perlmod

DESCRIPTION

       Packages

       Perl 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, "eval", or file, whichever
       comes first (the same scope as the my() and local() opera­
       tors).  Unqualified dynamic identifiers will be in this
       namespace, except for those few identifiers that if
       unqualified, default to the main package instead of the
       current one as described below.  A package statement
       affects only dynamic variables--including those you've
       used local() on--but not lexical variables created with
       my().  Typically it would be the first declaration in a
       file included by the "do", "require", or "use" operators.
       You can switch into a package in more than one place; it
       merely influences which symbol table is used by the com­
       piler for the rest of that block.  You can refer to vari­
       ables and filehandles in other packages by prefixing the
       identifier with the package name and a double colon:
       $Package::Variable.  If the package name is null, the
       "main" package is assumed.  That is, $::sail is equivalent
       to $main::sail.

       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 was going on.
       Because the old-fashioned syntax is still supported for
       backwards compatibility, if you try to use a string like
       "This is $owner's house", you'll be accessing $owner::s;
       that is, the $s variable in package "owner", which is
       probably not what you meant.  Use braces to disambiguate,
       as in "This is ${owner}'s house".

       Packages may themselves contain package separators, as in
       $OUTER::INNER::var.  This implies nothing about the order
       of name lookups, however.  There are no relative packages:
       all symbols are either local to the current package, or
       must be fully qualified from the outer package name down.
       For instance, there is nowhere within package "OUTER" that
       $INNER::var refers to $OUTER::INNER::var.  "INNER" refers
       to a totally separate global package.

       Only identifiers starting with letters (or underscore) are
       cate private variables and method names.  However, vari­
       ables and functions named with a single "_", such as $_
       and "sub _", are still forced into the package "main".
       See also "Technical Note on the Syntax of Variable Names"
       in perlvar.

       "eval"ed strings are compiled in the package in which the
       eval() was compiled.  (Assignments to $SIG{}, however,
       assume the signal handler specified is in the "main" pack­
       age.  Qualify the signal handler name if you wish to have
       a signal handler in a package.)  For an example, examine
       perldb.pl in the Perl library.  It initially switches to
       the "DB" package so that the debugger doesn't interfere
       with variables in the program you are trying to debug.  At
       various points, however, it temporarily switches back to
       the "main" package to evaluate various expressions in the
       context of the "main" package (or wherever you came from).
       See perldebug.

       The special symbol "__PACKAGE__" contains the current
       package, but cannot (easily) be used to construct variable
       names.

       See perlsub for other scoping issues related to my() and
       local(), and perlref regarding closures.

       Symbol Tables

       The symbol table for a package happens to be stored in the
       hash of that name with two colons appended.  The main sym­
       bol table's name is thus %main::, or %:: for short.  Like­
       wise the symbol table for the nested package mentioned
       earlier is named %OUTER::INNER::.

       The value in each entry of the hash is what you are refer­
       ring to when you use the *name typeglob notation.  In
       fact, the following have the same effect, though the first
       is more efficient because it does the symbol table lookups
       at compile time:

           local *main::foo    = *main::bar;
           local $main::{foo}  = $main::{bar};

       (Be sure to note the vast difference between the second
       line above and "local $main::foo = $main::bar". The former
       is accessing the hash %main::, which is the symbol table
       of package "main". The latter is simply assigning scalar
       $bar in package "main" to scalar $foo of the same pack­
       age.)

       You can use this to print out all the variables in a pack­
       age, for instance.  The standard but antiquated dumpvar.pl
           *dick = \$richard;

       Which makes $richard and $dick the same variable, but
       leaves @richard and @dick as separate arrays.  Tricky, eh?

       There is one subtle difference between the following
       statements:

           *foo = *bar;
           *foo = \$bar;

       "*foo = *bar" makes the typeglobs themselves synonymous
       while "*foo = \$bar" makes the SCALAR portions of two dis­
       tinct typeglobs refer to the same scalar value. This means
       that the following code:

           $bar = 1;
           *foo = \$bar;       # Make $foo an alias for $bar

           {
               local $bar = 2; # Restrict changes to block
               print $foo;     # Prints '1'!
           }

       Would print '1', because $foo holds a reference to the
       original $bar -- the one that was stuffed away by
       "local()" and which will be restored when the block ends.
       Because variables are accessed through the typeglob, you
       can use "*foo = *bar" to create an alias which can be
       localized. (But be aware that this means you can't have a
       separate @foo and @bar, etc.)

       What makes all of this important is that the Exporter mod­
       ule uses glob aliasing as the import/export mechanism.
       Whether or not you can properly localize a variable that
       has been exported from a module depends on how it was
       exported:

           @EXPORT = qw($FOO); # Usual form, can't be localized
           @EXPORT = qw(*FOO); # Can be localized

       You can work around the first case by using the fully
       qualified name ($Package::FOO) where you need a local
       value, or by overriding it by saying "*FOO = *Pack­
       age::FOO" in your script.

       The "*x = \$y" 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 assign­
       ing to dynamic variables, not lexicals.

               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 deref­
       erence variables explicitly.

       Another use of symbol tables is for making "constant"
       scalars.

           *PI = \3.14159265358979;

       Now you cannot alter $PI, which is probably a good thing
       all in all.  This isn't the same as a constant subroutine,
       which is subject to optimization at compile-time.  A con­
       stant subroutine is one prototyped to take no arguments
       and to return a constant expression.  See perlsub for
       details on these.  The "use constant" pragma is a conve­
       nient shorthand for these.

       You can say *foo{PACKAGE} and *foo{NAME} to find out what
       name and package the *foo symbol table entry comes from.
       This may be useful in a subroutine that gets passed type­
       globs as arguments:

           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 *foo{THING} notation can also be used to obtain refer­
       ences to the individual elements of *foo.  See perlref.

       Subroutine definitions (and declarations, for that matter)
       need not necessarily be situated in the package whose sym­
       bol table they occupy.  You can define a subroutine out­
       side 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 com­
       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 perlobj).

       Package Constructors and Destructors

       Four special subroutines act as package constructors and
       destructors.  These are the "BEGIN", "CHECK", "INIT", and
       "END" routines.  The "sub" is optional for these routines.

       A "BEGIN" subroutine is executed as soon as possible, that
       is, the moment it is completely defined, even before the
       rest of the containing file is parsed.  You may have mul­
       tiple "BEGIN" blocks within a file--they will execute in
       order of definition.  Because a "BEGIN" block executes
       immediately, it can pull in definitions of subroutines and
       such from other files in time to be visible to the rest of
       the file.  Once a "BEGIN" has run, it is immediately unde­
       fined and any code it used is returned to Perl's memory
       pool.  This means you can't ever explicitly call a
       "BEGIN".

       An "END" subroutine is executed as late as possible, that
       is, after perl has finished running the program and just
       before the interpreter is being exited, even if it is
       exiting as a result of a die() function.  (But not if it's
       polymorphing into another program via "exec", or being
       blown out of the water by a signal--you have to trap that
       yourself (if you can).)  You may have multiple "END"
       blocks within a file--they will execute in reverse order
       of definition; that is: last in, first out (LIFO).  "END"
       blocks are not executed when you run perl with the "-c"
       "CHECK" blocks are used in the Perl compiler suite to save
       the compiled state of the program.

       "INIT" blocks are run just before the Perl runtime begins
       execution, in "first in, first out" (FIFO) order. For
       example, the code generators documented in perlcc make use
       of "INIT" blocks to initialize and resolve pointers to
       XSUBs.

       When you use the -n and -p switches to Perl, "BEGIN" and
       "END" work just as they do in awk, as a degenerate case.
       Both "BEGIN" and "CHECK" blocks are run when you use the
       -c switch for a compile-only syntax check, although your
       main code is not.

       Perl Classes

       There 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 meth­
       ods from another class (package) by listing the other
       package name(s) in its global @ISA array (which must be a
       package global, not a lexical).

       For more on this, see perltoot and perlobj.

       Perl Modules

       A 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 mod­
       ules 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 defi­
       nition and make its semantics available implicitly through
       method calls on the class and its objects, without explic­
       itly 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

           # 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 Exporter and the perlmod­
       lib for details on mechanics and style issues in module
       creation.

       Perl modules are included into your program by saying

           use Module;

           use Module ();

       is exactly equivalent to

           BEGIN { require Module; }

       All Perl module files have the extension .pm.  The "use"
       operator assumes this so you don't have to spell out "Mod­
       ule.pm" in quotes.  This also helps to differentiate new
       modules from old .pl and .ph files.  Module names are also
       capitalized unless they're functioning as pragmas; pragmas
       are in effect compiler directives, and are sometimes
       called "pragmatic modules" (or even "pragmata" if you're a
       classicist).

       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 "Some::Mod­
       ule", are translated into your system's directory separa­
       tor, usually "/".   The second case does not, and would
       have to be specified literally.  The other difference is
       that seeing the first "require" clues in the compiler that
       uses of indirect object notation involving "SomeModule",
       as in "$ob = purge SomeModule", are method calls, not
       function calls.  (Yes, this really can make a difference.)

       Because the "use" statement implies a "BEGIN" block, the
       importing of semantics happens as soon as the "use" state­
       ment is compiled, before the rest of the file is compiled.
       This is how it is able to function as a pragma mechanism,
       and also how modules are able to declare subroutines that
       are then visible as list or unary operators for the rest
       of the current file.  This will not work if you use
       "require" instead of "use".  With "require" you can get
       into this problem:

           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, "use Module ()" is recommended over "require
       Module", because it determines module availability at com­
       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 respon­
       sibility 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 "use POSIX" to get it
       all.

       Making your module threadsafe

       Since 5.6.0, Perl has had support for a new type of
       threads called interpreter threads (ithreads). These
       threads can be used explicitly and implicitly.

       Ithreads work by cloning the data tree so that no data is
       shared between different threads. These threads can be
       used by using the "threads" module or by doing fork() on
       win32 (fake fork() support). When a thread is cloned all
       Perl data is cloned, however non-Perl data cannot be
       cloned automatically.  Perl after 5.7.2 has support for
       the "CLONE" special subroutine .  In "CLONE" you can do
       whatever you need to do, like for example handle the
       cloning of non-Perl data, if necessary.  "CLONE" will be
       executed once for every package that has it defined (or
       inherits it).  It will be called in the context of the new
       thread, so all modifications are made in the new area.

       If you want to CLONE all objects you will need to keep
       track of them per package. This is simply done using a
       hash and Scalar::Util::weaken().

SEE ALSO

       See perlmodlib for general style issues related to build­
       ing Perl modules and classes, as well as descriptions of
       the standard library and CPAN, Exporter for how Perl's
       standard import/export mechanism works, perltoot and perl­
       tooc for an in-depth tutorial on creating classes, perlobj
       for a hard-core reference document on objects, perlsub for
       an explanation of functions and scoping, and perlxstut and
       perlguts for more information on writing extension mod­
       ules.

perl v5.8.1                 2003-09-02                 PERLMOD(1)