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 and love.
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.
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).
This system is used by Moose, which supplies a powerful class builder system built entirely on top of "Class::MOP".
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.
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.
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 class's metaclass.
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 classes.
"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 this document.
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.
See Class::MOP::Class for more details.
See Class::MOP::Attribute for more details.
See Class::MOP::Method for more details.
See Class::MOP::Instance for more details.
Class::MOP::get_code_info($code)
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 $code reference is from.
Class::MOP::class_of($instance_or_class_name)
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 returned.
You should almost certainly be using "Moose::Util::find_meta" instead.
Class::MOP::get_all_metaclasses
This will return a hash of all the metaclass instances that have been cached by Class::MOP::Class, keyed by the package name.
Class::MOP::get_all_metaclass_instances
This will return a list of all the metaclass instances that have been cached by Class::MOP::Class.
Class::MOP::get_all_metaclass_names
This will return a list of all the metaclass names that have been cached by Class::MOP::Class.
Class::MOP::get_metaclass_by_name($name)
This will return a cached Class::MOP::Class instance, or nothing if no metaclass exists with that $name.
Class::MOP::store_metaclass_by_name($name, $meta)
This will store a metaclass in the cache at the supplied $key.
Class::MOP::weaken_metaclass($name)
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 in $name.
Class::MOP::metaclass_is_weak($name)
Returns true if the metaclass for $name has been weakened (via "weaken_metaclass").
Class::MOP::does_metaclass_exist($name)
This will return true of there exists a metaclass stored in the $name key, and return false otherwise.
Class::MOP::remove_metaclass_by_name($name)
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.
<http://scg.unibe.ch/archive/papers/Duca05ySafeMetaclassTrait.pdf>
Please report any bugs to "bug-class-mop@rt.cpan.org", or through the web interface at <http://rt.cpan.org>.
You can also discuss feature requests or possible bugs on the Moose mailing list (moose@perl.org) or on IRC at <irc://irc.perl.org/#moose>.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.