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Class::C3(3)                         User Contributed Perl Documentation                        Class::C3(3)



NAME
       Class::C3 - A pragma to use the C3 method resolution order algortihm

SYNOPSIS
           # NOTE - DO NOT USE Class::C3 directly as a user, use MRO::Compat instead!
           package A;
           use Class::C3;
           sub hello { 'A::hello' }

           package B;
           use base 'A';
           use Class::C3;

           package C;
           use base 'A';
           use Class::C3;

           sub hello { 'C::hello' }

           package D;
           use base ('B', 'C');
           use Class::C3;

           # Classic Diamond MI pattern
           #    <A>
           #   /   \
           # <B>   <C>
           #   \   /
           #    <D>

           package main;

           # initializez the C3 module
           # (formerly called in INIT)
           Class::C3::initialize();

           print join ', ' => Class::C3::calculateMRO('Diamond_D') # prints D, B, C, A

           print D->hello() # prints 'C::hello' instead of the standard p5 'A::hello'

           D->can('hello')->();          # can() also works correctly
           UNIVERSAL::can('D', 'hello'); # as does UNIVERSAL::can()

DESCRIPTION
       This is pragma to change Perl 5's standard method resolution order from depth-first left-to-right
       (a.k.a - pre-order) to the more sophisticated C3 method resolution order.

       NOTE: YOU SHOULD NOT USE THIS MODULE DIRECTLY - The feature provided is integrated into perl version
       >= 5.9.5, and you should use MRO::Compat instead, which will use the core implementation in newer
       perls, but fallback to using this implementation on older perls.

   What is C3?
       C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple
       inheritance. It was first introduced in the langauge Dylan (see links in the "SEE ALSO" section), and
       then later adopted as the prefered MRO (Method Resolution Order) for the new-style classes in Python
       2.3. Most recently it has been adopted as the 'canonical' MRO for Perl 6 classes, and the default MRO
       for Parrot objects as well.

   How does C3 work.
       C3 works by always preserving local precendence ordering. This essentially means that no class will
       appear before any of its subclasses. Take the classic diamond inheritance pattern for instance:

            <A>
           /   \
         <B>   <C>
           \   /
            <D>

       The standard Perl 5 MRO would be (D, B, A, C). The result being that A appears before C, even though
       C is the subclass of A. The C3 MRO algorithm however, produces the following MRO (D, B, C, A), which
       does not have this same issue.

       This example is fairly trival, for more complex examples and a deeper explaination, see the links in
       the "SEE ALSO" section.

   How does this module work?
       This module uses a technique similar to Perl 5's method caching. When "Class::C3::initialize" is
       called, this module calculates the MRO of all the classes which called "use Class::C3". It then
       gathers information from the symbol tables of each of those classes, and builds a set of method
       aliases for the correct dispatch ordering. Once all these C3-based method tables are created, it then
       adds the method aliases into the local classes symbol table.

       The end result is actually classes with pre-cached method dispatch. However, this caching does not do
       well if you start changing your @ISA or messing with class symbol tables, so you should consider your
       classes to be effectively closed. See the CAVEATS section for more details.

OPTIONAL LOWERCASE PRAGMA
       This release also includes an optional module c3 in the opt/ folder. I did not include this in the
       regular install since lowercase module names are considered "bad" by some people. However I think
       that code looks much nicer like this:

         package MyClass;
         use c3;

       The the more clunky:

         package MyClass;
         use Class::C3;

       But hey, it's your choice, thats why it is optional.

FUNCTIONS
       calculateMRO ($class)
           Given a $class this will return an array of class names in the proper C3 method resolution order.

       initialize
           This must be called to initalize the C3 method dispatch tables, this module will not work if you
           do not do this. It is advised to do this as soon as possible after loading any classes which use
           C3. Here is a quick code example:

             package Foo;
             use Class::C3;
             # ... Foo methods here

             package Bar;
             use Class::C3;
             use base 'Foo';
             # ... Bar methods here

             package main;

             Class::C3::initialize(); # now it is safe to use Foo and Bar

           This function used to be called automatically for you in the INIT phase of the perl compiler, but
           that lead to warnings if this module was required at runtime. After discussion with my user base
           (the DBIx::Class folks), we decided that calling this in INIT was more of an annoyance than a
           convience. I apologize to anyone this causes problems for (although i would very suprised if I
           had any other users other than the DBIx::Class folks). The simplest solution of course is to
           define your own INIT method which calls this function.

           NOTE:

           If "initialize" detects that "initialize" has already been executed, it will "uninitialize" and
           clear the MRO cache first.

       uninitialize
           Calling this function results in the removal of all cached methods, and the restoration of the
           old Perl 5 style dispatch order (depth-first, left-to-right).

       reinitialize
           This is an alias for "initialize" above.

METHOD REDISPATCHING
       It is always useful to be able to re-dispatch your method call to the "next most applicable method".
       This module provides a pseudo package along the lines of "SUPER::" or "NEXT::" which will re-dispatch
       the method along the C3 linearization. This is best shown with an example.

         # a classic diamond MI pattern ...
         #    <A>
         #   /   \
         # <B>   <C>
         #   \   /
         #    <D>

         package A;
         use c3;
         sub foo { 'A::foo' }

         package B;
         use base 'A';
         use c3;
         sub foo { 'B::foo => ' . (shift)->next::method() }

         package C;
         use base 'A';
         use c3;
         sub foo { 'C::foo => ' . (shift)->next::method() }

         package D;
         use base ('B', 'C');
         use c3;
         sub foo { 'D::foo => ' . (shift)->next::method() }

         print D->foo; # prints out "D::foo => B::foo => C::foo => A::foo"

       A few things to note. First, we do not require you to add on the method name to the "next::method"
       call (this is unlike "NEXT::" and "SUPER::" which do require that). This helps to enforce the rule
       that you cannot dispatch to a method of a different name (this is how "NEXT::" behaves as well).

       The next thing to keep in mind is that you will need to pass all arguments to "next::method".  It can
       not automatically use the current @_.

       If "next::method" cannot find a next method to re-dispatch the call to, it will throw an exception.
       You can use "next::can" to see if "next::method" will succeed before you call it like so:

         $self->next::method(@_) if $self->next::can;

       Additionally, you can use "maybe::next::method" as a shortcut to only call the next method if it
       exists.  The previous example could be simply written as:

         $self->maybe::next::method(@_);

       There are some caveats about using "next::method", see below for those.

CAVEATS
       This module used to be labeled as experimental, however it has now been pretty heavily tested by the
       good folks over at DBIx::Class and I am confident this module is perfectly usable for whatever your
       needs might be.

       But there are still caveats, so here goes ...

       Use of "SUPER::".
           The idea of "SUPER::" under multiple inheritance is ambigious, and generally not recomended
           anyway.  However, its use in conjuntion with this module is very much not recommended, and in
           fact very discouraged. The recommended approach is to instead use the supplied "next::method"
           feature, see more details on its usage above.

       Changing @ISA.
           It is the author's opinion that changing @ISA at runtime is pure insanity anyway. However, people
           do it, so I must caveat. Any changes to the @ISA will not be reflected in the MRO calculated by
           this module, and therefore probably won't even show up. If you do this, you will need to call
           "reinitialize" in order to recalulate all method dispatch tables. See the "reinitialize"
           documentation and an example in t/2__reinitialize.t for more information.

       Adding/deleting methods from class symbol tables.
           This module calculates the MRO for each requested class by interogating the symbol tables of said
           classes.  So any symbol table manipulation which takes place after our INIT phase is run will not
           be reflected in the calculated MRO. Just as with changing the @ISA, you will need to call
           "reinitialize" for any changes you make to take effect.

       Calling "next::method" from methods defined outside the class
           There is an edge case when using "next::method" from within a subroutine which was created in a
           different module than the one it is called from. It sounds complicated, but it really isn't. Here
           is an example which will not work correctly:

             *Foo::foo = sub { (shift)->next::method(@_) };

           The problem exists because the anonymous subroutine being assigned to the glob *Foo::foo will
           show up in the call stack as being called "__ANON__" and not "foo" as you might expect. Since
           "next::method" uses "caller" to find the name of the method it was called in, it will fail in
           this case.

           But fear not, there is a simple solution. The module "Sub::Name" will reach into the perl
           internals and assign a name to an anonymous subroutine for you. Simply do this:

             use Sub::Name 'subname';
             *Foo::foo = subname 'Foo::foo' => sub { (shift)->next::method(@_) };

           and things will Just Work. Of course this is not always possible to do, but to be honest, I just
           can't manage to find a workaround for it, so until someone gives me a working patch this will be
           a known limitation of this module.

COMPATIBILITY
       If your software requires Perl 5.9.5 or higher, you do not need Class::C3, you can simply "use mro
       'c3'", and not worry about "initialize()", avoid some of the above caveats, and get the best possible
       performance.  See mro for more details.

       If your software is meant to work on earlier Perls, use Class::C3 as documented here.  Class::C3 will
       detect Perl 5.9.5+ and take advantage of the core support when available.

Class::C3::XS
       This module will load Class::C3::XS if it's installed and you are running on a Perl version older
       than 5.9.5.  The optional module will be automatically installed for you if a C compiler is
       available, as it results in significant performance improvements (but unlike the 5.9.5+ core support,
       it still has all of the same caveats as Class::C3).

CODE COVERAGE
       Devel::Cover was reporting 94.4% overall test coverage earlier in this module's life.  Currently, the
       test suite does things that break under coverage testing, but it is fair to assume the coverage is
       still close to that value.

SEE ALSO
   The original Dylan paper
       http://www.webcom.com/haahr/dylan/linearization-oopsla96.html
       <http://www.webcom.com/haahr/dylan/linearization-oopsla96.html>

   The prototype Perl 6 Object Model uses C3
       http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/
       <http://svn.openfoundry.org/pugs/perl5/Perl6-MetaModel/>

   Parrot now uses C3
       http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631
       <http://aspn.activestate.com/ASPN/Mail/Message/perl6-internals/2746631>
       <http://use.perl.org/~autrijus/journal/25768>

   Python 2.3 MRO related links
       <http://www.python.org/2.3/mro.html>
       <http://www.python.org/2.2.2/descrintro.html#mro>

   C3 for TinyCLOS
       http://www.call-with-current-continuation.org/eggs/c3.html <http://www .call-with-current-
       continuation.org/eggs/c3.html>

ACKNOWLEGEMENTS
       Thanks to Matt S. Trout for using this module in his module DBIx::Class and finding many bugs and
       providing fixes.
       Thanks to Justin Guenther for making "next::method" more robust by handling calls inside "eval" and
       anon-subs.
       Thanks to Robert Norris for adding support for "next::can" and "maybe::next::method".

AUTHOR
       Stevan Little, <stevan@iinteractive.com>

       Brandon L. Black, <blblack@gmail.com>

COPYRIGHT AND LICENSE
       Copyright 2005, 2006 by Infinity Interactive, Inc.

       <http://www.iinteractive.com>

       This library is free software; you can redistribute it and/or modify it under the same terms as Perl
       itself.



perl v5.16.2                                     2012-05-12                                     Class::C3(3)