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# Tie::Hash

Perl 5 version 18.2 documentation
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# Tie::Hash

- NAME
- SYNOPSIS
- DESCRIPTION
- Inheriting from *Tie::StdHash*
- Inheriting from *Tie::ExtraHash*
- SCALAR, UNTIE and DESTROY
- 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 perltie 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.

Page index

- NAME
- SYNOPSIS
- DESCRIPTION
- Inheriting from *Tie::StdHash*
- Inheriting from *Tie::ExtraHash*
- SCALAR, UNTIE and DESTROY
- MORE INFORMATION