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| 128 | .rm #[ #] #H #V #F C |
| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "Exporter 3" |
| 132 | .TH Exporter 3 "2001-09-21" "perl v5.8.8" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | Exporter \- Implements default import method for modules |
| 135 | .SH "SYNOPSIS" |
| 136 | .IX Header "SYNOPSIS" |
| 137 | In module YourModule.pm: |
| 138 | .PP |
| 139 | .Vb 4 |
| 140 | \& package YourModule; |
| 141 | \& require Exporter; |
| 142 | \& @ISA = qw(Exporter); |
| 143 | \& @EXPORT_OK = qw(munge frobnicate); # symbols to export on request |
| 144 | .Ve |
| 145 | .PP |
| 146 | or |
| 147 | .PP |
| 148 | .Vb 3 |
| 149 | \& package YourModule; |
| 150 | \& use Exporter 'import'; # gives you Exporter's import() method directly |
| 151 | \& @EXPORT_OK = qw(munge frobnicate); # symbols to export on request |
| 152 | .Ve |
| 153 | .PP |
| 154 | In other files which wish to use YourModule: |
| 155 | .PP |
| 156 | .Vb 2 |
| 157 | \& use ModuleName qw(frobnicate); # import listed symbols |
| 158 | \& frobnicate ($left, $right) # calls YourModule::frobnicate |
| 159 | .Ve |
| 160 | .SH "DESCRIPTION" |
| 161 | .IX Header "DESCRIPTION" |
| 162 | The Exporter module implements an \f(CW\*(C`import\*(C'\fR method which allows a module |
| 163 | to export functions and variables to its users' namespaces. Many modules |
| 164 | use Exporter rather than implementing their own \f(CW\*(C`import\*(C'\fR method because |
| 165 | Exporter provides a highly flexible interface, with an implementation optimised |
| 166 | for the common case. |
| 167 | .PP |
| 168 | Perl automatically calls the \f(CW\*(C`import\*(C'\fR method when processing a |
| 169 | \&\f(CW\*(C`use\*(C'\fR statement for a module. Modules and \f(CW\*(C`use\*(C'\fR are documented |
| 170 | in perlfunc and perlmod. Understanding the concept of |
| 171 | modules and how the \f(CW\*(C`use\*(C'\fR statement operates is important to |
| 172 | understanding the Exporter. |
| 173 | .Sh "How to Export" |
| 174 | .IX Subsection "How to Export" |
| 175 | The arrays \f(CW@EXPORT\fR and \f(CW@EXPORT_OK\fR in a module hold lists of |
| 176 | symbols that are going to be exported into the users name space by |
| 177 | default, or which they can request to be exported, respectively. The |
| 178 | symbols can represent functions, scalars, arrays, hashes, or typeglobs. |
| 179 | The symbols must be given by full name with the exception that the |
| 180 | ampersand in front of a function is optional, e.g. |
| 181 | .PP |
| 182 | .Vb 2 |
| 183 | \& @EXPORT = qw(afunc $scalar @array); # afunc is a function |
| 184 | \& @EXPORT_OK = qw(&bfunc %hash *typeglob); # explicit prefix on &bfunc |
| 185 | .Ve |
| 186 | .PP |
| 187 | If you are only exporting function names it is recommended to omit the |
| 188 | ampersand, as the implementation is faster this way. |
| 189 | .Sh "Selecting What To Export" |
| 190 | .IX Subsection "Selecting What To Export" |
| 191 | Do \fBnot\fR export method names! |
| 192 | .PP |
| 193 | Do \fBnot\fR export anything else by default without a good reason! |
| 194 | .PP |
| 195 | Exports pollute the namespace of the module user. If you must export |
| 196 | try to use \f(CW@EXPORT_OK\fR in preference to \f(CW@EXPORT\fR and avoid short or |
| 197 | common symbol names to reduce the risk of name clashes. |
| 198 | .PP |
| 199 | Generally anything not exported is still accessible from outside the |
| 200 | module using the ModuleName::item_name (or \f(CW$blessed_ref\fR\->method) |
| 201 | syntax. By convention you can use a leading underscore on names to |
| 202 | informally indicate that they are 'internal' and not for public use. |
| 203 | .PP |
| 204 | (It is actually possible to get private functions by saying: |
| 205 | .PP |
| 206 | .Vb 3 |
| 207 | \& my $subref = sub { ... }; |
| 208 | \& $subref->(@args); # Call it as a function |
| 209 | \& $obj->$subref(@args); # Use it as a method |
| 210 | .Ve |
| 211 | .PP |
| 212 | However if you use them for methods it is up to you to figure out |
| 213 | how to make inheritance work.) |
| 214 | .PP |
| 215 | As a general rule, if the module is trying to be object oriented |
| 216 | then export nothing. If it's just a collection of functions then |
| 217 | \&\f(CW@EXPORT_OK\fR anything but use \f(CW@EXPORT\fR with caution. For function and |
| 218 | method names use barewords in preference to names prefixed with |
| 219 | ampersands for the export lists. |
| 220 | .PP |
| 221 | Other module design guidelines can be found in perlmod. |
| 222 | .Sh "How to Import" |
| 223 | .IX Subsection "How to Import" |
| 224 | In other files which wish to use your module there are three basic ways for |
| 225 | them to load your module and import its symbols: |
| 226 | .ie n .IP """use ModuleName;""" 4 |
| 227 | .el .IP "\f(CWuse ModuleName;\fR" 4 |
| 228 | .IX Item "use ModuleName;" |
| 229 | This imports all the symbols from ModuleName's \f(CW@EXPORT\fR into the namespace |
| 230 | of the \f(CW\*(C`use\*(C'\fR statement. |
| 231 | .ie n .IP """use ModuleName ();""" 4 |
| 232 | .el .IP "\f(CWuse ModuleName ();\fR" 4 |
| 233 | .IX Item "use ModuleName ();" |
| 234 | This causes perl to load your module but does not import any symbols. |
| 235 | .ie n .IP """use ModuleName qw(...);""" 4 |
| 236 | .el .IP "\f(CWuse ModuleName qw(...);\fR" 4 |
| 237 | .IX Item "use ModuleName qw(...);" |
| 238 | This imports only the symbols listed by the caller into their namespace. |
| 239 | All listed symbols must be in your \f(CW@EXPORT\fR or \f(CW@EXPORT_OK\fR, else an error |
| 240 | occurs. The advanced export features of Exporter are accessed like this, |
| 241 | but with list entries that are syntactically distinct from symbol names. |
| 242 | .PP |
| 243 | Unless you want to use its advanced features, this is probably all you |
| 244 | need to know to use Exporter. |
| 245 | .SH "Advanced features" |
| 246 | .IX Header "Advanced features" |
| 247 | .Sh "Specialised Import Lists" |
| 248 | .IX Subsection "Specialised Import Lists" |
| 249 | If any of the entries in an import list begins with !, : or / then |
| 250 | the list is treated as a series of specifications which either add to |
| 251 | or delete from the list of names to import. They are processed left to |
| 252 | right. Specifications are in the form: |
| 253 | .PP |
| 254 | .Vb 4 |
| 255 | \& [!]name This name only |
| 256 | \& [!]:DEFAULT All names in @EXPORT |
| 257 | \& [!]:tag All names in $EXPORT_TAGS{tag} anonymous list |
| 258 | \& [!]/pattern/ All names in @EXPORT and @EXPORT_OK which match |
| 259 | .Ve |
| 260 | .PP |
| 261 | A leading ! indicates that matching names should be deleted from the |
| 262 | list of names to import. If the first specification is a deletion it |
| 263 | is treated as though preceded by :DEFAULT. If you just want to import |
| 264 | extra names in addition to the default set you will still need to |
| 265 | include :DEFAULT explicitly. |
| 266 | .PP |
| 267 | e.g., Module.pm defines: |
| 268 | .PP |
| 269 | .Vb 3 |
| 270 | \& @EXPORT = qw(A1 A2 A3 A4 A5); |
| 271 | \& @EXPORT_OK = qw(B1 B2 B3 B4 B5); |
| 272 | \& %EXPORT_TAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]); |
| 273 | .Ve |
| 274 | .PP |
| 275 | .Vb 2 |
| 276 | \& Note that you cannot use tags in @EXPORT or @EXPORT_OK. |
| 277 | \& Names in EXPORT_TAGS must also appear in @EXPORT or @EXPORT_OK. |
| 278 | .Ve |
| 279 | .PP |
| 280 | An application using Module can say something like: |
| 281 | .PP |
| 282 | .Vb 1 |
| 283 | \& use Module qw(:DEFAULT :T2 !B3 A3); |
| 284 | .Ve |
| 285 | .PP |
| 286 | Other examples include: |
| 287 | .PP |
| 288 | .Vb 2 |
| 289 | \& use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET); |
| 290 | \& use POSIX qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/); |
| 291 | .Ve |
| 292 | .PP |
| 293 | Remember that most patterns (using //) will need to be anchored |
| 294 | with a leading ^, e.g., \f(CW\*(C`/^EXIT/\*(C'\fR rather than \f(CW\*(C`/EXIT/\*(C'\fR. |
| 295 | .PP |
| 296 | You can say \f(CW\*(C`BEGIN { $Exporter::Verbose=1 }\*(C'\fR to see how the |
| 297 | specifications are being processed and what is actually being imported |
| 298 | into modules. |
| 299 | .Sh "Exporting without using Exporter's import method" |
| 300 | .IX Subsection "Exporting without using Exporter's import method" |
| 301 | Exporter has a special method, 'export_to_level' which is used in situations |
| 302 | where you can't directly call Exporter's import method. The export_to_level |
| 303 | method looks like: |
| 304 | .PP |
| 305 | .Vb 1 |
| 306 | \& MyPackage->export_to_level($where_to_export, $package, @what_to_export); |
| 307 | .Ve |
| 308 | .PP |
| 309 | where \f(CW$where_to_export\fR is an integer telling how far up the calling stack |
| 310 | to export your symbols, and \f(CW@what_to_export\fR is an array telling what |
| 311 | symbols *to* export (usually this is \f(CW@_\fR). The \f(CW$package\fR argument is |
| 312 | currently unused. |
| 313 | .PP |
| 314 | For example, suppose that you have a module, A, which already has an |
| 315 | import function: |
| 316 | .PP |
| 317 | .Vb 1 |
| 318 | \& package A; |
| 319 | .Ve |
| 320 | .PP |
| 321 | .Vb 2 |
| 322 | \& @ISA = qw(Exporter); |
| 323 | \& @EXPORT_OK = qw ($b); |
| 324 | .Ve |
| 325 | .PP |
| 326 | .Vb 4 |
| 327 | \& sub import |
| 328 | \& { |
| 329 | \& $A::b = 1; # not a very useful import method |
| 330 | \& } |
| 331 | .Ve |
| 332 | .PP |
| 333 | and you want to Export symbol \f(CW$A::b\fR back to the module that called |
| 334 | package A. Since Exporter relies on the import method to work, via |
| 335 | inheritance, as it stands \fIExporter::import()\fR will never get called. |
| 336 | Instead, say the following: |
| 337 | .PP |
| 338 | .Vb 3 |
| 339 | \& package A; |
| 340 | \& @ISA = qw(Exporter); |
| 341 | \& @EXPORT_OK = qw ($b); |
| 342 | .Ve |
| 343 | .PP |
| 344 | .Vb 5 |
| 345 | \& sub import |
| 346 | \& { |
| 347 | \& $A::b = 1; |
| 348 | \& A->export_to_level(1, @_); |
| 349 | \& } |
| 350 | .Ve |
| 351 | .PP |
| 352 | This will export the symbols one level 'above' the current package \- ie: to |
| 353 | the program or module that used package A. |
| 354 | .PP |
| 355 | Note: Be careful not to modify \f(CW@_\fR at all before you call export_to_level |
| 356 | \&\- or people using your package will get very unexplained results! |
| 357 | .Sh "Exporting without inheriting from Exporter" |
| 358 | .IX Subsection "Exporting without inheriting from Exporter" |
| 359 | By including Exporter in your \f(CW@ISA\fR you inherit an Exporter's \fIimport()\fR method |
| 360 | but you also inherit several other helper methods which you probably don't |
| 361 | want. To avoid this you can do |
| 362 | .PP |
| 363 | .Vb 2 |
| 364 | \& package YourModule; |
| 365 | \& use Exporter qw( import ); |
| 366 | .Ve |
| 367 | .PP |
| 368 | which will export Exporter's own \fIimport()\fR method into YourModule. |
| 369 | Everything will work as before but you won't need to include Exporter in |
| 370 | \&\f(CW@YourModule::ISA\fR. |
| 371 | .Sh "Module Version Checking" |
| 372 | .IX Subsection "Module Version Checking" |
| 373 | The Exporter module will convert an attempt to import a number from a |
| 374 | module into a call to \f(CW$module_name\fR\->require_version($value). This can |
| 375 | be used to validate that the version of the module being used is |
| 376 | greater than or equal to the required version. |
| 377 | .PP |
| 378 | The Exporter module supplies a default require_version method which |
| 379 | checks the value of \f(CW$VERSION\fR in the exporting module. |
| 380 | .PP |
| 381 | Since the default require_version method treats the \f(CW$VERSION\fR number as |
| 382 | a simple numeric value it will regard version 1.10 as lower than |
| 383 | 1.9. For this reason it is strongly recommended that you use numbers |
| 384 | with at least two decimal places, e.g., 1.09. |
| 385 | .Sh "Managing Unknown Symbols" |
| 386 | .IX Subsection "Managing Unknown Symbols" |
| 387 | In some situations you may want to prevent certain symbols from being |
| 388 | exported. Typically this applies to extensions which have functions |
| 389 | or constants that may not exist on some systems. |
| 390 | .PP |
| 391 | The names of any symbols that cannot be exported should be listed |
| 392 | in the \f(CW@EXPORT_FAIL\fR array. |
| 393 | .PP |
| 394 | If a module attempts to import any of these symbols the Exporter |
| 395 | will give the module an opportunity to handle the situation before |
| 396 | generating an error. The Exporter will call an export_fail method |
| 397 | with a list of the failed symbols: |
| 398 | .PP |
| 399 | .Vb 1 |
| 400 | \& @failed_symbols = $module_name->export_fail(@failed_symbols); |
| 401 | .Ve |
| 402 | .PP |
| 403 | If the export_fail method returns an empty list then no error is |
| 404 | recorded and all the requested symbols are exported. If the returned |
| 405 | list is not empty then an error is generated for each symbol and the |
| 406 | export fails. The Exporter provides a default export_fail method which |
| 407 | simply returns the list unchanged. |
| 408 | .PP |
| 409 | Uses for the export_fail method include giving better error messages |
| 410 | for some symbols and performing lazy architectural checks (put more |
| 411 | symbols into \f(CW@EXPORT_FAIL\fR by default and then take them out if someone |
| 412 | actually tries to use them and an expensive check shows that they are |
| 413 | usable on that platform). |
| 414 | .Sh "Tag Handling Utility Functions" |
| 415 | .IX Subsection "Tag Handling Utility Functions" |
| 416 | Since the symbols listed within \f(CW%EXPORT_TAGS\fR must also appear in either |
| 417 | \&\f(CW@EXPORT\fR or \f(CW@EXPORT_OK\fR, two utility functions are provided which allow |
| 418 | you to easily add tagged sets of symbols to \f(CW@EXPORT\fR or \f(CW@EXPORT_OK:\fR |
| 419 | .PP |
| 420 | .Vb 1 |
| 421 | \& %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]); |
| 422 | .Ve |
| 423 | .PP |
| 424 | .Vb 2 |
| 425 | \& Exporter::export_tags('foo'); # add aa, bb and cc to @EXPORT |
| 426 | \& Exporter::export_ok_tags('bar'); # add aa, cc and dd to @EXPORT_OK |
| 427 | .Ve |
| 428 | .PP |
| 429 | Any names which are not tags are added to \f(CW@EXPORT\fR or \f(CW@EXPORT_OK\fR |
| 430 | unchanged but will trigger a warning (with \f(CW\*(C`\-w\*(C'\fR) to avoid misspelt tags |
| 431 | names being silently added to \f(CW@EXPORT\fR or \f(CW@EXPORT_OK\fR. Future versions |
| 432 | may make this a fatal error. |
| 433 | .Sh "Generating combined tags" |
| 434 | .IX Subsection "Generating combined tags" |
| 435 | If several symbol categories exist in \f(CW%EXPORT_TAGS\fR, it's usually |
| 436 | useful to create the utility \*(L":all\*(R" to simplify \*(L"use\*(R" statements. |
| 437 | .PP |
| 438 | The simplest way to do this is: |
| 439 | .PP |
| 440 | .Vb 1 |
| 441 | \& %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]); |
| 442 | .Ve |
| 443 | .PP |
| 444 | .Vb 4 |
| 445 | \& # add all the other ":class" tags to the ":all" class, |
| 446 | \& # deleting duplicates |
| 447 | \& { |
| 448 | \& my %seen; |
| 449 | .Ve |
| 450 | .PP |
| 451 | .Vb 3 |
| 452 | \& push @{$EXPORT_TAGS{all}}, |
| 453 | \& grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}} foreach keys %EXPORT_TAGS; |
| 454 | \& } |
| 455 | .Ve |
| 456 | .PP |
| 457 | \&\s-1CGI\s0.pm creates an \*(L":all\*(R" tag which contains some (but not really |
| 458 | all) of its categories. That could be done with one small |
| 459 | change: |
| 460 | .PP |
| 461 | .Vb 4 |
| 462 | \& # add some of the other ":class" tags to the ":all" class, |
| 463 | \& # deleting duplicates |
| 464 | \& { |
| 465 | \& my %seen; |
| 466 | .Ve |
| 467 | .PP |
| 468 | .Vb 4 |
| 469 | \& push @{$EXPORT_TAGS{all}}, |
| 470 | \& grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}} |
| 471 | \& foreach qw/html2 html3 netscape form cgi internal/; |
| 472 | \& } |
| 473 | .Ve |
| 474 | .PP |
| 475 | Note that the tag names in \f(CW%EXPORT_TAGS\fR don't have the leading ':'. |
| 476 | .ie n .Sh """AUTOLOAD""ed Constants" |
| 477 | .el .Sh "\f(CWAUTOLOAD\fPed Constants" |
| 478 | .IX Subsection "AUTOLOADed Constants" |
| 479 | Many modules make use of \f(CW\*(C`AUTOLOAD\*(C'\fRing for constant subroutines to |
| 480 | avoid having to compile and waste memory on rarely used values (see |
| 481 | perlsub for details on constant subroutines). Calls to such |
| 482 | constant subroutines are not optimized away at compile time because |
| 483 | they can't be checked at compile time for constancy. |
| 484 | .PP |
| 485 | Even if a prototype is available at compile time, the body of the |
| 486 | subroutine is not (it hasn't been \f(CW\*(C`AUTOLOAD\*(C'\fRed yet). perl needs to |
| 487 | examine both the \f(CW\*(C`()\*(C'\fR prototype and the body of a subroutine at |
| 488 | compile time to detect that it can safely replace calls to that |
| 489 | subroutine with the constant value. |
| 490 | .PP |
| 491 | A workaround for this is to call the constants once in a \f(CW\*(C`BEGIN\*(C'\fR block: |
| 492 | .PP |
| 493 | .Vb 1 |
| 494 | \& package My ; |
| 495 | .Ve |
| 496 | .PP |
| 497 | .Vb 1 |
| 498 | \& use Socket ; |
| 499 | .Ve |
| 500 | .PP |
| 501 | .Vb 3 |
| 502 | \& foo( SO_LINGER ); ## SO_LINGER NOT optimized away; called at runtime |
| 503 | \& BEGIN { SO_LINGER } |
| 504 | \& foo( SO_LINGER ); ## SO_LINGER optimized away at compile time. |
| 505 | .Ve |
| 506 | .PP |
| 507 | This forces the \f(CW\*(C`AUTOLOAD\*(C'\fR for \f(CW\*(C`SO_LINGER\*(C'\fR to take place before |
| 508 | \&\s-1SO_LINGER\s0 is encountered later in \f(CW\*(C`My\*(C'\fR package. |
| 509 | .PP |
| 510 | If you are writing a package that \f(CW\*(C`AUTOLOAD\*(C'\fRs, consider forcing |
| 511 | an \f(CW\*(C`AUTOLOAD\*(C'\fR for any constants explicitly imported by other packages |
| 512 | or which are usually used when your package is \f(CW\*(C`use\*(C'\fRd. |