7 use Scalar::Util 'blessed';
10 our $VERSION = '0.14';
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 }
32 # state tracking for initialize()/uninitialize()
33 our $_initialized = 0;
37 # skip if the caller is main::
38 # since that is clearly not relevant
39 return if $class eq 'main';
40 return if $TURN_OFF_C3;
41 # make a note to calculate $class
43 $MRO{$class} = undef unless exists $MRO{$class};
49 # why bother if we don't have anything ...
50 return unless keys %MRO;
53 $MRO{$_} = undef foreach keys %MRO;
55 _calculate_method_dispatch_tables();
56 _apply_method_dispatch_tables();
57 %next::METHOD_CACHE = ();
62 # why bother if we don't have anything ...
63 return unless keys %MRO;
64 _remove_method_dispatch_tables();
65 %next::METHOD_CACHE = ();
69 sub reinitialize { goto &initialize }
71 ## functions for applying C3 to classes
73 sub _calculate_method_dispatch_tables {
75 foreach my $class (keys %MRO) {
76 _calculate_method_dispatch_table($class, \%merge_cache);
80 sub _calculate_method_dispatch_table {
81 my ($class, $merge_cache) = @_;
83 my @MRO = calculateMRO($class, $merge_cache);
84 $MRO{$class} = { MRO => \@MRO };
85 my $has_overload_fallback = 0;
88 # we do @MRO[1 .. $#MRO] here because it
89 # makes no sense to interogate the class
90 # which you are calculating for.
91 foreach my $local (@MRO[1 .. $#MRO]) {
92 # if overload has tagged this module to
93 # have use "fallback", then we want to
95 $has_overload_fallback = ${"${local}::()"}
96 if defined ${"${local}::()"};
97 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
98 # skip if already overriden in local class
99 next unless !defined *{"${class}::$method"}{CODE};
100 $methods{$method} = {
101 orig => "${local}::$method",
102 code => \&{"${local}::$method"}
103 } unless exists $methods{$method};
106 # now stash them in our %MRO table
107 $MRO{$class}->{methods} = \%methods;
108 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
111 sub _apply_method_dispatch_tables {
112 foreach my $class (keys %MRO) {
113 _apply_method_dispatch_table($class);
117 sub _apply_method_dispatch_table {
120 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
121 if $MRO{$class}->{has_overload_fallback};
122 foreach my $method (keys %{$MRO{$class}->{methods}}) {
123 if ( $method =~ /^\(/ ) {
124 my $orig = $MRO{$class}->{methods}->{$method}->{orig};
125 ${"${class}::$method"} = $$orig if defined $$orig;
127 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
131 sub _remove_method_dispatch_tables {
132 foreach my $class (keys %MRO) {
133 _remove_method_dispatch_table($class);
137 sub _remove_method_dispatch_table {
140 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
141 foreach my $method (keys %{$MRO{$class}->{methods}}) {
142 delete ${"${class}::"}{$method}
143 if defined *{"${class}::${method}"}{CODE} &&
144 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
148 ## functions for calculating C3 MRO
151 my ($class, $merge_cache) = @_;
152 return Algorithm::C3::merge($class, sub {
158 package # hide me from PAUSE
164 use Scalar::Util 'blessed';
166 our $VERSION = '0.05';
171 my $indirect = caller() =~ /^(?:next|maybe::next)$/;
172 my $level = $indirect ? 2 : 1;
174 my ($method_caller, $label, @label);
175 while ($method_caller = (caller($level++))[3]) {
176 @label = (split '::', $method_caller);
179 $label eq '(eval)' ||
180 $label eq '__ANON__';
182 my $caller = join '::' => @label;
184 my $class = blessed($self) || $self;
186 my $method = $METHOD_CACHE{"$class|$caller|$label"} ||= do {
188 my @MRO = Class::C3::calculateMRO($class);
191 while ($current = shift @MRO) {
192 last if $caller eq $current;
197 foreach my $class (@MRO) {
198 next if (defined $Class::C3::MRO{$class} &&
199 defined $Class::C3::MRO{$class}{methods}{$label});
200 last if (defined ($found = *{$class . '::' . $label}{CODE}));
206 return $method if $indirect;
208 die "No next::method '$label' found for $self" if !$method;
213 sub can { method($_[0]) }
215 package # hide me from PAUSE
221 our $VERSION = '0.01';
223 sub method { (next::method($_[0]) || return)->(@_) }
233 Class::C3 - A pragma to use the C3 method resolution order algortihm
239 sub hello { 'A::hello' }
249 sub hello { 'C::hello' }
255 # Classic Diamond MI pattern
264 # initializez the C3 module
265 # (formerly called in INIT)
266 Class::C3::initialize();
268 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
270 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
272 D->can('hello')->(); # can() also works correctly
273 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
277 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
278 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
282 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
283 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
284 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
285 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
286 default MRO for Parrot objects as well.
288 =head2 How does C3 work.
290 C3 works by always preserving local precendence ordering. This essentially means that no class will
291 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
299 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
300 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
301 (D, B, C, A), which does not have this same issue.
303 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
304 the L<SEE ALSO> section.
306 =head2 How does this module work?
308 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
309 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
310 gathers information from the symbol tables of each of those classes, and builds a set of method
311 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
312 then adds the method aliases into the local classes symbol table.
314 The end result is actually classes with pre-cached method dispatch. However, this caching does not
315 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
316 your classes to be effectively closed. See the L<CAVEATS> section for more details.
318 =head1 OPTIONAL LOWERCASE PRAGMA
320 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
321 the regular install since lowercase module names are considered I<"bad"> by some people. However I
322 think that code looks much nicer like this:
332 But hey, it's your choice, thats why it is optional.
338 =item B<calculateMRO ($class)>
340 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
344 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
345 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
346 use C3. Here is a quick code example:
350 # ... Foo methods here
355 # ... Bar methods here
359 Class::C3::initialize(); # now it is safe to use Foo and Bar
361 This function used to be called automatically for you in the INIT phase of the perl compiler, but
362 that lead to warnings if this module was required at runtime. After discussion with my user base
363 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
364 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
365 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
366 your own INIT method which calls this function.
370 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
371 clear the MRO cache first.
373 =item B<uninitialize>
375 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
376 style dispatch order (depth-first, left-to-right).
378 =item B<reinitialize>
380 This is an alias for L</initialize> above.
384 =head1 METHOD REDISPATCHING
386 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
387 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
388 method along the C3 linearization. This is best show with an examples.
390 # a classic diamond MI pattern ...
404 sub foo { 'B::foo => ' . (shift)->next::method() }
409 sub foo { 'C::foo => ' . (shift)->next::method() }
414 sub foo { 'D::foo => ' . (shift)->next::method() }
416 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
418 A few things to note. First, we do not require you to add on the method name to the C<next::method>
419 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
420 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
422 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
423 not automatically use the current C<@_>.
425 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
426 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
428 $self->next::method(@_) if $self->next::can;
430 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
431 The previous example could be simply written as:
433 $self->maybe::next::method(@_);
435 There are some caveats about using C<next::method>, see below for those.
439 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
440 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
441 whatever your needs might be.
443 But there are still caveats, so here goes ...
447 =item Use of C<SUPER::>.
449 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
450 However, it's use in conjuntion with this module is very much not recommended, and in fact very
451 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
452 more details on it's usage above.
454 =item Changing C<@ISA>.
456 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
457 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
458 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
459 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
460 in F<t/20_reinitialize.t> for more information.
462 =item Adding/deleting methods from class symbol tables.
464 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
465 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
466 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
467 changes you make to take effect.
469 =item Calling C<next::method> from methods defined outside the class
471 There is an edge case when using C<next::method> from within a subroutine which was created in a different
472 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
473 will not work correctly:
475 *Foo::foo = sub { (shift)->next::method(@_) };
477 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
478 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
479 uses C<caller> to find the name of the method it was called in, it will fail in this case.
481 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
482 assign a name to an anonymous subroutine for you. Simply do this:
484 use Sub::Name 'subname';
485 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
487 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
488 manage to find a workaround for it, so until someone gives me a working patch this will be a known
489 limitation of this module.
495 I use B<Devel::Cover> to test the code coverage of my tests, below is the B<Devel::Cover> report on this
498 ---------------------------- ------ ------ ------ ------ ------ ------ ------
499 File stmt bran cond sub pod time total
500 ---------------------------- ------ ------ ------ ------ ------ ------ ------
501 Class/C3.pm 98.3 84.4 80.0 96.2 100.0 98.4 94.4
502 ---------------------------- ------ ------ ------ ------ ------ ------ ------
503 Total 98.3 84.4 80.0 96.2 100.0 98.4 94.4
504 ---------------------------- ------ ------ ------ ------ ------ ------ ------
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.