7 use Scalar::Util 'blessed';
10 our $VERSION = '0.12';
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 }
34 # skip if the caller is main::
35 # since that is clearly not relevant
36 return if $class eq 'main';
37 return if $TURN_OFF_C3;
38 # make a note to calculate $class
40 $MRO{$class} = undef unless exists $MRO{$class};
46 # why bother if we don't have anything ...
47 return unless keys %MRO;
48 _calculate_method_dispatch_tables();
49 _apply_method_dispatch_tables();
50 %next::METHOD_CACHE = ();
54 # why bother if we don't have anything ...
55 return unless keys %MRO;
56 _remove_method_dispatch_tables();
57 %next::METHOD_CACHE = ();
62 # clean up the %MRO before we re-initialize
63 $MRO{$_} = undef foreach keys %MRO;
67 ## functions for applying C3 to classes
69 sub _calculate_method_dispatch_tables {
70 foreach my $class (keys %MRO) {
71 _calculate_method_dispatch_table($class);
75 sub _calculate_method_dispatch_table {
78 my @MRO = calculateMRO($class);
79 $MRO{$class} = { MRO => \@MRO };
80 my $has_overload_fallback = 0;
83 # we do @MRO[1 .. $#MRO] here because it
84 # makes no sense to interogate the class
85 # which you are calculating for.
86 foreach my $local (@MRO[1 .. $#MRO]) {
87 # if overload has tagged this module to
88 # have use "fallback", then we want to
90 $has_overload_fallback = ${"${local}::()"}
91 if defined ${"${local}::()"};
92 foreach my $method (grep { defined &{"${local}::$_"} } keys %{"${local}::"}) {
93 # skip if already overriden in local class
94 next unless !defined *{"${class}::$method"}{CODE};
96 orig => "${local}::$method",
97 code => \&{"${local}::$method"}
98 } unless exists $methods{$method};
101 # now stash them in our %MRO table
102 $MRO{$class}->{methods} = \%methods;
103 $MRO{$class}->{has_overload_fallback} = $has_overload_fallback;
106 sub _apply_method_dispatch_tables {
107 foreach my $class (keys %MRO) {
108 _apply_method_dispatch_table($class);
112 sub _apply_method_dispatch_table {
115 ${"${class}::()"} = $MRO{$class}->{has_overload_fallback}
116 if $MRO{$class}->{has_overload_fallback};
117 foreach my $method (keys %{$MRO{$class}->{methods}}) {
118 *{"${class}::$method"} = $MRO{$class}->{methods}->{$method}->{code};
122 sub _remove_method_dispatch_tables {
123 foreach my $class (keys %MRO) {
124 _remove_method_dispatch_table($class);
128 sub _remove_method_dispatch_table {
131 delete ${"${class}::"}{"()"} if $MRO{$class}->{has_overload_fallback};
132 foreach my $method (keys %{$MRO{$class}->{methods}}) {
133 delete ${"${class}::"}{$method}
134 if defined *{"${class}::${method}"}{CODE} &&
135 (*{"${class}::${method}"}{CODE} eq $MRO{$class}->{methods}->{$method}->{code});
139 ## functions for calculating C3 MRO
143 return Algorithm::C3::merge($class, sub {
149 package # hide me from PAUSE
155 use Scalar::Util 'blessed';
157 our $VERSION = '0.05';
162 my $indirect = caller() =~ /^(?:next|maybe::next)$/;
163 my $level = $indirect ? 2 : 1;
165 my ($method_caller, $label, @label);
166 while ($method_caller = (caller($level++))[3]) {
167 @label = (split '::', $method_caller);
170 $label eq '(eval)' ||
171 $label eq '__ANON__';
173 my $caller = join '::' => @label;
175 my $class = blessed($self) || $self;
177 my $method = $METHOD_CACHE{"$class|$caller|$label"} ||= do {
179 my @MRO = Class::C3::calculateMRO($class);
182 while ($current = shift @MRO) {
183 last if $caller eq $current;
188 foreach my $class (@MRO) {
189 next if (defined $Class::C3::MRO{$class} &&
190 defined $Class::C3::MRO{$class}{methods}{$label});
191 last if (defined ($found = *{$class . '::' . $label}{CODE}));
197 return $method if $indirect;
199 die "No next::method '$label' found for $self" if !$method;
204 sub can { method($_[0]) }
206 package # hide me from PAUSE
212 our $VERSION = '0.01';
214 sub method { (next::method($_[0]) || return)->(@_) }
224 Class::C3 - A pragma to use the C3 method resolution order algortihm
230 sub hello { 'A::hello' }
240 sub hello { 'C::hello' }
246 # Classic Diamond MI pattern
255 # initializez the C3 module
256 # (formerly called in INIT)
257 Class::C3::initialize();
259 print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A
261 print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'
263 D->can('hello')->(); # can() also works correctly
264 UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()
268 This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
269 (a.k.a - pre-order) to the more sophisticated C3 method resolution order.
273 C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
274 inheritence. It was first introduced in the langauge Dylan (see links in the L<SEE ALSO> section),
275 and then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in
276 Python 2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the
277 default MRO for Parrot objects as well.
279 =head2 How does C3 work.
281 C3 works by always preserving local precendence ordering. This essentially means that no class will
282 appear before any of it's subclasses. Take the classic diamond inheritence pattern for instance:
290 The standard Perl 5 MRO would be (D, B, A, C). The result being that B<A> appears before B<C>, even
291 though B<C> is the subclass of B<A>. The C3 MRO algorithm however, produces the following MRO
292 (D, B, C, A), which does not have this same issue.
294 This example is fairly trival, for more complex examples and a deeper explaination, see the links in
295 the L<SEE ALSO> section.
297 =head2 How does this module work?
299 This module uses a technique similar to Perl 5's method caching. When C<Class::C3::initialize> is
300 called, this module calculates the MRO of all the classes which called C<use Class::C3>. It then
301 gathers information from the symbol tables of each of those classes, and builds a set of method
302 aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it
303 then adds the method aliases into the local classes symbol table.
305 The end result is actually classes with pre-cached method dispatch. However, this caching does not
306 do well if you start changing your C<@ISA> or messing with class symbol tables, so you should consider
307 your classes to be effectively closed. See the L<CAVEATS> section for more details.
309 =head1 OPTIONAL LOWERCASE PRAGMA
311 This release also includes an optional module B<c3> in the F<opt/> folder. I did not include this in
312 the regular install since lowercase module names are considered I<"bad"> by some people. However I
313 think that code looks much nicer like this:
323 But hey, it's your choice, thats why it is optional.
329 =item B<calculateMRO ($class)>
331 Given a C<$class> this will return an array of class names in the proper C3 method resolution order.
335 This B<must be called> to initalize the C3 method dispatch tables, this module B<will not work> if
336 you do not do this. It is advised to do this as soon as possible B<after> loading any classes which
337 use C3. Here is a quick code example:
341 # ... Foo methods here
346 # ... Bar methods here
350 Class::C3::initialize(); # now it is safe to use Foo and Bar
352 This function used to be called automatically for you in the INIT phase of the perl compiler, but
353 that lead to warnings if this module was required at runtime. After discussion with my user base
354 (the L<DBIx::Class> folks), we decided that calling this in INIT was more of an annoyance than a
355 convience. I apologize to anyone this causes problems for (although i would very suprised if I had
356 any other users other than the L<DBIx::Class> folks). The simplest solution of course is to define
357 your own INIT method which calls this function.
360 This can B<not> be used to re-load the dispatch tables for all classes. Use C<reinitialize> for that.
362 =item B<uninitialize>
364 Calling this function results in the removal of all cached methods, and the restoration of the old Perl 5
365 style dispatch order (depth-first, left-to-right).
367 =item B<reinitialize>
369 This effectively calls C<uninitialize> followed by C<initialize> the result of which is a reloading of
370 B<all> the calculated C3 dispatch tables.
372 It should be noted that if you have a large class library, this could potentially be a rather costly
377 =head1 METHOD REDISPATCHING
379 It is always useful to be able to re-dispatch your method call to the "next most applicable method". This
380 module provides a pseudo package along the lines of C<SUPER::> or C<NEXT::> which will re-dispatch the
381 method along the C3 linearization. This is best show with an examples.
383 # a classic diamond MI pattern ...
397 sub foo { 'B::foo => ' . (shift)->next::method() }
402 sub foo { 'C::foo => ' . (shift)->next::method() }
407 sub foo { 'D::foo => ' . (shift)->next::method() }
409 print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"
411 A few things to note. First, we do not require you to add on the method name to the C<next::method>
412 call (this is unlike C<NEXT::> and C<SUPER::> which do require that). This helps to enforce the rule
413 that you cannot dispatch to a method of a different name (this is how C<NEXT::> behaves as well).
415 The next thing to keep in mind is that you will need to pass all arguments to C<next::method> it can
416 not automatically use the current C<@_>.
418 If C<next::method> cannot find a next method to re-dispatch the call to, it will throw an exception.
419 You can use C<next::can> to see if C<next::method> will succeed before you call it like so:
421 $self->next::method(@_) if $self->next::can;
423 Additionally, you can use C<maybe::next::method> as a shortcut to only call the next method if it exists.
424 The previous example could be simply written as:
426 $self->maybe::next::method(@_);
428 There are some caveats about using C<next::method>, see below for those.
432 This module used to be labeled as I<experimental>, however it has now been pretty heavily tested by
433 the good folks over at L<DBIx::Class> and I am confident this module is perfectly usable for
434 whatever your needs might be.
436 But there are still caveats, so here goes ...
440 =item Use of C<SUPER::>.
442 The idea of C<SUPER::> under multiple inheritence is ambigious, and generally not recomended anyway.
443 However, it's use in conjuntion with this module is very much not recommended, and in fact very
444 discouraged. The recommended approach is to instead use the supplied C<next::method> feature, see
445 more details on it's usage above.
447 =item Changing C<@ISA>.
449 It is the author's opinion that changing C<@ISA> at runtime is pure insanity anyway. However, people
450 do it, so I must caveat. Any changes to the C<@ISA> will not be reflected in the MRO calculated by this
451 module, and therefor probably won't even show up. If you do this, you will need to call C<reinitialize>
452 in order to recalulate B<all> method dispatch tables. See the C<reinitialize> documentation and an example
453 in F<t/20_reinitialize.t> for more information.
455 =item Adding/deleting methods from class symbol tables.
457 This module calculates the MRO for each requested class by interogatting the symbol tables of said classes.
458 So any symbol table manipulation which takes place after our INIT phase is run will not be reflected in
459 the calculated MRO. Just as with changing the C<@ISA>, you will need to call C<reinitialize> for any
460 changes you make to take effect.
462 =item Calling C<next::method> from methods defined outside the class
464 There is an edge case when using C<next::method> from within a subroutine which was created in a different
465 module than the one it is called from. It sounds complicated, but it really isn't. Here is an example which
466 will not work correctly:
468 *Foo::foo = sub { (shift)->next::method(@_) };
470 The problem exists because the anonymous subroutine being assigned to the glob C<*Foo::foo> will show up
471 in the call stack as being called C<__ANON__> and not C<foo> as you might expect. Since C<next::method>
472 uses C<caller> to find the name of the method it was called in, it will fail in this case.
474 But fear not, there is a simple solution. The module C<Sub::Name> will reach into the perl internals and
475 assign a name to an anonymous subroutine for you. Simply do this:
477 use Sub::Name 'subname';
478 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
480 and things will Just Work. Of course this is not always possible to do, but to be honest, I just can't
481 manage to find a workaround for it, so until someone gives me a working patch this will be a known
482 limitation of this module.
488 I use B<Devel::Cover> to test the code coverage of my tests, below is the B<Devel::Cover> report on this
491 ---------------------------- ------ ------ ------ ------ ------ ------ ------
492 File stmt bran cond sub pod time total
493 ---------------------------- ------ ------ ------ ------ ------ ------ ------
494 Class/C3.pm 98.3 84.4 80.0 96.2 100.0 98.4 94.4
495 ---------------------------- ------ ------ ------ ------ ------ ------ ------
496 Total 98.3 84.4 80.0 96.2 100.0 98.4 94.4
497 ---------------------------- ------ ------ ------ ------ ------ ------ ------
501 =head2 The original Dylan paper
505 =item L<http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
509 =head2 The prototype Perl 6 Object Model uses C3
513 =item L<http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
517 =head2 Parrot now uses C3
521 =item L<http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
523 =item L<http://use.perl.org/~autrijus/journal/25768>
527 =head2 Python 2.3 MRO related links
531 =item L<http://www.python.org/2.3/mro.html>
533 =item L<http://www.python.org/2.2.2/descrintro.html#mro>
537 =head2 C3 for TinyCLOS
541 =item L<http://www.call-with-current-continuation.org/eggs/c3.html>
545 =head1 ACKNOWLEGEMENTS
549 =item Thanks to Matt S. Trout for using this module in his module L<DBIx::Class>
550 and finding many bugs and providing fixes.
552 =item Thanks to Justin Guenther for making C<next::method> more robust by handling
553 calls inside C<eval> and anon-subs.
559 Stevan Little, E<lt>stevan@iinteractive.comE<gt>
561 =head1 COPYRIGHT AND LICENSE
563 Copyright 2005, 2006 by Infinity Interactive, Inc.
565 L<http://www.iinteractive.com>
567 This library is free software; you can redistribute it and/or modify
568 it under the same terms as Perl itself.