# Tie::Hash - man - phpMan

## NAME
    [Tie::Hash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown), [Tie::StdHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AStdHash/markdown), [Tie::ExtraHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AExtraHash/markdown) - base class definitions for
    tied hashes

## SYNOPSIS
        package NewHash;
        require [Tie::Hash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown);

        @ISA = qw([Tie::Hash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown));

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


        package NewStdHash;
        require [Tie::Hash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown);

        @ISA = qw([Tie::StdHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AStdHash/markdown));

        # 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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown);

        @ISA = qw([Tie::ExtraHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AExtraHash/markdown));

        # 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]};           #  $_[0]->[SUPER::DELETE](https://www.chedong.com/phpMan.php/perldoc/SUPER%3A%3ADELETE/markdown)($_[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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown) package provides a "new" method, as well as
    methods "TIEHASH", "EXISTS" and "CLEAR". The [Tie::StdHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AStdHash/markdown) and
    [Tie::ExtraHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AExtraHash/markdown) 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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown) 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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown) 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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown) does not implement this method (but [Tie::StdHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AStdHash/markdown) and
        [Tie::ExtraHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AExtraHash/markdown) do).

## Inheriting from [Tie::StdHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AStdHash/markdown)
    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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AStdHash/markdown)';

      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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AExtraHash/markdown)
    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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AExtraHash/markdown)';

      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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AExtraHash/markdown) does not need to overwrite this
    method.

"SCALAR", "UNTIE" and "DESTROY"
    The methods "UNTIE" and "DESTROY" are not defined in [Tie::Hash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown),
    [Tie::StdHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AStdHash/markdown), or [Tie::ExtraHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AExtraHash/markdown). 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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AStdHash/markdown) and [Tie::ExtraHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AExtraHash/markdown).

    If needed, these methods should be defined by the package inheriting
    from [Tie::Hash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown), [Tie::StdHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AStdHash/markdown), or [Tie::ExtraHash](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AExtraHash/markdown). 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](https://www.chedong.com/phpMan.php/perldoc/Tie%3A%3AHash/markdown), they serve as good
    working examples.