Hash::FieldHash - Lightweight field hash for inside-out objects

This document describes Hash::FieldHash version 0.15.

    use Hash::FieldHash qw(:all);

    fieldhash my %foo;

    fieldhashes \my(%bar, %baz);

    {
            my $o = Something->new();

            $foo{$o} = 42;

            print $foo{$o}; # => 42
    }
    # when $o is released, $foo{$o} is also deleted,
    # so %foo is empty in here.

    # in a class
    {
            package Foo;
            use Hash::FieldHash qw(:all);

            fieldhash my %bar, 'bar'; # make an accessor
    }

    my $obj = bless {}, 'Foo';
    $obj->bar(10); # does $bar{$obj} = 10

Hash::FieldHash provides the field hash mechanism which supports the inside-out technique.

You may know Hash::Util::FieldHash. It's a very useful module, but too complex to understand the functionality and only available in 5.10. H::U::F::Compat is available for pre-5.10, but it is too slow to use.

This is a better alternative to H::U::F with following features:

• Simpler interface

  Hash::FieldHash provides a few functions: fieldhash() and fieldhashes(). That's enough.

• Higher performance

  Hash::FieldHash is faster than Hash::Util::FieldHash, because its internals use simpler structures.

• Relic support

  Although Hash::FieldHash uses a new feature introduced in Perl 5.10, the uvar magic for hashes described in "GUTS" in Hash::Util::Fieldhash, it supports Perl 5.8 using the traditional tie-hash layer.

• fieldhash(%hash, ?$name, ?$package)

  Creates a field hash. The first argument must be a hash.

  Optional $name and $package indicate the name of the field, which will create rw-accessors, using the same name as $name.

  Returns nothing.

• fieldhashes(@hash_refs)

  Creates a number of field hashes. All the arguments must be hash references.

  Returns nothing.

• from_hash($object, \%fields)

  Fills the named fields associated with $object with %fields. The keys of %fields can be simple or fully qualified.

  Returns $object.

• to_hash($object, ?-fully_qualify)

  Serializes $object into a hash reference.

  If the -fully_qualify option is supplied , field keys are fully qualified.

  For example:

        package MyClass;
        use FieldHash qw(:all);
  
        fieldhash my %foo => 'foo';
  
        sub new{
                my $class = shift;
                my $self  = bless {}, $class;
                return from_hash($self, @_);
        }
  
        package MyDerivedClass;
        use parent -norequire => 'MyClass';
        use FieldHash qw(:all);
  
        fieldhash my %bar => 'bar';
  
        package main;
  
        my $o = MyDerivedClass->new(foo => 10, bar => 20);
        my $p = MyDerivedClass->new('MyClass::foo' => 10, 'MyDerivedClass::bar' => 20);
  
        use Data::Dumper;
        print Dumper($o->to_hash());
        # $VAR1 = { foo => 10, bar => 20 }
  
        print Dumper($o->to_hash(-fully_qualify));
        # $VAR1 = { 'MyClass::foo' => 10, 'MyDerived::bar' => 20 }
  

As Hash::Util::FieldHash does, Hash::FieldHash fully supports threading using the CLONE method.

Hash::FieldHash itself does not leak memory, but it may leak memory when you uses hash references as field hash keys because of an issue of perl 5.10.0.

Hash::FieldHash accepts only references and registered addresses as its keys, whereas Hash::Util::FieldHash accepts any type of scalars.

According to "The Generic Object" in Hash::Util::FieldHash, Non-reference keys in H::U::F are used for class fields. That is, all the fields defined by H::U::F act as both object fields and class fields by default. It seems confusing; if you do not want them to be class fields, you must check the type of $self explicitly. In addition, these class fields are never inherited. This behavior seems problematic, so Hash::FieldHash restricts the type of keys.

While Hash::Util::FieldHash uses refaddr as the IDs of field hash keys, Hash::FieldHash allocates arbitrary integers as the IDs.

The accessors fieldhash() creates are chainable accessors. That is, it returns the $object (i.e. $self) with a parameter, where as it returns the $value without it.

For example:

my $o = YourClass->new();
$o->foo(42);           # returns $o itself
my $value = $o->foo(); # retuns 42

Perl 5.8.5 or later, and a C compiler.

No bugs have been reported.

Please report any bugs or feature requests to the author.

Hash::Util::FieldHash.

Hash::Util::FieldHash::Compat.

"Magic Virtual Tables" in perlguts.

Class::Std describes the inside-out technique.

Fuji, Goro (gfx) <gfuji(at)cpan.org>.

Copyright (c) 2009-2010, Fuji, Goro. All rights reserved.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.