AutoLoader(3) AutoLoader(3)
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.
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 the section on
SYNOPSIS in the perlsub manpage. 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
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AutoLoader(3) AutoLoader(3)
package.
Because of this mechanism it is important that AutoLoader is always used
and not required.
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 $constname;
($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 [Toc] [Back]
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
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AutoLoader(3) AutoLoader(3)
such variables as package globals will not work properly under the
AutoLoader.
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).
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 machanism like AutoSplit be used to create the
individual files. the ExtUtils::MakeMaker manpage 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.
the SelfLoader manpage - an autoloader that doesn't use external files.
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