Class::MOP - A Meta Object Protocol for Perl 5
# ... This will come later, for now see # the other SYNOPSIS for more information
This module is an attempt to create a 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 for these tools to keep to the spirit of the Perl 5 object system that we all know and love.
A meta object protocol is an API to an object system.
To be more specific, it is a set of abstractions of the components of an object system (typically things like; classes, object, methods, object attributes, etc.). These abstractions can then be used to both 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. Explict MOPs however as less common, and depending on the language can vary from restrictive (Reflection in Java or C#) to wide open (CLOS is a perfect example).
This is not a class builder so much as it is a class builder builder. My intent is that an end user does 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 module is specifically for anyone who has ever created or wanted to create a module for the Class:: namespace. The tools which this module will provide will hopefully make it easier to do more complex things with Perl 5 classes by removing such barriers as the need to hack the symbol tables, or understand the fine details of method dispatch.
This module was designed to be as unintrusive as possible. Many of its features are accessible without any change to your existsing code at all. It is meant to be a compliment to your existing code and not an intrusion on your code base. Unlike many other Class:: modules, this module does not require you subclass it, or even that you use it in within your module's package.
use
The only features which requires 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 explict MOPs are performance drains. But this is not a universal truth at all, it is an side-effect of specific implementations. For instance, using Java reflection is much slower because the JVM cannot take advantage of any compiler optimizations, and the JVM 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 it's 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.
The protocol is divided into 3 main sub-protocols:
This provides a means of manipulating and introspecting a Perl 5 class. It handles all of 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.
This provides a consistent represenation for an attribute of a Perl 5 class. Since there are so many ways to create and handle atttributes in Perl 5 OO, this attempts to provide as much of a unified approach as possible, while giving the freedom and flexibility to subclass for specialization.
See Class::MOP::Attribute for more details.
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.
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, please email me and let me know, I would love to hear about them.
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 it's philosophy is very different from this module.
To start with, it provides wrappers around common Perl data types, and even extends those types with more specific subtypes. This module does not go into that area at all.
Class::Meta also seems to create it's own custom meta-object protocol, which is both more restrictive and more featureful than the vanilla Perl 5 one. This module attempts to model the existing Perl 5 MOP as it is.
It's introspection capabilities also seem to be heavily rooted in this custom MOP, so that you can only introspect classes which are already created with Class::Meta. This module does not make such restictions.
Now, all this said, Class::Meta is much more featureful than Class::MOP would ever try to be. But Class::MOP has some features which Class::Meta could not easily implement. It would be very possible to completely re-implement Class::Meta using Class::MOP and bring some of these features to Class::Meta though.
But in the end, this module's admitedly ambitious goals have no direct equal on CPAN since surely no one has been crazy enough to try something as silly as this ;) until now.
All complex software has bugs lurking in it, and this module is no exception. If you find a bug please either email me, or add the bug to cpan-RT.
Thanks to Rob for actually getting the development of this module kick-started.
Stevan Little <stevan@iinteractive.com>
Copyright 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.
To install Class::MOP, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Class::MOP
CPAN shell
perl -MCPAN -e shell install Class::MOP
For more information on module installation, please visit the detailed CPAN module installation guide.