NAME Class::InsideOut - a safe, simple inside-out object construction kit SYNOPSIS package My::Class; use Class::InsideOut ':std'; # public, private, register and id public name => my %name; # accessor: name() private ssn => my %ssn; # no accessor public age => my %age, { set_hook => sub { /^\d+$/ or die "must be an integer" } }; public initials => my %initials, { set_hook => sub { $_ = uc $_ } }; sub new { register( bless \(my $s), shift ); } sub greeting { my $self = shift; 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 unfinished and should not be used for any production purpose. It has been released to solicit peer review and feedback. NOTICE: Version 0.08 introduced a *BACKWARDS INCOMPATIBLE* syntax change to the "property" method. "property" currently requires two arguments, including a label for the property. This label is used to support accessor creation and introspection. Serialization with Storable appears to be working but may have unanticipated bugs if an object contains a complicated (i.e. circular) reference structure and could use some real-world testing. Property destruction support for various inheritance patterns (e.g. diamond) is in draft form. There is minimal argument checking or other error handling. I believe API's may have stabilized. The module will be declared "beta" when argument/error checks are added, additional accessor styles are written and singleton support for Storable has been added. Users' feedback would be greatly appreciated. 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 and generating flexible accessors. All other implementation details, including writing a constructor and managing inheritance, are left to the user to maximize flexibility. 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 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, however, 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 a 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 optional "id" alias to "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. 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. USAGE Importing "Class::InsideOut" use Class::InsideOut; No functions are imported by default -- all functions must be called using their fully qualified names: Class::InsideOut::property name => my %name; Class::InsideOut::register $self; Functions can be imported by including them as arguments with "use". For example: use Class::InsideOut qw( register property ); property name => my %name; register $self; As a shortcut, "Class::InsideOut" supports two tags for importing sets of functions: use Class::InsideOut ':std'; # id, private, public, register use Class::InsideoUT ':all'; # all functions In addition, "Class::InsideOut" automatically imports three critical methods into the namespace that uses it: "DESTROY", "STORABLE_freeze" and "STORABLE_thaw". These methods are intimately tied to correct functioning of the inside-out objects. They will be imported regardless of whether or not any other functions are requested with "use". This can only be avoided by explicitly doing no importing, either via "require" or passing an empty list to "use": use Class::InsideOut (); There is almost no circumstance under which this is a good idea. See "Object destruction" and "Serialization" for how to add customized behavior to these methods. Object properties Object properties are declared with the "property" function (or its special aliases "public" and "private"), which must be passed a label and a lexical (i.e. "my") hash. property name => my %name; property age => my %age; Properties are private by default and no accessors are created. Users are free to create accessors of any style. See "Property accessors" for how to have "Class::InsideOut" automatically generate accessors. 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; Tip: since "refaddr" (or "id") are function calls, it may be helpful to store the value once at the beginning of a method rather than call it repeatedly throughout. This is particularly true if it would be called within a loop. For example: property dsn => my %dsn; property dbh => my %dbh; sub dbi_connect { my $self = shift; my $id = refaddr $self; # calculate once and store # try up to 20 times for ( 1 .. 20 ) { $dbh{ $id } = DBI->connect( $dsn{ $id } ); return if $dbh{ $id }; } die "Couldn't connect to $dbh{ $id }"; } Property accessors property color => my %color, { privacy => 'public' }; $obj->color( "red" ); print $obj->color(); # prints "red" The "property" method supports an optional hash reference of options. If the *privacy* option is equal to *public*, an accessor will be created with the same name as the label. If the accessor is passed an argument, the property will be set to the argument. The accessor always returns the value of the property. Future versions of "Class::InsideOut" will support additional accessor styles. Default accessor options may be set using the "options" function and will affect all subsequent calls to "property". "Class::InsideOut" offers two aliases for "property" that additionally set the privacy property accordingly, overriding the defaults and any options provided: public height => my %height; private weight => my %weight; See the documentation of each for details. Tip: generated accessors will be very slightly slower than a hand-rolled one as the generated accessor holds a reference rather than accessing the lexical property hash directly. Accessor hooks "Class::InsideOut" supports custom subroutine hooks to modify the behavior of accessors. Hooks are passed as property options: "set_hook" and "get_hook". The "set_hook" option is called when the accessor is called with an argument. The hook subroutine receives the entire argument list. Just before the hook is called, $_ is locally aliased to the first argument for convenience. public age => my %age, { set_hook => sub { /^\d+$/ or die "must be an integer" } }; If the "set_hook" dies, the error is caught and rethrown with a preamble that includes the name of the accessor: $obj->age(3.5); # dies with "Argument to age() must be an integer at..." When the "set_hook" returns, the property is set equal to $_. This feature is useful for on-the-fly modification of the value that will be stored. public list => my %list, { set_hook => sub { $_ = [ @_ ] } # stores arguments in a reference }; *Note that the return value of the "set_hook" is ignored.* (This simplifies syntax in the more frequent case of validating input versus modifying input.) The "get_hook" option is called when the accessor is called without an argument. Just before the hook is called, $_ is set equal to the property value of the object for convenience. The hook is called in the same context (i.e. list versus scalar) as the accessor. *The return value of the hook is passed through as the return value of the accessor.* public list => my %list, { set_hook => sub { $_ = [ @_ ] }, # stores arguments in a reference get_hook => sub { @$_ } # return property as a list }; Because $_ is a copy, not an alias, of the property value, it can be modified directly, if necessary, without affecting the underlying property. Accessor hooks can be set as a global default with the "options" function, though they may still be overridden with options passed to specific properties. Object construction "Class::InsideOut" provides no constructor method 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 = \( 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 the class and 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. # Sample DEMOLISH: Count objects demolished (for whatever reason) 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 $d; } } Generally, any class that inherits from another should define its own "DEMOLISH" method. Foreign inheritance Because inside-out objects built with "Class::InsideOut" can use any type of reference for the object, inside-out objects can be built using other objects. This is useful to extend a superclass without regard for whether the superclass implement objects with a hash or array or other reference. use base 'IO::File'; sub new { my ($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 support for serialization with Storable by providing the "STORABLE_freeze" and "STORABLE_thaw" methods. "Storable" will use these methods to serialize. They should not be called directly. Due to limitations of "Storable", this serialization will only work for objects based on scalars, arrays or hashes. References to object within the object being frozen will result in clones upon thawing unless the other references are included in the same freeze operation. (See "Storable" for details.) # assume $alice and $bob are objects $alice->friends( $bob ); $bob->friends( $alice ); $alice2 = Storable::dclone( $alice ); # $bob was cloned, too, thanks to the reference die if $alice2->has_friend( $bob ); # get alice2's friend ($bob2) = $alice2->friends(); # preserved relationship between bob2 and alice2 die unless $bob2->has_friend( $alice ); "Class::InsideOut" also supports custom freeze and thaw hooks. When an object is frozen, if its class or any superclass provides "STORABLE_freeze_hook" functions, they are each called with the object as an argument *prior* to the rest of the freezing process. This allows for custom preparation for freezing, such as writing a cache to disk, closing network connections, or disconnecting database handles. Likewise, when a serialized object is thawed, if its class or any superclass provides "STORABLE_thaw_hook" functions, they are each called *after* the object has been thawed with the thawed object as an argument. User feedback on serialization needs and limitations is welcome. 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 that use memory addresses (e.g. "Class::InsideOut") are not fork-safe for Win32 on Perl 5.6. Win32 Perl 5.8 "fork" is supported. FUNCTIONS "id" $name{ id $object } = "Larry"; This is a shorter, mnemonic alias for "Scalar::Util::refaddr". It returns the memory address of an object (just like "refaddr") as the index to access the properties of an inside-out object. "options" Class::InsideOut::options( \%new_options ); %current_options = Class::InsideOut::options(); The "options" function sets default options for use with all subsquent "property" calls for the calling package. If called without arguments, this function will return the options currently in effect. When called with a hash reference of options, these will be joined with the existing defaults, overriding any options of the same name. Valid options include: * "privacy => 'public|private'" property rank => my %rank, { privacy => 'public' }; If the *privacy* option is equal to *public*, an accessor will be created with the same name as the label. If the accessor is passed an argument, the property will be set to the argument. The accessor always returns the value of the property. * "set_hook => \&code_ref" public age => my %age, { set_hook => sub { /^\d+$/ or die "must be an integer" } }; Defines an accessor hook for when values are set. The hook subroutine receives the entire argument list. $_ is locally aliased to the first argument for convenience. The property receives the value of $_. See "Accessor Hooks" for details. * "get_hook => \&code_ref" public list => my %list, { get_hook => sub { @$_ } }; Defines an accessor hook for when values are retrieved. $_ is locally aliased to the property value for the object. *The return value of the hook is passed through as the return value of the accessor.* See "Accessor Hooks" for details. "private" private weight => my %weight; private haircolor => my %hair_color, { %options }; This is an alias to "property" that also sets the privacy option to 'private'. It will override default options or options passed as an argument. "property" property name => my %name; property rank => my %rank, { %options }; Declares an inside-out property. Two arguments are required and a third is optional. The first is a label for the property; this label will be used for introspection and generating accessors and thus should be a valid perl identifier. The second argument must be the lexical hash that will be used to store data for that property. Note that the "my" keyword can be included as part of the argument rather than as a separate statement. The property will be tracked for memory cleanup during object destruction and for proper thread-safety. If a third, optional argument is provided, it must be a reference to a hash of options that will be applied to the property. Valid options are the same as listed for the "options" function and will override any default options that have been set. "public" public height => my %height; public age => my %age, { %options }; This is an alias to "property" that also sets the privacy option to 'public'. It will override default options or options passed as an argument. "register" register( bless $object, $class ); 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). 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, this 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. * Abigail-II. "Re: Tutorial: Introduction to Object-Oriented Programming". December 11, 2002. * demerphq. "Yet Another Perl Object Model (Inside Out Objects)". December 14, 2002. * xdg. "Threads and fork and CLONE, oh my!". August 11, 2005. * jdhedden. "Anti-inside-out-object-ism". December 9, 2005. 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 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. 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