NAME¶
mro - Method Resolution Order
SYNOPSIS¶
use mro; # enables next::method and friends globally
use mro 'dfs'; # enable DFS MRO for this class (Perl default)
use mro 'c3'; # enable C3 MRO for this class
DESCRIPTION¶
The "mro" namespace provides several utilities for dealing with method
resolution order and method caching in general.
These interfaces are only available in Perl 5.9.5 and higher. See MRO::Compat on
CPAN for a mostly forwards compatible implementation for older Perls.
OVERVIEW¶
It's possible to change the MRO of a given class either by using "use
mro" as shown in the synopsis, or by using the "mro::set_mro"
function below.
The special methods "next::method", "next::can", and
"maybe::next::method" are not available until this "mro"
module has been loaded via "use" or "require".
The C3 MRO¶
In addition to the traditional Perl default MRO (depth first search, called
"DFS" here), Perl now offers the C3 MRO as well. Perl's support for
C3 is based on the work done in Stevan Little's module Class::C3, and most of
the C3-related documentation here is ripped directly from there.
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, for instance,
the classic diamond inheritance pattern:
<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 order: (D, B, C, A), which
does not have this issue.
This example is fairly trivial; for more complex cases and a deeper explanation,
see the links in the "SEE ALSO" section.
Functions¶
mro::get_linear_isa($classname[, $type])¶
Returns an arrayref which is the linearized MRO of the given class. Uses
whichever MRO is currently in effect for that class by default, or the given
MRO (either "c3" or "dfs" if specified as $type).
The linearized MRO of a class is an ordered array of all of the classes one
would search when resolving a method on that class, starting with the class
itself.
If the requested class doesn't yet exist, this function will still succeed, and
return "[ $classname ]"
Note that "UNIVERSAL" (and any members of "UNIVERSAL"'s MRO)
are not part of the MRO of a class, even though all classes implicitly inherit
methods from "UNIVERSAL" and its parents.
mro::set_mro ($classname, $type)¶
Sets the MRO of the given class to the $type argument (either "c3" or
"dfs").
mro::get_mro($classname)¶
Returns the MRO of the given class (either "c3" or "dfs").
mro::get_isarev($classname)¶
Gets the "mro_isarev" for this class, returned as an arrayref of class
names. These are every class that "isa" the given class name, even
if the isa relationship is indirect. This is used internally by the MRO code
to keep track of method/MRO cache invalidations.
As with "mro::get_linear_isa" above, "UNIVERSAL" is special.
"UNIVERSAL" (and parents') isarev lists do not include every class
in existence, even though all classes are effectively descendants for method
inheritance purposes.
mro::is_universal($classname)¶
Returns a boolean status indicating whether or not the given classname is either
"UNIVERSAL" itself, or one of "UNIVERSAL"'s parents by
@ISA inheritance.
Any class for which this function returns true is "universal" in the
sense that all classes potentially inherit methods from it.
mro::invalidate_all_method_caches()¶
Increments "PL_sub_generation", which invalidates method caching in
all packages.
mro::method_changed_in($classname)¶
Invalidates the method cache of any classes dependent on the given class. This
is not normally necessary. The only known case where pure perl code can
confuse the method cache is when you manually install a new constant
subroutine by using a readonly scalar value, like the internals of constant
do. If you find another case, please report it so we can either fix it or
document the exception here.
mro::get_pkg_gen($classname)¶
Returns an integer which is incremented every time a real local method in the
package $classname changes, or the local @ISA of $classname is modified.
This is intended for authors of modules which do lots of class introspection, as
it allows them to very quickly check if anything important about the local
properties of a given class have changed since the last time they looked. It
does not increment on method/@ISA changes in superclasses.
It's still up to you to seek out the actual changes, and there might not
actually be any. Perhaps all of the changes since you last checked cancelled
each other out and left the package in the state it was in before.
This integer normally starts off at a value of 1 when a package stash is
instantiated. Calling it on packages whose stashes do not exist at all will
return 0. If a package stash is completely deleted (not a normal occurrence,
but it can happen if someone does something like "undef
%PkgName::"), the number will be reset to either 0 or 1, depending on how
completely the package was wiped out.
next::method¶
This is somewhat like "SUPER", but it uses the C3 method resolution
order to get better consistency in multiple inheritance situations. Note that
while inheritance in general follows whichever MRO is in effect for the given
class, "next::method" only uses the C3 MRO.
One generally uses it like so:
sub some_method {
my $self = shift;
my $superclass_answer = $self->next::method(@_);
return $superclass_answer + 1;
}
Note that you don't (re-)specify the method name. It forces you to always use
the same method name as the method you started in.
It can be called on an object or a class, of course.
The way it resolves which actual method to call is:
- 1.
- First, it determines the linearized C3 MRO of the object or class it is
being called on.
- 2.
- Then, it determines the class and method name of the context it was
invoked from.
- 3.
- Finally, it searches down the C3 MRO list until it reaches the
contextually enclosing class, then searches further down the MRO list for
the next method with the same name as the contextually enclosing
method.
Failure to find a next method will result in an exception being thrown (see
below for alternatives).
This is substantially different than the behavior of "SUPER" under
complex multiple inheritance. (This becomes obvious when one realizes that the
common superclasses in the C3 linearizations of a given class and one of its
parents will not always be ordered the same for both.)
Caveat: 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
*Foo::foo glob 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's 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.
next::can¶
This is similar to "next::method", but just returns either a code
reference or "undef" to indicate that no further methods of this
name exist.
maybe::next::method¶
In simple cases, it is equivalent to:
$self->next::method(@_) if $self->next::can;
But there are some cases where only this solution works (like "goto
&maybe::next::method");
SEE ALSO¶
The original Dylan paper¶
- <http://haahr.tempdomainname.com/dylan/linearization-oopsla96.html>
Pugs¶
The Pugs prototype Perl 6 Object Model uses C3
Parrot¶
Parrot now uses C3
- <http://use.perl.org/~autrijus/journal/25768>
- <http://www.python.org/2.3/mro.html>
- <http://www.python.org/2.2.2/descrintro.html#mro>
Class::C3¶
- Class::C3
AUTHOR¶
Brandon L. Black, <blblack@gmail.com>
Based on Stevan Little's Class::C3