AutoLoader(3)
NNAAMMEE
AutoLoader - load subroutines only on demand
SSYYNNOOPPSSIISS
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;
}
DDEESSCCRRIIPPTTIIOONN
The AAuuttooLLooaaddeerr module works with the AAuuttooSSpplliitt 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 AAuuttooLLooaaddeerr, the author of a module has to place the
definitions of subroutines to be autoloaded after an
__END__ token. (See the perldata manpage.) The AAuuttooSSpplliitt
module can then be run manually to extract the definitions
into individual files auto/funcname.al.
AAuuttooLLooaaddeerr implements an AUTOLOAD subroutine. When an
undefined subroutine in is called in a client module of
AAuuttooLLooaaddeerr, AAuuttooLLooaaddeerr'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, AAuuttooLLooaaddeerr will look for
perl subroutines PPOOSSIIXX 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 funtion, it is
defined, so future calls to the subroutine will bypass the
AUTOLOAD mechanism.
SSuubbrroouuttiinnee SSttuubbss
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 the section on SYNOPSIS
in the perlsub manpage. Such forward declaration creates
"subroutine stubs", which are place holders with no code.
The AutoSplit and AAuuttooLLooaaddeerr 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 AAuuttooLLooaaddeerr
is always used and not required.
UUssiinngg AAuuttooLLooaaddeerr's AUTOLOAD Subroutine
In order to use AAuuttooLLooaaddeerr's AUTOLOAD subroutine you must
explicitly import it:
use AutoLoader 'AUTOLOAD';
OOvveerrrriiddiinngg AAuuttooLLooaaddeerr'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 AAuuttooLLooaaddeerr's AUTOLOAD for the rest.
Such modules should not import AAuuttooLLooaaddeerr's AUTOLOAD
subroutine. Instead, they should define their own
AUTOLOAD subroutines along these lines:
use AutoLoader;
use Carp;
sub AUTOLOAD {
my $constname;
($constname = $AUTOLOAD) =~ s/.*:://;
my $val = constant($constname, @_ ? $_[0] : 0);
if ($! != 0) {
if ($! =~ /Invalid/) {
$AutoLoader::AUTOLOAD = $AUTOLOAD;
goto &AutoLoader::AUTOLOAD;
}
else {
croak "Your vendor has not defined constant $constname";
}
}
*$AUTOLOAD = sub { $val }; # same as: eval "sub $AUTOLOAD { $val }";
goto &$AUTOLOAD;
}
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 AAuuttooLLooaaddeerr at all.
PPaacckkaaggee LLeexxiiccaallss
Package lexicals declared with my in the main block of a
package using AAuuttooLLooaaddeerr 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
AAuuttooLLooaaddeerr.
The vars pragma (see the section on vars in the perlmod
manpage) 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).
AAuuttooLLooaaddeerr vs. SSeellffLLooaaddeerr
The AAuuttooLLooaaddeerr is similar in purpose to SSeellffLLooaaddeerr: both
delay the loading of subroutines.
SSeellffLLooaaddeerr 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,
SSeellffLLooaaddeerr suffers a startup speed disadvantage in the
one-time parsing of the lines after __DATA__, after which
routines are cached. SSeellffLLooaaddeerr can also handle multiple
packages in a file.
AAuuttooLLooaaddeerr only reads code as it is requested, and in many
cases should be faster, but requires a machanism like
AAuuttooSSpplliitt be used to create the individual files. the
ExtUtils::MakeMaker manpage will invoke AAuuttooSSpplliitt
automatically if AAuuttooLLooaaddeerr is used in a module source
file.
CCAAVVEEAATTSS
AutoLoaders prior to Perl 5.002 had a slightly different
interface. Any old modules which use AAuuttooLLooaaddeerr 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 AAuuttooLLooaaddeerr 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.
SSEEEE AALLSSOO
the SelfLoader manpage - an autoloader that doesn't use
external files.