NAME Class::InsideOut - a safe, simple inside-out object construction kit SYNOPSIS package My::Class; use Class::InsideOut qw( property register id ); use Scalar::Util qw( refaddr ); # declare a lexical property hash with 'my' property my %name; sub new { my $class = shift; my $self = \do {my $scalar}; bless $self, $class; # register the object for thread-safety register( $self ); } sub name { my $self = shift; if ( @_ ) { # use 'refaddr' to access properties for an object $name{ refaddr $self } = shift; return $self; } return $name{ refaddr $self }; } sub greeting { my $self = shift; # use 'id' as a mnemonic alias for 'refaddr' return "Hello, my name is " . $name { id $self }; } LIMITATIONS AND ROADMAP This is an alpha release for a work in progress. It is functional but incomplete and should not be used for any production purpose. It has been released to solicit peer review and feedback. Currently, serialization with Storable is experimental and may have unanticipated bugs, particularly relating to serializing references to other objects. Property destruction support for various inheritance patterns (e.g. diamond) is experimental and the API may change. There is minimal argument checking or other error handling. A future version will also add very basic accessor support. DESCRIPTION This is a simple, safe and streamlined toolkit for building inside-out objects. Unlike most other inside-out object building modules already on CPAN, this module aims for minimalism and robustness. It does not require derived classes to subclass it; uses no source filters, attributes or CHECK blocks; supports any underlying object type including foreign inheritance; does not leak memory; is overloading-safe; is thread-safe for Perl 5.8 or better; and should be mod_perl compatible. It provides the minimal support necessary for creating safe inside-out objects. All other implementation details, including writing a constructor and managing inheritance, are left to the user. Programmers seeking a more full-featured approach to inside-out objects are encouraged to explore Object::InsideOut. Other implementations are briefly noted in the "See Also" section. 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 the memory address of the blessed reference is used as the index, the reference can be of any type In exchange for these benefits, however, robust implementation of inside-out objects can be quite complex. `Class::InsideOut' manages that complexity. Philosophy of `Class::InsideOut' `Class::InsideOut' provides a minimalist 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 new syntax). 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' of the object (or the `id' alias to `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. This does requires some additional work, but maximizes freedom. `Class::InsideOut' is intended to be the 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. USAGE Importing `Class::InsideOut' use Class::InsideOut; By default, `Class::InsideOut' imports three critical methods: `DESTROY', `STORABLE_freeze' and `STORABLE_thaw'. These methods are intimately tied to correct functioning of the inside-out objects. No other functions are imported by default. Additional functions can be imported by including them as arguments with `use': use Class::InsideOut qw( register property id ); Note that `DESTROY' and `STORABLE_*' will still be imported even without an explicit request. This can only be avoided by explicitly doing no importing, via `require' or passing an empty list to `use': use Class::InsideOut (); There is almost no circumstance under which this is a good idea. Users seeking custom destruction behavior should see "Object destruction" and the description of the `DEMOLISH' method. Declaring and accessing object properties Object properties are declared with the `property' function, which must be passed a single lexical (i.e. `my') hash. property my %name; property my %age; Properties are private by default and no accessors are created. Users are free to create accessors of any style. Properties for an object are accessed through an index into the lexical hash based on the memory address of the object. This memory address *must* be obtained via `Scalar::Util::refaddr'. The alias `id' is available for brevity. $name{ refaddr $self } = "James"; $age { id $self } = 32; In the future, additional options will be supported to create accessors in various styles. Object construction `Class::InsideOut' provides no constructor function as there are many possible ways of constructing an inside-out object. Additionally, this avoids constraining users to any particular object initialization or superclass initialization approach. By using the memory address of the object as the index for properties, *any* type of reference can be used as the basis for an inside-out object with `Class::InsideOut'. sub new { my $class = shift; my $self = \do{ my $scalar }; # anonymous scalar # my $self = {}; # anonymous hash # my $self = []; # anonymous array # open my $self, "<", $filename; # filehandle reference register( bless $self, $class ); } However, to ensure that the inside-out objects are thread-safe, the `register' function *must* be called on the newly created object. See register for details. A more advanced technique uses another object, usually a superclass object, as the object reference. See "Foreign inheritance" for details. Object destruction `Class::InsideOut' automatically exports a customized `DESTROY' function. This function cleans up object property memory for all declared properties for all `Class::InsideOut' based classes in the `@ISA' array to avoid memory leaks or data collision. Additionally, if a user-supplied `DEMOLISH' function is available in the same package, it will be called with the object being destroyed as its argument. `DEMOLISH' can be used for custom destruction behavior such as updating class properties, closing sockets or closing database connections. Object properties will not be deleted until after `DEMOLISH' returns. my $objects_destroyed; sub DEMOLISH { $objects_destroyed++; } `DEMOLISH' will only be automatically called if it exists for an object's class. `DEMOLISH' will not be inherited and `DEMOLISH' will not be called automatically for any superclasses. `DEMOLISH' should manage any necessary calls to superclass `DEMOLISH' methods. As with `new', implementation details are left to the user based on the user's approach to object inheritance. Depending on how the inheritance chain is constructed and how `DEMOLISH' is being used, users may wish to entirely override superclass `DEMOLISH' methods, rely upon `SUPER::DEMOLISH', or may prefer to walk the entire `@ISA' tree: use Class::ISA; sub DEMOLISH { my $self = shift; # class specific demolish actions # DEMOLISH for all parent classes, but only once my @demolishers = map { $_->can("DEMOLISH") } Class::ISA::super_path( __PACKAGE__ ); for my $d ( @demolishers ) { $d->($self) if $d; } } Generally, any class that inherits from another should define its own `DEMOLISH' method. Foreign inheritance Because inside-out objects built with `Class::InsideOut' can use any type of reference for the object, inside-out objects can be built using other objects. This is of greatest utility when extending a superclass object, without regard for whether the superclass object is implemented with a hash or array or other reference. use base 'IO::File'; sub new { my ($class, $filename) = @_; my $self = IO::File->new( $filename ); register( bless $self, $class ); } In the example above, `IO::File' is a superclass. The object is an `IO::File' object, re-blessed into the inside-out class. The resulting object can be used directly anywhere an `IO::File' object would be, without interfering with any of its own inside-out functionality. Classes using foreign inheritance should provide a `DEMOLISH' function that calls the foreign class' destructor explicitly. Serialization `Class::InsideOut' has experimental support for serialization with Storable by providing the `STORABLE_freeze' and `STORABLE_thaw' methods. Due to limitations of Storable, this serialization will only work for objects based on scalars, arrays or hashes. Serialization of objects that themselves contain references to objects has not yet been tested. The semantics of a deep cloning `freeze(thaw(.))' operation has not been fully explored or tested. User feedback on serialization needs and limitations are encouraged. Thread-safety Because `Class::InsideOut' uses memory addresses as indices to object properties, special handling is necessary for use with threads. When a new thread is created, the Perl interpreter is cloned, and all objects in the new thread will have new memory addresses. Starting with Perl 5.8, if a `CLONE' function exists in a package, it will be called when a thread is created to provide custom responses to thread cloning. (See perlmod for details.) `Class::InsideOut' itself has a `CLONE' function that automatically fixes up properties in a new thread to reflect the new memory addresses for all classes created with `Class::InsideOut'. `register' must be called on all newly constructed inside-out objects to register them for use in `Class::InsideOut::CLONE'. Users are strongly encouraged not to define their own `CLONE' functions as they may interfere with the operation of `Class::InsideOut::CLONE' and leave objects in an undefined state. Future versions may support a user-defined CLONE hook, depending on demand. Note: `fork' on Perl for Win32 is emulated using threads since Perl 5.6. (See perlfork.) As Perl 5.6 did not support `CLONE', inside-out objects using memory addresses (e.g. `Class::InsideOut' are not fork-safe for Win32. FUNCTIONS `property' property my %name; Declares an inside-out property. The argument must be a lexical hash, though the `my' keyword can be included as part of the argument rather than as a separate statement. No accessor is created, but the property will be tracked for memory cleanup during object destruction and for proper thread-safety. `register' register $object; Registers an object for thread-safety. This should be called as part of a constructor on a object blessed into the current package. Returns the object (without modification). `id' $name{ id $object } = "Larry"; This is a shorter, mnemonic alias for `Scalar::Util::refaddr'. It returns the memory address of an object (just like `refaddr') as the index to access the properties of an inside-out object. SEE ALSO 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 foreign inheritance is handled via delegation, which imposes certain limitations. * Class::Std -- Despite the name, does not reflect best practices for inside-out objects. Does not provide thread-safety with CLONE, is not mod_perl safe and doesn't support foreign inheritance. * 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 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 BUGS Please report bugs or feature requests using the CPAN Request Tracker. Bugs can be sent by email to `bug-Class-InsideOut@rt.cpan.org' or submitted using the web interface at http://rt.cpan.org/NoAuth/Bugs.html?Dist=Class-InsideOut When submitting a bug or request, please include a test-file or a patch to an existing test-file that illustrates the bug or desired feature. AUTHOR David A. Golden (DAGOLDEN) dagolden@cpan.org http://dagolden.com/ COPYRIGHT AND LICENSE Copyright (c) 2006 by David A. Golden This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. The full text of the license can be found in the LICENSE file included with this module. 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