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@perl.org>
Maintainer of the CPAN release: Steffen Mueller <smueller@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.