1Class::C3(3) User Contributed Perl Documentation Class::C3(3)
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6 Class::C3 - A pragma to use the C3 method resolution order algorithm
7
9 # NOTE - DO NOT USE Class::C3 directly as a user, use MRO::Compat instead!
10 package ClassA;
11 use Class::C3;
12 sub hello { 'A::hello' }
13 package ClassB;
15 use base 'ClassA';
16 use Class::C3;
17 package ClassC;
19 use base 'ClassA';
20 use Class::C3;
21 sub hello { 'C::hello' }
23 package ClassD;
25 use base ('ClassB', 'ClassC');
26 use Class::C3;
27 # Classic Diamond MI pattern
29 # <A>
30 # / \
31 # <B> <C>
32 # \ /
33 # <D>
34 package main;
36 # initializez the C3 module
38 # (formerly called in INIT)
39 Class::C3::initialize();
40 print join ', ' => Class::C3::calculateMRO('ClassD'); # prints ClassD, ClassB, ClassC, ClassA
42 print ClassD->hello(); # prints 'C::hello' instead of the standard p5 'A::hello'
44 ClassD->can('hello')->(); # can() also works correctly
46 UNIVERSAL::can('ClassD', 'hello'); # as does UNIVERSAL::can()
47
49 This is pragma to change Perl 5's standard method resolution order from
50 depth-first left-to-right (a.k.a - pre-order) to the more sophisticated
51 C3 method resolution order.
52 NOTE: YOU SHOULD NOT USE THIS MODULE DIRECTLY - The feature provided is
54 integrated into perl version >= 5.9.5, and you should use MRO::Compat
55 instead, which will use the core implementation in newer perls, but
56 fallback to using this implementation on older perls.
57 What is C3?
59 C3 is the name of an algorithm which aims to provide a sane method
60 resolution order under multiple inheritance. It was first introduced in
61 the language Dylan (see links in the "SEE ALSO" section), and then
62 later adopted as the preferred MRO (Method Resolution Order) for the
63 new-style classes in Python 2.3. Most recently it has been adopted as
64 the 'canonical' MRO for Perl 6 classes, and the default MRO for Parrot
65 objects as well.
66 How does C3 work.
68 C3 works by always preserving local precedence ordering. This
69 essentially means that no class will appear before any of its
70 subclasses. Take the classic diamond inheritance pattern for instance:
71 <A>
73 / \
74 <B> <C>
75 \ /
76 <D>
77 The standard Perl 5 MRO would be (D, B, A, C). The result being that A
79 appears before C, even though C is the subclass of A. The C3 MRO
80 algorithm however, produces the following MRO (D, B, C, A), which does
81 not have this same issue.
82 This example is fairly trivial, for more complex examples and a deeper
84 explanation, see the links in the "SEE ALSO" section.
85 How does this module work?
87 This module uses a technique similar to Perl 5's method caching. When
88 "Class::C3::initialize" is called, this module calculates the MRO of
89 all the classes which called "use Class::C3". It then gathers
90 information from the symbol tables of each of those classes, and builds
91 a set of method aliases for the correct dispatch ordering. Once all
92 these C3-based method tables are created, it then adds the method
93 aliases into the local classes symbol table.
94 The end result is actually classes with pre-cached method dispatch.
96 However, this caching does not do well if you start changing your @ISA
97 or messing with class symbol tables, so you should consider your
98 classes to be effectively closed. See the CAVEATS section for more
99 details.
100
102 This release also includes an optional module c3 in the opt/ folder. I
103 did not include this in the regular install since lowercase module
104 names are considered "bad" by some people. However I think that code
105 looks much nicer like this:
106 package MyClass;
108 use c3;
109 This is more clunky:
111 package MyClass;
113 use Class::C3;
114 But hey, it's your choice, that's why it is optional.
116
118 calculateMRO ($class)
119 Given a $class this will return an array of class names in the
120 proper C3 method resolution order.
121 initialize
123 This must be called to initialize the C3 method dispatch tables,
124 this module will not work if you do not do this. It is advised to
125 do this as soon as possible after loading any classes which use C3.
126 Here is a quick code example:
127 package Foo;
129 use Class::C3;
130 # ... Foo methods here
131 package Bar;
133 use Class::C3;
134 use base 'Foo';
135 # ... Bar methods here
136 package main;
138 Class::C3::initialize(); # now it is safe to use Foo and Bar
140 This function used to be called automatically for you in the INIT
142 phase of the perl compiler, but that lead to warnings if this
143 module was required at runtime. After discussion with my user base
144 (the DBIx::Class folks), we decided that calling this in INIT was
145 more of an annoyance than a convenience. I apologize to anyone this
146 causes problems for (although I would be very surprised if I had
147 any other users other than the DBIx::Class folks). The simplest
148 solution of course is to define your own INIT method which calls
149 this function.
150 NOTE:
152 If "initialize" detects that "initialize" has already been
154 executed, it will "uninitialize" and clear the MRO cache first.
155 uninitialize
157 Calling this function results in the removal of all cached methods,
158 and the restoration of the old Perl 5 style dispatch order (depth-
159 first, left-to-right).
160 reinitialize
162 This is an alias for "initialize" above.
163
165 It is always useful to be able to re-dispatch your method call to the
166 "next most applicable method". This module provides a pseudo package
167 along the lines of "SUPER::" or "NEXT::" which will re-dispatch the
168 method along the C3 linearization. This is best shown with an example.
169 # a classic diamond MI pattern ...
171 # <A>
172 # / \
173 # <B> <C>
174 # \ /
175 # <D>
176 package ClassA;
178 use Class::C3;
179 sub foo { 'ClassA::foo' }
180 package ClassB;
182 use base 'ClassA';
183 use Class::C3;
184 sub foo { 'ClassB::foo => ' . (shift)->next::method() }
185 package ClassC;
187 use base 'ClassA';
188 use Class::C3;
189 sub foo { 'ClassC::foo => ' . (shift)->next::method() }
190 package ClassD;
192 use base ('ClassB', 'ClassC');
193 use Class::C3;
194 sub foo { 'ClassD::foo => ' . (shift)->next::method() }
195 print ClassD->foo; # prints out "ClassD::foo => ClassB::foo => ClassC::foo => ClassA::foo"
197 A few things to note. First, we do not require you to add on the method
199 name to the "next::method" call (this is unlike "NEXT::" and "SUPER::"
200 which do require that). This helps to enforce the rule that you cannot
201 dispatch to a method of a different name (this is how "NEXT::" behaves
202 as well).
203 The next thing to keep in mind is that you will need to pass all
205 arguments to "next::method". It can not automatically use the current
206 @_.
207 If "next::method" cannot find a next method to re-dispatch the call to,
209 it will throw an exception. You can use "next::can" to see if
210 "next::method" will succeed before you call it like so:
211 $self->next::method(@_) if $self->next::can;
213 Additionally, you can use "maybe::next::method" as a shortcut to only
215 call the next method if it exists. The previous example could be
216 simply written as:
217 $self->maybe::next::method(@_);
219 There are some caveats about using "next::method", see below for those.
221
223 This module used to be labeled as experimental, however it has now been
224 pretty heavily tested by the good folks over at DBIx::Class and I am
225 confident this module is perfectly usable for whatever your needs might
226 be.
227 But there are still caveats, so here goes ...
229 Use of "SUPER::".
231 The idea of "SUPER::" under multiple inheritance is ambiguous, and
232 generally not recommended anyway. However, its use in conjunction
233 with this module is very much not recommended, and in fact very
234 discouraged. The recommended approach is to instead use the
235 supplied "next::method" feature, see more details on its usage
236 above.
237 Changing @ISA.
239 It is the author's opinion that changing @ISA at runtime is pure
240 insanity anyway. However, people do it, so I must caveat. Any
241 changes to the @ISA will not be reflected in the MRO calculated by
242 this module, and therefore probably won't even show up. If you do
243 this, you will need to call "reinitialize" in order to recalculate
244 all method dispatch tables. See the "reinitialize" documentation
245 and an example in t/20_reinitialize.t for more information.
246 Adding/deleting methods from class symbol tables.
248 This module calculates the MRO for each requested class by
249 interrogating the symbol tables of said classes. So any symbol
250 table manipulation which takes place after our INIT phase is run
251 will not be reflected in the calculated MRO. Just as with changing
252 the @ISA, you will need to call "reinitialize" for any changes you
253 make to take effect.
254 Calling "next::method" from methods defined outside the class
256 There is an edge case when using "next::method" from within a
257 subroutine which was created in a different module than the one it
258 is called from. It sounds complicated, but it really isn't. Here is
259 an example which will not work correctly:
260 *Foo::foo = sub { (shift)->next::method(@_) };
262 The problem exists because the anonymous subroutine being assigned
264 to the glob *Foo::foo will show up in the call stack as being
265 called "__ANON__" and not "foo" as you might expect. Since
266 "next::method" uses "caller" to find the name of the method it was
267 called in, it will fail in this case.
268 But fear not, there is a simple solution. The module "Sub::Name"
270 will reach into the perl internals and assign a name to an
271 anonymous subroutine for you. Simply do this:
272 use Sub::Name 'subname';
274 *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };
275 and things will Just Work. Of course this is not always possible to
277 do, but to be honest, I just can't manage to find a workaround for
278 it, so until someone gives me a working patch this will be a known
279 limitation of this module.
280
282 If your software requires Perl 5.9.5 or higher, you do not need
283 Class::C3, you can simply "use mro 'c3'", and not worry about
284 "initialize()", avoid some of the above caveats, and get the best
285 possible performance. See mro for more details.
286 If your software is meant to work on earlier Perls, use Class::C3 as
288 documented here. Class::C3 will detect Perl 5.9.5+ and take advantage
289 of the core support when available.
290
292 This module will load Class::C3::XS if it's installed and you are
293 running on a Perl version older than 5.9.5. The optional module will
294 be automatically installed for you if a C compiler is available, as it
295 results in significant performance improvements (but unlike the 5.9.5+
296 core support, it still has all of the same caveats as Class::C3).
297
299 Devel::Cover was reporting 94.4% overall test coverage earlier in this
300 module's life. Currently, the test suite does things that break under
301 coverage testing, but it is fair to assume the coverage is still close
302 to that value.
303
305 The original Dylan paper
306 <https://web.archive.org/web/20000817033012id_/http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>
307 The prototype Perl 6 Object Model uses C3
309 <http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>
310 Parrot now uses C3
312 <http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
313 <http://use.perl.org/~autrijus/journal/25768>
314 Python 2.3 MRO related links
316 <http://www.python.org/2.3/mro.html>
317 <http://www.python.org/2.2.2/descrintro.html#mro>
318 C3 for TinyCLOS
320 <http://www.call-with-current-continuation.org/eggs/c3.html>
321
323 Thanks to Matt S. Trout for using this module in his module DBIx::Class
324 and finding many bugs and providing fixes.
325 Thanks to Justin Guenther for making "next::method" more robust by
326 handling calls inside "eval" and anon-subs.
327 Thanks to Robert Norris for adding support for "next::can" and
328 "maybe::next::method".
329
331 Stevan Little, <stevan@iinteractive.com>
332 Brandon L. Black, <blblack@gmail.com>
334
336 Copyright 2005, 2006 by Infinity Interactive, Inc.
337 <http://www.iinteractive.com>
339 This library is free software; you can redistribute it and/or modify it
341 under the same terms as Perl itself.
342perl v5.36.0 2022-07-22 Class::C3(3)