tie::hash(3)
NAME
Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class defi
nitions 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 %{$_[0]};
# TIEHANDLE should return a reference to the actual
storage
sub DELETE { ... }
package NewExtraHash;
require Tie::Hash;
@ISA = (Tie::ExtraHash);
# All methods provided by default, define only those
needing overrides
# Accessors access the storage in %{$_[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] of $_[2].0};
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 for
gets 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, class
name". Associates a new hash instance with the speci
fied 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.
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 overwritten "TIEHANDLE" method
should return a hash reference, and the remaining methods
should operate on the hash referenced by the first argu
ment: - package ReportHash;
our @ISA = 'Tie::StdHash'; - sub TIEHASH {
my $storage = bless {}, shift;
warn "New ReportHash created, stored in $storage.0;
$storage - }
sub STORE {warn "Storing data with key $_[1] at $_[0].0;
$_[0]{$_[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 "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 $storage.0;
[$storage, @_] - }
sub STORE {warn "Storing data with key $_[1] at $_[0].0;
$_[0][0]{$_[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::Extra Hash.
MORE INFORMATION
- The packages relating to various DBM-related implementa
tions (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.