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Class::InsideOut::Manual::About.3pm

NAME

Class::InsideOut::Manual::About - guide to this and other implementations of the inside-out technique

VERSION

This documentation refers to version 1.10

DESCRIPTION

This manual provides an overview of the inside-out technique and its application within "Class::InsideOut" and other modules. It also provides a list of references for further study.

Inside-out object basics

Inside-out objects use the blessed reference as an index into lexical data structures holding object properties, rather than using the blessed reference itself as a data structure.

   $self->{ name }        = "Larry"; # classic, hash-based object
   $name{ refaddr $self } = "Larry"; # inside-out
 
 

The inside-out approach offers three major benefits:

*

Enforced encapsulation: object properties cannot be accessed directly from ouside the lexical scope that declared them

*

Making the property name part of a lexical variable rather than a hash-key means that typos in the name will be caught as compile-time errors (if using strict)

*

If the memory address of the blessed reference is used as the index, the reference can be of any type

In exchange for these benefits, robust implementation of inside-out objects can be quite complex. "Class::InsideOut" manages that complexity.

Philosophy of Class::InsideOut

"Class::InsideOut" provides a set of tools for building safe inside-out classes with maximum flexibility.

It aims to offer minimal restrictions beyond those necessary for robustness of the inside-out technique. All capabilities necessary for robustness should be automatic. Anything that can be optional should be. The design should not introduce new restrictions unrelated to inside-out objects, such as attributes and "CHECK" blocks that cause problems for "mod_perl" or the use of source filters for syntatic sugar.

As a result, only a few things are mandatory:

*

Properties must be based on hashes and declared via "property"

*

Property hashes must be keyed on the "Scalar::Util::refaddr"

*

"register" must be called on all new objects

All other implementation details, including constructors, initializers and class inheritance management are left to the user (though a very simple constructor is available as a convenience). This does requires some additional work, but maximizes freedom. "Class::InsideOut" is intended to be a base class providing only fundamental features. Subclasses of "Class::InsideOut" could be written that build upon it to provide particular styles of constructor, destructor and inheritance support.

Other modules on CPAN

*

Object::InsideOut --- This is perhaps the most full-featured, robust implementation of inside-out objects currently on CPAN. It is highly recommended if a more full-featured inside-out object builder is needed. Its array-based mode is faster than hash-based implementations, but black-box inheritance is handled via delegation, which imposes certain limitations.

*

Class::Std --- Despite the name, this does not reflect currently known best practices for inside-out objects. Does not provide thread-safety with CLONE and doesn't support black-box inheritance. Has a robust inheritance/initialization system.

*

Class::BuildMethods --- Generates accessors with encapsulated storage using a flyweight inside-out variant. Lexicals properties are hidden; accessors must be used everywhere. Not thread-safe.

*

Lexical::Attributes --- The original inside-out implementation, but missing some key features like thread-safety. Also, uses source filters to provide Perl-6-like object syntax. Not thread-safe.

*

Class::MakeMethods::Templates::InsideOut --- Not a very robust implementation. Not thread-safe. Not overloading-safe. Has a steep learning curve for the Class::MakeMethods system.

*

Object::LocalVars --- My own original thought experiment with 'outside-in' objects and local variable aliasing. Not safe for any production use and offers very weak encapsulation.

References for further study

Much of the Perl community discussion of inside-out objects has taken place on Perlmonks (<http://perlmonks.org>). My scratchpad there has a fairly comprehensive list of articles (<http://perlmonks.org/index.pl?node_id=360998>). Some of the more informative articles include:

*

Abigail-II. ``Re: Where/When is OO useful?''. July 1, 2002. <http://perlmonks.org/index.pl?node_id=178518>

*

Abigail-II. ``Re: Tutorial: Introduction to Object-Oriented Programming''. December 11, 2002. <http://perlmonks.org/index.pl?node_id=219131>

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demerphq. ``Yet Another Perl Object Model (Inside Out Objects)''. December 14, 2002. <http://perlmonks.org/index.pl?node_id=219924>

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xdg. ``Threads and fork and CLONE, oh my!''. August 11, 2005. <http://perlmonks.org/index.pl?node_id=483162>

*

jdhedden. ``Anti-inside-out-object-ism''. December 9, 2005. <http://perlmonks.org/index.pl?node_id=515650>

SEE ALSO

*

Class::InsideOut

*

Class::InsideOut::Manual::Advanced

AUTHOR

David A. Golden (DAGOLDEN)

COPYRIGHT AND LICENSE

Copyright (c) 2006, 2007 by David A. Golden

Licensed under the Apache License, Version 2.0 (the ``License''); you may not use this file except in compliance with the License. You may obtain a copy of the License at L<http://www.apache.org/licenses/LICENSE-2.0>

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an ``AS IS'' BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.