Hash::FieldHash(3pm) User Contributed Perl Documentation Hash::FieldHash(3pm)

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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:

"Hash::FieldHash" provides a few functions: fieldhash() and fieldhashes(). That's enough.
"Hash::FieldHash" is faster than "Hash::Util::FieldHash", because its internals use simpler structures.
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.

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.

2024-04-01 perl v5.38.2