Edit File: Hash.pm
package Tie::Hash; our $VERSION = '1.05'; =head1 NAME Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes =head1 SYNOPSIS package NewHash; require Tie::Hash; @ISA = qw(Tie::Hash); sub DELETE { ... } # Provides needed method sub CLEAR { ... } # Overrides inherited method package NewStdHash; require Tie::Hash; @ISA = qw(Tie::StdHash); # All methods provided by default, define # only those needing overrides # Accessors access the storage in %{$_[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; my $pkg_new = $pkg -> can ('new'); if ($pkg_new and $pkg ne __PACKAGE__) { my $my_new = __PACKAGE__ -> can ('new'); if ($pkg_new == $my_new) { # # Prevent recursion # croak "$pkg must define either a TIEHASH() or a new() method"; } 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;
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