| Class::C3 - A pragma to use the C3 method resolution order algortihm |
Class::C3 - A pragma to use the C3 method resolution order algortihm
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()
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.
C3 is the name of an algorithm which aims to provide a sane method resolution order under multiple inheritence. 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.
C3 works by always preserving local precendence ordering. This essentially means that no class will appear before any of it's subclasses. Take the classic diamond inheritence 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.
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.
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.
$class this will return an array of class names in the proper C3 method resolution order.
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 the DBIx::Class manpage 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 the DBIx::Class manpage 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.
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 show with an examples.
# 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 B;
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.
This module used to be labeled as experimental, however it has now been pretty heavily tested by the good folks over at the DBIx::Class manpage and I am confident this module is perfectly usable for whatever your needs might be.
But there are still caveats, so here goes ...
SUPER::.SUPER:: under multiple inheritence is ambigious, and generally not recomended anyway.
However, it's 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 it's usage above.
@ISA.@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 therefor 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/20_reinitialize.t for more information.
@ISA, you will need to call reinitialize for any
changes you make to take effect.
next::method from methods defined outside the classnext::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.
If your software requires Perl 5.9.5 or higher, you do not need the Class::C3 manpage, 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 the Class::C3 manpage as documented here. the Class::C3 manpage will detect Perl 5.9.5+ and take advantage of the core support when available.
This module will load the Class::C3::XS manpage if it's installed and you are running on a Perl version older than 5.9.5. Installing this is recommended when possible, as it results in significant performance improvements (but unlike the 5.9.5+ core support, it still has all of the same caveats as the Class::C3 manpage).
the Devel::Cover manpage 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.
next::method more robust by handling
calls inside eval and anon-subs.next::can and
maybe::next::method.
Stevan Little, <stevan@iinteractive.com>
Brandon L. Black, <blblack@gmail.com>
Copyright 2005, 2006 by Infinity Interactive, Inc.
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
| Class::C3 - A pragma to use the C3 method resolution order algortihm |