


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


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 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 the section on SYNOPSIS
       in the perlsub manpage.  Such forward declaration creates
       "subroutine stubs", which are place holders with no code.




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


       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.




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


       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 mechanism 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.




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


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






















































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