SelfLoader(3pm) Perl Programmers Reference Guide SelfLoader(3pm)
NAME
SelfLoader - load functions only on demand
SYNOPSIS
package FOOBAR;
use SelfLoader;
... (initializing code)
__DATA__
sub {....
DESCRIPTION
This module tells its users that functions in the FOOBAR package are to
be autoloaded from after the "__DATA__" token. See also "Autoloading"
in perlsub.
The __DATA__ token
The "__DATA__" token tells the perl compiler that the perl code for
compilation is finished. Everything after the "__DATA__" token is
available for reading via the filehandle FOOBAR::DATA, where FOOBAR is
the name of the current package when the "__DATA__" token is reached.
This works just the same as "__END__" does in package 'main', but for
other modules data after "__END__" is not automatically retrievable,
whereas data after "__DATA__" is. The "__DATA__" token is not
recognized in versions of perl prior to 5.001m.
Note that it is possible to have "__DATA__" tokens in the same package
in multiple files, and that the last "__DATA__" token in a given
package that is encountered by the compiler is the one accessible by
the filehandle. This also applies to "__END__" and main, i.e. if the
'main' program has an "__END__", but a module 'require'd (_not_ 'use'd)
by that program has a 'package main;' declaration followed by an
'"__DATA__"', then the "DATA" filehandle is set to access the data
after the "__DATA__" in the module, _not_ the data after the "__END__"
token in the 'main' program, since the compiler encounters the
'require'd file later.
SelfLoader autoloading
The SelfLoader works by the user placing the "__DATA__" token after
perl code which needs to be compiled and run at 'require' time, but
before subroutine declarations that can be loaded in later - usually
because they may never be called.
The SelfLoader will read from the FOOBAR::DATA filehandle to load in
the data after "__DATA__", and load in any subroutine when it is
called. The costs are the one-time parsing of the data after
"__DATA__", and a load delay for the _first_ call of any autoloaded
function. The benefits (hopefully) are a speeded up compilation phase,
with no need to load functions which are never used.
The SelfLoader will stop reading from "__DATA__" if it encounters the
"__END__" token - just as you would expect. If the "__END__" token is
present, and is followed by the token DATA, then the SelfLoader leaves
the FOOBAR::DATA filehandle open on the line after that token.
The SelfLoader exports the "AUTOLOAD" subroutine to the package using
the SelfLoader, and this loads the called subroutine when it is first
called.
There is no advantage to putting subroutines which will _always_ be
called after the "__DATA__" token.
Autoloading and package lexicals
A 'my $pack_lexical' statement makes the variable $pack_lexical local
_only_ to the file up to the "__DATA__" token. Subroutines declared
elsewhere _cannot_ see these types of variables, just as if you
declared subroutines in the package but in another file, they cannot
see these variables.
So specifically, autoloaded functions cannot see package lexicals (this
applies to both the SelfLoader and the Autoloader). The "vars" pragma
provides an alternative to defining package-level globals that will be
visible to autoloaded routines. See the documentation on vars in the
pragma section of perlmod.
SelfLoader and AutoLoader
The SelfLoader can replace the AutoLoader - just change 'use
AutoLoader' to 'use SelfLoader' (though note that the SelfLoader
exports the AUTOLOAD function - but if you have your own AUTOLOAD and
are using the AutoLoader too, you probably know what you're doing), and
the "__END__" token to "__DATA__". You will need perl version 5.001m or
later to use this (version 5.001 with all patches up to patch m).
There is no need to inherit from the SelfLoader.
The SelfLoader works similarly to the AutoLoader, but picks up the subs
from after the "__DATA__" instead of in the 'lib/auto' directory.
There is a maintenance gain in not needing to run AutoSplit on the
module at installation, and a runtime gain in not needing to keep
opening and closing files to load subs. There is a runtime loss in
needing to parse the code after the "__DATA__". Details of the
AutoLoader and another view of these distinctions can be found in that
module's documentation.
__DATA__, __END__, and the FOOBAR::DATA filehandle.
This section is only relevant if you want to use the "FOOBAR::DATA"
together with the SelfLoader.
Data after the "__DATA__" token in a module is read using the
FOOBAR::DATA filehandle. "__END__" can still be used to denote the end
of the "__DATA__" section if followed by the token DATA - this is
supported by the SelfLoader. The "FOOBAR::DATA" filehandle is left open
if an "__END__" followed by a DATA is found, with the filehandle
positioned at the start of the line after the "__END__" token. If no
"__END__" token is present, or an "__END__" token with no DATA token on
the same line, then the filehandle is closed.
The SelfLoader reads from wherever the current position of the
"FOOBAR::DATA" filehandle is, until the EOF or "__END__". This means
that if you want to use that filehandle (and ONLY if you want to), you
should either
1. Put all your subroutine declarations immediately after the
"__DATA__" token and put your own data after those declarations, using
the "__END__" token to mark the end of subroutine declarations. You
must also ensure that the SelfLoader reads first by calling
'SelfLoader->load_stubs();', or by using a function which is
selfloaded;
or
2. You should read the "FOOBAR::DATA" filehandle first, leaving the
handle open and positioned at the first line of subroutine
declarations.
You could conceivably do both.
Classes and inherited methods.
For modules which are not classes, this section is not relevant. This
section is only relevant if you have methods which could be inherited.
A subroutine stub (or forward declaration) looks like
sub stub;
i.e. it is a subroutine declaration without the body of the subroutine.
For modules which are not classes, there is no real need for stubs as
far as autoloading is concerned.
For modules which ARE classes, and need to handle inherited methods,
stubs are needed to ensure that the method inheritance mechanism works
properly. You can load the stubs into the module at 'require' time, by
adding the statement 'SelfLoader->load_stubs();' to the module to do
this.
The alternative is to put the stubs in before the "__DATA__" token
BEFORE releasing the module, and for this purpose the
"Devel::SelfStubber" module is available. However this does require
the extra step of ensuring that the stubs are in the module. If this is
done I strongly recommend that this is done BEFORE releasing the module
- it should NOT be done at install time in general.
Multiple packages and fully qualified subroutine names
Subroutines in multiple packages within the same file are supported -
but you should note that this requires exporting the
"SelfLoader::AUTOLOAD" to every package which requires it. This is done
automatically by the SelfLoader when it first loads the subs into the
cache, but you should really specify it in the initialization before
the "__DATA__" by putting a 'use SelfLoader' statement in each package.
Fully qualified subroutine names are also supported. For example,
__DATA__
sub foo::bar {23}
package baz;
sub dob {32}
will all be loaded correctly by the SelfLoader, and the SelfLoader will
ensure that the packages 'foo' and 'baz' correctly have the SelfLoader
"AUTOLOAD" method when the data after "__DATA__" is first parsed.
AUTHOR
"SelfLoader" is maintained by the perl5-porters. Please direct any
questions to the canonical mailing list. Anything that is applicable to
the CPAN release can be sent to its maintainer, though.
Author and Maintainer: The Perl5-Porters <perl5-porters AT perl.org>
Maintainer of the CPAN release: Steffen Mueller <smueller AT cpan.org>
COPYRIGHT AND LICENSE
This package has been part of the perl core since the first release of
perl5. It has been released separately to CPAN so older installations
can benefit from bug fixes.
This package has the same copyright and license as the perl core:
Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2000, 2001, 2002, 2003, 2004, 2005, 2006 by Larry Wall and others
All rights reserved.
This program is free software; you can redistribute it and/or modify
it under the terms of either:
a) the GNU General Public License as published by the Free
Software Foundation; either version 1, or (at your option) any
later version, or
b) the "Artistic License" which comes with this Kit.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See either
the GNU General Public License or the Artistic License for more details.
You should have received a copy of the Artistic License with this
Kit, in the file named "Artistic". If not, I'll be glad to provide one.
You should also have received a copy of the GNU General Public License
along with this program in the file named "Copying". If not, write to the
Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
MA 02110-1301, USA or visit their web page on the internet at
http://www.gnu.org/copyleft/gpl.html.
For those of you that choose to use the GNU General Public License,
my interpretation of the GNU General Public License is that no Perl
script falls under the terms of the GPL unless you explicitly put
said script under the terms of the GPL yourself. Furthermore, any
object code linked with perl does not automatically fall under the
terms of the GPL, provided such object code only adds definitions
of subroutines and variables, and does not otherwise impair the
resulting interpreter from executing any standard Perl script. I
consider linking in C subroutines in this manner to be the moral
equivalent of defining subroutines in the Perl language itself. You
may sell such an object file as proprietary provided that you provide
or offer to provide the Perl source, as specified by the GNU General
Public License. (This is merely an alternate way of specifying input
to the program.) You may also sell a binary produced by the dumping of
a running Perl script that belongs to you, provided that you provide or
offer to provide the Perl source as specified by the GPL. (The
fact that a Perl interpreter and your code are in the same binary file
is, in this case, a form of mere aggregation.) This is my interpretation
of the GPL. If you still have concerns or difficulties understanding
my intent, feel free to contact me. Of course, the Artistic License
spells all this out for your protection, so you may prefer to use that.
perl v5.16.3 2013-03-04 SelfLoader(3pm)
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