UNTIE and DESTROY
Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes
package NewHash;
require Tie::Hash;
@ISA = (Tie::Hash);
sub DELETE { ... } # Provides needed method
sub CLEAR { ... } # Overrides inherited method
package NewStdHash;
require Tie::Hash;
@ISA = (Tie::StdHash);
# All methods provided by default, define only those needing overrides
# Accessors access the storage in %{$_[0]};
# TIEHANDLE should return a reference to the actual storage
sub DELETE { ... }
package NewExtraHash;
require Tie::Hash;
@ISA = (Tie::ExtraHash);
# All methods provided by default, define only those needing overrides
# Accessors access the storage in %{$_[0][0]};
# TIEHANDLE 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"};
This module provides some skeletal methods for hash-tying classes. See
perltie for a list of the functions required in order to tie a hash
to a package. The basic Tie::Hash package provides a new method, as well
as methods TIEHASH, EXISTS and CLEAR. The Tie::StdHash and
Tie::ExtraHash packages
provide most methods for hashes described in perltie (the exceptions
are UNTIE and DESTROY). They cause tied hashes to behave exactly like standard hashes,
and allow for selective overwriting of methods. Tie::Hash grandfathers the
new method: it is used if TIEHASH is not defined
in the case a class forgets to include a TIEHASH method.
For developers wishing to write their own tied hashes, the required methods are briefly defined below. See the perltie section for more detailed descriptive, as well as example code:
tie %hash, classname. Associates a new
hash instance with the specified class. LIST would represent additional
arguments (along the lines of AnyDBM_File and compatriots) needed to
complete the association.
Verify that key exists with the tied hash this.
The Tie::Hash implementation is a stub that simply croaks.
The accessor methods assume that the actual storage for the data in the tied
hash is in the hash referenced by tied(%tiedhash). Thus overwritten
TIEHANDLE 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]
}
The accessor methods assume that the actual storage for the data in the tied
hash is in the hash referenced by (tied(%tiedhash))[0]. Thus overwritten
TIEHANDLE method should return an array reference with the first
element being a hash reference, and the remaining methods should operate on the
hash %{ $_[0]->[0] }:
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";
$_[0][0]{$_[1]} = $_[2]
}
The default TIEHANDLE method stores "extra" arguments to tie() starting
from offset 1 in the array referenced by tied(%tiedhash); this is the
same storage algorithm as in TIEHASH subroutine above. Hence, a typical
package inheriting from Tie::ExtraHash does not need to overwrite this
method.
UNTIE and DESTROY
The methods UNTIE and DESTROY are not defined in Tie::Hash,
Tie::StdHash, or Tie::ExtraHash. Tied hashes do not require
presense of these methods, but if defined, the methods will be called in
proper time, see perltie.
If needed, these methods should be defined by the package inheriting from Tie::Hash, Tie::StdHash, or Tie::ExtraHash.
The packages relating to various DBM-related implementations (DB_File, NDBM_File, etc.) show examples of general tied hashes, as does the Config module. While these do not utilize Tie::Hash, they serve as good working examples.