


SelfLoader(3p)   Perl Programmers Reference Guide  SelfLoader(3p)


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
       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



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SelfLoader(3p)   Perl Programmers Reference Guide  SelfLoader(3p)


       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 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 __DATA__.



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SelfLoader(3p)   Perl Programmers Reference Guide  SelfLoader(3p)


       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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SelfLoader(3p)   Perl Programmers Reference Guide  SelfLoader(3p)


       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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