Class::InsideOut::Manual::About - guide to this and other implementations of the
inside-out technique
version 1.14
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 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.
"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.
- •
- 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.
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>
- •
- Class::InsideOut
- •
- Class::InsideOut::Manual::Advanced
David Golden <
[email protected]>
This software is Copyright (c) 2006 by David A. Golden.
This is free software, licensed under:
The Apache License, Version 2.0, January 2004