3 perlmod - Perl modules (packages and symbol tables)
9 Perl provides a mechanism for alternative namespaces to protect packages
10 from stomping on each other's variables. In fact, apart from certain
11 magical variables, there's really no such thing as a global variable
12 in Perl. The package statement declares the compilation unit as
13 being in the given namespace. The scope of the package declaration
14 is from the declaration itself through the end of the enclosing block,
15 C<eval>, C<sub>, or end of file, whichever comes first (the same scope
16 as the my() and local() operators). All further unqualified dynamic
17 identifiers will be in this namespace. A package statement affects
18 only dynamic variables--including those you've used local() on--but
19 I<not> lexical variables created with my(). Typically it would be
20 the first declaration in a file to be included by the C<require> or
21 C<use> operator. You can switch into a package in more than one place;
22 it influences merely which symbol table is used by the compiler for the
23 rest of that block. You can refer to variables and filehandles in other
24 packages by prefixing the identifier with the package name and a double
25 colon: C<$Package::Variable>. If the package name is null, the C<main>
26 package is assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
28 (The old package delimiter was a single quote, but double colon
29 is now the preferred delimiter, in part because it's more readable
30 to humans, and in part because it's more readable to B<emacs> macros.
31 It also makes C++ programmers feel like they know what's going on.)
33 Packages may be nested inside other packages: C<$OUTER::INNER::var>. This
34 implies nothing about the order of name lookups, however. All symbols
35 are either local to the current package, or must be fully qualified
36 from the outer package name down. For instance, there is nowhere
37 within package C<OUTER> that C<$INNER::var> refers to C<$OUTER::INNER::var>.
38 It would treat package C<INNER> as a totally separate global package.
40 Only identifiers starting with letters (or underscore) are stored in a
41 package's symbol table. All other symbols are kept in package C<main>,
42 including all of the punctuation variables like $_. In addition, the
43 identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC, and SIG are
44 forced to be in package C<main>, even when used for other purposes than
45 their builtin one. Note also that, if you have a package called C<m>,
46 C<s>, or C<y>, then you can't use the qualified form of an identifier
47 because it will be interpreted instead as a pattern match, a substitution,
50 (Variables beginning with underscore used to be forced into package
51 main, but we decided it was more useful for package writers to be able
52 to use leading underscore to indicate private variables and method names.
53 $_ is still global though.)
55 Eval()ed strings are compiled in the package in which the eval() was
56 compiled. (Assignments to C<$SIG{}>, however, assume the signal
57 handler specified is in the C<main> package. Qualify the signal handler
58 name if you wish to have a signal handler in a package.) For an
59 example, examine F<perldb.pl> in the Perl library. It initially switches
60 to the C<DB> package so that the debugger doesn't interfere with variables
61 in the script you are trying to debug. At various points, however, it
62 temporarily switches back to the C<main> package to evaluate various
63 expressions in the context of the C<main> package (or wherever you came
64 from). See L<perldebug>.
66 The special symbol C<__PACKAGE__> contains the current package, but cannot
67 (easily) be used to construct variables.
69 See L<perlsub> for other scoping issues related to my() and local(),
70 and L<perlref> regarding closures.
74 The symbol table for a package happens to be stored in the hash of that
75 name with two colons appended. The main symbol table's name is thus
76 C<%main::>, or C<%::> for short. Likewise symbol table for the nested
77 package mentioned earlier is named C<%OUTER::INNER::>.
79 The value in each entry of the hash is what you are referring to when you
80 use the C<*name> typeglob notation. In fact, the following have the same
81 effect, though the first is more efficient because it does the symbol
82 table lookups at compile time:
84 local *main::foo = *main::bar;
85 local $main::{foo} = $main::{bar};
87 You can use this to print out all the variables in a package, for
88 instance. Here is F<dumpvar.pl> from the Perl library:
93 local(*stab) = eval("*${package}::");
94 while (($key,$val) = each(%stab)) {
97 print "\$$key = '$entry'\n";
100 if (defined @entry) {
101 print "\@$key = (\n";
102 foreach $num ($[ .. $#entry) {
103 print " $num\t'",$entry[$num],"'\n";
108 if ($key ne "${package}::" && defined %entry) {
109 print "\%$key = (\n";
110 foreach $key (sort keys(%entry)) {
111 print " $key\t'",$entry{$key},"'\n";
118 Note that even though the subroutine is compiled in package C<dumpvar>,
119 the name of the subroutine is qualified so that its name is inserted into
120 package C<main>. While popular many years ago, this is now considered
121 very poor style; in general, you should be writing modules and using the
122 normal export mechanism instead of hammering someone else's namespace,
125 Assignment to a typeglob performs an aliasing operation, i.e.,
129 causes variables, subroutines, and file handles accessible via the
130 identifier C<richard> to also be accessible via the identifier C<dick>. If
131 you want to alias only a particular variable or subroutine, you can
132 assign a reference instead:
136 makes $richard and $dick the same variable, but leaves
137 @richard and @dick as separate arrays. Tricky, eh?
139 This mechanism may be used to pass and return cheap references
140 into or from subroutines if you won't want to copy the whole
144 *some_hash = fn( \%another_hash );
146 local *hashsym = shift;
147 # now use %hashsym normally, and you
148 # will affect the caller's %another_hash
149 my %nhash = (); # do what you want
153 On return, the reference will overwrite the hash slot in the
154 symbol table specified by the *some_hash typeglob. This
155 is a somewhat tricky way of passing around references cheaply
156 when you won't want to have to remember to dereference variables
159 Another use of symbol tables is for making "constant" scalars.
161 *PI = \3.14159265358979;
163 Now you cannot alter $PI, which is probably a good thing all in all.
164 This isn't the same as a constant subroutine (one prototyped to
165 take no arguments and to return a constant expression), which is
166 subject to optimization at compile-time. This isn't. See L<perlsub>
167 for details on these.
169 You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and
170 package the *foo symbol table entry comes from. This may be useful
171 in a subroutine which is passed typeglobs as arguments
173 sub identify_typeglob {
175 print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
177 identify_typeglob *foo;
178 identify_typeglob *bar::baz;
182 You gave me main::foo
185 The *foo{THING} notation can also be used to obtain references to the
186 individual elements of *foo, see L<perlref>.
188 =head2 Package Constructors and Destructors
190 There are two special subroutine definitions that function as package
191 constructors and destructors. These are the C<BEGIN> and C<END>
192 routines. The C<sub> is optional for these routines.
194 A C<BEGIN> subroutine is executed as soon as possible, that is, the moment
195 it is completely defined, even before the rest of the containing file
196 is parsed. You may have multiple C<BEGIN> blocks within a file--they
197 will execute in order of definition. Because a C<BEGIN> block executes
198 immediately, it can pull in definitions of subroutines and such from other
199 files in time to be visible to the rest of the file. Once a C<BEGIN>
200 has run, it is immediately undefined and any code it used is returned to
201 Perl's memory pool. This means you can't ever explicitly call a C<BEGIN>.
203 An C<END> subroutine is executed as late as possible, that is, when the
204 interpreter is being exited, even if it is exiting as a result of a
205 die() function. (But not if it's is being blown out of the water by a
206 signal--you have to trap that yourself (if you can).) You may have
207 multiple C<END> blocks within a file--they will execute in reverse
208 order of definition; that is: last in, first out (LIFO).
210 Inside an C<END> subroutine C<$?> contains the value that the script is
211 going to pass to C<exit()>. You can modify C<$?> to change the exit
212 value of the script. Beware of changing C<$?> by accident (e.g. by
213 running something via C<system>).
215 Note that when you use the B<-n> and B<-p> switches to Perl, C<BEGIN>
216 and C<END> work just as they do in B<awk>, as a degenerate case.
220 There is no special class syntax in Perl, but a package may function
221 as a class if it provides subroutines that function as methods. Such a
222 package may also derive some of its methods from another class package
223 by listing the other package name in its @ISA array.
225 For more on this, see L<perltoot> and L<perlobj>.
229 A module is just a package that is defined in a library file of
230 the same name, and is designed to be reusable. It may do this by
231 providing a mechanism for exporting some of its symbols into the symbol
232 table of any package using it. Or it may function as a class
233 definition and make its semantics available implicitly through method
234 calls on the class and its objects, without explicit exportation of any
235 symbols. Or it can do a little of both.
237 For example, to start a normal module called Some::Module, create
238 a file called Some/Module.pm and start with this template:
240 package Some::Module; # assumes Some/Module.pm
246 use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS);
248 # set the version for version checking
250 # if using RCS/CVS, this may be preferred
251 $VERSION = do { my @r = (q$Revision: 2.21 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r }; # must be all one line, for MakeMaker
254 @EXPORT = qw(&func1 &func2 &func4);
255 %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
257 # your exported package globals go here,
258 # as well as any optionally exported functions
259 @EXPORT_OK = qw($Var1 %Hashit &func3);
263 # non-exported package globals go here
264 use vars qw(@more $stuff);
266 # initalize package globals, first exported ones
270 # then the others (which are still accessible as $Some::Module::stuff)
274 # all file-scoped lexicals must be created before
275 # the functions below that use them.
277 # file-private lexicals go here
279 my %secret_hash = ();
281 # here's a file-private function as a closure,
282 # callable as &$priv_func; it cannot be prototyped.
283 my $priv_func = sub {
287 # make all your functions, whether exported or not;
288 # remember to put something interesting in the {} stubs
289 sub func1 {} # no prototype
290 sub func2() {} # proto'd void
291 sub func3($$) {} # proto'd to 2 scalars
293 # this one isn't exported, but could be called!
294 sub func4(\%) {} # proto'd to 1 hash ref
296 END { } # module clean-up code here (global destructor)
298 Then go on to declare and use your variables in functions
299 without any qualifications.
300 See L<Exporter> and the L<perlmodlib> for details on
301 mechanics and style issues in module creation.
303 Perl modules are included into your program by saying
311 This is exactly equivalent to
313 BEGIN { require "Module.pm"; import Module; }
317 BEGIN { require "Module.pm"; import Module LIST; }
323 is exactly equivalent to
325 BEGIN { require "Module.pm"; }
327 All Perl module files have the extension F<.pm>. C<use> assumes this so
328 that you don't have to spell out "F<Module.pm>" in quotes. This also
329 helps to differentiate new modules from old F<.pl> and F<.ph> files.
330 Module names are also capitalized unless they're functioning as pragmas,
331 "Pragmas" are in effect compiler directives, and are sometimes called
332 "pragmatic modules" (or even "pragmata" if you're a classicist).
334 Because the C<use> statement implies a C<BEGIN> block, the importation
335 of semantics happens at the moment the C<use> statement is compiled,
336 before the rest of the file is compiled. This is how it is able
337 to function as a pragma mechanism, and also how modules are able to
338 declare subroutines that are then visible as list operators for
339 the rest of the current file. This will not work if you use C<require>
340 instead of C<use>. With require you can get into this problem:
342 require Cwd; # make Cwd:: accessible
343 $here = Cwd::getcwd();
345 use Cwd; # import names from Cwd::
348 require Cwd; # make Cwd:: accessible
349 $here = getcwd(); # oops! no main::getcwd()
351 In general C<use Module ();> is recommended over C<require Module;>.
353 Perl packages may be nested inside other package names, so we can have
354 package names containing C<::>. But if we used that package name
355 directly as a filename it would makes for unwieldy or impossible
356 filenames on some systems. Therefore, if a module's name is, say,
357 C<Text::Soundex>, then its definition is actually found in the library
358 file F<Text/Soundex.pm>.
360 Perl modules always have a F<.pm> file, but there may also be dynamically
361 linked executables or autoloaded subroutine definitions associated with
362 the module. If so, these will be entirely transparent to the user of
363 the module. It is the responsibility of the F<.pm> file to load (or
364 arrange to autoload) any additional functionality. The POSIX module
365 happens to do both dynamic loading and autoloading, but the user can
366 say just C<use POSIX> to get it all.
368 For more information on writing extension modules, see L<perlxstut>
373 See L<perlmodlib> for general style issues related to building Perl
374 modules and classes as well as descriptions of the standard library and
375 CPAN, L<Exporter> for how Perl's standard import/export mechanism works,
376 L<perltoot> for an in-depth tutorial on creating classes, L<perlobj>
377 for a hard-core reference document on objects, and L<perlsub> for an
378 explanation of functions and scoping.