- NAME
- SYNOPSIS
- DESCRIPTION
- Inheriting from Tie::StdHash
- Inheriting from Tie::ExtraHash
SCALAR
,UNTIE
andDESTROY
- MORE INFORMATION
NAME
Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes
SYNOPSIS
package NewHash; require Tie::Hash;
@ISA = qw(Tie::Hash);
sub DELETE { ... } # Provides needed method sub CLEAR { ... } # Overrides inherited method
package NewStdHash; require Tie::Hash;
@ISA = qw(Tie::StdHash);
# All methods provided by default, define only those needing overrides # Accessors access the storage in %{$_[0]}; # TIEHASH should return a reference to the actual storage sub DELETE { ... }
package NewExtraHash; require Tie::Hash;
@ISA = qw(Tie::ExtraHash);
# All methods provided by default, define only those needing overrides # Accessors access the storage in %{$_[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
. 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. - 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 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 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
presence 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 Config module. While these do not utilize Tie::Hash, they serve as good working examples.