7 use Scalar::Util 'blessed';
10 our $VERSION = '0.14_01';
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{$class}->{methods}}) {
123 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
127 sub _remove_method_dispatch_tables {
128 foreach my $class (keys %MRO) {
129 _remove_method_dispatch_table($class);
133 sub _remove_method_dispatch_table {
136 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
137 foreach my $method (keys %{$MRO{$class}->{methods}}) {
138 delete ${"${class}::"}{$method}
139 if defined *{"${class}::${method}"}{CODE} &&
140 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
144 ## functions for calculating C3 MRO
147 my ($class, $merge_cache) = @_;
148 return Algorithm::C3::merge($class, sub {
154 package # hide me from PAUSE
160 use Scalar::Util 'blessed';
162 our $VERSION = '0.05';
167 my $indirect = caller() =~ /^(?:next|maybe::next)$/;
168 my $level = $indirect ? 2 : 1;
170 my ($method_caller, $label, @label);
171 while ($method_caller = (caller($level++))[3]) {
172 @label = (split '::', $method_caller);
175 $label eq '(eval)' ||
176 $label eq '__ANON__';
178 my $caller = join '::' => @label;
180 my $class = blessed($self) || $self;
182 my $method = $METHOD_CACHE{"$class|$caller|$label"} ||= do {
184 my @MRO = Class::C3::calculateMRO($class);
187 while ($current = shift @MRO) {
188 last if $caller eq $current;
193 foreach my $class (@MRO) {
194 next if (defined $Class::C3::MRO{$class} &&
195 defined $Class::C3::MRO{$class}{methods}{$label});
196 last if (defined ($found = *{$class . '::' . $label}{CODE}));
202 return $method if $indirect;
204 die "No next::method '$label' found for $self" if !$method;
209 sub can { method($_[0]) }
211 package # hide me from PAUSE
217 our $VERSION = '0.01';
219 sub method { (next::method($_[0]) || return)->(@_) }
229 Class::C3 - A pragma to use the C3 method resolution order algortihm
235 sub hello { 'A::hello' }
245 sub hello { 'C::hello' }
251 # Classic Diamond MI pattern
260 # initializez the C3 module
261 # (formerly called in INIT)
262 Class::C3::initialize();
264 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
266 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
268 D->can('hello')->(); # can() also works correctly
269 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
273 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
274 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
278 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
279 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
280 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
281 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
282 default MRO for Parrot objects as well.
284 =head2 How does C3 work.
286 C3 works by always preserving local precendence ordering. This essentially means that no class will
287 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
295 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
296 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
297 (D, B, C, A), which does not have this same issue.
299 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
300 the L<SEE ALSO> section.
302 =head2 How does this module work?
304 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
305 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
306 gathers information from the symbol tables of each of those classes, and builds a set of method
307 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
308 then adds the method aliases into the local classes symbol table.
310 The end result is actually classes with pre-cached method dispatch. However, this caching does not
311 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
312 your classes to be effectively closed. See the L<CAVEATS> section for more details.
314 =head1 OPTIONAL LOWERCASE PRAGMA
316 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
317 the regular install since lowercase module names are considered I<"bad"> by some people. However I
318 think that code looks much nicer like this:
328 But hey, it's your choice, thats why it is optional.
334 =item B<calculateMRO ($class)>
336 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
340 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
341 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
342 use C3. Here is a quick code example:
346 # ... Foo methods here
351 # ... Bar methods here
355 Class::C3::initialize(); # now it is safe to use Foo and Bar
357 This function used to be called automatically for you in the INIT phase of the perl compiler, but
358 that lead to warnings if this module was required at runtime. After discussion with my user base
359 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
360 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
361 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
362 your own INIT method which calls this function.
366 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
367 clear the MRO cache first.
369 =item B<uninitialize>
371 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
372 style dispatch order (depth-first, left-to-right).
374 =item B<reinitialize>
376 This is an alias for L</initialize> above.
380 =head1 METHOD REDISPATCHING
382 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
383 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
384 method along the C3 linearization. This is best show with an examples.
386 # a classic diamond MI pattern ...
400 sub foo { 'B::foo => ' . (shift)->next::method() }
405 sub foo { 'C::foo => ' . (shift)->next::method() }
410 sub foo { 'D::foo => ' . (shift)->next::method() }
412 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
414 A few things to note. First, we do not require you to add on the method name to the C<next::method>
415 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
416 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
418 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
419 not automatically use the current C<@_>.
421 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
422 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
424 $self->next::method(@_) if $self->next::can;
426 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
427 The previous example could be simply written as:
429 $self->maybe::next::method(@_);
431 There are some caveats about using C<next::method>, see below for those.
435 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
436 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
437 whatever your needs might be.
439 But there are still caveats, so here goes ...
443 =item Use of C<SUPER::>.
445 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
446 However, it's use in conjuntion with this module is very much not recommended, and in fact very
447 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
448 more details on it's usage above.
450 =item Changing C<@ISA>.
452 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
453 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
454 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
455 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
456 in F<t/20_reinitialize.t> for more information.
458 =item Adding/deleting methods from class symbol tables.
460 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
461 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
462 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
463 changes you make to take effect.
465 =item Calling C<next::method> from methods defined outside the class
467 There is an edge case when using C<next::method> from within a subroutine which was created in a different
468 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
469 will not work correctly:
471 *Foo::foo = sub { (shift)->next::method(@_) };
473 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
474 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
475 uses C<caller> to find the name of the method it was called in, it will fail in this case.
477 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
478 assign a name to an anonymous subroutine for you. Simply do this:
480 use Sub::Name 'subname';
481 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
483 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
484 manage to find a workaround for it, so until someone gives me a working patch this will be a known
485 limitation of this module.
491 I use B<Devel::Cover> to test the code coverage of my tests, below is the B<Devel::Cover> report on this
494 ---------------------------- ------ ------ ------ ------ ------ ------ ------
495 File stmt bran cond sub pod time total
496 ---------------------------- ------ ------ ------ ------ ------ ------ ------
497 Class/C3.pm 98.3 84.4 80.0 96.2 100.0 98.4 94.4
498 ---------------------------- ------ ------ ------ ------ ------ ------ ------
499 Total 98.3 84.4 80.0 96.2 100.0 98.4 94.4
500 ---------------------------- ------ ------ ------ ------ ------ ------ ------
504 =head2 The original Dylan paper
508 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
512 =head2 The prototype Perl 6 Object Model uses C3
516 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
520 =head2 Parrot now uses C3
524 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
526 =item L<http://use.perl.org/~autrijus/journal/25768>
530 =head2 Python 2.3 MRO related links
534 =item L<http://www.python.org/2.3/mro.html>
536 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
540 =head2 C3 for TinyCLOS
544 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
548 =head1 ACKNOWLEGEMENTS
552 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
553 and finding many bugs and providing fixes.
555 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
556 calls inside C<eval> and anon-subs.
558 =item Thanks to Robert Norris for adding support for C<next::can> and
559 C<maybe::next::method>.
565 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
567 Brandon L. Black, E<lt>blblack@gmail.comE<gt>
569 =head1 COPYRIGHT AND LICENSE
571 Copyright 2005, 2006 by Infinity Interactive, Inc.
573 L<http://www.iinteractive.com>
575 This library is free software; you can redistribute it and/or modify
576 it under the same terms as Perl itself.