7 use Scalar::Util 'blessed';
10 our $VERSION = '0.12';
12 # this is our global stash of both
13 # MRO's and method dispatch tables
14 # the structure basically looks like
18 # MRO => [ <class precendence list> ],
20 # orig => <original location of method>,
21 # code => \&<ref to original method>
23 # has_overload_fallback => (1 | 0)
28 # use these for debugging ...
29 sub _dump_MRO_table { %MRO }
34 # skip if the caller is main::
35 # since that is clearly not relevant
36 return if $class eq 'main';
37 return if $TURN_OFF_C3;
38 # make a note to calculate $class
40 $MRO{$class} = undef unless exists $MRO{$class};
46 # why bother if we don't have anything ...
47 return unless keys %MRO;
48 _calculate_method_dispatch_tables();
49 _apply_method_dispatch_tables();
50 %next::METHOD_CACHE = ();
54 # why bother if we don't have anything ...
55 return unless keys %MRO;
56 _remove_method_dispatch_tables();
57 %next::METHOD_CACHE = ();
62 # clean up the %MRO before we re-initialize
63 $MRO{$_} = undef foreach keys %MRO;
67 ## functions for applying C3 to classes
69 sub _calculate_method_dispatch_tables {
70 foreach my $class (keys %MRO) {
71 _calculate_method_dispatch_table($class);
75 sub _calculate_method_dispatch_table {
78 my @MRO = calculateMRO($class);
79 $MRO{$class} = { MRO => \@MRO };
80 my $has_overload_fallback = 0;
83 # we do @MRO[1 .. $#MRO] here because it
84 # makes no sense to interogate the class
85 # which you are calculating for.
86 foreach my $local (@MRO[1 .. $#MRO]) {
87 # if overload has tagged this module to
88 # have use "fallback", then we want to
90 $has_overload_fallback = ${"${local}::()"}
91 if defined ${"${local}::()"};
92 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
93 # skip if already overriden in local class
94 next unless !defined *{"${class}::$method"}{CODE};
96 orig => "${local}::$method",
97 code => \&{"${local}::$method"}
98 } unless exists $methods{$method};
101 # now stash them in our %MRO table
102 $MRO{$class}->{methods} = \%methods;
103 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
106 sub _apply_method_dispatch_tables {
107 foreach my $class (keys %MRO) {
108 _apply_method_dispatch_table($class);
112 sub _apply_method_dispatch_table {
115 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
116 if $MRO{$class}->{has_overload_fallback};
117 foreach my $method (keys %{$MRO{$class}->{methods}}) {
118 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
122 sub _remove_method_dispatch_tables {
123 foreach my $class (keys %MRO) {
124 _remove_method_dispatch_table($class);
128 sub _remove_method_dispatch_table {
131 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
132 foreach my $method (keys %{$MRO{$class}->{methods}}) {
133 delete ${"${class}::"}{$method}
134 if defined *{"${class}::${method}"}{CODE} &&
135 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
139 ## functions for calculating C3 MRO
143 return Algorithm::C3::merge($class, sub {
149 package # hide me from PAUSE
155 use Scalar::Util 'blessed';
157 our $VERSION = '0.05';
163 my ($method_caller, $label, @label);
164 while ($method_caller = (caller($level++))[3]) {
165 @label = (split '::', $method_caller);
168 $label eq '(eval)' ||
169 $label eq '__ANON__';
171 my $caller = join '::' => @label;
173 my $class = blessed($self) || $self;
175 goto &{ $METHOD_CACHE{"$class|$caller|$label"} ||= do {
177 my @MRO = Class::C3::calculateMRO($class);
180 while ($current = shift @MRO) {
181 last if $caller eq $current;
186 foreach my $class (@MRO) {
187 next if (defined $Class::C3::MRO{$class} &&
188 defined $Class::C3::MRO{$class}{methods}{$label});
189 last if (defined ($found = *{$class . '::' . $label}{CODE}));
192 die "No next::method '$label' found for $self" unless $found;
206 Class::C3 - A pragma to use the C3 method resolution order algortihm
212 sub hello { 'A::hello' }
222 sub hello { 'C::hello' }
228 # Classic Diamond MI pattern
237 # initializez the C3 module
238 # (formerly called in INIT)
239 Class::C3::initialize();
241 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
243 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
245 D->can('hello')->(); # can() also works correctly
246 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
250 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
251 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
255 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
256 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
257 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
258 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
259 default MRO for Parrot objects as well.
261 =head2 How does C3 work.
263 C3 works by always preserving local precendence ordering. This essentially means that no class will
264 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
272 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
273 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
274 (D, B, C, A), which does not have this same issue.
276 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
277 the L<SEE ALSO> section.
279 =head2 How does this module work?
281 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
282 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
283 gathers information from the symbol tables of each of those classes, and builds a set of method
284 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
285 then adds the method aliases into the local classes symbol table.
287 The end result is actually classes with pre-cached method dispatch. However, this caching does not
288 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
289 your classes to be effectively closed. See the L<CAVEATS> section for more details.
291 =head1 OPTIONAL LOWERCASE PRAGMA
293 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
294 the regular install since lowercase module names are considered I<"bad"> by some people. However I
295 think that code looks much nicer like this:
305 But hey, it's your choice, thats why it is optional.
311 =item B<calculateMRO ($class)>
313 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
317 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
318 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
319 use C3. Here is a quick code example:
323 # ... Foo methods here
328 # ... Bar methods here
332 Class::C3::initialize(); # now it is safe to use Foo and Bar
334 This function used to be called automatically for you in the INIT phase of the perl compiler, but
335 that lead to warnings if this module was required at runtime. After discussion with my user base
336 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
337 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
338 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
339 your own INIT method which calls this function.
342 This can B<not> be used to re-load the dispatch tables for all classes. Use C<reinitialize> for that.
344 =item B<uninitialize>
346 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
347 style dispatch order (depth-first, left-to-right).
349 =item B<reinitialize>
351 This effectively calls C<uninitialize> followed by C<initialize> the result of which is a reloading of
352 B<all> the calculated C3 dispatch tables.
354 It should be noted that if you have a large class library, this could potentially be a rather costly
359 =head1 METHOD REDISPATCHING
361 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
362 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
363 method along the C3 linearization. This is best show with an examples.
365 # a classic diamond MI pattern ...
379 sub foo { 'B::foo => ' . (shift)->next::method() }
384 sub foo { 'C::foo => ' . (shift)->next::method() }
389 sub foo { 'D::foo => ' . (shift)->next::method() }
391 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
393 A few things to note. First, we do not require you to add on the method name to the C<next::method>
394 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
395 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
397 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
398 not automatically use the current C<@_>.
400 There are some caveats about using C<next::method>, see below for those.
404 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
405 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
406 whatever your needs might be.
408 But there are still caveats, so here goes ...
412 =item Use of C<SUPER::>.
414 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
415 However, it's use in conjuntion with this module is very much not recommended, and in fact very
416 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
417 more details on it's usage above.
419 =item Changing C<@ISA>.
421 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
422 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
423 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
424 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
425 in F<t/20_reinitialize.t> for more information.
427 =item Adding/deleting methods from class symbol tables.
429 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
430 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
431 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
432 changes you make to take effect.
434 =item Calling C<next::method> from methods defined outside the class
436 There is an edge case when using C<next::method> from within a subroutine which was created in a different
437 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
438 will not work correctly:
440 *Foo::foo = sub { (shift)->next::method(@_) };
442 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
443 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
444 uses C<caller> to find the name of the method it was called in, it will fail in this case.
446 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
447 assign a name to an anonymous subroutine for you. Simply do this:
449 use Sub::Name 'subname';
450 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
452 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
453 manage to find a workaround for it, so until someone gives me a working patch this will be a known
454 limitation of this module.
460 I use B<Devel::Cover> to test the code coverage of my tests, below is the B<Devel::Cover> report on this
463 ---------------------------- ------ ------ ------ ------ ------ ------ ------
464 File stmt bran cond sub pod time total
465 ---------------------------- ------ ------ ------ ------ ------ ------ ------
466 Class/C3.pm 98.3 84.4 80.0 96.2 100.0 98.4 94.4
467 ---------------------------- ------ ------ ------ ------ ------ ------ ------
468 Total 98.3 84.4 80.0 96.2 100.0 98.4 94.4
469 ---------------------------- ------ ------ ------ ------ ------ ------ ------
473 =head2 The original Dylan paper
477 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
481 =head2 The prototype Perl 6 Object Model uses C3
485 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
489 =head2 Parrot now uses C3
493 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
495 =item L<http://use.perl.org/~autrijus/journal/25768>
499 =head2 Python 2.3 MRO related links
503 =item L<http://www.python.org/2.3/mro.html>
505 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
509 =head2 C3 for TinyCLOS
513 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
517 =head1 ACKNOWLEGEMENTS
521 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
522 and finding many bugs and providing fixes.
524 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
525 calls inside C<eval> and anon-subs.
531 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
533 =head1 COPYRIGHT AND LICENSE
535 Copyright 2005, 2006 by Infinity Interactive, Inc.
537 L<http://www.iinteractive.com>
539 This library is free software; you can redistribute it and/or modify
540 it under the same terms as Perl itself.