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;
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 $has_overload_fallback && 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 !defined &{"${class}::()"}
122 && defined $MRO{$class}->{has_overload_fallback};
123 foreach my $method (keys %{$MRO{$class}->{methods}}) {
124 if ( $method =~ /^\(/ ) {
125 my $orig = $MRO{$class}->{methods}->{$method}->{orig};
126 ${"${class}::$method"} = $$orig if defined $$orig;
128 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
132 sub _remove_method_dispatch_tables {
133 foreach my $class (keys %MRO) {
134 _remove_method_dispatch_table($class);
138 sub _remove_method_dispatch_table {
141 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
142 foreach my $method (keys %{$MRO{$class}->{methods}}) {
143 delete ${"${class}::"}{$method}
144 if defined *{"${class}::${method}"}{CODE} &&
145 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
149 ## functions for calculating C3 MRO
152 my ($class, $merge_cache) = @_;
153 return Algorithm::C3::merge($class, sub {
159 package # hide me from PAUSE
165 use Scalar::Util 'blessed';
167 our $VERSION = '0.05';
172 my $indirect = caller() =~ /^(?:next|maybe::next)$/;
173 my $level = $indirect ? 2 : 1;
175 my ($method_caller, $label, @label);
176 while ($method_caller = (caller($level++))[3]) {
177 @label = (split '::', $method_caller);
180 $label eq '(eval)' ||
181 $label eq '__ANON__';
183 my $caller = join '::' => @label;
185 my $class = blessed($self) || $self;
187 my $method = $METHOD_CACHE{"$class|$caller|$label"} ||= do {
189 my @MRO = Class::C3::calculateMRO($class);
192 while ($current = shift @MRO) {
193 last if $caller eq $current;
198 foreach my $class (@MRO) {
199 next if (defined $Class::C3::MRO{$class} &&
200 defined $Class::C3::MRO{$class}{methods}{$label});
201 last if (defined ($found = *{$class . '::' . $label}{CODE}));
207 return $method if $indirect;
209 die "No next::method '$label' found for $self" if !$method;
214 sub can { method($_[0]) }
216 package # hide me from PAUSE
222 our $VERSION = '0.01';
224 sub method { (next::method($_[0]) || return)->(@_) }
234 Class::C3 - A pragma to use the C3 method resolution order algortihm
240 sub hello { 'A::hello' }
250 sub hello { 'C::hello' }
256 # Classic Diamond MI pattern
265 # initializez the C3 module
266 # (formerly called in INIT)
267 Class::C3::initialize();
269 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
271 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
273 D->can('hello')->(); # can() also works correctly
274 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
278 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
279 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
283 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
284 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
285 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
286 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
287 default MRO for Parrot objects as well.
289 =head2 How does C3 work.
291 C3 works by always preserving local precendence ordering. This essentially means that no class will
292 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
300 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
301 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
302 (D, B, C, A), which does not have this same issue.
304 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
305 the L<SEE ALSO> section.
307 =head2 How does this module work?
309 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
310 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
311 gathers information from the symbol tables of each of those classes, and builds a set of method
312 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
313 then adds the method aliases into the local classes symbol table.
315 The end result is actually classes with pre-cached method dispatch. However, this caching does not
316 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
317 your classes to be effectively closed. See the L<CAVEATS> section for more details.
319 =head1 OPTIONAL LOWERCASE PRAGMA
321 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
322 the regular install since lowercase module names are considered I<"bad"> by some people. However I
323 think that code looks much nicer like this:
333 But hey, it's your choice, thats why it is optional.
339 =item B<calculateMRO ($class)>
341 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
345 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
346 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
347 use C3. Here is a quick code example:
351 # ... Foo methods here
356 # ... Bar methods here
360 Class::C3::initialize(); # now it is safe to use Foo and Bar
362 This function used to be called automatically for you in the INIT phase of the perl compiler, but
363 that lead to warnings if this module was required at runtime. After discussion with my user base
364 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
365 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
366 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
367 your own INIT method which calls this function.
371 If C<initialize> detects that C<initialize> has already been executed, it will L</uninitialize> and
372 clear the MRO cache first.
374 =item B<uninitialize>
376 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
377 style dispatch order (depth-first, left-to-right).
379 =item B<reinitialize>
381 This is an alias for L</initialize> above.
385 =head1 METHOD REDISPATCHING
387 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
388 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
389 method along the C3 linearization. This is best show with an examples.
391 # a classic diamond MI pattern ...
405 sub foo { 'B::foo => ' . (shift)->next::method() }
410 sub foo { 'C::foo => ' . (shift)->next::method() }
415 sub foo { 'D::foo => ' . (shift)->next::method() }
417 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
419 A few things to note. First, we do not require you to add on the method name to the C<next::method>
420 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
421 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
423 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
424 not automatically use the current C<@_>.
426 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
427 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
429 $self->next::method(@_) if $self->next::can;
431 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
432 The previous example could be simply written as:
434 $self->maybe::next::method(@_);
436 There are some caveats about using C<next::method>, see below for those.
440 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
441 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
442 whatever your needs might be.
444 But there are still caveats, so here goes ...
448 =item Use of C<SUPER::>.
450 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
451 However, it's use in conjuntion with this module is very much not recommended, and in fact very
452 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
453 more details on it's usage above.
455 =item Changing C<@ISA>.
457 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
458 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
459 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
460 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
461 in F<t/20_reinitialize.t> for more information.
463 =item Adding/deleting methods from class symbol tables.
465 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
466 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
467 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
468 changes you make to take effect.
470 =item Calling C<next::method> from methods defined outside the class
472 There is an edge case when using C<next::method> from within a subroutine which was created in a different
473 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
474 will not work correctly:
476 *Foo::foo = sub { (shift)->next::method(@_) };
478 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
479 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
480 uses C<caller> to find the name of the method it was called in, it will fail in this case.
482 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
483 assign a name to an anonymous subroutine for you. Simply do this:
485 use Sub::Name 'subname';
486 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
488 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
489 manage to find a workaround for it, so until someone gives me a working patch this will be a known
490 limitation of this module.
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.