SelfLoader(3p) Perl Programmers Reference Guide SelfLoader(3p)NAMESelfLoader - 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 pack-
age 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 retriev-
able, 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 subrou-
tine when it is called. The costs are the one-time parsing
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of the data after "__DATA__", and a load delay for the
_first_ call of any autoloaded function. The benefits (hope-
fully) 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 sub-
routine 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 vari-
ables.
So specifically, autoloaded functions cannot see package
lexicals (this applies to both the SelfLoader and the Auto-
loader). The "vars" pragma provides an alternative to defin-
ing package-level globals that will be visible to autoloaded
routines. See the documentation on vars in the pragma sec-
tion of perlmod.
SelfLoader and AutoLoader
The SelfLoader can replace the AutoLoader - just change 'use
AutoLoader' to 'use SelfLoader' (though note that the Sel-
fLoader 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
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parse the code after the "__DATA__". Details of the Auto-
Loader 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__" fol-
lowed 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 filehan-
dle (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 Sel-
fLoader 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.
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For modules which ARE classes, and need to handle inherited
methods, stubs are needed to ensure that the method inheri-
tance 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 Sel-
fLoader will ensure that the packages 'foo' and 'baz'
correctly have the SelfLoader "AUTOLOAD" method when the
data after "__DATA__" is first parsed.
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