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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 algorithm

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

	   package ClassB;
	   use base 'ClassA';
	   use Class::C3;

	   package ClassC;
	   use base 'ClassA';
	   use Class::C3;

	   sub hello { 'C::hello' }

	   package ClassD;
	   use base ('ClassB', 'ClassC');
	   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('ClassD'); # prints ClassD, ClassB, ClassC, ClassA

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

	   ClassD->can('hello')->();	      #	can() also works correctly
	   UNIVERSAL::can('ClassD', '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 language Dylan (see links in	the "SEE ALSO" section), and then
       later adopted as	the preferred 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 precedence 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 trivial, for more	complex	examples and a deeper
       explanation, 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;

       This is more clunky:

	 package MyClass;
	 use Class::C3;

       But hey,	it's your choice, that's 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 initialize 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 convenience. I apologize	to anyone this
	   causes problems for (although I would be very surprised 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 ClassA;
	 use Class::C3;
	 sub foo { 'ClassA::foo' }

	 package ClassB;
	 use base 'ClassA';
	 use Class::C3;
	 sub foo { 'ClassB::foo	=> ' . (shift)->next::method() }

	 package ClassC;
	 use base 'ClassA';
	 use Class::C3;
	 sub foo { 'ClassC::foo	=> ' . (shift)->next::method() }

	 package ClassD;
	 use base ('ClassB', 'ClassC');
	 use Class::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 ambiguous, and
	   generally not recommended anyway.  However, its use in conjunction
	   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 recalculate
	   all method dispatch tables. See the "reinitialize" documentation
	   and an example in t/20_reinitialize.t for more information.

       Adding/deleting methods from class symbol tables.
	   This	module calculates the MRO for each requested class by
	   interrogating 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
       <https://web.archive.org/web/20000817033012id_/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/>

   Parrot now uses C3
       <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>

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 v5.24.1			  2017-04-23			  Class::C3(3)

NAME | SYNOPSIS | DESCRIPTION | OPTIONAL LOWERCASE PRAGMA | FUNCTIONS | METHOD REDISPATCHING | CAVEATS | COMPATIBILITY | Class::C3::XS | CODE COVERAGE | SEE ALSO | ACKNOWLEGEMENTS | AUTHOR | COPYRIGHT AND LICENSE

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