162 lines
4.0 KiB
Perl
162 lines
4.0 KiB
Perl
package Tie::Hash;
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=head1 NAME
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Tie::Hash, Tie::StdHash - base class definitions for tied hashes
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=head1 SYNOPSIS
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package NewHash;
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require Tie::Hash;
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@ISA = (Tie::Hash);
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sub DELETE { ... } # Provides needed method
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sub CLEAR { ... } # Overrides inherited method
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package NewStdHash;
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require Tie::Hash;
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@ISA = (Tie::StdHash);
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# All methods provided by default, define only those needing overrides
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sub DELETE { ... }
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package main;
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tie %new_hash, 'NewHash';
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tie %new_std_hash, 'NewStdHash';
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=head1 DESCRIPTION
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This module provides some skeletal methods for hash-tying classes. See
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L<perltie> for a list of the functions required in order to tie a hash
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to a package. The basic B<Tie::Hash> package provides a C<new> method, as well
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as methods C<TIEHASH>, C<EXISTS> and C<CLEAR>. The B<Tie::StdHash> package
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provides most methods required for hashes in L<perltie>. It inherits from
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B<Tie::Hash>, and causes tied hashes to behave exactly like standard hashes,
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allowing for selective overloading of methods. The C<new> method is provided
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as grandfathering in the case a class forgets to include a C<TIEHASH> method.
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For developers wishing to write their own tied hashes, the required methods
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are briefly defined below. See the L<perltie> section for more detailed
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descriptive, as well as example code:
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=over
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=item TIEHASH classname, LIST
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The method invoked by the command C<tie %hash, classname>. Associates a new
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hash instance with the specified class. C<LIST> would represent additional
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arguments (along the lines of L<AnyDBM_File> and compatriots) needed to
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complete the association.
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=item STORE this, key, value
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Store datum I<value> into I<key> for the tied hash I<this>.
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=item FETCH this, key
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Retrieve the datum in I<key> for the tied hash I<this>.
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=item FIRSTKEY this
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Return the (key, value) pair for the first key in the hash.
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=item NEXTKEY this, lastkey
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Return the next key for the hash.
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=item EXISTS this, key
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Verify that I<key> exists with the tied hash I<this>.
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The B<Tie::Hash> implementation is a stub that simply croaks.
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=item DELETE this, key
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Delete the key I<key> from the tied hash I<this>.
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=item CLEAR this
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Clear all values from the tied hash I<this>.
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=back
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=head1 CAVEATS
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The L<perltie> documentation includes a method called C<DESTROY> as
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a necessary method for tied hashes. Neither B<Tie::Hash> nor B<Tie::StdHash>
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define a default for this method. This is a standard for class packages,
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but may be omitted in favor of a simple default.
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=head1 MORE INFORMATION
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The packages relating to various DBM-related implementations (F<DB_File>,
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F<NDBM_File>, etc.) show examples of general tied hashes, as does the
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L<Config> module. While these do not utilize B<Tie::Hash>, they serve as
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good working examples.
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=cut
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use Carp;
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use warnings::register;
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sub new {
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my $pkg = shift;
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$pkg->TIEHASH(@_);
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}
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# Grandfather "new"
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sub TIEHASH {
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my $pkg = shift;
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if (defined &{"${pkg}::new"}) {
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warnings::warn "WARNING: calling ${pkg}->new since ${pkg}->TIEHASH is missing"
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if warnings::enabled();
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$pkg->new(@_);
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}
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else {
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croak "$pkg doesn't define a TIEHASH method";
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}
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}
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sub EXISTS {
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my $pkg = ref $_[0];
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croak "$pkg doesn't define an EXISTS method";
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}
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sub CLEAR {
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my $self = shift;
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my $key = $self->FIRSTKEY(@_);
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my @keys;
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while (defined $key) {
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push @keys, $key;
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$key = $self->NEXTKEY(@_, $key);
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}
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foreach $key (@keys) {
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$self->DELETE(@_, $key);
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}
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}
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# The Tie::StdHash package implements standard perl hash behaviour.
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# It exists to act as a base class for classes which only wish to
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# alter some parts of their behaviour.
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package Tie::StdHash;
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@ISA = qw(Tie::Hash);
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sub TIEHASH { bless {}, $_[0] }
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sub STORE { $_[0]->{$_[1]} = $_[2] }
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sub FETCH { $_[0]->{$_[1]} }
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sub FIRSTKEY { my $a = scalar keys %{$_[0]}; each %{$_[0]} }
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sub NEXTKEY { each %{$_[0]} }
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sub EXISTS { exists $_[0]->{$_[1]} }
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sub DELETE { delete $_[0]->{$_[1]} }
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sub CLEAR { %{$_[0]} = () }
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1;
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