SelfLoader(3) Perl Programmers Reference Guide SelfLoader(3)
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 the section on Autoloading in the perlsub
manpage.
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
retreivable , 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
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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 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 maintainance 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__.
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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.
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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.
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