AutoLoader - load subroutines only on demand
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 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.
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 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 $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 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).
You can stop using AutoLoader by simply
- no AutoLoader;
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 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.
SelfLoader - an autoloader that doesn't use external files.