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| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "Tie::Hash 3" |
| 132 | .TH Tie::Hash 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | Tie::Hash, Tie::StdHash, Tie::ExtraHash \- base class definitions for tied hashes |
| 135 | .SH "SYNOPSIS" |
| 136 | .IX Header "SYNOPSIS" |
| 137 | .Vb 2 |
| 138 | \& package NewHash; |
| 139 | \& require Tie::Hash; |
| 140 | .Ve |
| 141 | .PP |
| 142 | .Vb 1 |
| 143 | \& @ISA = (Tie::Hash); |
| 144 | .Ve |
| 145 | .PP |
| 146 | .Vb 2 |
| 147 | \& sub DELETE { ... } # Provides needed method |
| 148 | \& sub CLEAR { ... } # Overrides inherited method |
| 149 | .Ve |
| 150 | .PP |
| 151 | .Vb 2 |
| 152 | \& package NewStdHash; |
| 153 | \& require Tie::Hash; |
| 154 | .Ve |
| 155 | .PP |
| 156 | .Vb 1 |
| 157 | \& @ISA = (Tie::StdHash); |
| 158 | .Ve |
| 159 | .PP |
| 160 | .Vb 4 |
| 161 | \& # All methods provided by default, define only those needing overrides |
| 162 | \& # Accessors access the storage in %{$_[0]}; |
| 163 | \& # TIEHASH should return a reference to the actual storage |
| 164 | \& sub DELETE { ... } |
| 165 | .Ve |
| 166 | .PP |
| 167 | .Vb 2 |
| 168 | \& package NewExtraHash; |
| 169 | \& require Tie::Hash; |
| 170 | .Ve |
| 171 | .PP |
| 172 | .Vb 1 |
| 173 | \& @ISA = (Tie::ExtraHash); |
| 174 | .Ve |
| 175 | .PP |
| 176 | .Vb 8 |
| 177 | \& # All methods provided by default, define only those needing overrides |
| 178 | \& # Accessors access the storage in %{$_[0][0]}; |
| 179 | \& # TIEHASH should return an array reference with the first element being |
| 180 | \& # the reference to the actual storage |
| 181 | \& sub DELETE { |
| 182 | \& $_[0][1]->('del', $_[0][0], $_[1]); # Call the report writer |
| 183 | \& delete $_[0][0]->{$_[1]}; # $_[0]->SUPER::DELETE($_[1]) |
| 184 | \& } |
| 185 | .Ve |
| 186 | .PP |
| 187 | .Vb 1 |
| 188 | \& package main; |
| 189 | .Ve |
| 190 | .PP |
| 191 | .Vb 4 |
| 192 | \& tie %new_hash, 'NewHash'; |
| 193 | \& tie %new_std_hash, 'NewStdHash'; |
| 194 | \& tie %new_extra_hash, 'NewExtraHash', |
| 195 | \& sub {warn "Doing \eU$_[1]\eE of $_[2].\en"}; |
| 196 | .Ve |
| 197 | .SH "DESCRIPTION" |
| 198 | .IX Header "DESCRIPTION" |
| 199 | This module provides some skeletal methods for hash-tying classes. See |
| 200 | perltie for a list of the functions required in order to tie a hash |
| 201 | to a package. The basic \fBTie::Hash\fR package provides a \f(CW\*(C`new\*(C'\fR method, as well |
| 202 | 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 |
| 203 | \&\fBTie::ExtraHash\fR packages |
| 204 | provide most methods for hashes described in perltie (the exceptions |
| 205 | are \f(CW\*(C`UNTIE\*(C'\fR and \f(CW\*(C`DESTROY\*(C'\fR). They cause tied hashes to behave exactly like standard hashes, |
| 206 | and allow for selective overwriting of methods. \fBTie::Hash\fR grandfathers the |
| 207 | \&\f(CW\*(C`new\*(C'\fR method: it is used if \f(CW\*(C`TIEHASH\*(C'\fR is not defined |
| 208 | in the case a class forgets to include a \f(CW\*(C`TIEHASH\*(C'\fR method. |
| 209 | .PP |
| 210 | For developers wishing to write their own tied hashes, the required methods |
| 211 | are briefly defined below. See the perltie section for more detailed |
| 212 | descriptive, as well as example code: |
| 213 | .IP "\s-1TIEHASH\s0 classname, \s-1LIST\s0" 4 |
| 214 | .IX Item "TIEHASH classname, LIST" |
| 215 | The method invoked by the command \f(CW\*(C`tie %hash, classname\*(C'\fR. Associates a new |
| 216 | hash instance with the specified class. \f(CW\*(C`LIST\*(C'\fR would represent additional |
| 217 | arguments (along the lines of AnyDBM_File and compatriots) needed to |
| 218 | complete the association. |
| 219 | .IP "\s-1STORE\s0 this, key, value" 4 |
| 220 | .IX Item "STORE this, key, value" |
| 221 | Store datum \fIvalue\fR into \fIkey\fR for the tied hash \fIthis\fR. |
| 222 | .IP "\s-1FETCH\s0 this, key" 4 |
| 223 | .IX Item "FETCH this, key" |
| 224 | Retrieve the datum in \fIkey\fR for the tied hash \fIthis\fR. |
| 225 | .IP "\s-1FIRSTKEY\s0 this" 4 |
| 226 | .IX Item "FIRSTKEY this" |
| 227 | Return the first key in the hash. |
| 228 | .IP "\s-1NEXTKEY\s0 this, lastkey" 4 |
| 229 | .IX Item "NEXTKEY this, lastkey" |
| 230 | Return the next key in the hash. |
| 231 | .IP "\s-1EXISTS\s0 this, key" 4 |
| 232 | .IX Item "EXISTS this, key" |
| 233 | Verify that \fIkey\fR exists with the tied hash \fIthis\fR. |
| 234 | .Sp |
| 235 | The \fBTie::Hash\fR implementation is a stub that simply croaks. |
| 236 | .IP "\s-1DELETE\s0 this, key" 4 |
| 237 | .IX Item "DELETE this, key" |
| 238 | Delete the key \fIkey\fR from the tied hash \fIthis\fR. |
| 239 | .IP "\s-1CLEAR\s0 this" 4 |
| 240 | .IX Item "CLEAR this" |
| 241 | Clear all values from the tied hash \fIthis\fR. |
| 242 | .IP "\s-1SCALAR\s0 this" 4 |
| 243 | .IX Item "SCALAR this" |
| 244 | Returns what evaluating the hash in scalar context yields. |
| 245 | .Sp |
| 246 | \&\fBTie::Hash\fR does not implement this method (but \fBTie::StdHash\fR |
| 247 | and \fBTie::ExtraHash\fR do). |
| 248 | .SH "Inheriting from \fBTie::StdHash\fP" |
| 249 | .IX Header "Inheriting from Tie::StdHash" |
| 250 | The accessor methods assume that the actual storage for the data in the tied |
| 251 | hash is in the hash referenced by \f(CW\*(C`tied(%tiedhash)\*(C'\fR. Thus overwritten |
| 252 | \&\f(CW\*(C`TIEHASH\*(C'\fR method should return a hash reference, and the remaining methods |
| 253 | should operate on the hash referenced by the first argument: |
| 254 | .PP |
| 255 | .Vb 2 |
| 256 | \& package ReportHash; |
| 257 | \& our @ISA = 'Tie::StdHash'; |
| 258 | .Ve |
| 259 | .PP |
| 260 | .Vb 9 |
| 261 | \& sub TIEHASH { |
| 262 | \& my $storage = bless {}, shift; |
| 263 | \& warn "New ReportHash created, stored in $storage.\en"; |
| 264 | \& $storage |
| 265 | \& } |
| 266 | \& sub STORE { |
| 267 | \& warn "Storing data with key $_[1] at $_[0].\en"; |
| 268 | \& $_[0]{$_[1]} = $_[2] |
| 269 | \& } |
| 270 | .Ve |
| 271 | .SH "Inheriting from \fBTie::ExtraHash\fP" |
| 272 | .IX Header "Inheriting from Tie::ExtraHash" |
| 273 | The accessor methods assume that the actual storage for the data in the tied |
| 274 | hash is in the hash referenced by \f(CW\*(C`(tied(%tiedhash))\->[0]\*(C'\fR. Thus overwritten |
| 275 | \&\f(CW\*(C`TIEHASH\*(C'\fR method should return an array reference with the first |
| 276 | element being a hash reference, and the remaining methods should operate on the |
| 277 | hash \f(CW\*(C`%{ $_[0]\->[0] }\*(C'\fR: |
| 278 | .PP |
| 279 | .Vb 2 |
| 280 | \& package ReportHash; |
| 281 | \& our @ISA = 'Tie::ExtraHash'; |
| 282 | .Ve |
| 283 | .PP |
| 284 | .Vb 10 |
| 285 | \& sub TIEHASH { |
| 286 | \& my $class = shift; |
| 287 | \& my $storage = bless [{}, @_], $class; |
| 288 | \& warn "New ReportHash created, stored in $storage.\en"; |
| 289 | \& $storage; |
| 290 | \& } |
| 291 | \& sub STORE { |
| 292 | \& warn "Storing data with key $_[1] at $_[0].\en"; |
| 293 | \& $_[0][0]{$_[1]} = $_[2] |
| 294 | \& } |
| 295 | .Ve |
| 296 | .PP |
| 297 | The default \f(CW\*(C`TIEHASH\*(C'\fR method stores \*(L"extra\*(R" arguments to \fItie()\fR starting |
| 298 | from offset 1 in the array referenced by \f(CW\*(C`tied(%tiedhash)\*(C'\fR; this is the |
| 299 | same storage algorithm as in \s-1TIEHASH\s0 subroutine above. Hence, a typical |
| 300 | package inheriting from \fBTie::ExtraHash\fR does not need to overwrite this |
| 301 | method. |
| 302 | .ie n .SH """SCALAR""\fP, \f(CW""UNTIE""\fP and \f(CW""DESTROY""" |
| 303 | .el .SH "\f(CWSCALAR\fP, \f(CWUNTIE\fP and \f(CWDESTROY\fP" |
| 304 | .IX Header "SCALAR, UNTIE and DESTROY" |
| 305 | The methods \f(CW\*(C`UNTIE\*(C'\fR and \f(CW\*(C`DESTROY\*(C'\fR are not defined in \fBTie::Hash\fR, |
| 306 | \&\fBTie::StdHash\fR, or \fBTie::ExtraHash\fR. Tied hashes do not require |
| 307 | presence of these methods, but if defined, the methods will be called in |
| 308 | proper time, see perltie. |
| 309 | .PP |
| 310 | \&\f(CW\*(C`SCALAR\*(C'\fR is only defined in \fBTie::StdHash\fR and \fBTie::ExtraHash\fR. |
| 311 | .PP |
| 312 | If needed, these methods should be defined by the package inheriting from |
| 313 | \&\fBTie::Hash\fR, \fBTie::StdHash\fR, or \fBTie::ExtraHash\fR. See \*(L"\s-1SCALAR\s0\*(R" in pertie |
| 314 | to find out what happens when \f(CW\*(C`SCALAR\*(C'\fR does not exist. |
| 315 | .SH "MORE INFORMATION" |
| 316 | .IX Header "MORE INFORMATION" |
| 317 | The packages relating to various DBM-related implementations (\fIDB_File\fR, |
| 318 | \&\fINDBM_File\fR, etc.) show examples of general tied hashes, as does the |
| 319 | Config module. While these do not utilize \fBTie::Hash\fR, they serve as |
| 320 | good working examples. |