/usr/share/perl/5.22.1/Tie/Hash.pod is in perl-doc 5.22.1-9ubuntu0.6.
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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
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