AutoLoader(3pm) Perl Programmers Reference Guide AutoLoader(3pm)
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 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 exam-ple, example,
ple, if POSIX.pm is located in /usr/local/lib/perl5/POSIX.pm, AutoLoader will look for perl subrou-tines subroutines
tines 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 (presum-ably) (presumably)
ably) 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 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 "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 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 subrou-tine subroutine
tine (because it doesn't have any AutoSplit subroutines), then that module 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 visi-ble visible
ble 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 explic-itly explicitly
itly 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 hierar-chy hierarchy
chy 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.
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 pack-age package
age 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.
perl v5.8.8 2001-09-21 AutoLoader(3pm)
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