17 elsif($C3_XS or not defined $C3_XS) {
20 eval { require Class::C3::XS };
25 die $error if $error !~ /\blocate\b/;
29 Carp::croak( "XS explicitly requested but Class::C3::XS is not available" );
32 require Algorithm::C3;
33 require Class::C3::next;
41 # this is our global stash of both
42 # MRO's and method dispatch tables
43 # the structure basically looks like
47 # MRO => [ <class precendence list> ],
49 # orig => <original location of method>,
50 # code => \&<ref to original method>
52 # has_overload_fallback => (1 | 0)
57 # use these for debugging ...
58 sub _dump_MRO_table { %MRO }
61 # state tracking for initialize()/uninitialize()
62 our $_initialized = 0;
66 # skip if the caller is main::
67 # since that is clearly not relevant
68 return if $class eq 'main';
70 return if $TURN_OFF_C3;
71 mro::set_mro($class, 'c3') if $C3_IN_CORE;
73 # make a note to calculate $class
75 $MRO{$class} = undef unless exists $MRO{$class};
80 # This prevents silly warnings when Class::C3 is
81 # used explicitly along with MRO::Compat under 5.9.5+
83 { no warnings 'redefine';
86 %next::METHOD_CACHE = ();
87 # why bother if we don't have anything ...
88 return unless keys %MRO;
90 mro::set_mro($_, 'c3') for keys %MRO;
95 $MRO{$_} = undef foreach keys %MRO;
97 _calculate_method_dispatch_tables();
98 _apply_method_dispatch_tables();
104 # why bother if we don't have anything ...
105 %next::METHOD_CACHE = ();
106 return unless keys %MRO;
108 mro::set_mro($_, 'dfs') for keys %MRO;
111 _remove_method_dispatch_tables();
116 sub reinitialize { goto &initialize }
118 } # end of "no warnings 'redefine'"
120 ## functions for applying C3 to classes
122 sub _calculate_method_dispatch_tables {
123 return if $C3_IN_CORE;
125 foreach my $class (keys %MRO) {
126 _calculate_method_dispatch_table($class, \%merge_cache);
130 sub _calculate_method_dispatch_table {
131 return if $C3_IN_CORE;
132 my ($class, $merge_cache) = @_;
134 my @MRO = calculateMRO($class, $merge_cache);
135 $MRO{$class} = { MRO => \@MRO };
136 my $has_overload_fallback;
139 # we do @MRO[1 .. $#MRO] here because it
140 # makes no sense to interogate the class
141 # which you are calculating for.
142 foreach my $local (@MRO[1 .. $#MRO]) {
143 # if overload has tagged this module to
144 # have use "fallback", then we want to
146 $has_overload_fallback = ${"${local}::()"}
147 if !defined $has_overload_fallback && defined ${"${local}::()"};
148 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
149 # skip if already overriden in local class
150 next unless !defined *{"${class}::$method"}{CODE};
151 $methods{$method} = {
152 orig => "${local}::$method",
153 code => \&{"${local}::$method"}
154 } unless exists $methods{$method};
157 # now stash them in our %MRO table
158 $MRO{$class}->{methods} = \%methods;
159 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
162 sub _apply_method_dispatch_tables {
163 return if $C3_IN_CORE;
164 foreach my $class (keys %MRO) {
165 _apply_method_dispatch_table($class);
169 sub _apply_method_dispatch_table {
170 return if $C3_IN_CORE;
173 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
174 if !defined &{"${class}::()"}
175 && defined $MRO{$class}->{has_overload_fallback};
176 foreach my $method (keys %{$MRO{$class}->{methods}}) {
177 if ( $method =~ /^\(/ ) {
178 my $orig = $MRO{$class}->{methods}->{$method}->{orig};
179 ${"${class}::$method"} = $$orig if defined $$orig;
181 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
185 sub _remove_method_dispatch_tables {
186 return if $C3_IN_CORE;
187 foreach my $class (keys %MRO) {
188 _remove_method_dispatch_table($class);
192 sub _remove_method_dispatch_table {
193 return if $C3_IN_CORE;
196 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
197 foreach my $method (keys %{$MRO{$class}->{methods}}) {
198 delete ${"${class}::"}{$method}
199 if defined *{"${class}::${method}"}{CODE} &&
200 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
205 my ($class, $merge_cache) = @_;
207 return Algorithm::C3::merge($class, sub {
213 # Method overrides to support 5.9.5+ or Class::C3::XS
215 sub _core_calculateMRO { @{mro::get_linear_isa($_[0], 'c3')} }
218 no warnings 'redefine';
219 *Class::C3::calculateMRO = \&_core_calculateMRO;
222 no warnings 'redefine';
223 *Class::C3::calculateMRO = \&Class::C3::XS::calculateMRO;
224 *Class::C3::_calculate_method_dispatch_table
225 = \&Class::C3::XS::_calculate_method_dispatch_table;
236 Class::C3 - A pragma to use the C3 method resolution order algortihm
240 # NOTE - DO NOT USE Class::C3 directly as a user, use MRO::Compat instead!
243 sub hello { 'A::hello' }
253 sub hello { 'C::hello' }
259 # Classic Diamond MI pattern
268 # initializez the C3 module
269 # (formerly called in INIT)
270 Class::C3::initialize();
272 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
274 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
276 D->can('hello')->(); # can() also works correctly
277 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
281 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
282 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
284 B<NOTE:> YOU SHOULD NOT USE THIS MODULE DIRECTLY - The feature provided
285 is integrated into perl version >= 5.9.5, and you should use L<MRO::Compat>
286 instead, which will use the core implementation in newer perls, but fallback
287 to using this implementation on older perls.
291 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
292 inheritance. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
293 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
294 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
295 default MRO for Parrot objects as well.
297 =head2 How does C3 work.
299 C3 works by always preserving local precendence ordering. This essentially means that no class will
300 appear before any of its subclasses. Take the classic diamond inheritance pattern for instance:
308 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
309 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
310 (D, B, C, A), which does not have this same issue.
312 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
313 the L<SEE ALSO> section.
315 =head2 How does this module work?
317 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
318 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
319 gathers information from the symbol tables of each of those classes, and builds a set of method
320 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
321 then adds the method aliases into the local classes symbol table.
323 The end result is actually classes with pre-cached method dispatch. However, this caching does not
324 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
325 your classes to be effectively closed. See the L<CAVEATS> section for more details.
327 =head1 OPTIONAL LOWERCASE PRAGMA
329 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
330 the regular install since lowercase module names are considered I<"bad"> by some people. However I
331 think that code looks much nicer like this:
341 But hey, it's your choice, thats why it is optional.
347 =item B<calculateMRO ($class)>
349 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
353 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
354 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
355 use C3. Here is a quick code example:
359 # ... Foo methods here
364 # ... Bar methods here
368 Class::C3::initialize(); # now it is safe to use Foo and Bar
370 This function used to be called automatically for you in the INIT phase of the perl compiler, but
371 that lead to warnings if this module was required at runtime. After discussion with my user base
372 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
373 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
374 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
375 your own INIT method which calls this function.
379 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
380 clear the MRO cache first.
382 =item B<uninitialize>
384 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
385 style dispatch order (depth-first, left-to-right).
387 =item B<reinitialize>
389 This is an alias for L</initialize> above.
393 =head1 METHOD REDISPATCHING
395 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
396 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
397 method along the C3 linearization. This is best shown with an example.
399 # a classic diamond MI pattern ...
413 sub foo { 'B::foo => ' . (shift)->next::method() }
418 sub foo { 'C::foo => ' . (shift)->next::method() }
423 sub foo { 'D::foo => ' . (shift)->next::method() }
425 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
427 A few things to note. First, we do not require you to add on the method name to the C<next::method>
428 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
429 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
431 The next thing to keep in mind is that you will need to pass all arguments to C<next::method>. It can
432 not automatically use the current C<@_>.
434 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
435 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
437 $self->next::method(@_) if $self->next::can;
439 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
440 The previous example could be simply written as:
442 $self->maybe::next::method(@_);
444 There are some caveats about using C<next::method>, see below for those.
448 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
449 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
450 whatever your needs might be.
452 But there are still caveats, so here goes ...
456 =item Use of C<SUPER::>.
458 The idea of C<SUPER::> under multiple inheritance is ambigious, and generally not recomended anyway.
459 However, its use in conjuntion with this module is very much not recommended, and in fact very
460 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
461 more details on its usage above.
463 =item Changing C<@ISA>.
465 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
466 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
467 module, and therefore probably won't even show up. If you do this, you will need to call C<reinitialize>
468 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
469 in F<t/20_reinitialize.t> for more information.
471 =item Adding/deleting methods from class symbol tables.
473 This module calculates the MRO for each requested class by interogating the symbol tables of said classes.
474 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
475 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
476 changes you make to take effect.
478 =item Calling C<next::method> from methods defined outside the class
480 There is an edge case when using C<next::method> from within a subroutine which was created in a different
481 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
482 will not work correctly:
484 *Foo::foo = sub { (shift)->next::method(@_) };
486 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
487 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
488 uses C<caller> to find the name of the method it was called in, it will fail in this case.
490 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
491 assign a name to an anonymous subroutine for you. Simply do this:
493 use Sub::Name 'subname';
494 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
496 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
497 manage to find a workaround for it, so until someone gives me a working patch this will be a known
498 limitation of this module.
504 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.
506 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.
510 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. The optional module will be automatically installed for you if a C compiler is available, 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>).
514 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.
518 =head2 The original Dylan paper
522 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
526 =head2 The prototype Perl 6 Object Model uses C3
530 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
534 =head2 Parrot now uses C3
538 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
540 =item L<http://use.perl.org/~autrijus/journal/25768>
544 =head2 Python 2.3 MRO related links
548 =item L<http://www.python.org/2.3/mro.html>
550 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
554 =head2 C3 for TinyCLOS
558 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
562 =head1 ACKNOWLEGEMENTS
566 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
567 and finding many bugs and providing fixes.
569 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
570 calls inside C<eval> and anon-subs.
572 =item Thanks to Robert Norris for adding support for C<next::can> and
573 C<maybe::next::method>.
579 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
581 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
583 =head1 COPYRIGHT AND LICENSE
585 Copyright 2005, 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.