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.\" ========================================================================
.\"
.IX Title "Tie::Hash 3"
.TH Tie::Hash 3 "2002-11-24" "perl v5.8.0" "Perl Programmers Reference Guide"
.SH "NAME"
Tie::Hash, Tie::StdHash, Tie::ExtraHash \- base class definitions for tied hashes
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 2
\&    package NewHash;
\&    require Tie::Hash;
.Ve
.PP
.Vb 1
\&    @ISA = (Tie::Hash);
.Ve
.PP
.Vb 2
\&    sub DELETE { ... }          # Provides needed method
\&    sub CLEAR { ... }           # Overrides inherited method
.Ve
.PP
.Vb 2
\&    package NewStdHash;
\&    require Tie::Hash;
.Ve
.PP
.Vb 1
\&    @ISA = (Tie::StdHash);
.Ve
.PP
.Vb 4
\&    # All methods provided by default, define only those needing overrides
\&    # Accessors access the storage in %{$_[0]};
\&    # TIEHANDLE should return a reference to the actual storage
\&    sub DELETE { ... }
.Ve
.PP
.Vb 2
\&    package NewExtraHash;
\&    require Tie::Hash;
.Ve
.PP
.Vb 1
\&    @ISA = (Tie::ExtraHash);
.Ve
.PP
.Vb 7
\&    # All methods provided by default, define only those needing overrides
\&    # Accessors access the storage in %{$_[0][0]};
\&    # TIEHANDLE 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]) }
.Ve
.PP
.Vb 1
\&    package main;
.Ve
.PP
.Vb 4
\&    tie %new_hash, 'NewHash';
\&    tie %new_std_hash, 'NewStdHash';
\&    tie %new_extra_hash, 'NewExtraHash',
\&        sub {warn "Doing \eU$_[1]\eE of $_[2].\en"};
.Ve
.SH "DESCRIPTION"
.IX Header "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 \fBTie::Hash\fR package provides a \f(CW\*(C`new\*(C'\fR method, as well
as methods \f(CW\*(C`TIEHASH\*(C'\fR, \f(CW\*(C`EXISTS\*(C'\fR and \f(CW\*(C`CLEAR\*(C'\fR. The \fBTie::StdHash\fR and
\&\fBTie::ExtraHash\fR packages
provide most methods for hashes described in perltie (the exceptions
are \f(CW\*(C`UNTIE\*(C'\fR and \f(CW\*(C`DESTROY\*(C'\fR).  They cause tied hashes to behave exactly like standard hashes,
and allow for selective overwriting of methods.  \fBTie::Hash\fR grandfathers the
\&\f(CW\*(C`new\*(C'\fR method: it is used if \f(CW\*(C`TIEHASH\*(C'\fR is not defined
in the case a class forgets to include a \f(CW\*(C`TIEHASH\*(C'\fR method.
.PP
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:
.IP "\s-1TIEHASH\s0 classname, \s-1LIST\s0" 4
.IX Item "TIEHASH classname, LIST"
The method invoked by the command \f(CW\*(C`tie %hash, classname\*(C'\fR. Associates a new
hash instance with the specified class. \f(CW\*(C`LIST\*(C'\fR would represent additional
arguments (along the lines of AnyDBM_File and compatriots) needed to
complete the association.
.IP "\s-1STORE\s0 this, key, value" 4
.IX Item "STORE this, key, value"
Store datum \fIvalue\fR into \fIkey\fR for the tied hash \fIthis\fR.
.IP "\s-1FETCH\s0 this, key" 4
.IX Item "FETCH this, key"
Retrieve the datum in \fIkey\fR for the tied hash \fIthis\fR.
.IP "\s-1FIRSTKEY\s0 this" 4
.IX Item "FIRSTKEY this"
Return the first key in the hash.
.IP "\s-1NEXTKEY\s0 this, lastkey" 4
.IX Item "NEXTKEY this, lastkey"
Return the next key in the hash.
.IP "\s-1EXISTS\s0 this, key" 4
.IX Item "EXISTS this, key"
Verify that \fIkey\fR exists with the tied hash \fIthis\fR.
.Sp
The \fBTie::Hash\fR implementation is a stub that simply croaks.
.IP "\s-1DELETE\s0 this, key" 4
.IX Item "DELETE this, key"
Delete the key \fIkey\fR from the tied hash \fIthis\fR.
.IP "\s-1CLEAR\s0 this" 4
.IX Item "CLEAR this"
Clear all values from the tied hash \fIthis\fR.
.SH "Inheriting from \fBTie::StdHash\fP"
.IX Header "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 \f(CW\*(C`tied(%tiedhash)\*(C'\fR.  Thus overwritten
\&\f(CW\*(C`TIEHANDLE\*(C'\fR method should return a hash reference, and the remaining methods
should operate on the hash referenced by the first argument:
.PP
.Vb 2
\&  package ReportHash;
\&  our @ISA = 'Tie::StdHash';
.Ve
.PP
.Vb 9
\&  sub TIEHASH  {
\&    my $storage = bless {}, shift;
\&    warn "New ReportHash created, stored in $storage.\en";
\&    $storage
\&  }
\&  sub STORE    {
\&    warn "Storing data with key $_[1] at $_[0].\en";
\&    $_[0]{$_[1]} = $_[2]
\&  }
.Ve
.SH "Inheriting from \fBTie::ExtraHash\fP"
.IX Header "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 \f(CW\*(C`(tied(%tiedhash))[0]\*(C'\fR.  Thus overwritten
\&\f(CW\*(C`TIEHANDLE\*(C'\fR method should return an array reference with the first
element being a hash reference, and the remaining methods should operate on the
hash \f(CW\*(C`%{ $_[0]\->[0] }\*(C'\fR:
.PP
.Vb 2
\&  package ReportHash;
\&  our @ISA = 'Tie::StdHash';
.Ve
.PP
.Vb 9
\&  sub TIEHASH  {
\&    my $storage = bless {}, shift;
\&    warn "New ReportHash created, stored in $storage.\en";
\&    [$storage, @_]
\&  }
\&  sub STORE    {
\&    warn "Storing data with key $_[1] at $_[0].\en";
\&    $_[0][0]{$_[1]} = $_[2]
\&  }
.Ve
.PP
The default \f(CW\*(C`TIEHANDLE\*(C'\fR method stores \*(L"extra\*(R" arguments to \fItie()\fR starting
from offset 1 in the array referenced by \f(CW\*(C`tied(%tiedhash)\*(C'\fR; this is the
same storage algorithm as in \s-1TIEHASH\s0 subroutine above.  Hence, a typical
package inheriting from \fBTie::ExtraHash\fR does not need to overwrite this
method.
.ie n .SH """UNTIE""\fP and \f(CW""DESTROY"""
.el .SH "\f(CWUNTIE\fP and \f(CWDESTROY\fP"
.IX Header "UNTIE and DESTROY"
The methods \f(CW\*(C`UNTIE\*(C'\fR and \f(CW\*(C`DESTROY\*(C'\fR are not defined in \fBTie::Hash\fR,
\&\fBTie::StdHash\fR, or \fBTie::ExtraHash\fR.  Tied hashes do not require
presense of these methods, but if defined, the methods will be called in
proper time, see perltie.
.PP
If needed, these methods should be defined by the package inheriting from
\&\fBTie::Hash\fR, \fBTie::StdHash\fR, or \fBTie::ExtraHash\fR.
.SH "MORE INFORMATION"
.IX Header "MORE INFORMATION"
The packages relating to various DBM-related implementations (\fIDB_File\fR,
\&\fINDBM_File\fR, etc.) show examples of general tied hashes, as does the
Config module. While these do not utilize \fBTie::Hash\fR, they serve as
good working examples.