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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "fields 3" | |
132 | .TH fields 3 "2002-06-01" "perl v5.8.0" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | fields \- compile\-time class fields | |
135 | .SH "SYNOPSIS" | |
136 | .IX Header "SYNOPSIS" | |
137 | .Vb 14 | |
138 | \& { | |
139 | \& package Foo; | |
140 | \& use fields qw(foo bar _Foo_private); | |
141 | \& sub new { | |
142 | \& my Foo $self = shift; | |
143 | \& unless (ref $self) { | |
144 | \& $self = fields::new($self); | |
145 | \& $self->{_Foo_private} = "this is Foo's secret"; | |
146 | \& } | |
147 | \& $self->{foo} = 10; | |
148 | \& $self->{bar} = 20; | |
149 | \& return $self; | |
150 | \& } | |
151 | \& } | |
152 | .Ve | |
153 | .PP | |
154 | .Vb 2 | |
155 | \& my Foo $var = Foo::->new; | |
156 | \& $var->{foo} = 42; | |
157 | .Ve | |
158 | .PP | |
159 | .Vb 2 | |
160 | \& # this will generate a compile-time error | |
161 | \& $var->{zap} = 42; | |
162 | .Ve | |
163 | .PP | |
164 | .Vb 14 | |
165 | \& # subclassing | |
166 | \& { | |
167 | \& package Bar; | |
168 | \& use base 'Foo'; | |
169 | \& use fields qw(baz _Bar_private); # not shared with Foo | |
170 | \& sub new { | |
171 | \& my $class = shift; | |
172 | \& my $self = fields::new($class); | |
173 | \& $self->SUPER::new(); # init base fields | |
174 | \& $self->{baz} = 10; # init own fields | |
175 | \& $self->{_Bar_private} = "this is Bar's secret"; | |
176 | \& return $self; | |
177 | \& } | |
178 | \& } | |
179 | .Ve | |
180 | .SH "DESCRIPTION" | |
181 | .IX Header "DESCRIPTION" | |
182 | The \f(CW\*(C`fields\*(C'\fR pragma enables compile-time verified class fields. | |
183 | .PP | |
184 | \&\s-1NOTE:\s0 The current implementation keeps the declared fields in the \f(CW%FIELDS\fR | |
185 | hash of the calling package, but this may change in future versions. | |
186 | Do \fBnot\fR update the \f(CW%FIELDS\fR hash directly, because it must be created | |
187 | at compile-time for it to be fully useful, as is done by this pragma. | |
188 | .PP | |
189 | If a typed lexical variable holding a reference is used to access a | |
190 | hash element and a package with the same name as the type has declared | |
191 | class fields using this pragma, then the operation is turned into an | |
192 | array access at compile time. | |
193 | .PP | |
194 | The related \f(CW\*(C`base\*(C'\fR pragma will combine fields from base classes and any | |
195 | fields declared using the \f(CW\*(C`fields\*(C'\fR pragma. This enables field | |
196 | inheritance to work properly. | |
197 | .PP | |
198 | Field names that start with an underscore character are made private to | |
199 | the class and are not visible to subclasses. Inherited fields can be | |
200 | overridden but will generate a warning if used together with the \f(CW\*(C`\-w\*(C'\fR | |
201 | switch. | |
202 | .PP | |
203 | The effect of all this is that you can have objects with named fields | |
204 | which are as compact and as fast arrays to access. This only works | |
205 | as long as the objects are accessed through properly typed variables. | |
206 | If the objects are not typed, access is only checked at run time. | |
207 | .PP | |
208 | The following functions are supported: | |
209 | .IP "new" 8 | |
210 | .IX Item "new" | |
211 | \&\fIfields::new()\fR creates and blesses a pseudo-hash comprised of the fields | |
212 | declared using the \f(CW\*(C`fields\*(C'\fR pragma into the specified class. | |
213 | This makes it possible to write a constructor like this: | |
214 | .Sp | |
215 | .Vb 2 | |
216 | \& package Critter::Sounds; | |
217 | \& use fields qw(cat dog bird); | |
218 | .Ve | |
219 | .Sp | |
220 | .Vb 7 | |
221 | \& sub new { | |
222 | \& my Critter::Sounds $self = shift; | |
223 | \& $self = fields::new($self) unless ref $self; | |
224 | \& $self->{cat} = 'meow'; # scalar element | |
225 | \& @$self{'dog','bird'} = ('bark','tweet'); # slice | |
226 | \& return $self; | |
227 | \& } | |
228 | .Ve | |
229 | .IP "phash" 8 | |
230 | .IX Item "phash" | |
231 | \&\fIfields::phash()\fR can be used to create and initialize a plain (unblessed) | |
232 | pseudo\-hash. This function should always be used instead of creating | |
233 | pseudo-hashes directly. | |
234 | .Sp | |
235 | If the first argument is a reference to an array, the pseudo-hash will | |
236 | be created with keys from that array. If a second argument is supplied, | |
237 | it must also be a reference to an array whose elements will be used as | |
238 | the values. If the second array contains less elements than the first, | |
239 | the trailing elements of the pseudo-hash will not be initialized. | |
240 | This makes it particularly useful for creating a pseudo-hash from | |
241 | subroutine arguments: | |
242 | .Sp | |
243 | .Vb 3 | |
244 | \& sub dogtag { | |
245 | \& my $tag = fields::phash([qw(name rank ser_num)], [@_]); | |
246 | \& } | |
247 | .Ve | |
248 | .Sp | |
249 | \&\fIfields::phash()\fR also accepts a list of key-value pairs that will | |
250 | be used to construct the pseudo hash. Examples: | |
251 | .Sp | |
252 | .Vb 3 | |
253 | \& my $tag = fields::phash(name => "Joe", | |
254 | \& rank => "captain", | |
255 | \& ser_num => 42); | |
256 | .Ve | |
257 | .Sp | |
258 | .Vb 1 | |
259 | \& my $pseudohash = fields::phash(%args); | |
260 | .Ve | |
261 | .SH "SEE ALSO" | |
262 | .IX Header "SEE ALSO" | |
263 | base, | |
264 | \&\*(L"Pseudo\-hashes: Using an array as a hash\*(R" in perlref |