NAME¶
Class::InsideOut::Manual::About - guide to this and other implementations of the
inside-out technique
VERSION¶
version 1.13
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>
CONTRIBUTORS¶
- •
- Karen Etheridge <ether@cpan.org>
- •
- Toby Inkster <tonyink@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