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.\" ========================================================================
.\"
.IX Title "Tie::Hash 3"
.TH Tie::Hash 3 "2019-10-24" "perl v5.30.2" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.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;
\&
\&    @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]\->(\*(Aqdel\*(Aq, $_[0][0], $_[1]); # Call the report writer
\&      delete $_[0][0]\->{$_[1]};           #  $_[0]\->SUPER::DELETE($_[1])
\&    }
\&
\&
\&    package main;
\&
\&    tie %new_hash, \*(AqNewHash\*(Aq;
\&    tie %new_std_hash, \*(AqNewStdHash\*(Aq;
\&    tie %new_extra_hash, \*(AqNewExtraHash\*(Aq,
\&        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.
.IP "\s-1SCALAR\s0 this" 4
.IX Item "SCALAR this"
Returns what evaluating the hash in scalar context yields.
.Sp
\&\fBTie::Hash\fR does not implement this method (but \fBTie::StdHash\fR
and \fBTie::ExtraHash\fR do).
.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`TIEHASH\*(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 = \*(AqTie::StdHash\*(Aq;
\&
\&  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`TIEHASH\*(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 = \*(AqTie::ExtraHash\*(Aq;
\&
\&  sub TIEHASH  {
\&    my $class = shift;
\&    my $storage = bless [{}, @_], $class;
\&    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`TIEHASH\*(C'\fR method stores \*(L"extra\*(R" arguments to \fBtie()\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 """SCALAR"", ""UNTIE"" and ""DESTROY"""
.el .SH "\f(CWSCALAR\fP, \f(CWUNTIE\fP and \f(CWDESTROY\fP"
.IX Header "SCALAR, 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
presence of these methods, but if defined, the methods will be called in
proper time, see perltie.
.PP
\&\f(CW\*(C`SCALAR\*(C'\fR is only defined in \fBTie::StdHash\fR and \fBTie::ExtraHash\fR.
.PP
If needed, these methods should be defined by the package inheriting from
\&\fBTie::Hash\fR, \fBTie::StdHash\fR, or \fBTie::ExtraHash\fR. See \*(L"\s-1SCALAR\*(R"\s0 in perltie
to find out what happens when \f(CW\*(C`SCALAR\*(C'\fR does not exist.
.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.