Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class definitions for tied hashes
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"};
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).
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]
}
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.
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 perltie/``SCALAR''
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 utilize Tie::Hash, they serve as
good working examples.
|