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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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