phpman > man > AutoLoader(3pm)

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 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 "use"d and not "require"d.

  Using AutoLoader's AUTOLOAD Subroutine
    In order to use AutoLoader's AUTOLOAD subroutine you *must* explicitly 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 these lines:

        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 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

            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.

  Forcing AutoLoader to Load a Function
    Sometimes, it can be necessary or useful to make sure that a certain function is fully loaded by
    AutoLoader. This is the case, for example, when you need to wrap a function to inject debugging
    code. It is also helpful to force early loading of code before forking to make use of
    copy-on-write as much as possible.

    Starting with AutoLoader 5.73, you can call the "AutoLoader::autoload_sub" function with the
    fully-qualified name of the function to load from its .al file. The behaviour is exactly the
    same as if you called the function, triggering the regular "AUTOLOAD" mechanism, but it does not
    actually execute the autoloaded function.

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 AT perl.org>

    Maintainer of the CPAN release: Steffen Mueller <smueller AT 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, 2007, 2008, 2009,
            2011, 2012, 2013
            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.