AutoLoader

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 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 the
       perldata manpage.)  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 funtion, 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 the
       section on SYNOPSIS in the perlsub manpage.  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:

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

       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 the section on vars in the perlmod manpage) 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).

       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 machanism like AutoSplit be used to create
       the individual files.  the ExtUtils::MakeMaker manpage 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
       the SelfLoader manpage - an autoloader that doesn't use external files.















3rd Berkeley Distribution    perl 5.005, patch 02                AutoLoader(3)