7 our $VERSION = '0.15_01';
12 eval "require mro"; # XXX in the future, this should be a version check
14 die $@ if $@ !~ /locate/;
15 # eval "require Class::C3::XS";
17 # die $@ if $@ !~ /locate/;
18 eval "require Algorithm::C3; require Class::C3::next";
27 # this is our global stash of both
28 # MRO's and method dispatch tables
29 # the structure basically looks like
33 # MRO => [ <class precendence list> ],
35 # orig => <original location of method>,
36 # code => \&<ref to original method>
38 # has_overload_fallback => (1 | 0)
43 # use these for debugging ...
44 sub _dump_MRO_table { %MRO }
47 # state tracking for initialize()/uninitialize()
48 our $_initialized = 0;
52 # skip if the caller is main::
53 # since that is clearly not relevant
54 return if $class eq 'main';
56 return if $TURN_OFF_C3;
57 mro::set_mro($class, 'c3') if $C3_IN_CORE;
59 # make a note to calculate $class
61 $MRO{$class} = undef unless exists $MRO{$class};
67 %next::METHOD_CACHE = ();
68 # why bother if we don't have anything ...
69 return unless keys %MRO;
71 mro::set_mro($_, 'c3') for keys %MRO;
76 $MRO{$_} = undef foreach keys %MRO;
78 _calculate_method_dispatch_tables();
79 _apply_method_dispatch_tables();
85 # why bother if we don't have anything ...
86 %next::METHOD_CACHE = ();
87 return unless keys %MRO;
89 mro::set_mro($_, 'dfs') for keys %MRO;
92 _remove_method_dispatch_tables();
97 sub reinitialize { goto &initialize }
99 ## functions for applying C3 to classes
101 sub _calculate_method_dispatch_tables {
102 return if $C3_IN_CORE;
104 foreach my $class (keys %MRO) {
105 _calculate_method_dispatch_table($class, \%merge_cache);
109 sub _calculate_method_dispatch_table {
110 return if $C3_IN_CORE;
111 my ($class, $merge_cache) = @_;
113 my @MRO = calculateMRO($class, $merge_cache);
114 $MRO{$class} = { MRO => \@MRO };
115 my $has_overload_fallback = 0;
118 # we do @MRO[1 .. $#MRO] here because it
119 # makes no sense to interogate the class
120 # which you are calculating for.
121 foreach my $local (@MRO[1 .. $#MRO]) {
122 # if overload has tagged this module to
123 # have use "fallback", then we want to
125 $has_overload_fallback = ${"${local}::()"}
126 if defined ${"${local}::()"};
127 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
128 # skip if already overriden in local class
129 next unless !defined *{"${class}::$method"}{CODE};
130 $methods{$method} = {
131 orig => "${local}::$method",
132 code => \&{"${local}::$method"}
133 } unless exists $methods{$method};
136 # now stash them in our %MRO table
137 $MRO{$class}->{methods} = \%methods;
138 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
141 sub _apply_method_dispatch_tables {
142 return if $C3_IN_CORE;
143 foreach my $class (keys %MRO) {
144 _apply_method_dispatch_table($class);
148 sub _apply_method_dispatch_table {
149 return if $C3_IN_CORE;
152 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
153 if $MRO{$class}->{has_overload_fallback};
154 foreach my $method (keys %{$MRO{$class}->{methods}}) {
155 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
159 sub _remove_method_dispatch_tables {
160 return if $C3_IN_CORE;
161 foreach my $class (keys %MRO) {
162 _remove_method_dispatch_table($class);
166 sub _remove_method_dispatch_table {
167 return if $C3_IN_CORE;
170 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
171 foreach my $method (keys %{$MRO{$class}->{methods}}) {
172 delete ${"${class}::"}{$method}
173 if defined *{"${class}::${method}"}{CODE} &&
174 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
179 my ($class, $merge_cache) = @_;
181 return @{mro::get_linear_isa($class)} if $C3_IN_CORE;
183 return Algorithm::C3::merge($class, sub {
197 Class::C3 - A pragma to use the C3 method resolution order algortihm
203 sub hello { 'A::hello' }
213 sub hello { 'C::hello' }
219 # Classic Diamond MI pattern
228 # initializez the C3 module
229 # (formerly called in INIT)
230 Class::C3::initialize();
232 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
234 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
236 D->can('hello')->(); # can() also works correctly
237 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
239 =head1 SPECIAL NOTE FOR 0.15_01
241 To try this with the experimental perl core c3 patch,
242 download a recent copy perl-current:
244 http://mirrors.develooper.com/perl/APC/perl-current-snap/perl-current@30943.tar.bz2
246 apply the enclosed c3.patch, and install this perl:
248 sh Configure -Dusedevel -Dprefix=/where/I/want/it -d -e && make && make test && make install
250 then try your C3-using software against this perl + Class::C3 0.15_01.
254 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
255 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
259 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
260 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
261 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
262 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
263 default MRO for Parrot objects as well.
265 =head2 How does C3 work.
267 C3 works by always preserving local precendence ordering. This essentially means that no class will
268 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
276 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
277 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
278 (D, B, C, A), which does not have this same issue.
280 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
281 the L<SEE ALSO> section.
283 =head2 How does this module work?
285 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
286 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
287 gathers information from the symbol tables of each of those classes, and builds a set of method
288 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
289 then adds the method aliases into the local classes symbol table.
291 The end result is actually classes with pre-cached method dispatch. However, this caching does not
292 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
293 your classes to be effectively closed. See the L<CAVEATS> section for more details.
295 =head1 OPTIONAL LOWERCASE PRAGMA
297 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
298 the regular install since lowercase module names are considered I<"bad"> by some people. However I
299 think that code looks much nicer like this:
309 But hey, it's your choice, thats why it is optional.
315 =item B<calculateMRO ($class)>
317 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
321 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
322 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
323 use C3. Here is a quick code example:
327 # ... Foo methods here
332 # ... Bar methods here
336 Class::C3::initialize(); # now it is safe to use Foo and Bar
338 This function used to be called automatically for you in the INIT phase of the perl compiler, but
339 that lead to warnings if this module was required at runtime. After discussion with my user base
340 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
341 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
342 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
343 your own INIT method which calls this function.
347 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
348 clear the MRO cache first.
350 =item B<uninitialize>
352 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
353 style dispatch order (depth-first, left-to-right).
355 =item B<reinitialize>
357 This is an alias for L</initialize> above.
361 =head1 METHOD REDISPATCHING
363 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
364 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
365 method along the C3 linearization. This is best show with an examples.
367 # a classic diamond MI pattern ...
381 sub foo { 'B::foo => ' . (shift)->next::method() }
386 sub foo { 'C::foo => ' . (shift)->next::method() }
391 sub foo { 'D::foo => ' . (shift)->next::method() }
393 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
395 A few things to note. First, we do not require you to add on the method name to the C<next::method>
396 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
397 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
399 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
400 not automatically use the current C<@_>.
402 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
403 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
405 $self->next::method(@_) if $self->next::can;
407 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
408 The previous example could be simply written as:
410 $self->maybe::next::method(@_);
412 There are some caveats about using C<next::method>, see below for those.
416 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
417 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
418 whatever your needs might be.
420 But there are still caveats, so here goes ...
424 =item Use of C<SUPER::>.
426 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
427 However, it's use in conjuntion with this module is very much not recommended, and in fact very
428 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
429 more details on it's usage above.
431 =item Changing C<@ISA>.
433 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
434 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
435 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
436 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
437 in F<t/20_reinitialize.t> for more information.
439 =item Adding/deleting methods from class symbol tables.
441 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
442 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
443 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
444 changes you make to take effect.
446 =item Calling C<next::method> from methods defined outside the class
448 There is an edge case when using C<next::method> from within a subroutine which was created in a different
449 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
450 will not work correctly:
452 *Foo::foo = sub { (shift)->next::method(@_) };
454 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
455 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
456 uses C<caller> to find the name of the method it was called in, it will fail in this case.
458 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
459 assign a name to an anonymous subroutine for you. Simply do this:
461 use Sub::Name 'subname';
462 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
464 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
465 manage to find a workaround for it, so until someone gives me a working patch this will be a known
466 limitation of this module.
472 I use B<Devel::Cover> to test the code coverage of my tests, below is the B<Devel::Cover> report on this
475 ---------------------------- ------ ------ ------ ------ ------ ------ ------
476 File stmt bran cond sub pod time total
477 ---------------------------- ------ ------ ------ ------ ------ ------ ------
478 Class/C3.pm 98.3 84.4 80.0 96.2 100.0 98.4 94.4
479 ---------------------------- ------ ------ ------ ------ ------ ------ ------
480 Total 98.3 84.4 80.0 96.2 100.0 98.4 94.4
481 ---------------------------- ------ ------ ------ ------ ------ ------ ------
485 =head2 The original Dylan paper
489 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
493 =head2 The prototype Perl 6 Object Model uses C3
497 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
501 =head2 Parrot now uses C3
505 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
507 =item L<http://use.perl.org/~autrijus/journal/25768>
511 =head2 Python 2.3 MRO related links
515 =item L<http://www.python.org/2.3/mro.html>
517 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
521 =head2 C3 for TinyCLOS
525 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
529 =head1 ACKNOWLEGEMENTS
533 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
534 and finding many bugs and providing fixes.
536 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
537 calls inside C<eval> and anon-subs.
539 =item Thanks to Robert Norris for adding support for C<next::can> and
540 C<maybe::next::method>.
546 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
548 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
550 =head1 COPYRIGHT AND LICENSE
552 Copyright 2005, 2006 by Infinity Interactive, Inc.
554 L<http://www.iinteractive.com>
556 This library is free software; you can redistribute it and/or modify
557 it under the same terms as Perl itself.