---
name: "Class::InsideOut::Manual::About"
section: "3pm"
platform: "linux"
package: "libclass-insideout-perl"
version: "1.14-3"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/Class::InsideOut::Manual::About"
---

# Class::InsideOut::Manual::About(3pm)

guide to this and other implementations of the inside-out technique

## NAME

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

## VERSION

version 1.14

## 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 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\>

## SEE ALSO

- •

  Class::InsideOut

- •

  Class::InsideOut::Manual::Advanced

## AUTHOR

David Golden \<dagolden@cpan.org\>

## COPYRIGHT AND LICENSE

This software is Copyright (c) 2006 by David A. Golden.

This is free software, licensed under:

```
  The Apache License, Version 2.0, January 2004
```