# phpman > perldoc > Class::C3

## NAME
    [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown) - A pragma to use the C3 method resolution order algorithm

## SYNOPSIS
        # NOTE - DO NOT USE [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown) directly as a user, use [MRO::Compat](https://www.chedong.com/phpMan.php/perldoc/MRO%3A%3ACompat/markdown) instead!
        package ClassA;
        use [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);
        sub hello { '[A::hello](https://www.chedong.com/phpMan.php/perldoc/A%3A%3Ahello/markdown)' }

        package ClassB;
        use base 'ClassA';
        use [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);

        package ClassC;
        use base 'ClassA';
        use [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);

        sub hello { '[C::hello](https://www.chedong.com/phpMan.php/perldoc/C%3A%3Ahello/markdown)' }

        package ClassD;
        use base ('ClassB', 'ClassC');
        use [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);

        # Classic Diamond MI pattern
        #    <A>
        #   /   \
        # <B>   <C>
        #   \   /
        #    <D>

        package main;

        # initializez the C3 module
        # (formerly called in INIT)
        [Class::C3::initialize](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3%3A%3Ainitialize/markdown)();

        print join ', ' => [Class::C3::calculateMRO](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3%3A%3AcalculateMRO/markdown)('ClassD'); # prints ClassD, ClassB, ClassC, ClassA

        print ClassD->hello(); # prints '[C::hello](https://www.chedong.com/phpMan.php/perldoc/C%3A%3Ahello/markdown)' instead of the standard p5 '[A::hello](https://www.chedong.com/phpMan.php/perldoc/A%3A%3Ahello/markdown)'

        ClassD->can('hello')->();          # can() also works correctly
        [UNIVERSAL::can](https://www.chedong.com/phpMan.php/perldoc/UNIVERSAL%3A%3Acan/markdown)('ClassD', 'hello'); # as does [UNIVERSAL::can](https://www.chedong.com/phpMan.php/perldoc/UNIVERSAL%3A%3Acan/markdown)()

## 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](https://www.chedong.com/phpMan.php/perldoc/MRO%3A%3ACompat/markdown) 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](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3%3A%3Ainitialize/markdown)" is
    called, this module calculates the MRO of all the classes which called "use [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown)". 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](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);

    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](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);
          # ... Foo methods here

          package Bar;
          use [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);
          use base 'Foo';
          # ... Bar methods here

          package main;

          [Class::C3::initialize](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3%3A%3Ainitialize/markdown)(); # 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](https://www.chedong.com/phpMan.php/perldoc/DBIx%3A%3AClass/markdown) 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](https://www.chedong.com/phpMan.php/perldoc/DBIx%3A%3AClass/markdown) 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](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);
      sub foo { '[ClassA::foo](https://www.chedong.com/phpMan.php/perldoc/ClassA%3A%3Afoo/markdown)' }

      package ClassB;
      use base 'ClassA';
      use [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);
      sub foo { '[ClassB::foo](https://www.chedong.com/phpMan.php/perldoc/ClassB%3A%3Afoo/markdown) => ' . (shift)->[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)() }

      package ClassC;
      use base 'ClassA';
      use [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);
      sub foo { '[ClassC::foo](https://www.chedong.com/phpMan.php/perldoc/ClassC%3A%3Afoo/markdown) => ' . (shift)->[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)() }

      package ClassD;
      use base ('ClassB', 'ClassC');
      use [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown);
      sub foo { '[ClassD::foo](https://www.chedong.com/phpMan.php/perldoc/ClassD%3A%3Afoo/markdown) => ' . (shift)->[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)() }

      print ClassD->foo; # prints out "[ClassD::foo](https://www.chedong.com/phpMan.php/perldoc/ClassD%3A%3Afoo/markdown) => [ClassB::foo](https://www.chedong.com/phpMan.php/perldoc/ClassB%3A%3Afoo/markdown) => [ClassC::foo](https://www.chedong.com/phpMan.php/perldoc/ClassC%3A%3Afoo/markdown) => [ClassA::foo](https://www.chedong.com/phpMan.php/perldoc/ClassA%3A%3Afoo/markdown)"

    A few things to note. First, we do not require you to add on the method name to the
    "[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)" 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](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)". It
    can not automatically use the current @_.

    If "[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)" cannot find a next method to re-dispatch the call to, it will throw an
    exception. You can use "[next::can](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Acan/markdown)" to see if "[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)" will succeed before you call it like
    so:

      $self->[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)(@_) if $self->[next::can](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Acan/markdown);

    Additionally, you can use "[maybe::next::method](https://www.chedong.com/phpMan.php/perldoc/maybe%3A%3Anext%3A%3Amethod/markdown)" as a shortcut to only call the next method if it
    exists. The previous example could be simply written as:

      $self->[maybe::next::method](https://www.chedong.com/phpMan.php/perldoc/maybe%3A%3Anext%3A%3Amethod/markdown)(@_);

    There are some caveats about using "[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)", 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](https://www.chedong.com/phpMan.php/perldoc/DBIx%3A%3AClass/markdown) 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](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)" 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](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)" from methods defined outside the class
        There is an edge case when using "[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)" 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](https://www.chedong.com/phpMan.php/perldoc/Foo%3A%3Afoo/markdown) = sub { (shift)->[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)(@_) };

        The problem exists because the anonymous subroutine being assigned to the glob *[Foo::foo](https://www.chedong.com/phpMan.php/perldoc/Foo%3A%3Afoo/markdown)
        will show up in the call stack as being called "__ANON__" and not "foo" as you might expect.
        Since "[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)" 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](https://www.chedong.com/phpMan.php/perldoc/Sub%3A%3AName/markdown)" will reach into the perl
        internals and assign a name to an anonymous subroutine for you. Simply do this:

          use [Sub::Name](https://www.chedong.com/phpMan.php/perldoc/Sub%3A%3AName/markdown) 'subname';
          *[Foo::foo](https://www.chedong.com/phpMan.php/perldoc/Foo%3A%3Afoo/markdown) = subname '[Foo::foo](https://www.chedong.com/phpMan.php/perldoc/Foo%3A%3Afoo/markdown)' => sub { (shift)->[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)(@_) };

        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](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown), 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](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown) as documented here. [Class::C3](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown)
    will detect Perl 5.9.5+ and take advantage of the core support when available.

## [Class::C3::XS](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3%3A%3AXS/markdown)
    This module will load [Class::C3::XS](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3%3A%3AXS/markdown) 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](https://www.chedong.com/phpMan.php/perldoc/Class%3A%3AC3/markdown)).

## CODE COVERAGE
    [Devel::Cover](https://www.chedong.com/phpMan.php/perldoc/Devel%3A%3ACover/markdown) 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/haahr/dylan/linearization-o>
    opsla96.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](https://www.chedong.com/phpMan.php/perldoc/DBIx%3A%3AClass/markdown) and finding many bugs
    and providing fixes.
    Thanks to Justin Guenther for making "[next::method](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Amethod/markdown)" more robust by handling calls inside "eval"
    and anon-subs.
    Thanks to Robert Norris for adding support for "[next::can](https://www.chedong.com/phpMan.php/perldoc/next%3A%3Acan/markdown)" and "[maybe::next::method](https://www.chedong.com/phpMan.php/perldoc/maybe%3A%3Anext%3A%3Amethod/markdown)".

## 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.