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129 | .\" ======================================================================== | |
130 | .\" | |
131 | .IX Title "attributes 3" | |
132 | .TH attributes 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" | |
133 | .SH "NAME" | |
134 | attributes \- get/set subroutine or variable attributes | |
135 | .SH "SYNOPSIS" | |
136 | .IX Header "SYNOPSIS" | |
137 | .Vb 3 | |
138 | \& sub foo : method ; | |
139 | \& my ($x,@y,%z) : Bent = 1; | |
140 | \& my $s = sub : method { ... }; | |
141 | .Ve | |
142 | .PP | |
143 | .Vb 2 | |
144 | \& use attributes (); # optional, to get subroutine declarations | |
145 | \& my @attrlist = attributes::get(\e&foo); | |
146 | .Ve | |
147 | .PP | |
148 | .Vb 2 | |
149 | \& use attributes 'get'; # import the attributes::get subroutine | |
150 | \& my @attrlist = get \e&foo; | |
151 | .Ve | |
152 | .SH "DESCRIPTION" | |
153 | .IX Header "DESCRIPTION" | |
154 | Subroutine declarations and definitions may optionally have attribute lists | |
155 | associated with them. (Variable \f(CW\*(C`my\*(C'\fR declarations also may, but see the | |
156 | warning below.) Perl handles these declarations by passing some information | |
157 | about the call site and the thing being declared along with the attribute | |
158 | list to this module. In particular, the first example above is equivalent to | |
159 | the following: | |
160 | .PP | |
161 | .Vb 1 | |
162 | \& use attributes __PACKAGE__, \e&foo, 'method'; | |
163 | .Ve | |
164 | .PP | |
165 | The second example in the synopsis does something equivalent to this: | |
166 | .PP | |
167 | .Vb 6 | |
168 | \& use attributes (); | |
169 | \& my ($x,@y,%z); | |
170 | \& attributes::->import(__PACKAGE__, \e$x, 'Bent'); | |
171 | \& attributes::->import(__PACKAGE__, \e@y, 'Bent'); | |
172 | \& attributes::->import(__PACKAGE__, \e%z, 'Bent'); | |
173 | \& ($x,@y,%z) = 1; | |
174 | .Ve | |
175 | .PP | |
176 | Yes, that's a lot of expansion. | |
177 | .PP | |
178 | \&\fB\s-1WARNING\s0\fR: attribute declarations for variables are still evolving. | |
179 | The semantics and interfaces of such declarations could change in | |
180 | future versions. They are present for purposes of experimentation | |
181 | with what the semantics ought to be. Do not rely on the current | |
182 | implementation of this feature. | |
183 | .PP | |
184 | There are only a few attributes currently handled by Perl itself (or | |
185 | directly by this module, depending on how you look at it.) However, | |
186 | package-specific attributes are allowed by an extension mechanism. | |
187 | (See \*(L"Package\-specific Attribute Handling\*(R" below.) | |
188 | .PP | |
189 | The setting of subroutine attributes happens at compile time. | |
190 | Variable attributes in \f(CW\*(C`our\*(C'\fR declarations are also applied at compile time. | |
191 | However, \f(CW\*(C`my\*(C'\fR variables get their attributes applied at run\-time. | |
192 | This means that you have to \fIreach\fR the run-time component of the \f(CW\*(C`my\*(C'\fR | |
193 | before those attributes will get applied. For example: | |
194 | .PP | |
195 | .Vb 1 | |
196 | \& my $x : Bent = 42 if 0; | |
197 | .Ve | |
198 | .PP | |
199 | will neither assign 42 to \f(CW$x\fR \fInor\fR will it apply the \f(CW\*(C`Bent\*(C'\fR attribute | |
200 | to the variable. | |
201 | .PP | |
202 | An attempt to set an unrecognized attribute is a fatal error. (The | |
203 | error is trappable, but it still stops the compilation within that | |
204 | \&\f(CW\*(C`eval\*(C'\fR.) Setting an attribute with a name that's all lowercase | |
205 | letters that's not a built-in attribute (such as \*(L"foo\*(R") will result in | |
206 | a warning with \fB\-w\fR or \f(CW\*(C`use warnings 'reserved'\*(C'\fR. | |
207 | .Sh "Built-in Attributes" | |
208 | .IX Subsection "Built-in Attributes" | |
209 | The following are the built-in attributes for subroutines: | |
210 | .IP "locked" 4 | |
211 | .IX Item "locked" | |
212 | \&\fB5.005 threads only! The use of the \*(L"locked\*(R" attribute currently | |
213 | only makes sense if you are using the deprecated \*(L"Perl 5.005 threads\*(R" | |
214 | implementation of threads.\fR | |
215 | .Sp | |
216 | Setting this attribute is only meaningful when the subroutine or | |
217 | method is to be called by multiple threads. When set on a method | |
218 | subroutine (i.e., one marked with the \fBmethod\fR attribute below), | |
219 | Perl ensures that any invocation of it implicitly locks its first | |
220 | argument before execution. When set on a non-method subroutine, | |
221 | Perl ensures that a lock is taken on the subroutine itself before | |
222 | execution. The semantics of the lock are exactly those of one | |
223 | explicitly taken with the \f(CW\*(C`lock\*(C'\fR operator immediately after the | |
224 | subroutine is entered. | |
225 | .IP "method" 4 | |
226 | .IX Item "method" | |
227 | Indicates that the referenced subroutine is a method. | |
228 | This has a meaning when taken together with the \fBlocked\fR attribute, | |
229 | as described there. It also means that a subroutine so marked | |
230 | will not trigger the \*(L"Ambiguous call resolved as CORE::%s\*(R" warning. | |
231 | .IP "lvalue" 4 | |
232 | .IX Item "lvalue" | |
233 | Indicates that the referenced subroutine is a valid lvalue and can | |
234 | be assigned to. The subroutine must return a modifiable value such | |
235 | as a scalar variable, as described in perlsub. | |
236 | .PP | |
237 | For global variables there is \f(CW\*(C`unique\*(C'\fR attribute: see \*(L"our\*(R" in perlfunc. | |
238 | .Sh "Available Subroutines" | |
239 | .IX Subsection "Available Subroutines" | |
240 | The following subroutines are available for general use once this module | |
241 | has been loaded: | |
242 | .IP "get" 4 | |
243 | .IX Item "get" | |
244 | This routine expects a single parameter\*(--a reference to a | |
245 | subroutine or variable. It returns a list of attributes, which may be | |
246 | empty. If passed invalid arguments, it uses \fIdie()\fR (via Carp::croak) | |
247 | to raise a fatal exception. If it can find an appropriate package name | |
248 | for a class method lookup, it will include the results from a | |
249 | \&\f(CW\*(C`FETCH_\f(CItype\f(CW_ATTRIBUTES\*(C'\fR call in its return list, as described in | |
250 | \&\*(L"Package\-specific Attribute Handling\*(R" below. | |
251 | Otherwise, only built-in attributes will be returned. | |
252 | .IP "reftype" 4 | |
253 | .IX Item "reftype" | |
254 | This routine expects a single parameter\*(--a reference to a subroutine or | |
255 | variable. It returns the built-in type of the referenced variable, | |
256 | ignoring any package into which it might have been blessed. | |
257 | This can be useful for determining the \fItype\fR value which forms part of | |
258 | the method names described in \*(L"Package\-specific Attribute Handling\*(R" below. | |
259 | .PP | |
260 | Note that these routines are \fInot\fR exported by default. | |
261 | .Sh "Package-specific Attribute Handling" | |
262 | .IX Subsection "Package-specific Attribute Handling" | |
263 | \&\fB\s-1WARNING\s0\fR: the mechanisms described here are still experimental. Do not | |
264 | rely on the current implementation. In particular, there is no provision | |
265 | for applying package attributes to 'cloned' copies of subroutines used as | |
266 | closures. (See \*(L"Making References\*(R" in perlref for information on closures.) | |
267 | Package-specific attribute handling may change incompatibly in a future | |
268 | release. | |
269 | .PP | |
270 | When an attribute list is present in a declaration, a check is made to see | |
271 | whether an attribute 'modify' handler is present in the appropriate package | |
272 | (or its \f(CW@ISA\fR inheritance tree). Similarly, when \f(CW\*(C`attributes::get\*(C'\fR is | |
273 | called on a valid reference, a check is made for an appropriate attribute | |
274 | \&'fetch' handler. See \*(L"\s-1EXAMPLES\s0\*(R" to see how the \*(L"appropriate package\*(R" | |
275 | determination works. | |
276 | .PP | |
277 | The handler names are based on the underlying type of the variable being | |
278 | declared or of the reference passed. Because these attributes are | |
279 | associated with subroutine or variable declarations, this deliberately | |
280 | ignores any possibility of being blessed into some package. Thus, a | |
281 | subroutine declaration uses \*(L"\s-1CODE\s0\*(R" as its \fItype\fR, and even a blessed | |
282 | hash reference uses \*(L"\s-1HASH\s0\*(R" as its \fItype\fR. | |
283 | .PP | |
284 | The class methods invoked for modifying and fetching are these: | |
285 | .IP "\s-1FETCH_\s0\fItype\fR_ATTRIBUTES" 4 | |
286 | .IX Item "FETCH_type_ATTRIBUTES" | |
287 | This method receives a single argument, which is a reference to the | |
288 | variable or subroutine for which package-defined attributes are desired. | |
289 | The expected return value is a list of associated attributes. | |
290 | This list may be empty. | |
291 | .IP "\s-1MODIFY_\s0\fItype\fR_ATTRIBUTES" 4 | |
292 | .IX Item "MODIFY_type_ATTRIBUTES" | |
293 | This method is called with two fixed arguments, followed by the list of | |
294 | attributes from the relevant declaration. The two fixed arguments are | |
295 | the relevant package name and a reference to the declared subroutine or | |
296 | variable. The expected return value is a list of attributes which were | |
297 | not recognized by this handler. Note that this allows for a derived class | |
298 | to delegate a call to its base class, and then only examine the attributes | |
299 | which the base class didn't already handle for it. | |
300 | .Sp | |
301 | The call to this method is currently made \fIduring\fR the processing of the | |
302 | declaration. In particular, this means that a subroutine reference will | |
303 | probably be for an undefined subroutine, even if this declaration is | |
304 | actually part of the definition. | |
305 | .PP | |
306 | Calling \f(CW\*(C`attributes::get()\*(C'\fR from within the scope of a null package | |
307 | declaration \f(CW\*(C`package ;\*(C'\fR for an unblessed variable reference will | |
308 | not provide any starting package name for the 'fetch' method lookup. | |
309 | Thus, this circumstance will not result in a method call for package-defined | |
310 | attributes. A named subroutine knows to which symbol table entry it belongs | |
311 | (or originally belonged), and it will use the corresponding package. | |
312 | An anonymous subroutine knows the package name into which it was compiled | |
313 | (unless it was also compiled with a null package declaration), and so it | |
314 | will use that package name. | |
315 | .Sh "Syntax of Attribute Lists" | |
316 | .IX Subsection "Syntax of Attribute Lists" | |
317 | An attribute list is a sequence of attribute specifications, separated by | |
318 | whitespace or a colon (with optional whitespace). | |
319 | Each attribute specification is a simple | |
320 | name, optionally followed by a parenthesised parameter list. | |
321 | If such a parameter list is present, it is scanned past as for the rules | |
322 | for the \f(CW\*(C`q()\*(C'\fR operator. (See \*(L"Quote and Quote-like Operators\*(R" in perlop.) | |
323 | The parameter list is passed as it was found, however, and not as per \f(CW\*(C`q()\*(C'\fR. | |
324 | .PP | |
325 | Some examples of syntactically valid attribute lists: | |
326 | .PP | |
327 | .Vb 4 | |
328 | \& switch(10,foo(7,3)) : expensive | |
329 | \& Ugly('\e(") :Bad | |
330 | \& _5x5 | |
331 | \& locked method | |
332 | .Ve | |
333 | .PP | |
334 | Some examples of syntactically invalid attribute lists (with annotation): | |
335 | .PP | |
336 | .Vb 5 | |
337 | \& switch(10,foo() # ()-string not balanced | |
338 | \& Ugly('(') # ()-string not balanced | |
339 | \& 5x5 # "5x5" not a valid identifier | |
340 | \& Y2::north # "Y2::north" not a simple identifier | |
341 | \& foo + bar # "+" neither a colon nor whitespace | |
342 | .Ve | |
343 | .SH "EXPORTS" | |
344 | .IX Header "EXPORTS" | |
345 | .Sh "Default exports" | |
346 | .IX Subsection "Default exports" | |
347 | None. | |
348 | .Sh "Available exports" | |
349 | .IX Subsection "Available exports" | |
350 | The routines \f(CW\*(C`get\*(C'\fR and \f(CW\*(C`reftype\*(C'\fR are exportable. | |
351 | .Sh "Export tags defined" | |
352 | .IX Subsection "Export tags defined" | |
353 | The \f(CW\*(C`:ALL\*(C'\fR tag will get all of the above exports. | |
354 | .SH "EXAMPLES" | |
355 | .IX Header "EXAMPLES" | |
356 | Here are some samples of syntactically valid declarations, with annotation | |
357 | as to how they resolve internally into \f(CW\*(C`use attributes\*(C'\fR invocations by | |
358 | perl. These examples are primarily useful to see how the \*(L"appropriate | |
359 | package\*(R" is found for the possible method lookups for package-defined | |
360 | attributes. | |
361 | .IP "1." 4 | |
362 | Code: | |
363 | .Sp | |
364 | .Vb 3 | |
365 | \& package Canine; | |
366 | \& package Dog; | |
367 | \& my Canine $spot : Watchful ; | |
368 | .Ve | |
369 | .Sp | |
370 | Effect: | |
371 | .Sp | |
372 | .Vb 2 | |
373 | \& use attributes (); | |
374 | \& attributes::->import(Canine => \e$spot, "Watchful"); | |
375 | .Ve | |
376 | .IP "2." 4 | |
377 | Code: | |
378 | .Sp | |
379 | .Vb 2 | |
380 | \& package Felis; | |
381 | \& my $cat : Nervous; | |
382 | .Ve | |
383 | .Sp | |
384 | Effect: | |
385 | .Sp | |
386 | .Vb 2 | |
387 | \& use attributes (); | |
388 | \& attributes::->import(Felis => \e$cat, "Nervous"); | |
389 | .Ve | |
390 | .IP "3." 4 | |
391 | Code: | |
392 | .Sp | |
393 | .Vb 2 | |
394 | \& package X; | |
395 | \& sub foo : locked ; | |
396 | .Ve | |
397 | .Sp | |
398 | Effect: | |
399 | .Sp | |
400 | .Vb 1 | |
401 | \& use attributes X => \e&foo, "locked"; | |
402 | .Ve | |
403 | .IP "4." 4 | |
404 | Code: | |
405 | .Sp | |
406 | .Vb 2 | |
407 | \& package X; | |
408 | \& sub Y::x : locked { 1 } | |
409 | .Ve | |
410 | .Sp | |
411 | Effect: | |
412 | .Sp | |
413 | .Vb 1 | |
414 | \& use attributes Y => \e&Y::x, "locked"; | |
415 | .Ve | |
416 | .IP "5." 4 | |
417 | Code: | |
418 | .Sp | |
419 | .Vb 2 | |
420 | \& package X; | |
421 | \& sub foo { 1 } | |
422 | .Ve | |
423 | .Sp | |
424 | .Vb 2 | |
425 | \& package Y; | |
426 | \& BEGIN { *bar = \e&X::foo; } | |
427 | .Ve | |
428 | .Sp | |
429 | .Vb 2 | |
430 | \& package Z; | |
431 | \& sub Y::bar : locked ; | |
432 | .Ve | |
433 | .Sp | |
434 | Effect: | |
435 | .Sp | |
436 | .Vb 1 | |
437 | \& use attributes X => \e&X::foo, "locked"; | |
438 | .Ve | |
439 | .PP | |
440 | This last example is purely for purposes of completeness. You should not | |
441 | be trying to mess with the attributes of something in a package that's | |
442 | not your own. | |
443 | .SH "SEE ALSO" | |
444 | .IX Header "SEE ALSO" | |
445 | \&\*(L"Private Variables via \fImy()\fR\*(R" in perlsub and | |
446 | \&\*(L"Subroutine Attributes\*(R" in perlsub for details on the basic declarations; | |
447 | attrs for the obsolescent form of subroutine attribute specification | |
448 | which this module replaces; | |
449 | \&\*(L"use\*(R" in perlfunc for details on the normal invocation mechanism. |