MirOS Manual: Tie::Hash(3p)


Tie::Hash(3p)   Perl Programmers Reference Guide    Tie::Hash(3p)

NAME

     Tie::Hash, Tie::StdHash, Tie::ExtraHash - base class defini-
     tions for tied hashes

SYNOPSIS

         package NewHash;
         require Tie::Hash;

         @ISA = (Tie::Hash);

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

         package NewStdHash;
         require Tie::Hash;

         @ISA = (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 = (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 pack-
     age 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 perl-
     tie (the exceptions are "UNTIE" and "DESTROY").  They cause

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     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 argu-
         ments (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

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     "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 inherit-
     ing 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.

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     "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 pertie 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 util-
     ize Tie::Hash, they serve as good working examples.

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