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};
70 # This prevents silly warnings when Class::C3 is
71 # used explicitly along with MRO::Compat under 5.9.5+
73 { no warnings 'redefine';
76 %next::METHOD_CACHE = ();
77 # why bother if we don't have anything ...
78 return unless keys %MRO;
80 mro::set_mro($_, 'c3') for keys %MRO;
85 $MRO{$_} = undef foreach keys %MRO;
87 _calculate_method_dispatch_tables();
88 _apply_method_dispatch_tables();
94 # why bother if we don't have anything ...
95 %next::METHOD_CACHE = ();
96 return unless keys %MRO;
98 mro::set_mro($_, 'dfs') for keys %MRO;
101 _remove_method_dispatch_tables();
106 sub reinitialize { goto &initialize }
108 } # end of "no warnings 'redefine'"
110 ## functions for applying C3 to classes
112 sub _calculate_method_dispatch_tables {
113 return if $C3_IN_CORE;
115 foreach my $class (keys %MRO) {
116 _calculate_method_dispatch_table($class, \%merge_cache);
120 sub _calculate_method_dispatch_table {
121 return if $C3_IN_CORE;
122 my ($class, $merge_cache) = @_;
124 my @MRO = calculateMRO($class, $merge_cache);
125 $MRO{$class} = { MRO => \@MRO };
126 my $has_overload_fallback;
129 # we do @MRO[1 .. $#MRO] here because it
130 # makes no sense to interogate the class
131 # which you are calculating for.
132 foreach my $local (@MRO[1 .. $#MRO]) {
133 # if overload has tagged this module to
134 # have use "fallback", then we want to
136 $has_overload_fallback = ${"${local}::()"}
137 if !defined $has_overload_fallback && defined ${"${local}::()"};
138 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
139 # skip if already overriden in local class
140 next unless !defined *{"${class}::$method"}{CODE};
141 $methods{$method} = {
142 orig => "${local}::$method",
143 code => \&{"${local}::$method"}
144 } unless exists $methods{$method};
147 # now stash them in our %MRO table
148 $MRO{$class}->{methods} = \%methods;
149 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
152 sub _apply_method_dispatch_tables {
153 return if $C3_IN_CORE;
154 foreach my $class (keys %MRO) {
155 _apply_method_dispatch_table($class);
159 sub _apply_method_dispatch_table {
160 return if $C3_IN_CORE;
163 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
164 if !defined &{"${class}::()"}
165 && defined $MRO{$class}->{has_overload_fallback};
166 foreach my $method (keys %{$MRO{$class}->{methods}}) {
167 if ( $method =~ /^\(/ ) {
168 my $orig = $MRO{$class}->{methods}->{$method}->{orig};
169 ${"${class}::$method"} = $$orig if defined $$orig;
171 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
175 sub _remove_method_dispatch_tables {
176 return if $C3_IN_CORE;
177 foreach my $class (keys %MRO) {
178 _remove_method_dispatch_table($class);
182 sub _remove_method_dispatch_table {
183 return if $C3_IN_CORE;
186 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
187 foreach my $method (keys %{$MRO{$class}->{methods}}) {
188 delete ${"${class}::"}{$method}
189 if defined *{"${class}::${method}"}{CODE} &&
190 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
195 my ($class, $merge_cache) = @_;
197 return Algorithm::C3::merge($class, sub {
203 # Method overrides to support 5.9.5+ or Class::C3::XS
205 sub _core_calculateMRO { @{mro::get_linear_isa($_[0], 'c3')} }
208 no warnings 'redefine';
209 *Class::C3::calculateMRO = \&_core_calculateMRO;
212 no warnings 'redefine';
213 *Class::C3::calculateMRO = \&Class::C3::XS::calculateMRO;
214 *Class::C3::_calculate_method_dispatch_table
215 = \&Class::C3::XS::_calculate_method_dispatch_table;
226 Class::C3 - A pragma to use the C3 method resolution order algortihm
232 sub hello { 'A::hello' }
242 sub hello { 'C::hello' }
248 # Classic Diamond MI pattern
257 # initializez the C3 module
258 # (formerly called in INIT)
259 Class::C3::initialize();
261 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
263 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
265 D->can('hello')->(); # can() also works correctly
266 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
270 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
271 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
275 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
276 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
277 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
278 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
279 default MRO for Parrot objects as well.
281 =head2 How does C3 work.
283 C3 works by always preserving local precendence ordering. This essentially means that no class will
284 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
292 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
293 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
294 (D, B, C, A), which does not have this same issue.
296 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
297 the L<SEE ALSO> section.
299 =head2 How does this module work?
301 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
302 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
303 gathers information from the symbol tables of each of those classes, and builds a set of method
304 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
305 then adds the method aliases into the local classes symbol table.
307 The end result is actually classes with pre-cached method dispatch. However, this caching does not
308 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
309 your classes to be effectively closed. See the L<CAVEATS> section for more details.
311 =head1 OPTIONAL LOWERCASE PRAGMA
313 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
314 the regular install since lowercase module names are considered I<"bad"> by some people. However I
315 think that code looks much nicer like this:
325 But hey, it's your choice, thats why it is optional.
331 =item B<calculateMRO ($class)>
333 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
337 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
338 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
339 use C3. Here is a quick code example:
343 # ... Foo methods here
348 # ... Bar methods here
352 Class::C3::initialize(); # now it is safe to use Foo and Bar
354 This function used to be called automatically for you in the INIT phase of the perl compiler, but
355 that lead to warnings if this module was required at runtime. After discussion with my user base
356 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
357 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
358 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
359 your own INIT method which calls this function.
363 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
364 clear the MRO cache first.
366 =item B<uninitialize>
368 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
369 style dispatch order (depth-first, left-to-right).
371 =item B<reinitialize>
373 This is an alias for L</initialize> above.
377 =head1 METHOD REDISPATCHING
379 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
380 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
381 method along the C3 linearization. This is best show with an examples.
383 # a classic diamond MI pattern ...
397 sub foo { 'B::foo => ' . (shift)->next::method() }
402 sub foo { 'C::foo => ' . (shift)->next::method() }
407 sub foo { 'D::foo => ' . (shift)->next::method() }
409 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
411 A few things to note. First, we do not require you to add on the method name to the C<next::method>
412 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
413 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
415 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
416 not automatically use the current C<@_>.
418 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
419 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
421 $self->next::method(@_) if $self->next::can;
423 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
424 The previous example could be simply written as:
426 $self->maybe::next::method(@_);
428 There are some caveats about using C<next::method>, see below for those.
432 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
433 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
434 whatever your needs might be.
436 But there are still caveats, so here goes ...
440 =item Use of C<SUPER::>.
442 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
443 However, it's use in conjuntion with this module is very much not recommended, and in fact very
444 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
445 more details on it's usage above.
447 =item Changing C<@ISA>.
449 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
450 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
451 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
452 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
453 in F<t/20_reinitialize.t> for more information.
455 =item Adding/deleting methods from class symbol tables.
457 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
458 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
459 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
460 changes you make to take effect.
462 =item Calling C<next::method> from methods defined outside the class
464 There is an edge case when using C<next::method> from within a subroutine which was created in a different
465 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
466 will not work correctly:
468 *Foo::foo = sub { (shift)->next::method(@_) };
470 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
471 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
472 uses C<caller> to find the name of the method it was called in, it will fail in this case.
474 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
475 assign a name to an anonymous subroutine for you. Simply do this:
477 use Sub::Name 'subname';
478 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
480 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
481 manage to find a workaround for it, so until someone gives me a working patch this will be a known
482 limitation of this module.
488 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.
490 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.
494 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>).
498 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.
502 =head2 The original Dylan paper
506 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
510 =head2 The prototype Perl 6 Object Model uses C3
514 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
518 =head2 Parrot now uses C3
522 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
524 =item L<http://use.perl.org/~autrijus/journal/25768>
528 =head2 Python 2.3 MRO related links
532 =item L<http://www.python.org/2.3/mro.html>
534 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
538 =head2 C3 for TinyCLOS
542 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
546 =head1 ACKNOWLEGEMENTS
550 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
551 and finding many bugs and providing fixes.
553 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
554 calls inside C<eval> and anon-subs.
556 =item Thanks to Robert Norris for adding support for C<next::can> and
557 C<maybe::next::method>.
563 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
565 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
567 =head1 COPYRIGHT AND LICENSE
569 Copyright 2005, 2006 by Infinity Interactive, Inc.
571 L<http://www.iinteractive.com>
573 This library is free software; you can redistribute it and/or modify
574 it under the same terms as Perl itself.