Tie::Hash
Tie::Hash(3) Perl Programmers Reference Guide Tie::Hash(3)
NAME
Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for
tied hashes
SYNOPSIS
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 %{$_[0]};
# TIEHASH should return a reference to the actual storage
sub DELETE { ... }
package NewExtraHash;
require Tie::Hash;
@ISA = (Tie::ExtraHash);
# All methods provided by default, define only those needing overrides
# Accessors access the storage in %{$_[0][0]};
# 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"};
DESCRIPTION
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:
TIEHASH classname, LIST
The method invoked by the command "tie %hash, classname". Asso-
ciates 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.
STORE this, key, value
Store datum value into key for the tied hash this.
FETCH this, key
Retrieve the datum in key for the tied hash this.
FIRSTKEY this
Return the first key in the hash.
NEXTKEY this, lastkey
Return the next key in the hash.
EXISTS this, key
Verify that key exists with the tied hash this.
The Tie::Hash implementation is a stub that simply croaks.
DELETE this, key
Delete the key key from the tied hash this.
CLEAR this
Clear all values from the tied hash this.
SCALAR this
Returns what evaluating the hash in scalar context yields.
Tie::Hash does not implement this method (but Tie::StdHash and
Tie::ExtraHash do).
Inheriting from Tie::StdHash
The accessor methods assume that the actual storage for the data in the
tied hash is in the hash referenced by "tied(%tiedhash)". Thus over-
written "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]
}
Inheriting from Tie::ExtraHash
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 "TIEHASH" 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::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 "TIEHASH" 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 typi-
cal package inheriting from Tie::ExtraHash does not need to overwrite
this method.
"SCALAR", "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.
"SCALAR" is only defined in Tie::StdHash and Tie::ExtraHash.
If needed, these methods should be defined by the package inheriting
from Tie::Hash, Tie::StdHash, or Tie::ExtraHash. See "SCALAR" in pertie
to find out what happens when "SCALAR" does not exist.
MORE INFORMATION
The packages relating to various DBM-related implementations (DB_File,
NDBM_File, etc.) show examples of general tied hashes, as does the Con-
fig module. While these do not utilize Tie::Hash, they serve as good
working examples.
perl v5.8.6 2001-09-21 Tie::Hash(3)
