7 our $VERSION = '0.15_05';
14 eval "require Class::C3::XS";
16 die $@ if $@ !~ /locate/;
17 eval "require Algorithm::C3; require Class::C3::next";
29 # this is our global stash of both
30 # MRO's and method dispatch tables
31 # the structure basically looks like
35 # MRO => [ <class precendence list> ],
37 # orig => <original location of method>,
38 # code => \&<ref to original method>
40 # has_overload_fallback => (1 | 0)
45 # use these for debugging ...
46 sub _dump_MRO_table { %MRO }
49 # state tracking for initialize()/uninitialize()
50 our $_initialized = 0;
54 # skip if the caller is main::
55 # since that is clearly not relevant
56 return if $class eq 'main';
58 return if $TURN_OFF_C3;
59 mro::set_mro($class, 'c3') if $C3_IN_CORE;
61 # make a note to calculate $class
63 $MRO{$class} = undef unless exists $MRO{$class};
69 %next::METHOD_CACHE = ();
70 # why bother if we don't have anything ...
71 return unless keys %MRO;
73 mro::set_mro($_, 'c3') for keys %MRO;
78 $MRO{$_} = undef foreach keys %MRO;
80 _calculate_method_dispatch_tables();
81 _apply_method_dispatch_tables();
87 # why bother if we don't have anything ...
88 %next::METHOD_CACHE = ();
89 return unless keys %MRO;
91 mro::set_mro($_, 'dfs') for keys %MRO;
94 _remove_method_dispatch_tables();
99 sub reinitialize { goto &initialize }
101 ## functions for applying C3 to classes
103 sub _calculate_method_dispatch_tables {
104 return if $C3_IN_CORE;
106 foreach my $class (keys %MRO) {
107 _calculate_method_dispatch_table($class, \%merge_cache);
111 sub _calculate_method_dispatch_table {
112 return if $C3_IN_CORE;
113 my ($class, $merge_cache) = @_;
115 my @MRO = calculateMRO($class, $merge_cache);
116 $MRO{$class} = { MRO => \@MRO };
117 my $has_overload_fallback;
120 # we do @MRO[1 .. $#MRO] here because it
121 # makes no sense to interogate the class
122 # which you are calculating for.
123 foreach my $local (@MRO[1 .. $#MRO]) {
124 # if overload has tagged this module to
125 # have use "fallback", then we want to
127 $has_overload_fallback = ${"${local}::()"}
128 if !defined $has_overload_fallback && defined ${"${local}::()"};
129 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
130 # skip if already overriden in local class
131 next unless !defined *{"${class}::$method"}{CODE};
132 $methods{$method} = {
133 orig => "${local}::$method",
134 code => \&{"${local}::$method"}
135 } unless exists $methods{$method};
138 # now stash them in our %MRO table
139 $MRO{$class}->{methods} = \%methods;
140 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
143 sub _apply_method_dispatch_tables {
144 return if $C3_IN_CORE;
145 foreach my $class (keys %MRO) {
146 _apply_method_dispatch_table($class);
150 sub _apply_method_dispatch_table {
151 return if $C3_IN_CORE;
154 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
155 if !defined &{"${class}::()"}
156 && defined $MRO{$class}->{has_overload_fallback};
157 foreach my $method (keys %{$MRO{$class}->{methods}}) {
158 if ( $method =~ /^\(/ ) {
159 my $orig = $MRO{$class}->{methods}->{$method}->{orig};
160 ${"${class}::$method"} = $$orig if defined $$orig;
162 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
166 sub _remove_method_dispatch_tables {
167 return if $C3_IN_CORE;
168 foreach my $class (keys %MRO) {
169 _remove_method_dispatch_table($class);
173 sub _remove_method_dispatch_table {
174 return if $C3_IN_CORE;
177 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
178 foreach my $method (keys %{$MRO{$class}->{methods}}) {
179 delete ${"${class}::"}{$method}
180 if defined *{"${class}::${method}"}{CODE} &&
181 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
186 my ($class, $merge_cache) = @_;
188 return Algorithm::C3::merge($class, sub {
194 sub _core_calculateMRO { @{mro::get_linear_isa($_[0])} }
197 no warnings 'redefine';
198 *Class::C3::calculateMRO = \&_core_calculateMRO;
201 no warnings 'redefine';
202 *Class::C3::calculateMRO = \&Class::C3::XS::calculateMRO;
213 Class::C3 - A pragma to use the C3 method resolution order algortihm
219 sub hello { 'A::hello' }
229 sub hello { 'C::hello' }
235 # Classic Diamond MI pattern
244 # initializez the C3 module
245 # (formerly called in INIT)
246 Class::C3::initialize();
248 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
250 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
252 D->can('hello')->(); # can() also works correctly
253 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
255 =head1 SPECIAL NOTE FOR 0.15_05
257 To try this with the new perl core c3 support,
258 download the most recent copy perl-current:
260 http://mirrors.develooper.com/perl/APC/perl-current-snap/
262 sh Configure -Dusedevel -Dprefix=/where/I/want/it -d -e && make && make test && make install
264 then try your C3-using software against this perl + Class::C3 0.15_05.
268 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
269 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
273 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
274 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
275 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
276 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
277 default MRO for Parrot objects as well.
279 =head2 How does C3 work.
281 C3 works by always preserving local precendence ordering. This essentially means that no class will
282 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
290 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
291 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
292 (D, B, C, A), which does not have this same issue.
294 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
295 the L<SEE ALSO> section.
297 =head2 How does this module work?
299 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
300 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
301 gathers information from the symbol tables of each of those classes, and builds a set of method
302 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
303 then adds the method aliases into the local classes symbol table.
305 The end result is actually classes with pre-cached method dispatch. However, this caching does not
306 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
307 your classes to be effectively closed. See the L<CAVEATS> section for more details.
309 =head1 OPTIONAL LOWERCASE PRAGMA
311 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
312 the regular install since lowercase module names are considered I<"bad"> by some people. However I
313 think that code looks much nicer like this:
323 But hey, it's your choice, thats why it is optional.
329 =item B<calculateMRO ($class)>
331 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
335 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
336 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
337 use C3. Here is a quick code example:
341 # ... Foo methods here
346 # ... Bar methods here
350 Class::C3::initialize(); # now it is safe to use Foo and Bar
352 This function used to be called automatically for you in the INIT phase of the perl compiler, but
353 that lead to warnings if this module was required at runtime. After discussion with my user base
354 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
355 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
356 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
357 your own INIT method which calls this function.
361 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
362 clear the MRO cache first.
364 =item B<uninitialize>
366 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
367 style dispatch order (depth-first, left-to-right).
369 =item B<reinitialize>
371 This is an alias for L</initialize> above.
375 =head1 METHOD REDISPATCHING
377 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
378 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
379 method along the C3 linearization. This is best show with an examples.
381 # a classic diamond MI pattern ...
395 sub foo { 'B::foo => ' . (shift)->next::method() }
400 sub foo { 'C::foo => ' . (shift)->next::method() }
405 sub foo { 'D::foo => ' . (shift)->next::method() }
407 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
409 A few things to note. First, we do not require you to add on the method name to the C<next::method>
410 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
411 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
413 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
414 not automatically use the current C<@_>.
416 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
417 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
419 $self->next::method(@_) if $self->next::can;
421 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
422 The previous example could be simply written as:
424 $self->maybe::next::method(@_);
426 There are some caveats about using C<next::method>, see below for those.
430 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
431 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
432 whatever your needs might be.
434 But there are still caveats, so here goes ...
438 =item Use of C<SUPER::>.
440 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
441 However, it's use in conjuntion with this module is very much not recommended, and in fact very
442 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
443 more details on it's usage above.
445 =item Changing C<@ISA>.
447 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
448 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
449 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
450 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
451 in F<t/20_reinitialize.t> for more information.
453 =item Adding/deleting methods from class symbol tables.
455 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
456 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
457 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
458 changes you make to take effect.
460 =item Calling C<next::method> from methods defined outside the class
462 There is an edge case when using C<next::method> from within a subroutine which was created in a different
463 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
464 will not work correctly:
466 *Foo::foo = sub { (shift)->next::method(@_) };
468 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
469 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
470 uses C<caller> to find the name of the method it was called in, it will fail in this case.
472 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
473 assign a name to an anonymous subroutine for you. Simply do this:
475 use Sub::Name 'subname';
476 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
478 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
479 manage to find a workaround for it, so until someone gives me a working patch this will be a known
480 limitation of this module.
486 If your software requires Perl 5.9.5 or higher, you do not need L<Class::C3>, you can simple 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.
488 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.
492 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>).
496 I use B<Devel::Cover> to test the code coverage of my tests, below is the B<Devel::Cover> report on this
499 ---------------------------- ------ ------ ------ ------ ------ ------ ------
500 File stmt bran cond sub pod time total
501 ---------------------------- ------ ------ ------ ------ ------ ------ ------
502 Class/C3.pm 98.3 84.4 80.0 96.2 100.0 98.4 94.4
503 ---------------------------- ------ ------ ------ ------ ------ ------ ------
504 Total 98.3 84.4 80.0 96.2 100.0 98.4 94.4
505 ---------------------------- ------ ------ ------ ------ ------ ------ ------
509 =head2 The original Dylan paper
513 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
517 =head2 The prototype Perl 6 Object Model uses C3
521 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
525 =head2 Parrot now uses C3
529 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
531 =item L<http://use.perl.org/~autrijus/journal/25768>
535 =head2 Python 2.3 MRO related links
539 =item L<http://www.python.org/2.3/mro.html>
541 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
545 =head2 C3 for TinyCLOS
549 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
553 =head1 ACKNOWLEGEMENTS
557 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
558 and finding many bugs and providing fixes.
560 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
561 calls inside C<eval> and anon-subs.
563 =item Thanks to Robert Norris for adding support for C<next::can> and
564 C<maybe::next::method>.
570 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
572 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
574 =head1 COPYRIGHT AND LICENSE
576 Copyright 2005, 2006 by Infinity Interactive, Inc.
578 L<http://www.iinteractive.com>
580 This library is free software; you can redistribute it and/or modify
581 it under the same terms as Perl itself.