Tie::Hash(3p) Perl Programmers Reference Guide Tie::Hash(3p)
Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class defini-
tions 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 %{$_[0]};
# TIEHASH should return a reference to the actual storage
sub DELETE { ... }
package NewExtraHash;
require Tie::Hash;
@ISA = (Tie::ExtraHash);
# All methods provided by default, define only those needing overrides
# Accessors access the storage in %{$_[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"};
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 pack-
age 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 perl-
tie (the exceptions are "UNTIE" and "DESTROY"). They cause
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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". Associates a new hash instance with the
specified class. "LIST" would represent additional argu-
ments (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
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"tied(%tiedhash)". Thus overwritten "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 typical package inherit-
ing 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 presence of these methods, but if defined, the
methods will be called in proper time, see perltie.
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"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.
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 util-
ize Tie::Hash, they serve as good working examples.
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