package Tie::Hash;

our $VERSION = '1.03';

=head1 NAME

Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes

=head1 SYNOPSIS

package NewHash;
require Tie::Hash;

@ISA = qw(Tie::Hash);

sub DELETE { ... } # Provides needed method
sub CLEAR { ... } # Overrides inherited method


package NewStdHash;
require Tie::Hash;

@ISA = qw(Tie::StdHash);

# All methods provided by default, define only those needing overrides
# Accessors access the storage in %{$_[0]};
# TIEHASH should return a reference to the actual storage
sub DELETE { ... }

package NewExtraHash;
require Tie::Hash;

@ISA = qw(Tie::ExtraHash);

# All methods provided by default, define only those needing overrides
# Accessors access the storage in %{$_[0][0]};
# TIEHASH should return an array reference with the first element being
# the reference to the actual storage
sub DELETE {
$_[0][1]->('del', $_[0][0], $_[1]); # Call the report writer
delete $_[0][0]->{$_[1]}; # $_[0]->SUPER::DELETE($_[1])
}


package main;

tie %new_hash, 'NewHash';
tie %new_std_hash, 'NewStdHash';
tie %new_extra_hash, 'NewExtraHash',
sub {warn "Doing \U$_[1]\E of $_[2].\n"};

=head1 DESCRIPTION

This module provides some skeletal methods for hash-tying classes. See
L<perltie> for a list of the functions required in order to tie a hash
to a package. The basic B<Tie::Hash> package provides a C<new> method, as well
as methods C<TIEHASH>, C<EXISTS> and C<CLEAR>. The B<Tie::StdHash> and
B<Tie::ExtraHash> packages
provide most methods for hashes described in L<perltie> (the exceptions
are C<UNTIE> and C<DESTROY>). They cause tied hashes to behave exactly like standard hashes,
and allow for selective overwriting of methods. B<Tie::Hash> grandfathers the
C<new> method: it is used if C<TIEHASH> is not defined
in the case a class forgets to include a C<TIEHASH> method.

For developers wishing to write their own tied hashes, the required methods
are briefly defined below. See the L<perltie> section for more detailed
descriptive, as well as example code:

=over 4

=item TIEHASH classname, LIST

The method invoked by the command C<tie %hash, classname>. Associates a new
hash instance with the specified class. C<LIST> would represent additional
arguments (along the lines of L<AnyDBM_File> and compatriots) needed to
complete the association.

=item STORE this, key, value

Store datum I<value> into I<key> for the tied hash I<this>.

=item FETCH this, key

Retrieve the datum in I<key> for the tied hash I<this>.

=item FIRSTKEY this

Return the first key in the hash.

=item NEXTKEY this, lastkey

Return the next key in the hash.

=item EXISTS this, key

Verify that I<key> exists with the tied hash I<this>.

The B<Tie::Hash> implementation is a stub that simply croaks.

=item DELETE this, key

Delete the key I<key> from the tied hash I<this>.

=item CLEAR this

Clear all values from the tied hash I<this>.

=item SCALAR this

Returns what evaluating the hash in scalar context yields.

B<Tie::Hash> does not implement this method (but B<Tie::StdHash>
and B<Tie::ExtraHash> do).

=back

=head1 Inheriting from B<Tie::StdHash>

The accessor methods assume that the actual storage for the data in the tied
hash is in the hash referenced by C<tied(%tiedhash)>. Thus overwritten
C<TIEHASH> method should return a hash reference, and the remaining methods
should operate on the hash referenced by the first argument:

package ReportHash;
our @ISA = 'Tie::StdHash';

sub TIEHASH {
my $storage = bless {}, shift;
warn "New ReportHash created, stored in $storage.\n";
$storage
}
sub STORE {
warn "Storing data with key $_[1] at $_[0].\n";
$_[0]{$_[1]} = $_[2]
}


=head1 Inheriting from B<Tie::ExtraHash>

The accessor methods assume that the actual storage for the data in the tied
hash is in the hash referenced by C<(tied(%tiedhash))-E<gt>[0]>. Thus overwritten
C<TIEHASH> method should return an array reference with the first
element being a hash reference, and the remaining methods should operate on the
hash C<< %{ $_[0]->[0] } >>:

package ReportHash;
our @ISA = 'Tie::ExtraHash';

sub TIEHASH {
my $class = shift;
my $storage = bless [{}, @_], $class;
warn "New ReportHash created, stored in $storage.\n";
$storage;
}
sub STORE {
warn "Storing data with key $_[1] at $_[0].\n";
$_[0][0]{$_[1]} = $_[2]
}

The default C<TIEHASH> method stores "extra" arguments to tie() starting
from offset 1 in the array referenced by C<tied(%tiedhash)>; this is the
same storage algorithm as in TIEHASH subroutine above. Hence, a typical
package inheriting from B<Tie::ExtraHash> does not need to overwrite this
method.

=head1 C<SCALAR>, C<UNTIE> and C<DESTROY>

The methods C<UNTIE> and C<DESTROY> are not defined in B<Tie::Hash>,
B<Tie::StdHash>, or B<Tie::ExtraHash>. Tied hashes do not require
presence of these methods, but if defined, the methods will be called in
proper time, see L<perltie>.

C<SCALAR> is only defined in B<Tie::StdHash> and B<Tie::ExtraHash>.

If needed, these methods should be defined by the package inheriting from
B<Tie::Hash>, B<Tie::StdHash>, or B<Tie::ExtraHash>. See L<perltie/"SCALAR">
to find out what happens when C<SCALAR> does not exist.

=head1 MORE INFORMATION

The packages relating to various DBM-related implementations (F<DB_File>,
F<NDBM_File>, etc.) show examples of general tied hashes, as does the
L<Config> module. While these do not utilize B<Tie::Hash>, they serve as
good working examples.

=cut

use Carp;
use warnings::register;

sub new {
my $pkg = shift;
$pkg->TIEHASH(@_);
}

# Grandfather "new"

sub TIEHASH {
my $pkg = shift;
if (defined &{"${pkg}::new"}) {
warnings::warnif("WARNING: calling ${pkg}->new since ${pkg}->TIEHASH is missing");
$pkg->new(@_);
}
else {
croak "$pkg doesn't define a TIEHASH method";
}
}

sub EXISTS {
my $pkg = ref $_[0];
croak "$pkg doesn't define an EXISTS method";
}

sub CLEAR {
my $self = shift;
my $key = $self->FIRSTKEY(@_);
my @keys;

while (defined $key) {
push @keys, $key;
$key = $self->NEXTKEY(@_, $key);
}
foreach $key (@keys) {
$self->DELETE(@_, $key);
}
}

# The Tie::StdHash package implements standard perl hash behaviour.
# It exists to act as a base class for classes which only wish to
# alter some parts of their behaviour.

package Tie::StdHash;
# @ISA = qw(Tie::Hash); # would inherit new() only

sub TIEHASH { bless {}, $_[0] }
sub STORE { $_[0]->{$_[1]} = $_[2] }
sub FETCH { $_[0]->{$_[1]} }
sub FIRSTKEY { my $a = scalar keys %{$_[0]}; each %{$_[0]} }
sub NEXTKEY { each %{$_[0]} }
sub EXISTS { exists $_[0]->{$_[1]} }
sub DELETE { delete $_[0]->{$_[1]} }
sub CLEAR { %{$_[0]} = () }
sub SCALAR { scalar %{$_[0]} }

package Tie::ExtraHash;

sub TIEHASH { my $p = shift; bless [{}, @_], $p }
sub STORE { $_[0][0]{$_[1]} = $_[2] }
sub FETCH { $_[0][0]{$_[1]} }
sub FIRSTKEY { my $a = scalar keys %{$_[0][0]}; each %{$_[0][0]} }
sub NEXTKEY { each %{$_[0][0]} }
sub EXISTS { exists $_[0][0]->{$_[1]} }
sub DELETE { delete $_[0][0]->{$_[1]} }
sub CLEAR { %{$_[0][0]} = () }
sub SCALAR { scalar %{$_[0][0]} }

1;