mro

mro(3perl)             Perl Programmers Reference Guide             mro(3perl)

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://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.19.3910&rep=rep1&type=pdf>

   Pugs
       The Pugs prototype Perl 6 Object Model uses C3

   Parrot
       Parrot now uses C3

       <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>

   Class::C3
       Class::C3

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

       Based on Stevan Little's Class::C3

perl v5.30.0                      2023-11-23                        mro(3perl)