SelfLoader

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 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__. 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.















3rd Berkeley Distribution    perl 5.005, patch 02                SelfLoader(3)