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 perl-
data.) 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 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 POSIX.pm is located in
/usr/local/lib/perl5/POSIX.pm, 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
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 "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
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 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.
CAVEATS
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
"chdir".
SEE ALSO
SelfLoader - an autoloader that doesn't use external files.
perl v5.8.6 2001-09-21 AutoLoader(3)
