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MRO::Compat - mro::* interface compatibility for Perls < 5.9.5


   package PPP;      use base qw/Exporter/;
   package X;        use base qw/PPP/;
   package Y;        use base qw/PPP/;
   package Z;        use base qw/PPP/;

   package FooClass; use base qw/X Y Z/;

   package main;
   use MRO::Compat;
   my $linear = mro::get_linear_isa('FooClass');
   print join(q{, }, @$linear);

   # Prints: FooClass, X, PPP, Exporter, Y, Z


The "mro" namespace provides several utilities for dealing with method resolution order and method caching in general in Perl 5.9.5 and higher.

This module provides those interfaces for earlier versions of Perl (back to 5.6.0 anyways).

It is a harmless no-op to use this module on 5.9.5+. That is to say, code which properly uses MRO::Compat will work unmodified on both older Perls and 5.9.5+.

If you're writing a piece of software that would like to use the parts of 5.9.5+'s mro:: interfaces that are supported here, and you want compatibility with older Perls, this is the module for you.

Some parts of this code will work better and/or faster with Class::C3::XS installed (which is an optional prereq of Class::C3, which is in turn a prereq of this package), but it's not a requirement.

This module never exports any functions. All calls must be fully qualified with the mro:: prefix.

The interface documentation here serves only as a quick reference of what the function basically does, and what differences between MRO::Compat and 5.9.5+ one should look out for. The main docs in 5.9.5's mro are the real interface docs, and contain a lot of other useful information.


mro::get_linear_isa($classname[, $type])

Returns an arrayref which is the linearized "ISA" 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 ISA of a class is a single ordered list of all of the classes that would be visited in the process of resolving a method on the given class, starting with itself. It does not include any duplicate entries.

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.


This allows the use mro 'dfs' and use mro 'c3' syntaxes, providing you use MRO::Compat first. Please see the "USING C3" section for additional details.

mro::set_mro($classname, $type)

Sets the mro of $classname to one of the types dfs or c3. Please see the "USING C3" section for additional details.


Returns the MRO of the given class (either c3 or dfs).

It considers any Class::C3-using class to have C3 MRO even before Class::C3::initialize() is called.


Returns an arrayref of classes who are subclasses of the given classname. In other words, classes in whose @ISA hierarchy we appear, no matter how indirectly.

This is much slower on pre-5.9.5 Perls with MRO::Compat than it is on 5.9.5+, as it has to search the entire package namespace.


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.


Increments PL_sub_generation, which invalidates method caching in all packages.

Please note that this is rarely necessary, unless you are dealing with a situation which is known to confuse Perl's method caching.


Invalidates the method cache of any classes dependent on the given class. In MRO::Compat on pre-5.9.5 Perls, this is an alias for mro::invalidate_all_method_caches above, as pre-5.9.5 Perls have no other way to do this. It will still enforce the requirement that you pass it a classname, for compatibility.

Please note that this is rarely necessary, unless you are dealing with a situation which is known to confuse Perl's method caching.


Returns an integer which is incremented every time a local method of or the @ISA of the given package changes on Perl 5.9.5+. On earlier Perls with this MRO::Compat module, it will probably increment a lot more often than necessary.


While this module makes the 5.9.5+ syntaxes use mro 'c3' and mro::set_mro("Foo", 'c3') available on older Perls, it does so merely by passing off the work to Class::C3.

It does not remove the need for you to call Class::C3::initialize(), Class::C3::reinitialize(), and/or Class::C3::uninitialize() at the appropriate times as documented in the Class::C3 docs. These three functions are always provided by MRO::Compat, either via Class::C3 itself on older Perls, or directly as no-ops on 5.9.5+.





Brandon L. Black, <>


Copyright 2007-2008 Brandon L. Black <>

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.