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.TH Tie::Hash 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide"
Tie::Hash, Tie::StdHash, Tie::ExtraHash \- base class definitions for tied hashes
\& sub DELETE { ... } # Provides needed method
\& sub CLEAR { ... } # Overrides inherited method
\& @ISA = (Tie::StdHash);
\& # All methods provided by default, define only those needing overrides
\& # Accessors access the storage in %{$_[0]};
\& # TIEHASH should return a reference to the actual storage
\& @ISA = (Tie::ExtraHash);
\& # All methods provided by default, define only those needing overrides
\& # Accessors access the storage in %{$_[0][0]};
\& # TIEHASH should return an array reference with the first element being
\& # the reference to the actual storage
\& $_[0][1]->('del', $_[0][0], $_[1]); # Call the report writer
\& delete $_[0][0]->{$_[1]}; # $_[0]->SUPER::DELETE($_[1])
\& tie %new_hash, 'NewHash';
\& tie %new_std_hash, 'NewStdHash';
\& tie %new_extra_hash, 'NewExtraHash',
\& sub {warn "Doing \eU$_[1]\eE of $_[2].\en"};
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.
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
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.
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
Clear all values from the tied hash \fIthis\fR.
.IP "\s-1SCALAR\s0 this" 4
Returns what evaluating the hash in scalar context yields.
\&\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:
\& our @ISA = 'Tie::StdHash';
\& my $storage = bless {}, shift;
\& warn "New ReportHash created, stored in $storage.\en";
\& warn "Storing data with key $_[1] at $_[0].\en";
.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:
\& our @ISA = 'Tie::ExtraHash';
\& my $storage = bless [{}, @_], $class;
\& warn "New ReportHash created, stored in $storage.\en";
\& warn "Storing data with key $_[1] at $_[0].\en";
\& $_[0][0]{$_[1]} = $_[2]
The default \f(CW\*(C`TIEHASH\*(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
.ie n .SH """SCALAR""\fP, \f(CW""UNTIE""\fP and \f(CW""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.
\&\f(CW\*(C`SCALAR\*(C'\fR is only defined in \fBTie::StdHash\fR and \fBTie::ExtraHash\fR.
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\s0\*(R" in pertie
to find out what happens when \f(CW\*(C`SCALAR\*(C'\fR does not exist.
.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
projects / OpenSPARC-T2-SAM / blob