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 { 'ClassD::foo => ' . (shift)->next::method() }
print ClassD->foo; # prints out "ClassD::foo => ClassB::foo => ClassC::foo => ClassA::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/haa
hr/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.