/ 
Alter-0.07
River stage zero No dependents
/ Alter

NAME

Alter - Alter Ego Objects

Synopsis

package MyClass;
use Alter ego => {}; # Alter ego of type hash

# Put data in it
my $obj = \ do { my $o };
ego( $obj)->{a} = 1;
ego( $obj)->{b} = 2;

# Retrieve it again
print ego( $obj)->{ b}, "\n"; # prints 2

package OtherClass;
defined( ego $obj) or die; # dies, OtherClass hasn't set an alter ego

# Direct access to the corona of alter egos
my $crown = Alter::corona $obj;

Functions

Basic Functions

The functions described here accept a first argument named $obj. Despite the name, $obj can be any reference, it doesn't have to be blessed (though it usually will be). It is a fatal error if it is not a reference or if the reference points to a read-only value.

ego($obj)

Retrieves the class-specific alter ego assigned to $obj by alter() or by autovivification if that is enabled. If neither is the case, an undefined value is returned. The class is the package into which the call to ego() is compiled.

alter($obj, $val)

Assigns $val to the reference $obj as an alter ego for the caller's class. The class is the package into which the call to alter is compiled. Returns $obj (not the value assigned).

Alter::corona( $obj)

Direct access to the corona of alter ego's of $obj. The corona is a hash keyed by class name in which the alter ego's of an object are stored. Unlike alter() and ego(), this function is not caller-sensitive. Returns a reference to the corona hash, which is created if necessary. This function is not exported, if needed it must be called fully qualified.

Alter::is_xs

Returns a true value if the XS implementation of Alter is active, false if the pure Perl fallback is in place.

Autovivification

You can set one of the types SCALAR, ARRAY, HASH or GLOB for autovivification of the alter ego. This is done by specifying the type in a use statement, as in

package MyClass;
use Alter 'ARRAY';

If the ego() function is later called from MyClass before an alter ego has been specified using alter(), a new array reference will be created and returned. Autovivification happens only once per class and object. (You would have to delete the class entry from the object's corona to make it happen again.)

The type specification can also be a referece of the appropriate type, so [] can be used for "ARRAY" and {} for "HASH" (globrefs and scalar refs can also be used, but are less attractive).

Type specification can be combined with function imports. Thus

package MyClass;
use Alter ego => {};

imports the ego() function and specifies a hash tape for autovivification. With autovivification you will usually not need to import the alter function at all.

Specifying "NOAUTO" in place of a type specification switches autovivification off for the current class. This is also the default.

Serialization Support

Serialization is supported for human inspection in Data::Dumper style and for disk storage and cloning in Storable style.

For Data::Dumper support Alter provides the class Alter::Dumper for use as a base class, which contains the single method Dumper. Dumper returns a string that represents a hash in Data::Dumper format. The hash shows all alter egos that have been created for the object, keyed by class. An additional key "(body)" (which can't be a class name) holds the actual body of the object. Formatting- and other options of Data::Dumper will change the appearance of the dump string, with the exception of $Data::Dumper::Purity, which will always be 1. Dumper can also be imported from Alter directly.

Note that eval()-ing a dump string will not restore the object, but rather create a hash as described. Re-creation of an object is only available through Storable.

For Storable support the class Alter::Storable is provided with the methods STORABLE_freeze, STORABLE_thaw and STORABLE_attach. The three functions are also exported by Alter Their interaction with Storable is described there.

Inheriting these methods allows Storable's own functions freeze() and thaw() to save and restore an object's alter egos along with the actual object body. Other Storable functions, like store, nstore, retrieve, etc. also become Alter-aware. There is one exception. Storable::dclone cannot be used on Alter-based objects. To clone an Alter-based object, Storable::thaw(Storable::freeze($obj) must be called explicitly.

Per default, both Alter::Dumper and Alter::Storable are made base classes of the current class (if necessary) by use Alter. If the function Dumper is imported, or if -dumper is specified, Alter::Dumper is not made a base class. If any of the functions STORABLE_freeze, STORABLE_thaw or STORABLE_attach is imported, or if -storable is specified, Alter::Storable is not made a base class.

Fallback Perl Implementation

Alter is properly an XS module and a suitable C compiler is required to build it. If compilation isn't possible, the XS part is replaced with a pure Perl implementation Alter::AlterXS_in_perl. That happens automatically at load time when loading the XS part fails. The boolean function Alter::is_xs tells (in the obvious way) which implementation is active. If, for some reason, you want to run the Perl fallback when the XS version is available, set the environment variable PERL_ALTER_NO_XS to a true value before Alter is loaded.

This fallback is not a full replacement for the XS implementation. Besides being markedly slower, it lacks key features in that it is not automatically garbage-collected and not thread-safe. Instead, Alter::AlterXS_in_perl provides a CLONE method for thread safety and a universal destructor Alter::Destructor::DESTROY for garbage collection. A class that uses the pure Perl implementation of Alter will obtain this destructor through inheritance (unless -destroy is specified with the use statement). So at the surface thread-safety and garbage-collection are retained. However, if you want to add your own destructor to a class, you must make sure that both (all) destructors are called as needed. Perl only calls the first one it meets on the @ISA tree and that's it.

Otherwise the fallback implementation works like the original. If compilation has problems, it should allow you to run test cases to help decide if it's worth trying. To make sure that production code doesn't inadvertently run with the Perl implementation

Alter::is_xs or die "XS implementation of Alter required";

can be used.

Exports

None by default, alter() and ego() upon request. Further available are STORABLE_freeze, STORABLE_thaw and STORABLE_attach as well as Dumper. :all imports all these functions.

Environment

The environment variable PERL_ALTER_NO_XS is inspected once at load time to decide whether to load the XS version of Alter or the pure Perl fallback. At run time it has no effect.

Description

The Alter module is meant to facilitate the creation of classes that support black-box inheritance, which is to say that an Alter based class can be a parent class for any other class, whether itself Alter based or not. Inside-out classes also have that property. Alter is thus an alternative to the inside-out technique of class construction. In some respects, Alter objects are easier to handle.

Alter objects support the same data model as traditional Perl objects. To each class, an Alter object presents an arbitrary reference, the object's alter ego. The type of reference and how it is used are the entirely the class's business. In particular, the common practice of using a hash whose keys represent object fields still applies, only each class sees its individual hash.

Alter based objects are garbage-collected and thread-safe without additional measures.

Alter also supports Data::Dumper and Storable in a generic way, so that Alter based objects can be easily be viewed and made persistent (within the limitations of the respective modules).

Alter works by giving every object a class-specific alter ego, which can be any scalar, for its (the classe's) specific needs for data storage. The alter ego is set by the alter() function (or by autovivification), usually once per class and object at initialization time. It is retrieved by the ego() function in terms of which a class will define its accessors.

That works by magically (in the technical sense of PERL_MAGIC_ext) assigning a hash keyed by classname, the corona, to every object that takes part in the game. The corona holds the individual alter ego's for each class. It is created when needed and stays with an object for its lifetime. It is subject to garbage collection when the object goes out of scope. Normally the corona is invisible to the user, but the Alter::corona() function (not exported) allows direct access if needed.

Example

The example first shows how a class Name is built from two classes First and Last which implement the first and last names separately. First treats its objects as hashes whereas Last uses them as arrays. Nevertheless, the code in Name that joins the two classes via subclassing is straightforward.

The second part of the example shows that Alter classes actually support black-box inheritance. Here, we use an object of class IO::File as the "carrier" object. This must be a globref to work. This object can be initialized to the class Name, which in part sees it as a hash, in another part as an array. Methods of both classes now work on the object.

#!/usr/local/bin/perl
use strict; use warnings; $| = 1;

# Show that class Name works
my $prof = Name->new( qw( Albert Einstein));
print $prof->fname, "\n";
print $prof->lname, "\n";
print $prof->name, "\n";


# Share an object with a foreign class
{
    package Named::Handle;
    use base 'IO::File';
    push our @ISA, qw( Name);

    sub new {
        my $class = shift;
        my ( $file, $first, $last) = @_;
        $class->IO::File::new( $file)->init( $first, $last);
    }

    sub init {
        my $nh = shift;
        $nh->Name::init( @_);
    }
}

my $nh = Named::Handle->new( '/dev/null', 'Bit', 'Bucket');
print "okay, at eof\n" if $nh->eof; # IO::File methods work
print $nh->name, "\n";      # ...as do Name methods

exit;

#######################################################################

{
    package First;
    use Alter qw( alter ego);

    sub new {
        my $class = shift;
        bless( \ my $o, $class)->init( @_);
    }

    sub init {
        my $f = shift;
        alter $f, { name => shift };
        $f;
    }

    sub fname {
        my $h = ego shift;
        @_ ? $h->{ name} = shift : $h->{ name};
    }
}

{
    package Last;
    use Alter qw( alter ego);

    sub new {
        my $class = shift;
        bless( \ my $o, $class)->init( @_);
    }

    sub init {
        my $l = shift;
        alter $l, [ shift];
        $l;
    }

    sub lname {
        my $l = ego( shift);
        @_ ? $l->[ 0] = shift : $l->[ 0];
    }
}

{
    package Name;
    use base 'First';
    use base 'Last';

    sub init {
        my $n = shift;
        $n->First::init( shift);
        $n->Last::init( shift);
    }

    sub name {
        my $n = shift;
        join ' ' => $n->fname, $n->lname;
    }
}

__END__

Thanks

Thanks to Abigail who invented the inside-out technique, showhing what the problem is with Perl inheritance and how it could be overcome with just a little stroke of genius.

Thanks also to Jerry Hedden for making me aware of the possibilities of ext magic on which this implementation of Alter is built.

Author

Anno Siegel, <anno4000@zrz.tu-berlin.de>

COPYRIGHT AND LICENSE

Copyright (C) 2007 by Anno Siegel

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.7 or, at your option, any later version of Perl 5 you may have available.