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David Golden


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


This documentation refers to version 1.10


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.

  • 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:



David A. Golden (DAGOLDEN)


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>

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