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Fix the test boilerplate in lib/if.t, so it can be run
[perl5.git] / pod / perlmod.pod
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1=head1 NAME
2
f102b883 3perlmod - Perl modules (packages and symbol tables)
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4
5=head1 DESCRIPTION
6
7=head2 Packages
8
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9Perl provides a mechanism for alternative namespaces to protect
10packages from stomping on each other's variables. In fact, there's
bc8df162 11really no such thing as a global variable in Perl. The package
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12statement declares the compilation unit as being in the given
13namespace. The scope of the package declaration is from the
14declaration itself through the end of the enclosing block, C<eval>,
15or file, whichever comes first (the same scope as the my() and
16local() operators). Unqualified dynamic identifiers will be in
17this namespace, except for those few identifiers that if unqualified,
18default to the main package instead of the current one as described
19below. A package statement affects only dynamic variables--including
20those you've used local() on--but I<not> lexical variables created
21with my(). Typically it would be the first declaration in a file
22included by the C<do>, C<require>, or C<use> operators. You can
23switch into a package in more than one place; it merely influences
24which symbol table is used by the compiler for the rest of that
25block. You can refer to variables and filehandles in other packages
26by prefixing the identifier with the package name and a double
27colon: C<$Package::Variable>. If the package name is null, the
28C<main> package is assumed. That is, C<$::sail> is equivalent to
29C<$main::sail>.
a0d0e21e 30
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31The old package delimiter was a single quote, but double colon is now the
32preferred delimiter, in part because it's more readable to humans, and
33in part because it's more readable to B<emacs> macros. It also makes C++
34programmers feel like they know what's going on--as opposed to using the
35single quote as separator, which was there to make Ada programmers feel
14c715f4 36like they knew what was going on. Because the old-fashioned syntax is still
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37supported for backwards compatibility, if you try to use a string like
38C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is,
39the $s variable in package C<owner>, which is probably not what you meant.
40Use braces to disambiguate, as in C<"This is ${owner}'s house">.
a0d0e21e 41
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42Packages may themselves contain package separators, as in
43C<$OUTER::INNER::var>. This implies nothing about the order of
44name lookups, however. There are no relative packages: all symbols
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45are either local to the current package, or must be fully qualified
46from the outer package name down. For instance, there is nowhere
19799a22 47within package C<OUTER> that C<$INNER::var> refers to
14c715f4 48C<$OUTER::INNER::var>. C<INNER> refers to a totally
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49separate global package.
50
51Only identifiers starting with letters (or underscore) are stored
52in a package's symbol table. All other symbols are kept in package
53C<main>, including all punctuation variables, like $_. In addition,
54when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV,
55ARGVOUT, ENV, INC, and SIG are forced to be in package C<main>,
14c715f4 56even when used for other purposes than their built-in ones. If you
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57have a package called C<m>, C<s>, or C<y>, then you can't use the
58qualified form of an identifier because it would be instead interpreted
59as a pattern match, a substitution, or a transliteration.
60
61Variables beginning with underscore used to be forced into package
a0d0e21e 62main, but we decided it was more useful for package writers to be able
cb1a09d0 63to use leading underscore to indicate private variables and method names.
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64However, variables and functions named with a single C<_>, such as
65$_ and C<sub _>, are still forced into the package C<main>. See also
cea6626f 66L<perlvar/"Technical Note on the Syntax of Variable Names">.
a0d0e21e 67
19799a22 68C<eval>ed strings are compiled in the package in which the eval() was
a0d0e21e 69compiled. (Assignments to C<$SIG{}>, however, assume the signal
748a9306 70handler specified is in the C<main> package. Qualify the signal handler
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71name if you wish to have a signal handler in a package.) For an
72example, examine F<perldb.pl> in the Perl library. It initially switches
73to the C<DB> package so that the debugger doesn't interfere with variables
19799a22 74in the program you are trying to debug. At various points, however, it
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75temporarily switches back to the C<main> package to evaluate various
76expressions in the context of the C<main> package (or wherever you came
77from). See L<perldebug>.
78
f102b883 79The special symbol C<__PACKAGE__> contains the current package, but cannot
14c715f4 80(easily) be used to construct variable names.
f102b883 81
5f05dabc 82See L<perlsub> for other scoping issues related to my() and local(),
f102b883 83and L<perlref> regarding closures.
cb1a09d0 84
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85=head2 Symbol Tables
86
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87The symbol table for a package happens to be stored in the hash of that
88name with two colons appended. The main symbol table's name is thus
5803be0d 89C<%main::>, or C<%::> for short. Likewise the symbol table for the nested
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90package mentioned earlier is named C<%OUTER::INNER::>.
91
92The value in each entry of the hash is what you are referring to when you
93use the C<*name> typeglob notation. In fact, the following have the same
94effect, though the first is more efficient because it does the symbol
95table lookups at compile time:
a0d0e21e 96
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97 local *main::foo = *main::bar;
98 local $main::{foo} = $main::{bar};
a0d0e21e 99
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100(Be sure to note the B<vast> difference between the second line above
101and C<local $main::foo = $main::bar>. The former is accessing the hash
102C<%main::>, which is the symbol table of package C<main>. The latter is
103simply assigning scalar C<$bar> in package C<main> to scalar C<$foo> of
104the same package.)
105
a0d0e21e 106You can use this to print out all the variables in a package, for
4375e838 107instance. The standard but antiquated F<dumpvar.pl> library and
19799a22 108the CPAN module Devel::Symdump make use of this.
a0d0e21e 109
cb1a09d0 110Assignment to a typeglob performs an aliasing operation, i.e.,
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111
112 *dick = *richard;
113
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114causes variables, subroutines, formats, and file and directory handles
115accessible via the identifier C<richard> also to be accessible via the
116identifier C<dick>. If you want to alias only a particular variable or
19799a22 117subroutine, assign a reference instead:
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118
119 *dick = \$richard;
120
5a964f20 121Which makes $richard and $dick the same variable, but leaves
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122@richard and @dick as separate arrays. Tricky, eh?
123
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124There is one subtle difference between the following statements:
125
126 *foo = *bar;
127 *foo = \$bar;
128
129C<*foo = *bar> makes the typeglobs themselves synonymous while
130C<*foo = \$bar> makes the SCALAR portions of two distinct typeglobs
131refer to the same scalar value. This means that the following code:
132
133 $bar = 1;
134 *foo = \$bar; # Make $foo an alias for $bar
135
136 {
137 local $bar = 2; # Restrict changes to block
138 print $foo; # Prints '1'!
139 }
140
141Would print '1', because C<$foo> holds a reference to the I<original>
142C<$bar> -- the one that was stuffed away by C<local()> and which will be
143restored when the block ends. Because variables are accessed through the
144typeglob, you can use C<*foo = *bar> to create an alias which can be
145localized. (But be aware that this means you can't have a separate
146C<@foo> and C<@bar>, etc.)
147
148What makes all of this important is that the Exporter module uses glob
149aliasing as the import/export mechanism. Whether or not you can properly
150localize a variable that has been exported from a module depends on how
151it was exported:
152
153 @EXPORT = qw($FOO); # Usual form, can't be localized
154 @EXPORT = qw(*FOO); # Can be localized
155
14c715f4 156You can work around the first case by using the fully qualified name
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157(C<$Package::FOO>) where you need a local value, or by overriding it
158by saying C<*FOO = *Package::FOO> in your script.
159
160The C<*x = \$y> mechanism may be used to pass and return cheap references
5803be0d 161into or from subroutines if you don't want to copy the whole
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162thing. It only works when assigning to dynamic variables, not
163lexicals.
cb1a09d0 164
5a964f20 165 %some_hash = (); # can't be my()
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166 *some_hash = fn( \%another_hash );
167 sub fn {
168 local *hashsym = shift;
169 # now use %hashsym normally, and you
170 # will affect the caller's %another_hash
171 my %nhash = (); # do what you want
5f05dabc 172 return \%nhash;
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173 }
174
5f05dabc 175On return, the reference will overwrite the hash slot in the
cb1a09d0 176symbol table specified by the *some_hash typeglob. This
c36e9b62 177is a somewhat tricky way of passing around references cheaply
5803be0d 178when you don't want to have to remember to dereference variables
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179explicitly.
180
19799a22 181Another use of symbol tables is for making "constant" scalars.
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182
183 *PI = \3.14159265358979;
184
bc8df162 185Now you cannot alter C<$PI>, which is probably a good thing all in all.
5a964f20 186This isn't the same as a constant subroutine, which is subject to
5803be0d 187optimization at compile-time. A constant subroutine is one prototyped
14c715f4 188to take no arguments and to return a constant expression. See
5803be0d 189L<perlsub> for details on these. The C<use constant> pragma is a
5a964f20 190convenient shorthand for these.
cb1a09d0 191
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192You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and
193package the *foo symbol table entry comes from. This may be useful
5a964f20 194in a subroutine that gets passed typeglobs as arguments:
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195
196 sub identify_typeglob {
197 my $glob = shift;
198 print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
199 }
200 identify_typeglob *foo;
201 identify_typeglob *bar::baz;
202
203This prints
204
205 You gave me main::foo
206 You gave me bar::baz
207
19799a22 208The C<*foo{THING}> notation can also be used to obtain references to the
5803be0d 209individual elements of *foo. See L<perlref>.
55497cff 210
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211Subroutine definitions (and declarations, for that matter) need
212not necessarily be situated in the package whose symbol table they
213occupy. You can define a subroutine outside its package by
214explicitly qualifying the name of the subroutine:
215
216 package main;
217 sub Some_package::foo { ... } # &foo defined in Some_package
218
219This is just a shorthand for a typeglob assignment at compile time:
220
221 BEGIN { *Some_package::foo = sub { ... } }
222
223and is I<not> the same as writing:
224
225 {
226 package Some_package;
227 sub foo { ... }
228 }
229
230In the first two versions, the body of the subroutine is
231lexically in the main package, I<not> in Some_package. So
232something like this:
233
234 package main;
235
236 $Some_package::name = "fred";
237 $main::name = "barney";
238
239 sub Some_package::foo {
240 print "in ", __PACKAGE__, ": \$name is '$name'\n";
241 }
242
243 Some_package::foo();
244
245prints:
246
247 in main: $name is 'barney'
248
249rather than:
250
251 in Some_package: $name is 'fred'
252
253This also has implications for the use of the SUPER:: qualifier
254(see L<perlobj>).
255
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256=head2 Package Constructors and Destructors
257
7d981616 258Four special subroutines act as package constructors and destructors.
7d30b5c4 259These are the C<BEGIN>, C<CHECK>, C<INIT>, and C<END> routines. The
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260C<sub> is optional for these routines. See the B<begincheck> program, at
261the end of this section, to see them in action.
a0d0e21e 262
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263A C<BEGIN> subroutine is executed as soon as possible, that is, the moment
264it is completely defined, even before the rest of the containing file
265is parsed. You may have multiple C<BEGIN> blocks within a file--they
266will execute in order of definition. Because a C<BEGIN> block executes
267immediately, it can pull in definitions of subroutines and such from other
268files in time to be visible to the rest of the file. Once a C<BEGIN>
269has run, it is immediately undefined and any code it used is returned to
270Perl's memory pool. This means you can't ever explicitly call a C<BEGIN>.
a0d0e21e 271
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272An C<END> subroutine is executed as late as possible, that is, after
273perl has finished running the program and just before the interpreter
274is being exited, even if it is exiting as a result of a die() function.
275(But not if it's polymorphing into another program via C<exec>, or
276being blown out of the water by a signal--you have to trap that yourself
277(if you can).) You may have multiple C<END> blocks within a file--they
278will execute in reverse order of definition; that is: last in, first
279out (LIFO). C<END> blocks are not executed when you run perl with the
db517d64 280C<-c> switch, or if compilation fails.
a0d0e21e 281
19799a22 282Inside an C<END> subroutine, C<$?> contains the value that the program is
c36e9b62 283going to pass to C<exit()>. You can modify C<$?> to change the exit
19799a22 284value of the program. Beware of changing C<$?> by accident (e.g. by
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285running something via C<system>).
286
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287C<CHECK> and C<INIT> blocks are useful to catch the transition between
288the compilation phase and the execution phase of the main program.
289
290C<CHECK> blocks are run just after the Perl compile phase ends and before
291the run time begins, in LIFO order. C<CHECK> blocks are used in
292the Perl compiler suite to save the compiled state of the program.
293
294C<INIT> blocks are run just before the Perl runtime begins execution, in
295"first in, first out" (FIFO) order. For example, the code generators
296documented in L<perlcc> make use of C<INIT> blocks to initialize and
297resolve pointers to XSUBs.
4f25aa18 298
19799a22 299When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and
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300C<END> work just as they do in B<awk>, as a degenerate case.
301Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c>
302switch for a compile-only syntax check, although your main code
303is not.
a0d0e21e 304
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305The B<begincheck> program makes it all clear, eventually:
306
307 #!/usr/bin/perl
308
309 # begincheck
310
311 print " 8. Ordinary code runs at runtime.\n";
312
313 END { print "14. So this is the end of the tale.\n" }
314 INIT { print " 5. INIT blocks run FIFO just before runtime.\n" }
315 CHECK { print " 4. So this is the fourth line.\n" }
316
317 print " 9. It runs in order, of course.\n";
318
319 BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" }
320 END { print "13. Read perlmod for the rest of the story.\n" }
321 CHECK { print " 3. CHECK blocks run LIFO at compilation's end.\n" }
322 INIT { print " 6. Run this again, using Perl's -c switch.\n" }
323
324 print "10. This is anti-obfuscated code.\n";
325
326 END { print "12. END blocks run LIFO at quitting time.\n" }
327 BEGIN { print " 2. So this line comes out second.\n" }
328 INIT { print " 7. You'll see the difference right away.\n" }
329
330 print "11. It merely _looks_ like it should be confusing.\n";
331
332 __END__
333
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334=head2 Perl Classes
335
19799a22 336There is no special class syntax in Perl, but a package may act
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337as a class if it provides subroutines to act as methods. Such a
338package may also derive some of its methods from another class (package)
14c715f4 339by listing the other package name(s) in its global @ISA array (which
5a964f20 340must be a package global, not a lexical).
4633a7c4 341
f102b883 342For more on this, see L<perltoot> and L<perlobj>.
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343
344=head2 Perl Modules
345
5803be0d 346A module is just a set of related functions in a library file, i.e.,
14c715f4 347a Perl package with the same name as the file. It is specifically
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348designed to be reusable by other modules or programs. It may do this
349by providing a mechanism for exporting some of its symbols into the
14c715f4 350symbol table of any package using it, or it may function as a class
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351definition and make its semantics available implicitly through
352method calls on the class and its objects, without explicitly
4375e838 353exporting anything. Or it can do a little of both.
a0d0e21e 354
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355For example, to start a traditional, non-OO module called Some::Module,
356create a file called F<Some/Module.pm> and start with this template:
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357
358 package Some::Module; # assumes Some/Module.pm
359
360 use strict;
9f1b1f2d 361 use warnings;
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362
363 BEGIN {
364 use Exporter ();
77ca0c92 365 our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
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366
367 # set the version for version checking
368 $VERSION = 1.00;
369 # if using RCS/CVS, this may be preferred
328fc025 370 $VERSION = sprintf "%d.%03d", q$Revision: 1.1 $ =~ /(\d+)/g;
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371
372 @ISA = qw(Exporter);
373 @EXPORT = qw(&func1 &func2 &func4);
374 %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
375
376 # your exported package globals go here,
377 # as well as any optionally exported functions
378 @EXPORT_OK = qw($Var1 %Hashit &func3);
379 }
77ca0c92 380 our @EXPORT_OK;
9607fc9c 381
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382 # exported package globals go here
383 our $Var1;
384 our %Hashit;
385
9607fc9c 386 # non-exported package globals go here
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387 our @more;
388 our $stuff;
9607fc9c 389
c2611fb3 390 # initialize package globals, first exported ones
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391 $Var1 = '';
392 %Hashit = ();
393
394 # then the others (which are still accessible as $Some::Module::stuff)
395 $stuff = '';
396 @more = ();
397
398 # all file-scoped lexicals must be created before
399 # the functions below that use them.
400
401 # file-private lexicals go here
402 my $priv_var = '';
403 my %secret_hash = ();
404
405 # here's a file-private function as a closure,
406 # callable as &$priv_func; it cannot be prototyped.
407 my $priv_func = sub {
408 # stuff goes here.
409 };
410
411 # make all your functions, whether exported or not;
412 # remember to put something interesting in the {} stubs
413 sub func1 {} # no prototype
414 sub func2() {} # proto'd void
415 sub func3($$) {} # proto'd to 2 scalars
416
417 # this one isn't exported, but could be called!
418 sub func4(\%) {} # proto'd to 1 hash ref
419
420 END { } # module clean-up code here (global destructor)
4633a7c4 421
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422 ## YOUR CODE GOES HERE
423
424 1; # don't forget to return a true value from the file
425
426Then go on to declare and use your variables in functions without
427any qualifications. See L<Exporter> and the L<perlmodlib> for
428details on mechanics and style issues in module creation.
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429
430Perl modules are included into your program by saying
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431
432 use Module;
433
434or
435
436 use Module LIST;
437
438This is exactly equivalent to
439
5a964f20 440 BEGIN { require Module; import Module; }
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441
442or
443
5a964f20 444 BEGIN { require Module; import Module LIST; }
a0d0e21e 445
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446As a special case
447
448 use Module ();
449
450is exactly equivalent to
451
5a964f20 452 BEGIN { require Module; }
cb1a09d0 453
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454All Perl module files have the extension F<.pm>. The C<use> operator
455assumes this so you don't have to spell out "F<Module.pm>" in quotes.
456This also helps to differentiate new modules from old F<.pl> and
457F<.ph> files. Module names are also capitalized unless they're
458functioning as pragmas; pragmas are in effect compiler directives,
459and are sometimes called "pragmatic modules" (or even "pragmata"
460if you're a classicist).
a0d0e21e 461
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462The two statements:
463
464 require SomeModule;
14c715f4 465 require "SomeModule.pm";
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466
467differ from each other in two ways. In the first case, any double
468colons in the module name, such as C<Some::Module>, are translated
469into your system's directory separator, usually "/". The second
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470case does not, and would have to be specified literally. The other
471difference is that seeing the first C<require> clues in the compiler
472that uses of indirect object notation involving "SomeModule", as
473in C<$ob = purge SomeModule>, are method calls, not function calls.
474(Yes, this really can make a difference.)
475
476Because the C<use> statement implies a C<BEGIN> block, the importing
477of semantics happens as soon as the C<use> statement is compiled,
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478before the rest of the file is compiled. This is how it is able
479to function as a pragma mechanism, and also how modules are able to
19799a22 480declare subroutines that are then visible as list or unary operators for
a0d0e21e 481the rest of the current file. This will not work if you use C<require>
19799a22 482instead of C<use>. With C<require> you can get into this problem:
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483
484 require Cwd; # make Cwd:: accessible
54310121 485 $here = Cwd::getcwd();
a0d0e21e 486
5f05dabc 487 use Cwd; # import names from Cwd::
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488 $here = getcwd();
489
490 require Cwd; # make Cwd:: accessible
491 $here = getcwd(); # oops! no main::getcwd()
492
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493In general, C<use Module ()> is recommended over C<require Module>,
494because it determines module availability at compile time, not in the
495middle of your program's execution. An exception would be if two modules
496each tried to C<use> each other, and each also called a function from
14c715f4 497that other module. In that case, it's easy to use C<require> instead.
cb1a09d0 498
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499Perl packages may be nested inside other package names, so we can have
500package names containing C<::>. But if we used that package name
5803be0d 501directly as a filename it would make for unwieldy or impossible
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502filenames on some systems. Therefore, if a module's name is, say,
503C<Text::Soundex>, then its definition is actually found in the library
504file F<Text/Soundex.pm>.
505
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506Perl modules always have a F<.pm> file, but there may also be
507dynamically linked executables (often ending in F<.so>) or autoloaded
5803be0d 508subroutine definitions (often ending in F<.al>) associated with the
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509module. If so, these will be entirely transparent to the user of
510the module. It is the responsibility of the F<.pm> file to load
511(or arrange to autoload) any additional functionality. For example,
512although the POSIX module happens to do both dynamic loading and
5803be0d 513autoloading, the user can say just C<use POSIX> to get it all.
a0d0e21e 514
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515=head2 Making your module threadsafe
516
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517Since 5.6.0, Perl has had support for a new type of threads called
518interpreter threads (ithreads). These threads can be used explicitly
519and implicitly.
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520
521Ithreads work by cloning the data tree so that no data is shared
14c715f4 522between different threads. These threads can be used by using the C<threads>
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523module or by doing fork() on win32 (fake fork() support). When a
524thread is cloned all Perl data is cloned, however non-Perl data cannot
14c715f4 525be cloned automatically. Perl after 5.7.2 has support for the C<CLONE>
cd0db8e5 526special subroutine . In C<CLONE> you can do whatever you need to do,
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527like for example handle the cloning of non-Perl data, if necessary.
528C<CLONE> will be executed once for every package that has it defined
529(or inherits it). It will be called in the context of the new thread,
530so all modifications are made in the new area.
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531
532If you want to CLONE all objects you will need to keep track of them per
533package. This is simply done using a hash and Scalar::Util::weaken().
534
f102b883 535=head1 SEE ALSO
cb1a09d0 536
f102b883 537See L<perlmodlib> for general style issues related to building Perl
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538modules and classes, as well as descriptions of the standard library
539and CPAN, L<Exporter> for how Perl's standard import/export mechanism
890a53b9 540works, L<perltoot> and L<perltooc> for an in-depth tutorial on
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541creating classes, L<perlobj> for a hard-core reference document on
542objects, L<perlsub> for an explanation of functions and scoping,
543and L<perlxstut> and L<perlguts> for more information on writing
544extension modules.