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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 "Autoloading" in perlsub.

       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



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


       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 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), 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



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


       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.



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


       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.




















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