


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 subrou-
       tines 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 check-
       ing 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 SYNOPSIS entry 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 pack-
       age.

       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 sub-
       routine.  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 mod-
       ule 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 vari-
       ables as package globals will not work properly under the
       AutoLoader.

       The "vars" pragma (see the vars entry in the perlmod man-
       page) may be used in such situations as an alternative to
       explicitly qualifying all globals with the package names-
       pace.  Variables pre-declared with this pragma will be
       visible to any autoloaded routines (but will not be invis-
       ible 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.  the
       ExtUtils::MakeMaker manpage will invoke AutoSplit automat-
       ically 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



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


       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 rela-
       tive paths, and the program does "chdir".

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
















































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