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AutoLoader(3)          Perl Programmers Reference Guide          AutoLoader(3)



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 sub-
       routines 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 perl-
       sub.  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 cre-
       ates 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 quali-
       fying 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 han-
       dle 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 rel-
       ative paths, and the program does "chdir".

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



perl v5.8.8                       2001-09-21                     AutoLoader(3)