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269 lines
7.4 KiB
Perl
269 lines
7.4 KiB
Perl
package Tie::Hash;
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our $VERSION = '1.04';
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=head1 NAME
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Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes
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=head1 SYNOPSIS
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package NewHash;
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require Tie::Hash;
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@ISA = qw(Tie::Hash);
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sub DELETE { ... } # Provides needed method
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sub CLEAR { ... } # Overrides inherited method
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package NewStdHash;
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require Tie::Hash;
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@ISA = qw(Tie::StdHash);
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# All methods provided by default, define only those needing overrides
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# Accessors access the storage in %{$_[0]};
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# TIEHASH should return a reference to the actual storage
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sub DELETE { ... }
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package NewExtraHash;
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require Tie::Hash;
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@ISA = qw(Tie::ExtraHash);
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# All methods provided by default, define only those needing overrides
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# Accessors access the storage in %{$_[0][0]};
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# TIEHASH should return an array reference with the first element being
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# the reference to the actual storage
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sub DELETE {
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$_[0][1]->('del', $_[0][0], $_[1]); # Call the report writer
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delete $_[0][0]->{$_[1]}; # $_[0]->SUPER::DELETE($_[1])
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}
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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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tie %new_extra_hash, 'NewExtraHash',
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sub {warn "Doing \U$_[1]\E of $_[2].\n"};
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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> and
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B<Tie::ExtraHash> packages
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provide most methods for hashes described in L<perltie> (the exceptions
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are C<UNTIE> and C<DESTROY>). They cause tied hashes to behave exactly like standard hashes,
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and allow for selective overwriting of methods. B<Tie::Hash> grandfathers the
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C<new> method: it is used if C<TIEHASH> is not defined
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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 4
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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 first key in the hash.
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=item NEXTKEY this, lastkey
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Return the next key in 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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=item SCALAR this
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Returns what evaluating the hash in scalar context yields.
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B<Tie::Hash> does not implement this method (but B<Tie::StdHash>
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and B<Tie::ExtraHash> do).
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=back
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=head1 Inheriting from B<Tie::StdHash>
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The accessor methods assume that the actual storage for the data in the tied
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hash is in the hash referenced by C<tied(%tiedhash)>. Thus overwritten
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C<TIEHASH> method should return a hash reference, and the remaining methods
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should operate on the hash referenced by the first argument:
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package ReportHash;
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our @ISA = 'Tie::StdHash';
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sub TIEHASH {
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my $storage = bless {}, shift;
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warn "New ReportHash created, stored in $storage.\n";
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$storage
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}
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sub STORE {
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warn "Storing data with key $_[1] at $_[0].\n";
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$_[0]{$_[1]} = $_[2]
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}
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=head1 Inheriting from B<Tie::ExtraHash>
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The accessor methods assume that the actual storage for the data in the tied
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hash is in the hash referenced by C<(tied(%tiedhash))-E<gt>[0]>. Thus overwritten
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C<TIEHASH> method should return an array reference with the first
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element being a hash reference, and the remaining methods should operate on the
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hash C<< %{ $_[0]->[0] } >>:
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package ReportHash;
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our @ISA = 'Tie::ExtraHash';
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sub TIEHASH {
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my $class = shift;
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my $storage = bless [{}, @_], $class;
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warn "New ReportHash created, stored in $storage.\n";
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$storage;
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}
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sub STORE {
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warn "Storing data with key $_[1] at $_[0].\n";
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$_[0][0]{$_[1]} = $_[2]
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}
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The default C<TIEHASH> method stores "extra" arguments to tie() starting
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from offset 1 in the array referenced by C<tied(%tiedhash)>; this is the
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same storage algorithm as in TIEHASH subroutine above. Hence, a typical
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package inheriting from B<Tie::ExtraHash> does not need to overwrite this
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method.
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=head1 C<SCALAR>, C<UNTIE> and C<DESTROY>
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The methods C<UNTIE> and C<DESTROY> are not defined in B<Tie::Hash>,
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B<Tie::StdHash>, or B<Tie::ExtraHash>. Tied hashes do not require
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presence of these methods, but if defined, the methods will be called in
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proper time, see L<perltie>.
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C<SCALAR> is only defined in B<Tie::StdHash> and B<Tie::ExtraHash>.
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If needed, these methods should be defined by the package inheriting from
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B<Tie::Hash>, B<Tie::StdHash>, or B<Tie::ExtraHash>. See L<perltie/"SCALAR">
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to find out what happens when C<SCALAR> does not exist.
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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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my $pkg_new = $pkg -> can ('new');
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if ($pkg_new and $pkg ne __PACKAGE__) {
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my $my_new = __PACKAGE__ -> can ('new');
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if ($pkg_new == $my_new) {
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#
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# Prevent recursion
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#
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croak "$pkg must define either a TIEHASH() or a new() method";
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}
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warnings::warnif ("WARNING: calling ${pkg}->new since " .
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"${pkg}->TIEHASH is missing");
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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); # would inherit new() only
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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 { %{$_[0]} = () }
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sub SCALAR { scalar %{$_[0]} }
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package Tie::ExtraHash;
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sub TIEHASH { my $p = shift; bless [{}, @_], $p }
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sub STORE { $_[0][0]{$_[1]} = $_[2] }
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sub FETCH { $_[0][0]{$_[1]} }
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sub FIRSTKEY { my $a = scalar keys %{$_[0][0]}; each %{$_[0][0]} }
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sub NEXTKEY { each %{$_[0][0]} }
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sub EXISTS { exists $_[0][0]->{$_[1]} }
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sub DELETE { delete $_[0][0]->{$_[1]} }
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sub CLEAR { %{$_[0][0]} = () }
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sub SCALAR { scalar %{$_[0][0]} }
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1;
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