Perl 5 version 8.8 documentation
- The __DATA__ token
- SelfLoader autoloading
- Autoloading and package lexicals
- SelfLoader and AutoLoader
- __DATA__, __END__, and the FOOBAR::DATA filehandle.
- Classes and inherited methods.
- Multiple packages and fully qualified subroutine names
SelfLoader - load functions only on demand
This module tells its users that functions in the FOOBAR package are to be
autoloaded from after the
token. See also
Autoloading in perlsub.
The __DATA__ token
token tells the perl compiler that the perl code
for compilation is finished. Everything after the
is available for reading via the filehandle FOOBAR::DATA,
where FOOBAR is the name of the current package when the
token is reached. This works just the same as
package 'main', but for other modules data after
automatically retrievable, whereas data after
token is not recognized in versions of perl prior to
Note that it is possible to have
tokens in the same package
in multiple files, and that the last
token in a given
package that is encountered by the compiler is the one accessible
by the filehandle. This also applies to
and main, i.e. if
the 'main' program has an
, but a module 'require'd (_not_ 'use'd)
by that program has a 'package main;' declaration followed by an '
filehandle is set to access the data after the
in the module, _not_ the data after the
token in the 'main'
program, since the compiler encounters the 'require'd file later.
The SelfLoader works by the user placing the
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
The SelfLoader will read from the FOOBAR::DATA filehandle to
load in the data after
, and load in any subroutine
when it is called. The costs are the one-time parsing of the
, 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
it encounters the
token - just as you would expect.
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
subroutine to the
package using the SelfLoader, and this loads the called
subroutine when it is first called.
Autoloading and package lexicals
A 'my $pack_lexical' statement makes the variable $pack_lexical
local _only_ to the file up to the
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).
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),
. 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
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
. Details of the AutoLoader and
another view of these distinctions can be found in that module's
__DATA__, __END__, and the FOOBAR::DATA filehandle.
This section is only relevant if you want to use
together with the SelfLoader.
Data after the
token in a module is read using the
can still be used to denote the end
section if followed by the token DATA - this is supported
by the SelfLoader. The
filehandle is left open if an
followed by a DATA is found, with the filehandle positioned at
the start of the line after the
token. If no
present, or an
token with no DATA token on the same line, then
the filehandle is closed.
The SelfLoader reads from wherever the current
position of the
filehandle is, until the
. 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
token and put your own data after those
declarations, using the
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;
2. You should read the
filehandle first, leaving
the handle open and positioned at the first line of subroutine
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
releasing the module, and for this purpose the
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
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
a 'use SelfLoader' statement in each package.
Fully qualified subroutine names are also supported. For example,
will all be loaded correctly by the SelfLoader, and the SelfLoader
will ensure that the packages 'foo' and 'baz' correctly have the
method when the data after