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

       AutoLoader - load subroutines only on demand

           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;

       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 perl-
       data.)  The AutoSplit module can then be run manually to extract the
       definitions into individual files auto/

       AutoLoader implements an AUTOLOAD subroutine.  When an undefined sub-
       routine 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 is located in
       /usr/local/lib/perl5/, AutoLoader will look for perl subrou-
       tines 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

       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 "SYNOP-
       SIS" 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 subrou-
       tines.  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

           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 situa-
       tions 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 auto-
       matically if AutoLoader is used in a module source file.

       AutoLoaders prior to Perl 5.002 had a slightly different interface.
       Any old modules which use AutoLoader should be changed to the new call-
       ing 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 correspond-
       ing 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

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

perl v5.8.6                       2001-09-21                     AutoLoader(3)