7 use Scalar::Util 'blessed';
11 our $VERSION = '0.14';
16 if(!$@ && &mro::get_mro_linear_c3) {
21 # this is our global stash of both
22 # MRO's and method dispatch tables
23 # the structure basically looks like
27 # MRO => [ <class precendence list> ],
29 # orig => <original location of method>,
30 # code => \&<ref to original method>
32 # has_overload_fallback => (1 | 0)
37 # use these for debugging ...
38 sub _dump_MRO_table { %MRO }
41 # state tracking for initialize()/uninitialize()
42 our $_initialized = 0;
46 # skip if the caller is main::
47 # since that is clearly not relevant
48 return if $class eq 'main';
49 return if $TURN_OFF_C3;
51 B::enable_c3mro($class);
53 # make a note to calculate $class
55 $MRO{$class} = undef unless exists $MRO{$class};
61 %next::METHOD_CACHE = ();
62 # why bother if we don't have anything ...
63 return unless keys %MRO;
65 mro::set_mro_c3($_) for keys %MRO;
70 $MRO{$_} = undef foreach keys %MRO;
72 _calculate_method_dispatch_tables();
73 _apply_method_dispatch_tables();
79 # why bother if we don't have anything ...
80 %next::METHOD_CACHE = ();
81 return unless keys %MRO;
83 mro::set_mro_dfs($_) for keys %MRO;
86 _remove_method_dispatch_tables();
91 sub reinitialize { goto &initialize }
93 ## functions for applying C3 to classes
95 sub _calculate_method_dispatch_tables {
96 return if $C3_IN_CORE;
98 foreach my $class (keys %MRO) {
99 _calculate_method_dispatch_table($class, \%merge_cache);
103 sub _calculate_method_dispatch_table {
104 return if $C3_IN_CORE;
105 my ($class, $merge_cache) = @_;
107 my @MRO = calculateMRO($class, $merge_cache);
108 $MRO{$class} = { MRO => \@MRO };
109 my $has_overload_fallback = 0;
112 # we do @MRO[1 .. $#MRO] here because it
113 # makes no sense to interogate the class
114 # which you are calculating for.
115 foreach my $local (@MRO[1 .. $#MRO]) {
116 # if overload has tagged this module to
117 # have use "fallback", then we want to
119 $has_overload_fallback = ${"${local}::()"}
120 if defined ${"${local}::()"};
121 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
122 # skip if already overriden in local class
123 next unless !defined *{"${class}::$method"}{CODE};
124 $methods{$method} = {
125 orig => "${local}::$method",
126 code => \&{"${local}::$method"}
127 } unless exists $methods{$method};
130 # now stash them in our %MRO table
131 $MRO{$class}->{methods} = \%methods;
132 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
135 sub _apply_method_dispatch_tables {
136 return if $C3_IN_CORE;
137 foreach my $class (keys %MRO) {
138 _apply_method_dispatch_table($class);
142 sub _apply_method_dispatch_table {
143 return if $C3_IN_CORE;
146 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
147 if $MRO{$class}->{has_overload_fallback};
148 foreach my $method (keys %{$MRO{$class}->{methods}}) {
149 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
153 sub _remove_method_dispatch_tables {
154 return if $C3_IN_CORE;
155 foreach my $class (keys %MRO) {
156 _remove_method_dispatch_table($class);
160 sub _remove_method_dispatch_table {
161 return if $C3_IN_CORE;
164 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
165 foreach my $method (keys %{$MRO{$class}->{methods}}) {
166 delete ${"${class}::"}{$method}
167 if defined *{"${class}::${method}"}{CODE} &&
168 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
172 ## functions for calculating C3 MRO
175 my ($class, $merge_cache) = @_;
177 return @{mro::get_mro_linear_c3($class)};
180 return Algorithm::C3::merge($class, sub {
187 package # hide me from PAUSE
193 use Scalar::Util 'blessed';
195 our $VERSION = '0.05';
201 my $class = blessed($self) || $self;
202 my $indirect = caller() =~ /^(?:next|maybe::next)$/;
203 my $level = $indirect ? 2 : 1;
205 my ($method_caller, $label, @label);
206 while ($method_caller = (caller($level++))[3]) {
207 @label = (split '::', $method_caller);
210 $label eq '(eval)' ||
211 $label eq '__ANON__';
216 # You would think we could do this, but we can't apparently :(
217 #if($Class::C3::C3_IN_CORE && mro::is_mro_c3($class)) {
218 # $method = $class->can('SUPER::' . $label);
221 my $caller = join '::' => @label;
223 $method = $METHOD_CACHE{"$class|$caller|$label"} ||= do {
225 my @MRO = Class::C3::calculateMRO($class);
228 while ($current = shift @MRO) {
229 last if $caller eq $current;
234 foreach my $class (@MRO) {
235 next if (defined $Class::C3::MRO{$class} &&
236 defined $Class::C3::MRO{$class}{methods}{$label});
237 last if (defined ($found = *{$class . '::' . $label}{CODE}));
244 return $method if $indirect;
246 die "No next::method '$label' found for $self" if !$method;
251 sub can { method($_[0]) }
253 package # hide me from PAUSE
259 our $VERSION = '0.01';
261 sub method { (next::method($_[0]) || return)->(@_) }
271 Class::C3 - A pragma to use the C3 method resolution order algortihm
277 sub hello { 'A::hello' }
287 sub hello { 'C::hello' }
293 # Classic Diamond MI pattern
302 # initializez the C3 module
303 # (formerly called in INIT)
304 Class::C3::initialize();
306 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
308 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
310 D->can('hello')->(); # can() also works correctly
311 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
315 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
316 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
320 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
321 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
322 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
323 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
324 default MRO for Parrot objects as well.
326 =head2 How does C3 work.
328 C3 works by always preserving local precendence ordering. This essentially means that no class will
329 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
337 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
338 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
339 (D, B, C, A), which does not have this same issue.
341 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
342 the L<SEE ALSO> section.
344 =head2 How does this module work?
346 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
347 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
348 gathers information from the symbol tables of each of those classes, and builds a set of method
349 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
350 then adds the method aliases into the local classes symbol table.
352 The end result is actually classes with pre-cached method dispatch. However, this caching does not
353 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
354 your classes to be effectively closed. See the L<CAVEATS> section for more details.
356 =head1 OPTIONAL LOWERCASE PRAGMA
358 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
359 the regular install since lowercase module names are considered I<"bad"> by some people. However I
360 think that code looks much nicer like this:
370 But hey, it's your choice, thats why it is optional.
376 =item B<calculateMRO ($class)>
378 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
382 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
383 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
384 use C3. Here is a quick code example:
388 # ... Foo methods here
393 # ... Bar methods here
397 Class::C3::initialize(); # now it is safe to use Foo and Bar
399 This function used to be called automatically for you in the INIT phase of the perl compiler, but
400 that lead to warnings if this module was required at runtime. After discussion with my user base
401 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
402 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
403 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
404 your own INIT method which calls this function.
408 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
409 clear the MRO cache first.
411 =item B<uninitialize>
413 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
414 style dispatch order (depth-first, left-to-right).
416 =item B<reinitialize>
418 This is an alias for L</initialize> above.
422 =head1 METHOD REDISPATCHING
424 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
425 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
426 method along the C3 linearization. This is best show with an examples.
428 # a classic diamond MI pattern ...
442 sub foo { 'B::foo => ' . (shift)->next::method() }
447 sub foo { 'C::foo => ' . (shift)->next::method() }
452 sub foo { 'D::foo => ' . (shift)->next::method() }
454 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
456 A few things to note. First, we do not require you to add on the method name to the C<next::method>
457 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
458 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
460 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
461 not automatically use the current C<@_>.
463 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
464 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
466 $self->next::method(@_) if $self->next::can;
468 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
469 The previous example could be simply written as:
471 $self->maybe::next::method(@_);
473 There are some caveats about using C<next::method>, see below for those.
477 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
478 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
479 whatever your needs might be.
481 But there are still caveats, so here goes ...
485 =item Use of C<SUPER::>.
487 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
488 However, it's use in conjuntion with this module is very much not recommended, and in fact very
489 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
490 more details on it's usage above.
492 =item Changing C<@ISA>.
494 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
495 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
496 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
497 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
498 in F<t/20_reinitialize.t> for more information.
500 =item Adding/deleting methods from class symbol tables.
502 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
503 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
504 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
505 changes you make to take effect.
507 =item Calling C<next::method> from methods defined outside the class
509 There is an edge case when using C<next::method> from within a subroutine which was created in a different
510 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
511 will not work correctly:
513 *Foo::foo = sub { (shift)->next::method(@_) };
515 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
516 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
517 uses C<caller> to find the name of the method it was called in, it will fail in this case.
519 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
520 assign a name to an anonymous subroutine for you. Simply do this:
522 use Sub::Name 'subname';
523 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
525 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
526 manage to find a workaround for it, so until someone gives me a working patch this will be a known
527 limitation of this module.
533 I use B<Devel::Cover> to test the code coverage of my tests, below is the B<Devel::Cover> report on this
536 ---------------------------- ------ ------ ------ ------ ------ ------ ------
537 File stmt bran cond sub pod time total
538 ---------------------------- ------ ------ ------ ------ ------ ------ ------
539 Class/C3.pm 98.3 84.4 80.0 96.2 100.0 98.4 94.4
540 ---------------------------- ------ ------ ------ ------ ------ ------ ------
541 Total 98.3 84.4 80.0 96.2 100.0 98.4 94.4
542 ---------------------------- ------ ------ ------ ------ ------ ------ ------
546 =head2 The original Dylan paper
550 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
554 =head2 The prototype Perl 6 Object Model uses C3
558 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
562 =head2 Parrot now uses C3
566 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
568 =item L<http://use.perl.org/~autrijus/journal/25768>
572 =head2 Python 2.3 MRO related links
576 =item L<http://www.python.org/2.3/mro.html>
578 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
582 =head2 C3 for TinyCLOS
586 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
590 =head1 ACKNOWLEGEMENTS
594 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
595 and finding many bugs and providing fixes.
597 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
598 calls inside C<eval> and anon-subs.
600 =item Thanks to Robert Norris for adding support for C<next::can> and
601 C<maybe::next::method>.
607 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
609 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
611 =head1 COPYRIGHT AND LICENSE
613 Copyright 2005, 2006 by Infinity Interactive, Inc.
615 L<http://www.iinteractive.com>
617 This library is free software; you can redistribute it and/or modify
618 it under the same terms as Perl itself.