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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "constant 3" | |
132 | .TH constant 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | constant \- Perl pragma to declare constants | |
135 | .SH "SYNOPSIS" | |
136 | .IX Header "SYNOPSIS" | |
137 | .Vb 2 | |
138 | \& use constant PI => 4 * atan2(1, 1); | |
139 | \& use constant DEBUG => 0; | |
140 | .Ve | |
141 | .PP | |
142 | .Vb 1 | |
143 | \& print "Pi equals ", PI, "...\en" if DEBUG; | |
144 | .Ve | |
145 | .PP | |
146 | .Vb 11 | |
147 | \& use constant { | |
148 | \& SEC => 0, | |
149 | \& MIN => 1, | |
150 | \& HOUR => 2, | |
151 | \& MDAY => 3, | |
152 | \& MON => 4, | |
153 | \& YEAR => 5, | |
154 | \& WDAY => 6, | |
155 | \& YDAY => 7, | |
156 | \& ISDST => 8, | |
157 | \& }; | |
158 | .Ve | |
159 | .PP | |
160 | .Vb 3 | |
161 | \& use constant WEEKDAYS => qw( | |
162 | \& Sunday Monday Tuesday Wednesday Thursday Friday Saturday | |
163 | \& ); | |
164 | .Ve | |
165 | .PP | |
166 | .Vb 1 | |
167 | \& print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\en"; | |
168 | .Ve | |
169 | .SH "DESCRIPTION" | |
170 | .IX Header "DESCRIPTION" | |
171 | This will declare a symbol to be a constant with the given value. | |
172 | .PP | |
173 | When you declare a constant such as \f(CW\*(C`PI\*(C'\fR using the method shown | |
174 | above, each machine your script runs upon can have as many digits | |
175 | of accuracy as it can use. Also, your program will be easier to | |
176 | read, more likely to be maintained (and maintained correctly), and | |
177 | far less likely to send a space probe to the wrong planet because | |
178 | nobody noticed the one equation in which you wrote \f(CW3.14195\fR. | |
179 | .PP | |
180 | When a constant is used in an expression, perl replaces it with its | |
181 | value at compile time, and may then optimize the expression further. | |
182 | In particular, any code in an \f(CW\*(C`if (CONSTANT)\*(C'\fR block will be optimized | |
183 | away if the constant is false. | |
184 | .SH "NOTES" | |
185 | .IX Header "NOTES" | |
186 | As with all \f(CW\*(C`use\*(C'\fR directives, defining a constant happens at | |
187 | compile time. Thus, it's probably not correct to put a constant | |
188 | declaration inside of a conditional statement (like \f(CW\*(C`if ($foo) | |
189 | { use constant ... }\*(C'\fR). | |
190 | .PP | |
191 | Constants defined using this module cannot be interpolated into | |
192 | strings like variables. However, concatenation works just fine: | |
193 | .PP | |
194 | .Vb 2 | |
195 | \& print "Pi equals PI...\en"; # WRONG: does not expand "PI" | |
196 | \& print "Pi equals ".PI."...\en"; # right | |
197 | .Ve | |
198 | .PP | |
199 | Even though a reference may be declared as a constant, the reference may | |
200 | point to data which may be changed, as this code shows. | |
201 | .PP | |
202 | .Vb 4 | |
203 | \& use constant ARRAY => [ 1,2,3,4 ]; | |
204 | \& print ARRAY->[1]; | |
205 | \& ARRAY->[1] = " be changed"; | |
206 | \& print ARRAY->[1]; | |
207 | .Ve | |
208 | .PP | |
209 | Dereferencing constant references incorrectly (such as using an array | |
210 | subscript on a constant hash reference, or vice versa) will be trapped at | |
211 | compile time. | |
212 | .PP | |
213 | Constants belong to the package they are defined in. To refer to a | |
214 | constant defined in another package, specify the full package name, as | |
215 | in \f(CW\*(C`Some::Package::CONSTANT\*(C'\fR. Constants may be exported by modules, | |
216 | and may also be called as either class or instance methods, that is, | |
217 | as \f(CW\*(C`Some::Package\->CONSTANT\*(C'\fR or as \f(CW\*(C`$obj\->CONSTANT\*(C'\fR where | |
218 | \&\f(CW$obj\fR is an instance of \f(CW\*(C`Some::Package\*(C'\fR. Subclasses may define | |
219 | their own constants to override those in their base class. | |
220 | .PP | |
221 | The use of all caps for constant names is merely a convention, | |
222 | although it is recommended in order to make constants stand out | |
223 | and to help avoid collisions with other barewords, keywords, and | |
224 | subroutine names. Constant names must begin with a letter or | |
225 | underscore. Names beginning with a double underscore are reserved. Some | |
226 | poor choices for names will generate warnings, if warnings are enabled at | |
227 | compile time. | |
228 | .Sh "List constants" | |
229 | .IX Subsection "List constants" | |
230 | Constants may be lists of more (or less) than one value. A constant | |
231 | with no values evaluates to \f(CW\*(C`undef\*(C'\fR in scalar context. Note that | |
232 | constants with more than one value do \fInot\fR return their last value in | |
233 | scalar context as one might expect. They currently return the number | |
234 | of values, but \fBthis may change in the future\fR. Do not use constants | |
235 | with multiple values in scalar context. | |
236 | .PP | |
237 | \&\fB\s-1NOTE:\s0\fR This implies that the expression defining the value of a | |
238 | constant is evaluated in list context. This may produce surprises: | |
239 | .PP | |
240 | .Vb 2 | |
241 | \& use constant TIMESTAMP => localtime; # WRONG! | |
242 | \& use constant TIMESTAMP => scalar localtime; # right | |
243 | .Ve | |
244 | .PP | |
245 | The first line above defines \f(CW\*(C`TIMESTAMP\*(C'\fR as a 9\-element list, as | |
246 | returned by \fIlocaltime()\fR in list context. To set it to the string | |
247 | returned by \fIlocaltime()\fR in scalar context, an explicit \f(CW\*(C`scalar\*(C'\fR | |
248 | keyword is required. | |
249 | .PP | |
250 | List constants are lists, not arrays. To index or slice them, they | |
251 | must be placed in parentheses. | |
252 | .PP | |
253 | .Vb 2 | |
254 | \& my @workdays = WEEKDAYS[1 .. 5]; # WRONG! | |
255 | \& my @workdays = (WEEKDAYS)[1 .. 5]; # right | |
256 | .Ve | |
257 | .Sh "Defining multiple constants at once" | |
258 | .IX Subsection "Defining multiple constants at once" | |
259 | Instead of writing multiple \f(CW\*(C`use constant\*(C'\fR statements, you may define | |
260 | multiple constants in a single statement by giving, instead of the | |
261 | constant name, a reference to a hash where the keys are the names of | |
262 | the constants to be defined. Obviously, all constants defined using | |
263 | this method must have a single value. | |
264 | .PP | |
265 | .Vb 4 | |
266 | \& use constant { | |
267 | \& FOO => "A single value", | |
268 | \& BAR => "This", "won't", "work!", # Error! | |
269 | \& }; | |
270 | .Ve | |
271 | .PP | |
272 | This is a fundamental limitation of the way hashes are constructed in | |
273 | Perl. The error messages produced when this happens will often be | |
274 | quite cryptic \*(-- in the worst case there may be none at all, and | |
275 | you'll only later find that something is broken. | |
276 | .PP | |
277 | When defining multiple constants, you cannot use the values of other | |
278 | constants defined in the same declaration. This is because the | |
279 | calling package doesn't know about any constant within that group | |
280 | until \fIafter\fR the \f(CW\*(C`use\*(C'\fR statement is finished. | |
281 | .PP | |
282 | .Vb 4 | |
283 | \& use constant { | |
284 | \& BITMASK => 0xAFBAEBA8, | |
285 | \& NEGMASK => ~BITMASK, # Error! | |
286 | \& }; | |
287 | .Ve | |
288 | .Sh "Magic constants" | |
289 | .IX Subsection "Magic constants" | |
290 | Magical values and references can be made into constants at compile | |
291 | time, allowing for way cool stuff like this. (These error numbers | |
292 | aren't totally portable, alas.) | |
293 | .PP | |
294 | .Vb 3 | |
295 | \& use constant E2BIG => ($! = 7); | |
296 | \& print E2BIG, "\en"; # something like "Arg list too long" | |
297 | \& print 0+E2BIG, "\en"; # "7" | |
298 | .Ve | |
299 | .PP | |
300 | You can't produce a tied constant by giving a tied scalar as the | |
301 | value. References to tied variables, however, can be used as | |
302 | constants without any problems. | |
303 | .SH "TECHNICAL NOTES" | |
304 | .IX Header "TECHNICAL NOTES" | |
305 | In the current implementation, scalar constants are actually | |
306 | inlinable subroutines. As of version 5.004 of Perl, the appropriate | |
307 | scalar constant is inserted directly in place of some subroutine | |
308 | calls, thereby saving the overhead of a subroutine call. See | |
309 | \&\*(L"Constant Functions\*(R" in perlsub for details about how and when this | |
310 | happens. | |
311 | .PP | |
312 | In the rare case in which you need to discover at run time whether a | |
313 | particular constant has been declared via this module, you may use | |
314 | this function to examine the hash \f(CW%constant::declared\fR. If the given | |
315 | constant name does not include a package name, the current package is | |
316 | used. | |
317 | .PP | |
318 | .Vb 8 | |
319 | \& sub declared ($) { | |
320 | \& use constant 1.01; # don't omit this! | |
321 | \& my $name = shift; | |
322 | \& $name =~ s/^::/main::/; | |
323 | \& my $pkg = caller; | |
324 | \& my $full_name = $name =~ /::/ ? $name : "${pkg}::$name"; | |
325 | \& $constant::declared{$full_name}; | |
326 | \& } | |
327 | .Ve | |
328 | .SH "BUGS" | |
329 | .IX Header "BUGS" | |
330 | In the current version of Perl, list constants are not inlined | |
331 | and some symbols may be redefined without generating a warning. | |
332 | .PP | |
333 | It is not possible to have a subroutine or a keyword with the same | |
334 | name as a constant in the same package. This is probably a Good Thing. | |
335 | .PP | |
336 | A constant with a name in the list \f(CW\*(C`STDIN STDOUT STDERR ARGV ARGVOUT | |
337 | ENV INC SIG\*(C'\fR is not allowed anywhere but in package \f(CW\*(C`main::\*(C'\fR, for | |
338 | technical reasons. | |
339 | .PP | |
340 | Unlike constants in some languages, these cannot be overridden | |
341 | on the command line or via environment variables. | |
342 | .PP | |
343 | You can get into trouble if you use constants in a context which | |
344 | automatically quotes barewords (as is true for any subroutine call). | |
345 | For example, you can't say \f(CW$hash{CONSTANT}\fR because \f(CW\*(C`CONSTANT\*(C'\fR will | |
346 | be interpreted as a string. Use \f(CW$hash{CONSTANT()}\fR or | |
347 | \&\f(CW$hash{+CONSTANT}\fR to prevent the bareword quoting mechanism from | |
348 | kicking in. Similarly, since the \f(CW\*(C`=>\*(C'\fR operator quotes a bareword | |
349 | immediately to its left, you have to say \f(CW\*(C`CONSTANT() => 'value'\*(C'\fR | |
350 | (or simply use a comma in place of the big arrow) instead of | |
351 | \&\f(CW\*(C`CONSTANT => 'value'\*(C'\fR. | |
352 | .SH "AUTHOR" | |
353 | .IX Header "AUTHOR" | |
354 | Tom Phoenix, <\fIrootbeer@redcat.com\fR>, with help from | |
355 | many other folks. | |
356 | .PP | |
357 | Multiple constant declarations at once added by Casey West, | |
358 | <\fIcasey@geeknest.com\fR>. | |
359 | .PP | |
360 | Documentation mostly rewritten by Ilmari Karonen, | |
361 | <\fIperl@itz.pp.sci.fi\fR>. | |
362 | .SH "COPYRIGHT" | |
363 | .IX Header "COPYRIGHT" | |
364 | Copyright (C) 1997, 1999 Tom Phoenix | |
365 | .PP | |
366 | This module is free software; you can redistribute it or modify it | |
367 | under the same terms as Perl itself. |