AutoLoader(3p) Perl Programmers Reference Guide AutoLoader(3p)NAMEAutoLoader - 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 indivi-
dual files auto/funcname.al.
AutoLoader implements an AUTOLOAD subroutine. When an unde-
fined subroutine in is called in a client module of Auto-
Loader, 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 subrou-
tine.
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 for-
ward declaration creates "subroutine stubs", which are place
holders with no code.
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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 subrou-
tine. Instead, they should define their own AUTOLOAD subrou-
tines 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 fall-
back 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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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, Sel-
fLoader 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 AutoS-
plit be used to create the individual files.
ExtUtils::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 con-
flicts when used to split a module.
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AutoLoader(3p) Perl Programmers Reference Guide AutoLoader(3p)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.
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