AutoLoader - load subroutines only on demand
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;
}
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 the perldata manpage.) 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 funtion, it is defined, so future calls to the subroutine will bypass the AUTOLOAD mechanism.
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. 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 used and not required.
In order to use AutoLoader's AUTOLOAD subroutine you must explicitly import it:
use AutoLoader 'AUTOLOAD';
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 $constname;
($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 AutoLoader at all.
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) 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).
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 machanism like AutoSplit be used to create the individual files. MakeMaker will invoke AutoSplit automatically if AutoLoader is used in a module source file.
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.
the SelfLoader manpage - an autoloader that doesn't use external files.
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.