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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. |