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Class-InsideOut-1.00
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/ Class::InsideOut::Manual::About

NAME

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

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. 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 foreign 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 foreign 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.

  • 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:

SEE ALSO

AUTHOR

David A. Golden (DAGOLDEN)

dagolden@cpan.org

http://dagolden.com/

COPYRIGHT AND LICENSE

Copyright (c) 2006 by David A. Golden

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

The full text of the license can be found in the LICENSE file included with this module.

DISCLAIMER OF WARRANTY

BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR, OR CORRECTION.

IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.