


SelfLoader(3p)   Perl Programmers Reference Guide  SelfLoader(3p)


NAME
       SelfLoader - load functions only on demand

SYNOPSIS
           package FOOBAR;
           use SelfLoader;

           ... (initializing code)

           __DATA__
           sub {....

DESCRIPTION
       This module tells its users that functions in the FOOBAR
       package are to be autoloaded from after the "__DATA__"
       token.  See also "Autoloading" in perlsub.

   The __DATA__ token
       The "__DATA__" token tells the perl compiler that the perl
       code for compilation is finished. Everything after the
       "__DATA__" token is available for reading via the
       filehandle FOOBAR::DATA, where FOOBAR is the name of the
       current package when the "__DATA__" token is reached. This
       works just the same as "__END__" does in package 'main',
       but for other modules data after "__END__" is not
       automatically retrievable, whereas data after "__DATA__"
       is.  The "__DATA__" token is not recognized in versions of
       perl prior to 5.001m.

       Note that it is possible to have "__DATA__" tokens in the
       same package in multiple files, and that the last
       "__DATA__" token in a given package that is encountered by
       the compiler is the one accessible by the filehandle. This
       also applies to "__END__" and main, i.e. if the 'main'
       program has an "__END__", but a module 'require'd (_not_
       'use'd) by that program has a 'package main;' declaration
       followed by an '"__DATA__"', then the "DATA" filehandle is
       set to access the data after the "__DATA__" in the module,
       _not_ the data after the "__END__" token in the 'main'
       program, since the compiler encounters the 'require'd file
       later.

   SelfLoader autoloading
       The SelfLoader works by the user placing the "__DATA__"
       token after perl code which needs to be compiled and run
       at 'require' time, but before subroutine declarations that
       can be loaded in later - usually because they may never be
       called.

       The SelfLoader will read from the FOOBAR::DATA filehandle
       to load in the data after "__DATA__", and load in any
       subroutine when it is called. The costs are the one-time
       parsing of the data after "__DATA__", and a load delay for
       the _first_ call of any autoloaded function. The benefits



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       (hopefully) are a speeded up compilation phase, with no
       need to load functions which are never used.

       The SelfLoader will stop reading from "__DATA__" if it
       encounters the "__END__" token - just as you would expect.
       If the "__END__" token is present, and is followed by the
       token DATA, then the SelfLoader leaves the FOOBAR::DATA
       filehandle open on the line after that token.

       The SelfLoader exports the "AUTOLOAD" subroutine to the
       package using the SelfLoader, and this loads the called
       subroutine when it is first called.

       There is no advantage to putting subroutines which will
       _always_ be called after the "__DATA__" token.

   Autoloading and package lexicals
       A 'my $pack_lexical' statement makes the variable
       $pack_lexical local _only_ to the file up to the
       "__DATA__" token. Subroutines declared elsewhere _cannot_
       see these types of variables, just as if you declared
       subroutines in the package but in another file, they
       cannot see these variables.

       So specifically, autoloaded functions cannot see package
       lexicals (this applies to both the SelfLoader and the
       Autoloader).  The "vars" pragma provides an alternative to
       defining package-level globals that will be visible to
       autoloaded routines. See the documentation on vars in the
       pragma section of perlmod.

   SelfLoader and AutoLoader
       The SelfLoader can replace the AutoLoader - just change
       'use AutoLoader' to 'use SelfLoader' (though note that the
       SelfLoader exports the AUTOLOAD function - but if you have
       your own AUTOLOAD and are using the AutoLoader too, you
       probably know what you're doing), and the "__END__" token
       to "__DATA__". You will need perl version 5.001m or later
       to use this (version 5.001 with all patches up to patch
       m).

       There is no need to inherit from the SelfLoader.

       The SelfLoader works similarly to the AutoLoader, but
       picks up the subs from after the "__DATA__" instead of in
       the 'lib/auto' directory.  There is a maintenance gain in
       not needing to run AutoSplit on the module at
       installation, and a runtime gain in not needing to keep
       opening and closing files to load subs. There is a runtime
       loss in needing to parse the code after the "__DATA__".
       Details of the AutoLoader and another view of these
       distinctions can be found in that module's documentation.





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   __DATA__, __END__, and the FOOBAR::DATA filehandle.
       This section is only relevant if you want to use the
       "FOOBAR::DATA" together with the SelfLoader.

       Data after the "__DATA__" token in a module is read using
       the FOOBAR::DATA filehandle. "__END__" can still be used
       to denote the end of the "__DATA__" section if followed by
       the token DATA - this is supported by the SelfLoader. The
       "FOOBAR::DATA" filehandle is left open if an "__END__"
       followed by a DATA is found, with the filehandle
       positioned at the start of the line after the "__END__"
       token. If no "__END__" token is present, or an "__END__"
       token with no DATA token on the same line, then the
       filehandle is closed.

       The SelfLoader reads from wherever the current position of
       the "FOOBAR::DATA" filehandle is, until the EOF or
       "__END__". This means that if you want to use that
       filehandle (and ONLY if you want to), you should either

       1. Put all your subroutine declarations immediately after
       the "__DATA__" token and put your own data after those
       declarations, using the "__END__" token to mark the end of
       subroutine declarations. You must also ensure that the
       SelfLoader reads first by  calling
       'SelfLoader->load_stubs();', or by using a function which
       is selfloaded;

       or

       2. You should read the "FOOBAR::DATA" filehandle first,
       leaving the handle open and positioned at the first line
       of subroutine declarations.

       You could conceivably do both.

   Classes and inherited methods.
       For modules which are not classes, this section is not
       relevant.  This section is only relevant if you have
       methods which could be inherited.

       A subroutine stub (or forward declaration) looks like

         sub stub;

       i.e. it is a subroutine declaration without the body of
       the subroutine. For modules which are not classes, there
       is no real need for stubs as far as autoloading is
       concerned.

       For modules which ARE classes, and need to handle
       inherited methods, stubs are needed to ensure that the
       method inheritance mechanism works properly. You can load
       the stubs into the module at 'require' time, by adding the



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       statement 'SelfLoader->load_stubs();' to the module to do
       this.

       The alternative is to put the stubs in before the
       "__DATA__" token BEFORE releasing the module, and for this
       purpose the "Devel::SelfStubber" module is available.
       However this does require the extra step of ensuring that
       the stubs are in the module. If this is done I strongly
       recommend that this is done BEFORE releasing the module -
       it should NOT be done at install time in general.

Multiple packages and fully qualified subroutine names
       Subroutines in multiple packages within the same file are
       supported - but you should note that this requires
       exporting the "SelfLoader::AUTOLOAD" to every package
       which requires it. This is done automatically by the
       SelfLoader when it first loads the subs into the cache,
       but you should really specify it in the initialization
       before the "__DATA__" by putting a 'use SelfLoader'
       statement in each package.

       Fully qualified subroutine names are also supported. For
       example,

          __DATA__
          sub foo::bar {23}
          package baz;
          sub dob {32}

       will all be loaded correctly by the SelfLoader, and the
       SelfLoader will ensure that the packages 'foo' and 'baz'
       correctly have the SelfLoader "AUTOLOAD" method when the
       data after "__DATA__" is first parsed.

AUTHOR
       "SelfLoader" is maintained by the perl5-porters. Please
       direct any questions to the canonical mailing list.
       Anything that is applicable to the CPAN release can be
       sent to its maintainer, though.

       Author and Maintainer: The Perl5-Porters
       <perl5-porters@perl.org>

       Maintainer of the CPAN release: Steffen Mueller
       <smueller@cpan.org>

COPYRIGHT AND LICENSE
       This package has been part of the perl core since the
       first release of perl5. It has been released separately to
       CPAN so older installations can benefit from bug fixes.

       This package has the same copyright and license as the
       perl core:




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                    Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999,
               2000, 2001, 2002, 2003, 2004, 2005, 2006 by Larry Wall and others

                                   All rights reserved.

           This program is free software; you can redistribute it and/or modify
           it under the terms of either:

               a) the GNU General Public License as published by the Free
               Software Foundation; either version 1, or (at your option) any
               later version, or

               b) the "Artistic License" which comes with this Kit.

           This program is distributed in the hope that it will be useful,
           but WITHOUT ANY WARRANTY; without even the implied warranty of
           MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See either
           the GNU General Public License or the Artistic License for more details.

           You should have received a copy of the Artistic License with this
           Kit, in the file named "Artistic".  If not, I'll be glad to provide one.

           You should also have received a copy of the GNU General Public License
           along with this program in the file named "Copying". If not, write to the
           Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
           02111-1307, USA or visit their web page on the internet at
           http://www.gnu.org/copyleft/gpl.html.

           For those of you that choose to use the GNU General Public License,
           my interpretation of the GNU General Public License is that no Perl
           script falls under the terms of the GPL unless you explicitly put
           said script under the terms of the GPL yourself.  Furthermore, any
           object code linked with perl does not automatically fall under the
           terms of the GPL, provided such object code only adds definitions
           of subroutines and variables, and does not otherwise impair the
           resulting interpreter from executing any standard Perl script.  I
           consider linking in C subroutines in this manner to be the moral
           equivalent of defining subroutines in the Perl language itself.  You
           may sell such an object file as proprietary provided that you provide
           or offer to provide the Perl source, as specified by the GNU General
           Public License.  (This is merely an alternate way of specifying input
           to the program.)  You may also sell a binary produced by the dumping of
           a running Perl script that belongs to you, provided that you provide or
           offer to provide the Perl source as specified by the GPL.  (The
           fact that a Perl interpreter and your code are in the same binary file
           is, in this case, a form of mere aggregation.)  This is my interpretation
           of the GPL.  If you still have concerns or difficulties understanding
           my intent, feel free to contact me.  Of course, the Artistic License
           spells all this out for your protection, so you may prefer to use that.








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