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Tie::Hash.3
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.\" -*- mode: troff; coding: utf-8 -*- .\" Automatically generated by Pod::Man 5.01 (Pod::Simple 3.43) .\" .\" Standard preamble: .\" ======================================================================== .de Sp \" Vertical space (when we can't use .PP) .if t .sp .5v .if n .sp .. .de Vb \" Begin verbatim text .ft CW .nf .ne \\$1 .. .de Ve \" End verbatim text .ft R .fi .. .\" \*(C` and \*(C' are quotes in nroff, nothing in troff, for use with C<>. .ie n \{\ . ds C` "" . ds C' "" 'br\} .el\{\ . ds C` . ds C' 'br\} .\" .\" Escape single quotes in literal strings from groff's Unicode transform. .ie \n(.g .ds Aq \(aq .el .ds Aq ' .\" .\" If the F register is >0, we'll generate index entries on stderr for .\" titles (.TH), headers (.SH), subsections (.SS), items (.Ip), and index .\" entries marked with X<> in POD. Of course, you'll have to process the .\" output yourself in some meaningful fashion. .\" .\" Avoid warning from groff about undefined register 'F'. .de IX .. .nr rF 0 .if \n(.g .if rF .nr rF 1 .if (\n(rF:(\n(.g==0)) \{\ . if \nF \{\ . de IX . tm Index:\\$1\t\\n%\t"\\$2" .. . if !\nF==2 \{\ . nr % 0 . nr F 2 . \} . \} .\} .rr rF .\" ======================================================================== .\" .IX Title "Tie::Hash 3" .TH Tie::Hash 3 2022-05-28 "perl v5.38.0" "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 has legacy support for 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 "TIEHASH classname, LIST" 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 "STORE this, key, value" 4 .IX Item "STORE this, key, value" Store datum \fIvalue\fR into \fIkey\fR for the tied hash \fIthis\fR. .IP "FETCH this, key" 4 .IX Item "FETCH this, key" Retrieve the datum in \fIkey\fR for the tied hash \fIthis\fR. .IP "FIRSTKEY this" 4 .IX Item "FIRSTKEY this" Return the first key in the hash. .IP "NEXTKEY this, lastkey" 4 .IX Item "NEXTKEY this, lastkey" Return the next key in the hash. .IP "EXISTS 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 "DELETE this, key" 4 .IX Item "DELETE this, key" Delete the key \fIkey\fR from the tied hash \fIthis\fR. .IP "CLEAR this" 4 .IX Item "CLEAR this" Clear all values from the tied hash \fIthis\fR. .IP "SCALAR 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(CWtied(%tiedhash)\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 "extra" arguments to \fBtie()\fR starting from offset 1 in the array referenced by \f(CWtied(%tiedhash)\fR; this is the same storage algorithm as in TIEHASH 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 "SCALAR" 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.