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
230 # NOTE - DO NOT USE Class::C3 directly as a user, use MRO::Compat instead!
233 sub hello { 'A::hello' }
243 sub hello { 'C::hello' }
249 # Classic Diamond MI pattern
258 # initializez the C3 module
259 # (formerly called in INIT)
260 Class::C3::initialize();
262 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
264 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
266 D->can('hello')->(); # can() also works correctly
267 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
271 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
272 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
274 B<NOTE:> YOU SHOULD NOT USE THIS MODULE DIRECTLY - The feature provided
275 is integrated into perl version >= 5.9.5, and you should use L<MRO::Compat>
276 instead, which will use the core implementation in newer perls, but fallback
277 to using this implementation on older perls.
281 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
282 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
283 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
284 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
285 default MRO for Parrot objects as well.
287 =head2 How does C3 work.
289 C3 works by always preserving local precendence ordering. This essentially means that no class will
290 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
298 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
299 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
300 (D, B, C, A), which does not have this same issue.
302 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
303 the L<SEE ALSO> section.
305 =head2 How does this module work?
307 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
308 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
309 gathers information from the symbol tables of each of those classes, and builds a set of method
310 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
311 then adds the method aliases into the local classes symbol table.
313 The end result is actually classes with pre-cached method dispatch. However, this caching does not
314 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
315 your classes to be effectively closed. See the L<CAVEATS> section for more details.
317 =head1 OPTIONAL LOWERCASE PRAGMA
319 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
320 the regular install since lowercase module names are considered I<"bad"> by some people. However I
321 think that code looks much nicer like this:
331 But hey, it's your choice, thats why it is optional.
337 =item B<calculateMRO ($class)>
339 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
343 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
344 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
345 use C3. Here is a quick code example:
349 # ... Foo methods here
354 # ... Bar methods here
358 Class::C3::initialize(); # now it is safe to use Foo and Bar
360 This function used to be called automatically for you in the INIT phase of the perl compiler, but
361 that lead to warnings if this module was required at runtime. After discussion with my user base
362 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
363 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
364 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
365 your own INIT method which calls this function.
369 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
370 clear the MRO cache first.
372 =item B<uninitialize>
374 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
375 style dispatch order (depth-first, left-to-right).
377 =item B<reinitialize>
379 This is an alias for L</initialize> above.
383 =head1 METHOD REDISPATCHING
385 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
386 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
387 method along the C3 linearization. This is best show with an examples.
389 # a classic diamond MI pattern ...
403 sub foo { 'B::foo => ' . (shift)->next::method() }
408 sub foo { 'C::foo => ' . (shift)->next::method() }
413 sub foo { 'D::foo => ' . (shift)->next::method() }
415 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
417 A few things to note. First, we do not require you to add on the method name to the C<next::method>
418 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
419 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
421 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
422 not automatically use the current C<@_>.
424 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
425 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
427 $self->next::method(@_) if $self->next::can;
429 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
430 The previous example could be simply written as:
432 $self->maybe::next::method(@_);
434 There are some caveats about using C<next::method>, see below for those.
438 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
439 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
440 whatever your needs might be.
442 But there are still caveats, so here goes ...
446 =item Use of C<SUPER::>.
448 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
449 However, it's use in conjuntion with this module is very much not recommended, and in fact very
450 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
451 more details on it's usage above.
453 =item Changing C<@ISA>.
455 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
456 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
457 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
458 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
459 in F<t/20_reinitialize.t> for more information.
461 =item Adding/deleting methods from class symbol tables.
463 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
464 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
465 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
466 changes you make to take effect.
468 =item Calling C<next::method> from methods defined outside the class
470 There is an edge case when using C<next::method> from within a subroutine which was created in a different
471 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
472 will not work correctly:
474 *Foo::foo = sub { (shift)->next::method(@_) };
476 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
477 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
478 uses C<caller> to find the name of the method it was called in, it will fail in this case.
480 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
481 assign a name to an anonymous subroutine for you. Simply do this:
483 use Sub::Name 'subname';
484 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
486 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
487 manage to find a workaround for it, so until someone gives me a working patch this will be a known
488 limitation of this module.
494 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.
496 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.
500 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>).
504 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.
508 =head2 The original Dylan paper
512 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
516 =head2 The prototype Perl 6 Object Model uses C3
520 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
524 =head2 Parrot now uses C3
528 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
530 =item L<http://use.perl.org/~autrijus/journal/25768>
534 =head2 Python 2.3 MRO related links
538 =item L<http://www.python.org/2.3/mro.html>
540 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
544 =head2 C3 for TinyCLOS
548 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
552 =head1 ACKNOWLEGEMENTS
556 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
557 and finding many bugs and providing fixes.
559 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
560 calls inside C<eval> and anon-subs.
562 =item Thanks to Robert Norris for adding support for C<next::can> and
563 C<maybe::next::method>.
569 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
571 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
573 =head1 COPYRIGHT AND LICENSE
575 Copyright 2005, 2006 by Infinity Interactive, Inc.
577 L<http://www.iinteractive.com>
579 This library is free software; you can redistribute it and/or modify
580 it under the same terms as Perl itself.