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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "Scalar::Util 3" | |
132 | .TH Scalar::Util 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | Scalar::Util \- A selection of general\-utility scalar subroutines | |
135 | .SH "SYNOPSIS" | |
136 | .IX Header "SYNOPSIS" | |
137 | .Vb 2 | |
138 | \& use Scalar::Util qw(blessed dualvar isweak readonly refaddr reftype tainted | |
139 | \& weaken isvstring looks_like_number set_prototype); | |
140 | .Ve | |
141 | .SH "DESCRIPTION" | |
142 | .IX Header "DESCRIPTION" | |
143 | \&\f(CW\*(C`Scalar::Util\*(C'\fR contains a selection of subroutines that people have | |
144 | expressed would be nice to have in the perl core, but the usage would | |
145 | not really be high enough to warrant the use of a keyword, and the size | |
146 | so small such that being individual extensions would be wasteful. | |
147 | .PP | |
148 | By default \f(CW\*(C`Scalar::Util\*(C'\fR does not export any subroutines. The | |
149 | subroutines defined are | |
150 | .IP "blessed \s-1EXPR\s0" 4 | |
151 | .IX Item "blessed EXPR" | |
152 | If \s-1EXPR\s0 evaluates to a blessed reference the name of the package | |
153 | that it is blessed into is returned. Otherwise \f(CW\*(C`undef\*(C'\fR is returned. | |
154 | .Sp | |
155 | .Vb 2 | |
156 | \& $scalar = "foo"; | |
157 | \& $class = blessed $scalar; # undef | |
158 | .Ve | |
159 | .Sp | |
160 | .Vb 2 | |
161 | \& $ref = []; | |
162 | \& $class = blessed $ref; # undef | |
163 | .Ve | |
164 | .Sp | |
165 | .Vb 2 | |
166 | \& $obj = bless [], "Foo"; | |
167 | \& $class = blessed $obj; # "Foo" | |
168 | .Ve | |
169 | .IP "dualvar \s-1NUM\s0, \s-1STRING\s0" 4 | |
170 | .IX Item "dualvar NUM, STRING" | |
171 | Returns a scalar that has the value \s-1NUM\s0 in a numeric context and the | |
172 | value \s-1STRING\s0 in a string context. | |
173 | .Sp | |
174 | .Vb 3 | |
175 | \& $foo = dualvar 10, "Hello"; | |
176 | \& $num = $foo + 2; # 12 | |
177 | \& $str = $foo . " world"; # Hello world | |
178 | .Ve | |
179 | .IP "isvstring \s-1EXPR\s0" 4 | |
180 | .IX Item "isvstring EXPR" | |
181 | If \s-1EXPR\s0 is a scalar which was coded as a vstring the result is true. | |
182 | .Sp | |
183 | .Vb 3 | |
184 | \& $vs = v49.46.48; | |
185 | \& $fmt = isvstring($vs) ? "%vd" : "%s"; #true | |
186 | \& printf($fmt,$vs); | |
187 | .Ve | |
188 | .IP "isweak \s-1EXPR\s0" 4 | |
189 | .IX Item "isweak EXPR" | |
190 | If \s-1EXPR\s0 is a scalar which is a weak reference the result is true. | |
191 | .Sp | |
192 | .Vb 4 | |
193 | \& $ref = \e$foo; | |
194 | \& $weak = isweak($ref); # false | |
195 | \& weaken($ref); | |
196 | \& $weak = isweak($ref); # true | |
197 | .Ve | |
198 | .Sp | |
199 | \&\fB\s-1NOTE\s0\fR: Copying a weak reference creates a normal, strong, reference. | |
200 | .Sp | |
201 | .Vb 2 | |
202 | \& $copy = $ref; | |
203 | \& $weak = isweak($ref); # false | |
204 | .Ve | |
205 | .IP "looks_like_number \s-1EXPR\s0" 4 | |
206 | .IX Item "looks_like_number EXPR" | |
207 | Returns true if perl thinks \s-1EXPR\s0 is a number. See | |
208 | \&\*(L"looks_like_number\*(R" in perlapi. | |
209 | .IP "openhandle \s-1FH\s0" 4 | |
210 | .IX Item "openhandle FH" | |
211 | Returns \s-1FH\s0 if \s-1FH\s0 may be used as a filehandle and is open, or \s-1FH\s0 is a tied | |
212 | handle. Otherwise \f(CW\*(C`undef\*(C'\fR is returned. | |
213 | .Sp | |
214 | .Vb 4 | |
215 | \& $fh = openhandle(*STDIN); # \e*STDIN | |
216 | \& $fh = openhandle(\e*STDIN); # \e*STDIN | |
217 | \& $fh = openhandle(*NOTOPEN); # undef | |
218 | \& $fh = openhandle("scalar"); # undef | |
219 | .Ve | |
220 | .IP "readonly \s-1SCALAR\s0" 4 | |
221 | .IX Item "readonly SCALAR" | |
222 | Returns true if \s-1SCALAR\s0 is readonly. | |
223 | .Sp | |
224 | .Vb 1 | |
225 | \& sub foo { readonly($_[0]) } | |
226 | .Ve | |
227 | .Sp | |
228 | .Vb 2 | |
229 | \& $readonly = foo($bar); # false | |
230 | \& $readonly = foo(0); # true | |
231 | .Ve | |
232 | .IP "refaddr \s-1EXPR\s0" 4 | |
233 | .IX Item "refaddr EXPR" | |
234 | If \s-1EXPR\s0 evaluates to a reference the internal memory address of | |
235 | the referenced value is returned. Otherwise \f(CW\*(C`undef\*(C'\fR is returned. | |
236 | .Sp | |
237 | .Vb 3 | |
238 | \& $addr = refaddr "string"; # undef | |
239 | \& $addr = refaddr \e$var; # eg 12345678 | |
240 | \& $addr = refaddr []; # eg 23456784 | |
241 | .Ve | |
242 | .Sp | |
243 | .Vb 2 | |
244 | \& $obj = bless {}, "Foo"; | |
245 | \& $addr = refaddr $obj; # eg 88123488 | |
246 | .Ve | |
247 | .IP "reftype \s-1EXPR\s0" 4 | |
248 | .IX Item "reftype EXPR" | |
249 | If \s-1EXPR\s0 evaluates to a reference the type of the variable referenced | |
250 | is returned. Otherwise \f(CW\*(C`undef\*(C'\fR is returned. | |
251 | .Sp | |
252 | .Vb 3 | |
253 | \& $type = reftype "string"; # undef | |
254 | \& $type = reftype \e$var; # SCALAR | |
255 | \& $type = reftype []; # ARRAY | |
256 | .Ve | |
257 | .Sp | |
258 | .Vb 2 | |
259 | \& $obj = bless {}, "Foo"; | |
260 | \& $type = reftype $obj; # HASH | |
261 | .Ve | |
262 | .IP "set_prototype \s-1CODEREF\s0, \s-1PROTOTYPE\s0" 4 | |
263 | .IX Item "set_prototype CODEREF, PROTOTYPE" | |
264 | Sets the prototype of the given function, or deletes it if \s-1PROTOTYPE\s0 is | |
265 | undef. Returns the \s-1CODEREF\s0. | |
266 | .Sp | |
267 | .Vb 1 | |
268 | \& set_prototype \e&foo, '$$'; | |
269 | .Ve | |
270 | .IP "tainted \s-1EXPR\s0" 4 | |
271 | .IX Item "tainted EXPR" | |
272 | Return true if the result of \s-1EXPR\s0 is tainted | |
273 | .Sp | |
274 | .Vb 2 | |
275 | \& $taint = tainted("constant"); # false | |
276 | \& $taint = tainted($ENV{PWD}); # true if running under -T | |
277 | .Ve | |
278 | .IP "weaken \s-1REF\s0" 4 | |
279 | .IX Item "weaken REF" | |
280 | \&\s-1REF\s0 will be turned into a weak reference. This means that it will not | |
281 | hold a reference count on the object it references. Also when the reference | |
282 | count on that object reaches zero, \s-1REF\s0 will be set to undef. | |
283 | .Sp | |
284 | This is useful for keeping copies of references , but you don't want to | |
285 | prevent the object being DESTROY-ed at its usual time. | |
286 | .Sp | |
287 | .Vb 6 | |
288 | \& { | |
289 | \& my $var; | |
290 | \& $ref = \e$var; | |
291 | \& weaken($ref); # Make $ref a weak reference | |
292 | \& } | |
293 | \& # $ref is now undef | |
294 | .Ve | |
295 | .Sp | |
296 | Note that if you take a copy of a scalar with a weakened reference, | |
297 | the copy will be a strong reference. | |
298 | .Sp | |
299 | .Vb 4 | |
300 | \& my $var; | |
301 | \& my $foo = \e$var; | |
302 | \& weaken($foo); # Make $foo a weak reference | |
303 | \& my $bar = $foo; # $bar is now a strong reference | |
304 | .Ve | |
305 | .Sp | |
306 | This may be less obvious in other situations, such as \f(CW\*(C`grep()\*(C'\fR, for instance | |
307 | when grepping through a list of weakened references to objects that may have | |
308 | been destroyed already: | |
309 | .Sp | |
310 | .Vb 1 | |
311 | \& @object = grep { defined } @object; | |
312 | .Ve | |
313 | .Sp | |
314 | This will indeed remove all references to destroyed objects, but the remaining | |
315 | references to objects will be strong, causing the remaining objects to never | |
316 | be destroyed because there is now always a strong reference to them in the | |
317 | \&\f(CW@object\fR array. | |
318 | .SH "KNOWN BUGS" | |
319 | .IX Header "KNOWN BUGS" | |
320 | There is a bug in perl5.6.0 with \s-1UV\s0's that are >= 1<<31. This will | |
321 | show up as tests 8 and 9 of dualvar.t failing | |
322 | .SH "COPYRIGHT" | |
323 | .IX Header "COPYRIGHT" | |
324 | Copyright (c) 1997\-2005 Graham Barr <gbarr@pobox.com>. All rights reserved. | |
325 | This program is free software; you can redistribute it and/or modify it | |
326 | under the same terms as Perl itself. | |
327 | .PP | |
328 | Except weaken and isweak which are | |
329 | .PP | |
330 | Copyright (c) 1999 Tuomas J. Lukka <lukka@iki.fi>. All rights reserved. | |
331 | This program is free software; you can redistribute it and/or modify it | |
332 | under the same terms as perl itself. | |
333 | .SH "BLATANT PLUG" | |
334 | .IX Header "BLATANT PLUG" | |
335 | The weaken and isweak subroutines in this module and the patch to the core Perl | |
336 | were written in connection with the APress book `Tuomas J. Lukka's Definitive | |
337 | Guide to Object-Oriented Programming in Perl', to avoid explaining why certain | |
338 | things would have to be done in cumbersome ways. |