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perlmod ()
  • >> perlmod (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perlmod (1) ( Разные man: Команды и прикладные программы пользовательского уровня )


         perlmod - Perl modules (packages and symbol tables)


         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()
         operators).  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 compiler for the rest of that
         block.  You can refer to variables 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's 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'.  It would
         treat package `INNER' as a totally separate global package.
         Only identifiers starting with letters (or underscore) are
         stored in a package's symbol table.  All other symbols are
         kept in package `main', including all punctuation variables,
         like $_.  In addition, when unqualified, the identifiers
         forced to be in package `main', even when used for other
         purposes than their built-in one.  If you have a package
         called `m', `s', or `y', then you can't use the qualified
         form of an identifier because it would be instead
         interpreted as a pattern match, a substitution, or a
         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.  $_ is still global
         though.  See also the Technical Note on the Syntax of
         Variable Names entry in the perlvar manpage.
         `eval'ed strings are compiled in the package in which the
         eval() was compiled.  (Assignments to `$SIG{}', however,
         assume the signal handler specified is in the `main'
         package.  Qualify the signal handler name if you wish to
         have a signal handler in a package.)  For an example,
         examine 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 the perldebug manpage.
         The special symbol `__PACKAGE__' contains the current
         package, but cannot (easily) be used to construct variables.
         See the perlsub manpage for other scoping issues related to
         my() and local(), and the perlref manpage regarding
         Symbol Tables
         The 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 `%main::', or `%::' for short.
         Likewise symbol table for the nested package mentioned
         earlier is named `%OUTER::INNER::'.
         The value in each entry of the hash is what you are
         referring 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};
         You can use this to print out all the variables in a
         package, for instance.  The standard but antequated library and the CPAN module Devel::Symdump make
         use of this.
         Assignment to a typeglob performs an aliasing operation,
             *dick = *richard;
         causes variables, subroutines, formats, and file and
         directory handles accessible via the identifier `richard'
         also to be accessible via the identifier `dick'.  If you
         want to alias only a particular variable or subroutine,
         assign a reference instead:
             *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 won'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 won't want to have to remember to dereference
         variables explicitly.
         Another use of symbol tables is for making "constant"
             *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.  This isn't.  A
         constant subroutine is one prototyped to take no arguments
         and to return a constant expression.  See the perlsub
         manpage for details on these.  The `use constant' pragma is
         a convenient 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
         typeglobs 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
         references to the individual elements of *foo, see the
         perlref manpage.
         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";
             in main: $name is 'barney'
         rather than:
             in Some_package: $name is 'fred'
         This also has implications for the use of the SUPER::
         qualifier (see the perlobj manpage).
         Package Constructors and Destructors
         Four special subroutines act as package constructors and
         destructors.  These are the `BEGIN', `CHECK', `INIT', and
         `END' routines.  The `sub' is optional for these routines.
         A `BEGIN' subroutine is executed as soon as possible, that
         is, the moment it is completely defined, even before the
         rest of the containing file is parsed.  You may have
         multiple `BEGIN' blocks within a file--they will execute in
         order of definition.  Because a `BEGIN' block executes
         immediately, it can pull in definitions of subroutines and
         such from other files in time to be visible to the rest of
         the file.  Once a `BEGIN' has run, it is immediately
         undefined and any code it used is returned to Perl's memory
         pool.  This means you can't ever explicitly call a `BEGIN'.
         An `END' subroutine is executed as late as possible, that
         is, after perl has finished running the program and just
         before the interpreter is being exited, even if it is
         exiting as a result of a die() function.  (But not if it's
         polymorphing into another program via `exec', or being blown
         out of the water by a signal--you have to trap that yourself
         (if you can).)  You may have multiple `END' blocks within a
         file--they will execute in reverse order of definition; that
         is: last in, first out (LIFO).  `END' blocks are not
         executed when you run perl with the `-c' switch.
         Inside an `END' subroutine, `$?' contains the value that the
         program is going to pass to `exit()'.  You can modify `$?'
         to change the exit value of the program.  Beware of changing
         `$?' by accident (e.g. by running something via `system').
         Similar to `BEGIN' blocks, `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 the perlcc manpage make use of `INIT' blocks to
         initialize and resolve pointers to XSUBs.
         Similar to `END' blocks, `CHECK' blocks are run just after
         the Perl compile phase ends and before the run time begins,
         in LIFO order.  `CHECK' blocks are again useful in the Perl
         compiler suite to save the compiled state of the program.
         When you use the -n and -p switches to Perl, `BEGIN' and
         `END' work just as they do in awk, as a degenerate case.  As
         currently implemented (and subject to change, since its
         inconvenient at best), both `BEGIN' and<END> 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 methods 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 the perltoot manpage and the perlobj
         Perl Modules
         A module is just a set of related function in a library file
         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
         exportating 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/ and start
         with this template:
             package Some::Module;  # assumes Some/
             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;
             # 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;
             use Module LIST;
         This is exactly equivalent to
             BEGIN { require Module; import Module; }
             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 `use'
         operator assumes this so you don't have to spell out
         "" 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
         The two statements:
             require SomeModule;
             require "";
         differ from each other in two ways.  In the first case, any
         double colons in the module name, such as `Some::Module',
         are translated into your system's directory separator,
         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'
         statement 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
         compile time, not in the middle of your program's execution.
         An exception would be if two modules each tried to `use'
         each other, and each also called a function from that other
         module.  In that case, it's easy to use `require's instead.
         Perl packages may be nested inside other package names, so
         we can have package names containing `::'.  But if we used
         that package name directly as a filename it would makes for
         unwieldy or impossible filenames on some systems.
         Therefore, if a module's name is, say, `Text::Soundex', then
         its definition is actually found in the library file
         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, but the user can say just `use POSIX' to
         get it all.


         See 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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