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AutoLoader

Perl 5 version 10.1 documentation
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AutoLoader

NAME

AutoLoader - load subroutines only on demand

SYNOPSIS

  1.     package Foo;
  2.     use AutoLoader 'AUTOLOAD';   # import the default AUTOLOAD subroutine
  4.     package Bar;
  5.     use AutoLoader;              # don't import AUTOLOAD, define our own
  6.     sub AUTOLOAD {
  7.         ...
  8.         $AutoLoader::AUTOLOAD = "...";
  9.         goto &AutoLoader::AUTOLOAD;
  10.     }

DESCRIPTION

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

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

AutoLoader implements an AUTOLOAD subroutine. When an undefined subroutine in is called in a client module of AutoLoader, AutoLoader's AUTOLOAD subroutine attempts to locate the subroutine in a file with a name related to the location of the file from which the client module was read. As an example, if POSIX.pm is located in /usr/local/lib/perl5/POSIX.pm, AutoLoader will look for perl subroutines POSIX in /usr/local/lib/perl5/auto/POSIX/*.al, where the .al file has the same name as the subroutine, sans package. If such 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, so future calls to the subroutine will bypass the AUTOLOAD mechanism.

Subroutine Stubs

In order for object method lookup and/or prototype checking to operate correctly 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 "subroutine stubs", which are place holders with no code.

The AutoSplit and AutoLoader modules automate the creation of forward declarations. The AutoSplit module creates an 'index' file containing forward declarations of all the AutoSplit subroutines. When the AutoLoader module is 'use'd it loads these declarations into its callers package.

Because of this mechanism it is important that AutoLoader is always used and not required.

Using AutoLoader's AUTOLOAD Subroutine

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

  1.     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 these lines:

  1.     use AutoLoader;
  2.     use Carp;
  4.     sub AUTOLOAD {
  5.         my $sub = $AUTOLOAD;
  6.         (my $constname = $sub) =~ s/.*:://;
  7.         my $val = constant($constname, @_ ? $_[0] : 0);
  8.         if ($! != 0) {
  9.             if ($! =~ /Invalid/ || $!{EINVAL}) {
  10.                 $AutoLoader::AUTOLOAD = $sub;
  11.                 goto &AutoLoader::AUTOLOAD;
  12.             }
  13.             else {
  14.                 croak "Your vendor has not defined constant $constname";
  15.             }
  16.         }
  17.         *$sub = sub { $val }; # same as: eval "sub $sub { $val }";
  18.         goto &$sub;
  19.     }

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

Package Lexicals

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

The vars pragma (see vars in perlmod) may be used in such situations as an alternative to explicitly qualifying all globals with the package namespace. Variables pre-declared with this pragma will be visible to any autoloaded routines (but will not be invisible outside the package, unfortunately).

Not Using AutoLoader

You can stop using AutoLoader by simply

  1. 	no AutoLoader;

AutoLoader vs. SelfLoader

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

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

AutoLoader only reads code as it is requested, and in many cases should be faster, but requires a mechanism like AutoSplit be used to create the individual files. ExtUtils::MakeMaker will invoke AutoSplit automatically if AutoLoader is used in a module source file.

CAVEATS

AutoLoaders prior to Perl 5.002 had a slightly different interface. Any old modules which use AutoLoader should be changed to the new calling style. Typically this just means changing a require to a use, adding the explicit 'AUTOLOAD' import if needed, and removing AutoLoader from @ISA .

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

AutoLoader may fail to find the autosplit files (or even find the wrong ones) in cases where @INC contains relative paths, and the program does chdir.

SEE ALSO

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

AUTHOR

AutoLoader 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:

  1.              Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999,
  2.         2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008
  3.         by Larry Wall and others
  4.     
  5. 			    All rights reserved.
  6.     
  7.     This program is free software; you can redistribute it and/or modify
  8.     it under the terms of either:
  9.     
  10. 	a) the GNU General Public License as published by the Free
  11. 	Software Foundation; either version 1, or (at your option) any
  12. 	later version, or
  13.     
  14. 	b) the "Artistic License" which comes with this Kit.
  15.     
  16.     This program is distributed in the hope that it will be useful,
  17.     but WITHOUT ANY WARRANTY; without even the implied warranty of
  18.     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See either
  19.     the GNU General Public License or the Artistic License for more details.
  20.     
  21.     You should have received a copy of the Artistic License with this
  22.     Kit, in the file named "Artistic".  If not, I'll be glad to provide one.
  23.     
  24.     You should also have received a copy of the GNU General Public License
  25.     along with this program in the file named "Copying". If not, write to the 
  26.     Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 
  27.     02111-1307, USA or visit their web page on the internet at
  28.     http://www.gnu.org/copyleft/gpl.html.
  29.     
  30.     For those of you that choose to use the GNU General Public License,
  31.     my interpretation of the GNU General Public License is that no Perl
  32.     script falls under the terms of the GPL unless you explicitly put
  33.     said script under the terms of the GPL yourself.  Furthermore, any
  34.     object code linked with perl does not automatically fall under the
  35.     terms of the GPL, provided such object code only adds definitions
  36.     of subroutines and variables, and does not otherwise impair the
  37.     resulting interpreter from executing any standard Perl script.  I
  38.     consider linking in C subroutines in this manner to be the moral
  39.     equivalent of defining subroutines in the Perl language itself.  You
  40.     may sell such an object file as proprietary provided that you provide
  41.     or offer to provide the Perl source, as specified by the GNU General
  42.     Public License.  (This is merely an alternate way of specifying input
  43.     to the program.)  You may also sell a binary produced by the dumping of
  44.     a running Perl script that belongs to you, provided that you provide or
  45.     offer to provide the Perl source as specified by the GPL.  (The
  46.     fact that a Perl interpreter and your code are in the same binary file
  47.     is, in this case, a form of mere aggregation.)  This is my interpretation
  48.     of the GPL.  If you still have concerns or difficulties understanding
  49.     my intent, feel free to contact me.  Of course, the Artistic License
  50.     spells all this out for your protection, so you may prefer to use that.
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