NAMEperlmod - Perl modules (packages and symbol tables)
DESCRIPTION
PackagesPerl provides a mechanism for alternative namespaces to protect
packages from stomping on each other's variables. In fact, there's
really no such thing as a global variable in Perl. The package
statement declares the compilation unit as being in the given
namespace. The scope of the package declaration is from the
declaration itself through the end of the enclosing block, The old package delimiter was a single quote, but double colon is now the
preferred delimiter, in part because it's more readable to humans, and
in part because it's more readable to emacs macros. It also makes C++
programmers feel like they know what's going on--as opposed to using the
single quote as separator, which was there to make Ada programmers feel
like they knew what's going on. Because the old-fashioned syntax is still
supported for backwards compatibility, if you try to use a string like
Packages may themselves contain package separators, as in
Only identifiers starting with letters (or underscore) are stored
in a package's symbol table. All other symbols are kept in package
Variables beginning with underscore used to be forced into package main, but we decided it was more useful for package writers to be able to use leading underscore to indicate private variables and method names. $_ is still global though. See also Technical Note on the Syntax of Variable Names in the perlvar manpage.
The special symbol See the perlsub manpage for other scoping issues related to
Symbol TablesThe symbol table for a package happens to be stored in the hash of that
name with two colons appended. The main symbol table's name is thus
The value in each entry of the hash is what you are referring to when you
use the local *main::foo = *main::bar; local $main::{foo} = $main::{bar}; (Be sure to note the vast difference between the second line above
and You can use this to print out all the variables in a package, for instance. The standard but antiquated dumpvar.pl library and the CPAN module Devel::Symdump make use of this. Assignment to a typeglob performs an aliasing operation, i.e., *dick = *richard; causes variables, subroutines, formats, and file and directory handles
accessible via the identifier *dick = \$richard; Which makes $richard and $dick the same variable, but leaves @richard and @dick as separate arrays. Tricky, eh? This mechanism may be used to pass and return cheap references into or from subroutines if you don't want to copy the whole thing. It only works when assigning to dynamic variables, not lexicals. %some_hash = (); # can't be my() *some_hash = fn( \%another_hash ); sub fn { local *hashsym = shift; # now use %hashsym normally, and you # will affect the caller's %another_hash my %nhash = (); # do what you want return \%nhash; } On return, the reference will overwrite the hash slot in the symbol table specified by the *some_hash typeglob. This is a somewhat tricky way of passing around references cheaply when you don't want to have to remember to dereference variables explicitly. Another use of symbol tables is for making ``constant'' scalars. *PI = \3.14159265358979; Now you cannot alter You can say sub identify_typeglob { my $glob = shift; print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n"; } identify_typeglob *foo; identify_typeglob *bar::baz; This prints You gave me main::foo You gave me bar::baz The Subroutine definitions (and declarations, for that matter) need not necessarily be situated in the package whose symbol table they occupy. You can define a subroutine outside its package by explicitly qualifying the name of the subroutine: package main; sub Some_package::foo { ... } # &foo defined in Some_package This is just a shorthand for a typeglob assignment at compile time: BEGIN { *Some_package::foo = sub { ... } } and is not the same as writing: { package Some_package; sub foo { ... } } In the first two versions, the body of the subroutine is lexically in the main package, not in Some_package. So something like this: package main; $Some_package::name = "fred"; $main::name = "barney"; sub Some_package::foo { print "in ", __PACKAGE__, ": \$name is '$name'\n"; } Some_package::foo(); prints: in main: $name is 'barney' rather than: in Some_package: $name is 'fred' This also has implications for the use of the SUPER:: qualifier (see the perlobj manpage).
Package Constructors and DestructorsFour special subroutines act as package constructors and destructors.
These are the A An Inside an Similar to Similar to When you use the -n and -p switches to Perl,
Perl ClassesThere is no special class syntax in Perl, but a package may act
as a class if it provides subroutines to act as methods. Such a
package may also derive some of its methods from another class (package)
by listing the other package For more on this, see the perltoot manpage and the perlobj manpage.
Perl ModulesA module is just a set of related functions in a library file, i.e., a Perl package with the same name as the file. It is specifically designed to be reusable by other modules or programs. It may do this by providing a mechanism for exporting some of its symbols into the symbol table of any package using it. Or it may function as a class definition and make its semantics available implicitly through method calls on the class and its objects, without explicitly exporting anything. Or it can do a little of both. For example, to start a traditional, non-OO module called Some::Module, create a file called Some/Module.pm and start with this template: package Some::Module; # assumes Some/Module.pm use strict; use warnings; BEGIN { use Exporter (); our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); # set the version for version checking $VERSION = 1.00; # if using RCS/CVS, this may be preferred $VERSION = do { my @r = (q$Revision: 2.21 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r }; # must be all one line, for MakeMaker @ISA = qw(Exporter); @EXPORT = qw(&func1 &func2 &func4); %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], # your exported package globals go here, # as well as any optionally exported functions @EXPORT_OK = qw($Var1 %Hashit &func3); } our @EXPORT_OK; # exported package globals go here our $Var1; our %Hashit; # non-exported package globals go here our @more; our $stuff; # initialize package globals, first exported ones $Var1 = ''; %Hashit = (); # then the others (which are still accessible as $Some::Module::stuff) $stuff = ''; @more = (); # all file-scoped lexicals must be created before # the functions below that use them. # file-private lexicals go here my $priv_var = ''; my %secret_hash = (); # here's a file-private function as a closure, # callable as &$priv_func; it cannot be prototyped. my $priv_func = sub { # stuff goes here. }; # make all your functions, whether exported or not; # remember to put something interesting in the {} stubs sub func1 {} # no prototype sub func2() {} # proto'd void sub func3($$) {} # proto'd to 2 scalars # this one isn't exported, but could be called! sub func4(\%) {} # proto'd to 1 hash ref END { } # module clean-up code here (global destructor) ## YOUR CODE GOES HERE 1; # don't forget to return a true value from the file Then go on to declare and use your variables in functions without any qualifications. See the Exporter manpage and the the perlmodlib manpage for details on mechanics and style issues in module creation. Perl modules are included into your program by saying use Module; or use Module LIST; This is exactly equivalent to BEGIN { require Module; import Module; } or BEGIN { require Module; import Module LIST; } As a special case use Module (); is exactly equivalent to BEGIN { require Module; } All Perl module files have the extension .pm. The The two statements: require SomeModule; require "SomeModule.pm"; differ from each other in two ways. In the first case, any double
colons in the module name, such as Because the require Cwd; # make Cwd:: accessible $here = Cwd::getcwd(); use Cwd; # import names from Cwd:: $here = getcwd(); require Cwd; # make Cwd:: accessible $here = getcwd(); # oops! no main::getcwd() In general, Perl packages may be nested inside other package names, so we can have
package names containing Perl modules always have a .pm file, but there may also be
dynamically linked executables (often ending in .so) or autoloaded
subroutine definitions (often ending in .al) associated with the
module. If so, these will be entirely transparent to the user of
the module. It is the responsibility of the .pm file to load
(or arrange to autoload) any additional functionality. For example,
although the POSIX module happens to do both dynamic loading and
autoloading, the user can say just
SEE ALSOSee the perlmodlib manpage for general style issues related to building Perl modules and classes, as well as descriptions of the standard library and CPAN, the Exporter manpage for how Perl's standard import/export mechanism works, the perltoot manpage and the perltootc manpage for an in-depth tutorial on creating classes, the perlobj manpage for a hard-core reference document on objects, the perlsub manpage for an explanation of functions and scoping, and the perlxstut manpage and the perlguts manpage for more information on writing extension modules.
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