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 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 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 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 "SYNOPSIS" in perl
sub. 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 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. - Package Lexicals
- Package lexicals declared with "my" in the main block of a
package using AutoLoader will not be visible to autoloaded 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 "vars" in perlmod) may be used in
such situations as an alternative to explicitly qualifying
all globals with the package namespace. Variables predeclared 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. 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.