Manpages - Tie_Hash.3perl

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 SCALAR in perltie 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.