class);
152 sub _apply_method_dispatch_table {
153 return if $C3_IN_CORE;
156 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
157 if !defined &{"${class}::()"}
158 && defined $MRO{$class}->{has_overload_fallback};
159 foreach my $method (keys %{$MRO{$class}->{methods}}) {
160 if ( $method =~ /^\(/ ) {
161 my $orig = $MRO{$class}->{methods}->{$method}->{orig};
162 ${"${class}::$method"} = $$orig if defined $$orig;
164 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
168 sub _remove_method_dispatch_tables {
169 return if $C3_IN_CORE;
170 foreach my $class (keys %MRO) {
171 _remove_method_dispatch_table($class);
175 sub _remove_method_dispatch_table {
176 return if $C3_IN_CORE;
179 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
180 foreach my $method (keys %{$MRO{$class}->{methods}}) {
181 delete ${"${class}::"}{$method}
182 if defined *{"${class}::${method}"}{CODE} &&
183 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
188 my ($class, $merge_cache) = @_;
190 return Algorithm::C3::merge($class, sub {
196 # Method overrides to support 5.9.5+ or Class::C3::XS
198 sub _core_calculateMRO { @{mro::get_linear_isa($_[0], 'c3')} }
201 no warnings 'redefine';
202 *Class::C3::calculateMRO = \&_core_calculateMRO;
205 no warnings 'redefine';
206 *Class::C3::calculateMRO = \&Class::C3::XS::calculateMRO;
207 *Class::C3::_calculate_method_dispatch_table
208 = \&Class::C3::XS::_calculate_method_dispatch_table;
219 Class::C3 - A pragma to use the C3 method resolution order algortihm
225 sub hello { 'A::hello' }
235 sub hello { 'C::hello' }
241 # Classic Diamond MI pattern
250 # initializez the C3 module
251 # (formerly called in INIT)
252 Class::C3::initialize();
254 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
256 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
258 D->can('hello')->(); # can() also works correctly
259 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
263 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
264 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
268 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
269 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
270 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
271 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
272 default MRO for Parrot objects as well.
274 =head2 How does C3 work.
276 C3 works by always preserving local precendence ordering. This essentially means that no class will
277 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
285 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
286 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
287 (D, B, C, A), which does not have this same issue.
289 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
290 the L<SEE ALSO> section.
292 =head2 How does this module work?
294 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
295 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
296 gathers information from the symbol tables of each of those classes, and builds a set of method
297 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
298 then adds the method aliases into the local classes symbol table.
300 The end result is actually classes with pre-cached method dispatch. However, this caching does not
301 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
302 your classes to be effectively closed. See the L<CAVEATS> section for more details.
304 =head1 OPTIONAL LOWERCASE PRAGMA
306 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
307 the regular install since lowercase module names are considered I<"bad"> by some people. However I
308 think that code looks much nicer like this:
318 But hey, it's your choice, thats why it is optional.
324 =item B<calculateMRO ($class)>
326 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
330 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
331 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
332 use C3. Here is a quick code example:
336 # ... Foo methods here
341 # ... Bar methods here
345 Class::C3::initialize(); # now it is safe to use Foo and Bar
347 This function used to be called automatically for you in the INIT phase of the perl compiler, but
348 that lead to warnings if this module was required at runtime. After discussion with my user base
349 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
350 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
351 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
352 your own INIT method which calls this function.
356 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
357 clear the MRO cache first.
359 =item B<uninitialize>
361 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
362 style dispatch order (depth-first, left-to-right).
364 =item B<reinitialize>
366 This is an alias for L</initialize> above.
370 =head1 METHOD REDISPATCHING
372 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
373 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
374 method along the C3 linearization. This is best show with an examples.
376 # a classic diamond MI pattern ...
390 sub foo { 'B::foo => ' . (shift)->next::method() }
395 sub foo { 'C::foo => ' . (shift)->next::method() }
400 sub foo { 'D::foo => ' . (shift)->next::method() }
402 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
404 A few things to note. First, we do not require you to add on the method name to the C<next::method>
405 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
406 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
408 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
409 not automatically use the current C<@_>.
411 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
412 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
414 $self->next::method(@_) if $self->next::can;
416 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
417 The previous example could be simply written as:
419 $self->maybe::next::method(@_);
421 There are some caveats about using C<next::method>, see below for those.
425 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
426 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
427 whatever your needs might be.
429 But there are still caveats, so here goes ...
433 =item Use of C<SUPER::>.
435 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
436 However, it's use in conjuntion with this module is very much not recommended, and in fact very
437 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
438 more details on it's usage above.
440 =item Changing C<@ISA>.
442 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
443 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
444 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
445 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
446 in F<t/20_reinitialize.t> for more information.
448 =item Adding/deleting methods from class symbol tables.
450 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
451 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
452 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
453 changes you make to take effect.
455 =item Calling C<next::method> from methods defined outside the class
457 There is an edge case when using C<next::method> from within a subroutine which was created in a different
458 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
459 will not work correctly:
461 *Foo::foo = sub { (shift)->next::method(@_) };
463 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
464 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
465 uses C<caller> to find the name of the method it was called in, it will fail in this case.
467 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
468 assign a name to an anonymous subroutine for you. Simply do this:
470 use Sub::Name 'subname';
471 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
473 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
474 manage to find a workaround for it, so until someone gives me a working patch this will be a known
475 limitation of this module.
481 If your software requires Perl 5.9.5 or higher, you do not need L<Class::C3>, you can simply C<use mro 'c3'>, and not worry about C<initialize()>, avoid some of the above caveats, and get the best possible performance. See L<mro> for more details.
483 If your software is meant to work on earlier Perls, use L<Class::C3> as documented here. L<Class::C3> will detect Perl 5.9.5+ and take advantage of the core support when available.
487 This module will load L<Class::C3::XS> if it's installed and you are running on a Perl version older than 5.9.5. Installing this is recommended when possible, as it results in significant performance improvements (but unlike the 5.9.5+ core support, it still has all of the same caveats as L<Class::C3>).
491 L<Devel::Cover> was reporting 94.4% overall test coverage earlier in this module's life. Currently, the test suite does things that break under coverage testing, but it is fair to assume the coverage is still close to that value.
495 =head2 The original Dylan paper
499 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
503 =head2 The prototype Perl 6 Object Model uses C3
507 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
511 =head2 Parrot now uses C3
515 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
517 =item L<http://use.perl.org/~autrijus/journal/25768>
521 =head2 Python 2.3 MRO related links
525 =item L<http://www.python.org/2.3/mro.html>
527 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
531 =head2 C3 for TinyCLOS
535 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
539 =head1 ACKNOWLEGEMENTS
543 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
544 and finding many bugs and providing fixes.
546 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
547 calls inside C<eval> and anon-subs.
549 =item Thanks to Robert Norris for adding support for C<next::can> and
550 C<maybe::next::method>.
556 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
558 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
560 =head1 COPYRIGHT AND LICENSE
562 Copyright 2005, 2006 by Infinity Interactive, Inc.
564 L<http://www.iinteractive.com>
566 This library is free software; you can redistribute it and/or modify
567 it under the same terms as Perl itself.