Class::MOP(3pm)

1Class::MOP(3)         User Contributed Perl Documentation        Class::MOP(3)
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NAME

6       Class::MOP - A Meta Object Protocol for Perl 5
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DESCRIPTON

9       This module is an attempt to create a meta object protocol for the Perl
10       5 object system. It makes no attempt to change the behavior or charac‐
11       teristics of the Perl 5 object system, only to create a protocol for
12       its manipulation and introspection.
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14       That said, it does attempt to create the tools for building a rich set
15       of extensions to the Perl 5 object system. Every attempt has been made
16       for these tools to keep to the spirit of the Perl 5 object system that
17       we all know and love.
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19       This documentation is admittedly sparse on details, as time permits I
20       will try to improve them. For now, I suggest looking at the items
21       listed in the "SEE ALSO" section for more information. In particular
22       the book "The Art of the Meta Object Protocol" was very influential in
23       the development of this system.
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25       What is a Meta Object Protocol?
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27       A meta object protocol is an API to an object system.
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29       To be more specific, it is a set of abstractions of the components of
30       an object system (typically things like; classes, object, methods,
31       object attributes, etc.). These abstractions can then be used to both
32       inspect and manipulate the object system which they describe.
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34       It can be said that there are two MOPs for any object system; the
35       implicit MOP, and the explicit MOP. The implicit MOP handles things
36       like method dispatch or inheritance, which happen automatically as part
37       of how the object system works. The explicit MOP typically handles the
38       introspection/reflection features of the object system.  All object
39       systems have implicit MOPs, without one, they would not work. Explict
40       MOPs however as less common, and depending on the language can vary
41       from restrictive (Reflection in Java or C#) to wide open (CLOS is a
42       perfect example).
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44       Yet Another Class Builder!! Why?
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46       This is not a class builder so much as it is a class builder builder.
47       My intent is that an end user does not use this module directly, but
48       instead this module is used by module authors to build extensions and
49       features onto the Perl 5 object system.
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51       Who is this module for?
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53       This module is specifically for anyone who has ever created or wanted
54       to create a module for the Class:: namespace. The tools which this mod‐
55       ule will provide will hopefully make it easier to do more complex
56       things with Perl 5 classes by removing such barriers as the need to
57       hack the symbol tables, or understand the fine details of method dis‐
58       patch.
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60       What changes do I have to make to use this module?
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62       This module was designed to be as unintrusive as possible. Many of its
63       features are accessible without any change to your existsing code at
64       all. It is meant to be a compliment to your existing code and not an
65       intrusion on your code base. Unlike many other Class:: modules, this
66       module does not require you subclass it, or even that you "use" it in
67       within your module's package.
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69       The only features which requires additions to your code are the
70       attribute handling and instance construction features, and these are
71       both completely optional features. The only reason for this is because
72       Perl 5's object system does not actually have these features built in.
73       More information about this feature can be found below.
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75       A Note about Performance?
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77       It is a common misconception that explict MOPs are performance drains.
78       But this is not a universal truth at all, it is an side-effect of spe‐
79       cific implementations. For instance, using Java reflection is much
80       slower because the JVM cannot take advantage of any compiler optimiza‐
81       tions, and the JVM has to deal with much more runtime type information
82       as well. Reflection in C# is marginally better as it was designed into
83       the language and runtime (the CLR). In contrast, CLOS (the Common Lisp
84       Object System) was built to support an explicit MOP, and so performance
85       is tuned for it.
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87       This library in particular does it's absolute best to avoid putting any
88       drain at all upon your code's performance. In fact, by itself it does
89       nothing to affect your existing code. So you only pay for what you
90       actually use.
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92       About Metaclass compatibility
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94       This module makes sure that all metaclasses created are both upwards
95       and downwards compatible. The topic of metaclass compatibility is
96       highly esoteric and is something only encountered when doing deep and
97       involved metaclass hacking. There are two basic kinds of metaclass
98       incompatibility; upwards and downwards.
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100       Upwards metaclass compatibility means that the metaclass of a given
101       class is either the same as (or a subclass of) all of the class's
102       ancestors.
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104       Downward metaclass compatibility means that the metaclasses of a given
105       class's anscestors are all either the same as (or a subclass of) that
106       metaclass.
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108       Here is a diagram showing a set of two classes ("A" and "B") and two
109       metaclasses ("Meta::A" and "Meta::B") which have correct metaclass com‐
110       patibility both upwards and downwards.
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112           +---------+     +---------+
113           ⎪ Meta::A ⎪<----⎪ Meta::B ⎪      <....... (instance of  )
114           +---------+     +---------+      <------- (inherits from)
115                ^               ^
116                :               :
117           +---------+     +---------+
118           ⎪    A    ⎪<----⎪    B    ⎪
119           +---------+     +---------+
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121       As I said this is a highly esoteric topic and one you will only run
122       into if you do a lot of subclassing of Class::MOP::Class. If you are
123       interested in why this is an issue see the paper Uniform and safe meta‐
124       class composition linked to in the "SEE ALSO" section of this document.
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126       Using custom metaclasses
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128       Always use the metaclass pragma when using a custom metaclass, this
129       will ensure the proper initialization order and not accidentely create
130       an incorrect type of metaclass for you. This is a very rare problem,
131       and one which can only occur if you are doing deep metaclass program‐
132       ming. So in other words, don't worry about it.
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PROTOCOLS

135       The protocol is divided into 3 main sub-protocols:
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137       The Class protocol
138           This provides a means of manipulating and introspecting a Perl 5
139           class. It handles all of symbol table hacking for you, and provides
140           a rich set of methods that go beyond simple package introspection.
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142           See Class::MOP::Class for more details.
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144       The Attribute protocol
145           This provides a consistent represenation for an attribute of a Perl
146           5 class. Since there are so many ways to create and handle att‐
147           tributes in Perl 5 OO, this attempts to provide as much of a uni‐
148           fied approach as possible, while giving the freedom and flexibility
149           to subclass for specialization.
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151           See Class::MOP::Attribute for more details.
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153       The Method protocol
154           This provides a means of manipulating and introspecting methods in
155           the Perl 5 object system. As with attributes, there are many ways
156           to approach this topic, so we try to keep it pretty basic, while
157           still making it possible to extend the system in many ways.
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159           See Class::MOP::Method for more details.
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FUNCTIONS

162       Utility functions
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164       load_class ($class_name)
165           This will load a given $class_name and if it does not have an
166           already initialized metaclass, then it will intialize one for it.
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168       is_class_loaded ($class_name)
169           This will return a boolean depending on if the $class_name has been
170           loaded.
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172           NOTE: This does a basic check of the symbol table to try and deter‐
173           mine as best it can if the $class_name is loaded, it is probably
174           correct about 99% of the time.
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176       Metaclass cache functions
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178       Class::MOP holds a cache of metaclasses, the following are functions
179       (not methods) which can be used to access that cache. It is not recom‐
180       mended that you mess with this, bad things could happen. But if you are
181       brave and willing to risk it, go for it.
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183       get_all_metaclasses
184           This will return an hash of all the metaclass instances that have
185           been cached by Class::MOP::Class keyed by the package name.
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187       get_all_metaclass_instances
188           This will return an array of all the metaclass instances that have
189           been cached by Class::MOP::Class.
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191       get_all_metaclass_names
192           This will return an array of all the metaclass names that have been
193           cached by Class::MOP::Class.
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195       get_metaclass_by_name ($name)
196       store_metaclass_by_name ($name, $meta)
197       weaken_metaclass ($name)
198       does_metaclass_exist ($name)
199       remove_metaclass_by_name ($name)
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SIMILAR MODULES

246       As I have said above, this module is a class-builder-builder, so it is
247       not the same thing as modules like Class::Accessor and Class::Method‐
248       Maker. That being said there are very few modules on CPAN with similar
249       goals to this module. The one I have found which is most like this mod‐
250       ule is Class::Meta, although it's philosophy and the MOP it creates are
251       very different from this modules.
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BUGS

254       All complex software has bugs lurking in it, and this module is no
255       exception. If you find a bug please either email me, or add the bug to
256       cpan-RT.
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ACKNOWLEDGEMENTS

259       Rob Kinyon
260           Thanks to Rob for actually getting the development of this module
261           kick-started.
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AUTHORS

264       Stevan Little <stevan@iinteractive.com>
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266       with contributions from:
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268       Brandon (blblack) Black
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270       Guillermo (groditi) Roditi
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272       Matt (mst) Trout
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274       Rob (robkinyon) Kinyon
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276       Yuval (nothingmuch) Kogman
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COPYRIGHT AND LICENSE

279       Copyright 2006, 2007 by Infinity Interactive, Inc.
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281       <http://www.iinteractive.com>
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283       This library is free software; you can redistribute it and/or modify it
284       under the same terms as Perl itself.
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288perl v5.8.8                       2007-11-07                     Class::MOP(3)