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RedHat 9 (Linux i386) - man page for autoloader (redhat section 3pm)

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

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 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 subrou-
       tine.  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 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 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 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, Self-
       Loader 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.  ExtU-
       tils::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 relative paths, and the program does "chdir".

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

perl v5.8.0				    2002-06-01				  AutoLoader(3pm)


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