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