phpman > perldoc > Tie::Hash(3perl)

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.