SelfLoader(3) Perl Programmers Reference Guide SelfLoader(3)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 the Autoloading entry in the perlsub man
page.
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 filehan
dle 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 sub
routine 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 sub
routines 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 the perlmod manpage.
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 installa
tion, 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 "FOO
BAR::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 posi
tioned 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 file
handle (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 'Self
Loader->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 meth
ods 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 con
cerned.
For modules which ARE classes, and need to handle inher
ited 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 export
ing 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.
2001-02-22 perl v5.6.1 SelfLoader(3)
