Class::InsideOut::ManuUserAContributed Perl Class::InsideOut::Manual::About(3)NAMEClass::InsideOut::Manual::About - guide to this and other
implementations of the inside-out technique
VERSION
This documentation refers to version 1.10
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 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.
· 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 A. Golden (DAGOLDEN)
COPYRIGHT AND LICENSE
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>
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
implied. See the License for the specific language governing
permissions and limitations under the License.
perl v5.14.02009-08-25Class::InsideOut::Manual::About(3)
