Section: User Contributed Perl Documentation (3)
Class::MOP - A Meta Object Protocol for Perl 5
This module is a fully functioning meta object protocol for the
Perl 5 object system. It makes no attempt to change the behavior or
characteristics of the Perl 5 object system, only to create a
protocol for its manipulation and introspection.
That said, it does attempt to create the tools for building a rich set
of extensions to the Perl 5 object system. Every attempt has been made
to abide by the spirit of the Perl 5 object system that we all know
This documentation is sparse on conceptual details. We suggest looking
at the items listed in the ``SEE ALSO'' section for more
information. In particular the book ``The Art of the Meta Object
Protocol'' was very influential in the development of this system.
What is a Meta Object Protocol?
A meta object protocol is an API
to an object system.
To be more specific, it abstracts the components of an object system
(classes, object, methods, object attributes, etc.). These
abstractions can then be used to inspect and manipulate the object
system which they describe.
It can be said that there are two MOPs for any object system; the
implicit MOP and the explicit MOP. The implicit MOP handles things
like method dispatch or inheritance, which happen automatically as
part of how the object system works. The explicit MOP typically
handles the introspection/reflection features of the object system.
All object systems have implicit MOPs. Without one, they would not
work. Explicit MOPs are much less common, and depending on the
language can vary from restrictive (Reflection in Java or C#) to wide
open (CLOS is a perfect example).
Yet Another Class Builder! Why?
This is not
a class builder so much as a class builder
. The intent is that an end user will not use this module
directly, but instead this module is used by module authors to build
extensions and features onto the Perl 5 object system.
This system is used by Moose, which supplies a powerful class
builder system built entirely on top of "Class::MOP".
Who is this module for?
This module is for anyone who has ever created or wanted to create a
module for the Class:: namespace. The tools which this module provides
make doing complex Perl 5 wizardry simpler, by removing such barriers
as the need to hack symbol tables, or understand the fine details of
What changes do I have to make to use this module?
This module was designed to be as unobtrusive as possible. Many of its
features are accessible without any
change to your existing
code. It is meant to be a complement to your existing code and not an
intrusion on your code base. Unlike many other Class::
this module does not
require you subclass it, or even that you
it in within your module's package.
The only features which require additions to your code are the
attribute handling and instance construction features, and these are
both completely optional features. The only reason for this is because
Perl 5's object system does not actually have these features built
in. More information about this feature can be found below.
It is a common misconception that explicit MOPs are a performance hit.
This is not a universal truth, it is a side-effect of some specific
implementations. For instance, using Java reflection is slow because
cannot take advantage of any compiler optimizations, and the
has to deal with much more runtime type information as well.
Reflection in C# is marginally better as it was designed into the
language and runtime (the CLR). In contrast, CLOS (the Common Lisp
Object System) was built to support an explicit MOP, and so
performance is tuned for it.
This library in particular does its absolute best to avoid putting
any drain at all upon your code's performance. In fact, by itself
it does nothing to affect your existing code. So you only pay for what
you actually use.
About Metaclass compatibility
This module makes sure that all metaclasses created are both upwards
and downwards compatible. The topic of metaclass compatibility is
highly esoteric and is something only encountered when doing deep and
involved metaclass hacking. There are two basic kinds of metaclass
incompatibility; upwards and downwards.
Upwards metaclass compatibility means that the metaclass of a
given class is either the same as (or a subclass of) all of the
metaclasses of the class's ancestors.
Downward metaclass compatibility means that the metaclasses of a
given class's ancestors are all the same as (or a subclass of) that
Here is a diagram showing a set of two classes ("A" and "B") and
two metaclasses ("Meta::A" and "Meta::B") which have correct
metaclass compatibility both upwards and downwards.
| Meta::A |<----| Meta::B | <....... (instance of )
+---------+ +---------+ <------- (inherits from)
| A |<----| B |
In actuality, all of a class's metaclasses must be compatible,
not just the class metaclass. That includes the instance, attribute,
and method metaclasses, as well as the constructor and destructor
"Class::MOP" will attempt to fix some simple types of
incompatibilities. If all the metaclasses for the parent class are
subclasses of the child's metaclasses then we can simply replace
the child's metaclasses with the parent's. In addition, if the child
is missing a metaclass that the parent has, we can also just make the
child use the parent's metaclass.
As I said this is a highly esoteric topic and one you will only run
into if you do a lot of subclassing of Class::MOP::Class. If you
are interested in why this is an issue see the paper Uniform and
safe metaclass composition linked to in the ``SEE ALSO'' section of
Using custom metaclasses
Always use the metaclass pragma when using a custom metaclass, this
will ensure the proper initialization order and not accidentally
create an incorrect type of metaclass for you. This is a very rare
problem, and one which can only occur if you are doing deep metaclass
programming. So in other words, don't worry about it.
Note that if you're using Moose we encourage you to not use the
metaclass pragma, and instead use Moose::Util::MetaRole to apply
roles to a class's metaclasses. This topic is covered at length in
various Moose::Cookbook recipes.
The meta-object protocol is divided into 4 main sub-protocols:
The Class protocol
This provides a means of manipulating and introspecting a Perl 5
class. It handles symbol table hacking for you, and provides a rich
set of methods that go beyond simple package introspection.
See Class::MOP::Class for more details.
The Attribute protocol
This provides a consistent representation for an attribute of a Perl 5
class. Since there are so many ways to create and handle attributes in
Perl 5 OO,
the Attribute protocol provide as much of a unified
approach as possible. Of course, you are always free to extend this
protocol by subclassing the appropriate classes.
See Class::MOP::Attribute for more details.
The Method protocol
This provides a means of manipulating and introspecting methods in the
Perl 5 object system. As with attributes, there are many ways to
approach this topic, so we try to keep it pretty basic, while still
making it possible to extend the system in many ways.
See Class::MOP::Method for more details.
The Instance protocol
This provides a layer of abstraction for creating object instances.
Since the other layers use this protocol, it is relatively easy to
change the type of your instances from the default hash reference to
some other type of reference. Several examples are provided in the
directory included in this distribution.
See Class::MOP::Instance for more details.
Note that this module does not export any constants or functions.
Note that these are all called as functions, not methods
This function returns two values, the name of the package the $code
is from and the name of the $code itself. This is used by several
elements of the MOP to determine where a given $code reference is
This will return the metaclass of the given instance or class name. If the
class lacks a metaclass, no metaclass will be initialized, and "undef" will be
You should almost certainly be using
Metaclass cache functions
holds a cache of metaclasses. The following are functions
) which can be used to access that cache. It is not
recommended that you mess with these. Bad things could happen, but if
you are brave and willing to risk it: go for it!
This will return a hash of all the metaclass instances that have
been cached by Class::MOP::Class, keyed by the package name.
This will return a list of all the metaclass instances that have
been cached by Class::MOP::Class.
This will return a list of all the metaclass names that have
been cached by Class::MOP::Class.
This will return a cached Class::MOP::Class instance, or nothing
if no metaclass exists with that $name.
This will store a metaclass in the cache at the supplied $key.
In rare cases (e.g. anonymous metaclasses) it is desirable to
store a weakened reference in the metaclass cache. This
function will weaken the reference to the metaclass stored
Returns true if the metaclass for $name has been weakened
This will return true of there exists a metaclass stored in the
$name key, and return false otherwise.
This will remove the metaclass stored in the $name key.
Some utility functions (such as "Class::MOP::load_class") that were
previously defined in "Class::MOP" regarding loading of classes have been
extracted to Class::Load. Please see Class::Load for documentation.
There are very few books out on Meta Object Protocols and Metaclasses
because it is such an esoteric topic. The following books are really
the only ones I have found. If you know of any more,
email me and let me know, I would love to hear about them.
- The Art of the Meta Object Protocol
- Advances in Object-Oriented Metalevel Architecture and Reflection
- Putting MetaClasses to Work
- Smalltalk: The Language
- "Uniform and safe metaclass composition"
An excellent paper by the people who brought us the original Traits paper.
This paper is on how Traits can be used to do safe metaclass composition,
and offers an excellent introduction section which delves into the topic of
- "Safe Metaclass Programming"
This paper seems to precede the above paper, and propose a mix-in based
approach as opposed to the Traits based approach. Both papers have similar
information on the metaclass compatibility problem space.
- The Perl 6 MetaModel work in the Pugs project
- CPAN Module Review of Class::MOP
As I have said above, this module is a class-builder-builder, so it is
not the same thing as modules like Class::Accessor and
Class::MethodMaker. That being said there are very few modules on CPAN
with similar goals to this module. The one I have found which is most
like this module is Class::Meta, although its philosophy and the MOP
creates are very different from this modules.
All complex software has bugs lurking in it, and this module is no
Please report any bugs to "email@example.com", or through the
web interface at <http://rt.cpan.org>.
You can also discuss feature requests or possible bugs on the Moose
mailing list (firstname.lastname@example.org) or on IRC at
- Rob Kinyon
Thanks to Rob for actually getting the development of this module kick-started.
Stevan Little <email@example.com>
Dave Rolsky <firstname.lastname@example.org>
Jesse Luehrs <email@example.com>
Shawn M Moore <firstname.lastname@example.org>
יובל קוג'מן (Yuval Kogman) <email@example.com>
Karen Etheridge <firstname.lastname@example.org>
Florian Ragwitz <email@example.com>
Hans Dieter Pearcey <firstname.lastname@example.org>
Chris Prather <email@example.com>
Matt S Trout <firstname.lastname@example.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2006 by Infinity Interactive, Inc.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.