17 eval "require Class::C3::XS";
23 die $error if $error !~ /\blocate\b/;
24 require Algorithm::C3;
25 require Class::C3::next;
30 # this is our global stash of both
31 # MRO's and method dispatch tables
32 # the structure basically looks like
36 # MRO => [ <class precendence list> ],
38 # orig => <original location of method>,
39 # code => \&<ref to original method>
41 # has_overload_fallback => (1 | 0)
46 # use these for debugging ...
47 sub _dump_MRO_table { %MRO }
50 # state tracking for initialize()/uninitialize()
51 our $_initialized = 0;
55 # skip if the caller is main::
56 # since that is clearly not relevant
57 return if $class eq 'main';
59 return if $TURN_OFF_C3;
60 mro::set_mro($class, 'c3') if $C3_IN_CORE;
62 # make a note to calculate $class
64 $MRO{$class} = undef unless exists $MRO{$class};
70 %next::METHOD_CACHE = ();
71 # why bother if we don't have anything ...
72 return unless keys %MRO;
74 mro::set_mro($_, 'c3') for keys %MRO;
79 $MRO{$_} = undef foreach keys %MRO;
81 _calculate_method_dispatch_tables();
82 _apply_method_dispatch_tables();
88 # why bother if we don't have anything ...
89 %next::METHOD_CACHE = ();
90 return unless keys %MRO;
92 mro::set_mro($_, 'dfs') for keys %MRO;
95 _remove_method_dispatch_tables();
100 sub reinitialize { goto &initialize }
102 ## functions for applying C3 to classes
104 sub _calculate_method_dispatch_tables {
105 return if $C3_IN_CORE;
107 foreach my $class (keys %MRO) {
108 _calculate_method_dispatch_table($class, \%merge_cache);
112 sub _calculate_method_dispatch_table {
113 return if $C3_IN_CORE;
114 my ($class, $merge_cache) = @_;
116 my @MRO = calculateMRO($class, $merge_cache);
117 $MRO{$class} = { MRO => \@MRO };
118 my $has_overload_fallback;
121 # we do @MRO[1 .. $#MRO] here because it
122 # makes no sense to interogate the class
123 # which you are calculating for.
124 foreach my $local (@MRO[1 .. $#MRO]) {
125 # if overload has tagged this module to
126 # have use "fallback", then we want to
128 $has_overload_fallback = ${"${local}::()"}
129 if !defined $has_overload_fallback && defined ${"${local}::()"};
130 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
131 # skip if already overriden in local class
132 next unless !defined *{"${class}::$method"}{CODE};
133 $methods{$method} = {
134 orig => "${local}::$method",
135 code => \&{"${local}::$method"}
136 } unless exists $methods{$method};
139 # now stash them in our %MRO table
140 $MRO{$class}->{methods} = \%methods;
141 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
144 sub _apply_method_dispatch_tables {
145 return if $C3_IN_CORE;
146 foreach my $class (keys %MRO) {
147 _apply_method_dispatch_table($class);
151 sub _apply_method_dispatch_table {
152 return if $C3_IN_CORE;
155 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
156 if !defined &{"${class}::()"}
157 && defined $MRO{$class}->{has_overload_fallback};
158 foreach my $method (keys %{$MRO{$class}->{methods}}) {
159 if ( $method =~ /^\(/ ) {
160 my $orig = $MRO{$class}->{methods}->{$method}->{orig};
161 ${"${class}::$method"} = $$orig if defined $$orig;
163 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
167 sub _remove_method_dispatch_tables {
168 return if $C3_IN_CORE;
169 foreach my $class (keys %MRO) {
170 _remove_method_dispatch_table($class);
174 sub _remove_method_dispatch_table {
175 return if $C3_IN_CORE;
178 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
179 foreach my $method (keys %{$MRO{$class}->{methods}}) {
180 delete ${"${class}::"}{$method}
181 if defined *{"${class}::${method}"}{CODE} &&
182 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
187 my ($class, $merge_cache) = @_;
189 return Algorithm::C3::merge($class, sub {
195 # Method overrides to support 5.9.5+ or Class::C3::XS
197 sub _core_calculateMRO { @{mro::get_linear_isa($_[0], 'c3')} }
200 no warnings 'redefine';
201 *Class::C3::calculateMRO = \&_core_calculateMRO;
204 no warnings 'redefine';
205 *Class::C3::calculateMRO = \&Class::C3::XS::calculateMRO;
206 *Class::C3::_calculate_method_dispatch_table
207 = \&Class::C3::XS::_calculate_method_dispatch_table;
218 Class::C3 - A pragma to use the C3 method resolution order algortihm
224 sub hello { 'A::hello' }
234 sub hello { 'C::hello' }
240 # Classic Diamond MI pattern
249 # initializez the C3 module
250 # (formerly called in INIT)
251 Class::C3::initialize();
253 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
255 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
257 D->can('hello')->(); # can() also works correctly
258 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
262 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
263 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
267 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
268 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
269 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
270 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
271 default MRO for Parrot objects as well.
273 =head2 How does C3 work.
275 C3 works by always preserving local precendence ordering. This essentially means that no class will
276 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
284 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
285 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
286 (D, B, C, A), which does not have this same issue.
288 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
289 the L<SEE ALSO> section.
291 =head2 How does this module work?
293 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
294 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
295 gathers information from the symbol tables of each of those classes, and builds a set of method
296 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
297 then adds the method aliases into the local classes symbol table.
299 The end result is actually classes with pre-cached method dispatch. However, this caching does not
300 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
301 your classes to be effectively closed. See the L<CAVEATS> section for more details.
303 =head1 OPTIONAL LOWERCASE PRAGMA
305 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
306 the regular install since lowercase module names are considered I<"bad"> by some people. However I
307 think that code looks much nicer like this:
317 But hey, it's your choice, thats why it is optional.
323 =item B<calculateMRO ($class)>
325 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
329 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
330 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
331 use C3. Here is a quick code example:
335 # ... Foo methods here
340 # ... Bar methods here
344 Class::C3::initialize(); # now it is safe to use Foo and Bar
346 This function used to be called automatically for you in the INIT phase of the perl compiler, but
347 that lead to warnings if this module was required at runtime. After discussion with my user base
348 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
349 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
350 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
351 your own INIT method which calls this function.
355 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
356 clear the MRO cache first.
358 =item B<uninitialize>
360 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
361 style dispatch order (depth-first, left-to-right).
363 =item B<reinitialize>
365 This is an alias for L</initialize> above.
369 =head1 METHOD REDISPATCHING
371 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
372 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
373 method along the C3 linearization. This is best show with an examples.
375 # a classic diamond MI pattern ...
389 sub foo { 'B::foo => ' . (shift)->next::method() }
394 sub foo { 'C::foo => ' . (shift)->next::method() }
399 sub foo { 'D::foo => ' . (shift)->next::method() }
401 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
403 A few things to note. First, we do not require you to add on the method name to the C<next::method>
404 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
405 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
407 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
408 not automatically use the current C<@_>.
410 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
411 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
413 $self->next::method(@_) if $self->next::can;
415 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
416 The previous example could be simply written as:
418 $self->maybe::next::method(@_);
420 There are some caveats about using C<next::method>, see below for those.
424 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
425 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
426 whatever your needs might be.
428 But there are still caveats, so here goes ...
432 =item Use of C<SUPER::>.
434 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
435 However, it's use in conjuntion with this module is very much not recommended, and in fact very
436 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
437 more details on it's usage above.
439 =item Changing C<@ISA>.
441 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
442 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
443 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
444 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
445 in F<t/20_reinitialize.t> for more information.
447 =item Adding/deleting methods from class symbol tables.
449 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
450 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
451 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
452 changes you make to take effect.
454 =item Calling C<next::method> from methods defined outside the class
456 There is an edge case when using C<next::method> from within a subroutine which was created in a different
457 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
458 will not work correctly:
460 *Foo::foo = sub { (shift)->next::method(@_) };
462 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
463 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
464 uses C<caller> to find the name of the method it was called in, it will fail in this case.
466 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
467 assign a name to an anonymous subroutine for you. Simply do this:
469 use Sub::Name 'subname';
470 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
472 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
473 manage to find a workaround for it, so until someone gives me a working patch this will be a known
474 limitation of this module.
480 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.
482 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.
486 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>).
490 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.
494 =head2 The original Dylan paper
498 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
502 =head2 The prototype Perl 6 Object Model uses C3
506 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
510 =head2 Parrot now uses C3
514 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
516 =item L<http://use.perl.org/~autrijus/journal/25768>
520 =head2 Python 2.3 MRO related links
524 =item L<http://www.python.org/2.3/mro.html>
526 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
530 =head2 C3 for TinyCLOS
534 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
538 =head1 ACKNOWLEGEMENTS
542 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
543 and finding many bugs and providing fixes.
545 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
546 calls inside C<eval> and anon-subs.
548 =item Thanks to Robert Norris for adding support for C<next::can> and
549 C<maybe::next::method>.
555 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
557 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
559 =head1 COPYRIGHT AND LICENSE
561 Copyright 2005, 2006 by Infinity Interactive, Inc.
563 L<http://www.iinteractive.com>
565 This library is free software; you can redistribute it and/or modify
566 it under the same terms as Perl itself.