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Programming Perl

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The Standard Perl Library
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7.2.72 Tie::Hash, Tie::StdHash - Base Class Definitions for Tied Hashes

package NewHash;
require Tie::Hash;

@ISA = (Tie::Hash);

sub DELETE { ... }          # Provides additional method
sub CLEAR { ... }           # Overrides inherited method

package NewStdHash;
require Tie::Hash;

@ISA = (Tie::StdHash);

sub DELETE { ... }

package main;

tie %new_hash, "NewHash";
tie %new_std_hash, "NewStdHash";

This module provides some skeletal methods for hash-tying classes. (See Chapter 5 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 package provides most methods required for hashes. It inherits from Tie::Hash, and causes tied hashes to behave exactly like standard hashes, allowing for selective overloading of methods. The new() method is provided as grandfathering in 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. (Chapter 5 not only documents these methods, but also has sample code.)

TIEHASH ClassName, LIST

The method invoked by the command:

tie %hash, ClassName, LIST

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 value into key for the tied hash this.

FETCH this, key

Retrieve the value associated with key for the tied hash this.

FIRSTKEY this

Return the key/value pair for the first key in hash this.

NEXTKEY this, lastkey

Return the next key/value pair for the hash.

EXISTS this, key

Verify that key exists with the tied hash this.

DELETE this, key

Delete key from the tied hash this.

CLEAR this

Clear all values from the tied hash this.

Chapter 5 includes a method called DESTROY() as a "necessary" method for tied hashes. However, it is not actually required, and neither Tie::Hash nor Tie::StdHash defines a default for this method.

7.2.72.1 See also

The library modules relating to various DBM-related implementations (DB_File, GDBM_File, NDBM_File, ODBM_File, and SDBM_File) show examples of general tied hashes, as does the Config module. While these modules do not utilize Tie::Hash, they serve as good working examples.


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