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# NAME

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

# VERSION

This document describes Hash::FieldHash version 0.15.

# SYNOPSIS

       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

# DESCRIPTION

`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](https://metacpan.org/pod/Hash::Util::Fieldhash#GUTS),
   it supports Perl 5.8 using the traditional tie-hash layer.

# INTERFACE

## Exportable functions

- `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 }

# ROBUSTNESS

## Thread support

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

## Memory leaks

`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.

# NOTES

## The type of field hash keys

`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](https://metacpan.org/pod/Hash::Util::FieldHash#The-Generic-Object),
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.

## The ID of field hash keys

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

## What accessors return

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

# DEPENDENCIES

Perl 5.8.5 or later, and a C compiler.

# BUGS

No bugs have been reported.

Please report any bugs or feature requests to the author.

# SEE ALSO

[Hash::Util::FieldHash](https://metacpan.org/pod/Hash::Util::FieldHash).

[Hash::Util::FieldHash::Compat](https://metacpan.org/pod/Hash::Util::FieldHash::Compat).

["Magic Virtual Tables" in perlguts](https://metacpan.org/pod/perlguts#Magic-Virtual-Tables).

[Class::Std](https://metacpan.org/pod/Class::Std) describes the inside-out technique.

# AUTHOR

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

# LICENSE AND COPYRIGHT

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.

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