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




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 (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.

   __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 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:

       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., 51 Franklin St, Fifth Floor,
       Boston, MA 02110-1301, 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.



perl v5.24.0                      2016-03-19                   SelfLoader(3pm)

perl 5.24 - Generated Mon Nov 21 19:09:37 CST 2016
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