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AutoLoader

Section: User Contributed Perl Documentation (3)
Updated: 2009-09-21
Index 

NAME

AutoLoader - load subroutines only on demand 

SYNOPSIS

    package Foo;    use AutoLoader 'AUTOLOAD';   # import the default AUTOLOAD subroutine    package Bar;    use AutoLoader;              # don't import AUTOLOAD, define our own    sub AUTOLOAD {        ...        $AutoLoader::AUTOLOAD = "...";        goto &AutoLoader::AUTOLOAD;    }
 

DESCRIPTION

The AutoLoader module works with the AutoSplit module and the"__END__" token to defer the loading of some subroutines until they areused rather than loading them all at once.

To use AutoLoader, the author of a module has to place thedefinitions of subroutines to be autoloaded after an "__END__" token.(See perldata.) The AutoSplit module can then be run manually toextract the definitions into individual files auto/funcname.al.

AutoLoader implements an AUTOLOAD subroutine. When an undefinedsubroutine in is called in a client module of AutoLoader,AutoLoader's AUTOLOAD subroutine attempts to locate the subroutine in afile with a name related to the location of the file from which theclient module was read. As an example, if POSIX.pm is located in/usr/local/lib/perl5/POSIX.pm, AutoLoader will look for perlsubroutines POSIX in /usr/local/lib/perl5/auto/POSIX/*.al, wherethe ".al" file has the same name as the subroutine, sans package. Ifsuch a file exists, AUTOLOAD will read and evaluate it,thus (presumably) defining the needed subroutine. AUTOLOAD will then"goto" the newly defined subroutine.

Once this process completes for a given function, it is defined, sofuture calls to the subroutine will bypass the AUTOLOAD mechanism. 

Subroutine Stubs

In order for object method lookup and/or prototype checking to operatecorrectly even when methods have not yet been defined it is necessary to``forward declare'' each subroutine (as in "sub NAME;"). See``SYNOPSIS'' in perlsub. Such forward declaration creates ``subroutinestubs'', which are place holders with no code.

The AutoSplit and AutoLoader modules automate the creation of forwarddeclarations. The AutoSplit module creates an 'index' file containingforward declarations of all the AutoSplit subroutines. When theAutoLoader module is 'use'd it loads these declarations into its callerspackage.

Because of this mechanism it is important that AutoLoader is always"use"d and not "require"d. 

Using AutoLoader's AUTOLOAD Subroutine

In order to use AutoLoader's AUTOLOAD subroutine you mustexplicitly import it:

    use AutoLoader 'AUTOLOAD';
 

Overriding AutoLoader's AUTOLOAD Subroutine

Some modules, mainly extensions, provide their own AUTOLOAD subroutines.They typically need to check for some special cases (such as constants)and then fallback to AutoLoader's AUTOLOAD for the rest.

Such modules should not import AutoLoader's AUTOLOAD subroutine.Instead, they should define their own AUTOLOAD subroutines along theselines:

    use AutoLoader;    use Carp;    sub AUTOLOAD {        my $sub = $AUTOLOAD;        (my $constname = $sub) =~ s/.*:://;        my $val = constant($constname, @_ ? $_[0] : 0);        if ($! != 0) {            if ($! =~ /Invalid/ || $!{EINVAL}) {                $AutoLoader::AUTOLOAD = $sub;                goto &AutoLoader::AUTOLOAD;            }            else {                croak "Your vendor has not defined constant $constname";            }        }        *$sub = sub { $val }; # same as: eval "sub $sub { $val }";        goto &$sub;    }

If any module's own AUTOLOAD subroutine has no need to fallback to theAutoLoader's AUTOLOAD subroutine (because it doesn't have any AutoSplitsubroutines), then that module should not use AutoLoader at all. 

Package Lexicals

Package lexicals declared with "my" in the main block of a packageusing AutoLoader will not be visible to auto-loaded subroutines, due tothe fact that the given scope ends at the "__END__" marker. A moduleusing such variables as package globals will not work properly under theAutoLoader.

The "vars" pragma (see ``vars'' in perlmod) may be used in suchsituations as an alternative to explicitly qualifying all globals withthe package namespace. Variables pre-declared with this pragma will bevisible to any autoloaded routines (but will not be invisible outsidethe package, unfortunately). 

Not Using AutoLoader

You can stop using AutoLoader by simply

        no AutoLoader;
 

AutoLoader vs. SelfLoader

The AutoLoader is similar in purpose to SelfLoader: both delay theloading of subroutines.

SelfLoader uses the "__DATA__" marker rather than "__END__".While this avoids the use of a hierarchy of disk files and theassociated open/close for each routine loaded, SelfLoader suffers astartup speed disadvantage in the one-time parsing of the lines after"__DATA__", after which routines are cached. SelfLoader can alsohandle multiple packages in a file.

AutoLoader only reads code as it is requested, and in many casesshould be faster, but requires a mechanism like AutoSplit be used tocreate the individual files. ExtUtils::MakeMaker will invokeAutoSplit automatically if AutoLoader is used in a module sourcefile. 

CAVEATS

AutoLoaders prior to Perl 5.002 had a slightly different interface. Anyold modules which use AutoLoader should be changed to the new callingstyle. Typically this just means changing a require to a use, addingthe explicit 'AUTOLOAD' import if needed, and removing AutoLoaderfrom @ISA.

On systems with restrictions on file name length, the file correspondingto a subroutine may have a shorter name that the routine itself. Thiscan lead to conflicting file names. The AutoSplit package warns ofthese potential conflicts when used to split a module.

AutoLoader may fail to find the autosplit files (or even find the wrongones) in cases where @INC contains relative paths, and the programdoes "chdir". 

SEE ALSO

SelfLoader - an autoloader that doesn't use external files. 

AUTHOR

"AutoLoader" is maintained by the perl5-porters. Please directany questions to the canonical mailing list. Anything thatis applicable to the CPAN release can be sent to its maintainer,though.

Author and Maintainer: The Perl5-Porters <perl5-portersAATTperl.org>

Maintainer of the CPAN release: Steffen Mueller <smuellerAATTcpan.org> 

COPYRIGHT AND LICENSE

This package has been part of the perl core since the first releaseof perl5. It has been released separately to CPAN so older installationscan 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, 2007, 2008, 2009        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.


 

Index

NAME
SYNOPSIS
DESCRIPTION
Subroutine Stubs
Using AutoLoader's AUTOLOAD Subroutine
Overriding AutoLoader's AUTOLOAD Subroutine
Package Lexicals
Not Using AutoLoader
AutoLoader vs. SelfLoader
CAVEATS
SEE ALSO
AUTHOR
COPYRIGHT AND LICENSE

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