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SelfLoader(3) - SelfLoader - load functions only on demand - man 3 SelfLoader

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



NAME
       SelfLoader - load(7,n) functions only on demand

SYNOPSIS
           package FOOBAR;
           use SelfLoader;

           ... (initializing code)

           __DATA__
           sub {....

DESCRIPTION
       This module tells its users(1,5) that functions in(1,8) the FOOBAR package are to
       be autoloaded from after the "__DATA__" token.  See also "Autoloading"
       in(1,8) 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 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(1,8) package 'main', but for
       other modules data after "__END__" is not automatically retrievable,
       whereas data after "__DATA__" is.  The "__DATA__" token is not recog-
       nized in(1,8) versions of perl prior to 5.001m.

       Note that it is possible to have "__DATA__" tokens in(1,8) the same package
       in(1,8) multiple files, and that the last "__DATA__" token in(1,8) a given pack-
       age that is encountered by the compiler is the one accessible by the
       filehandle. This also applies to "__END__" and main, i.e. if(3,n) 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(7,n,1 builtins) to access(2,5) the data
       after the "__DATA__" in(1,8) the module, _not_ the data after the "__END__"
       token in(1,8) the 'main' program, since the compiler encounters the
       'require'd file(1,n) 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(1,2,n), but
       before subroutine declarations that can be loaded in(1,8) later - usually
       because they may never be called.

       The SelfLoader will read(2,n,1 builtins) from the FOOBAR::DATA filehandle to load(7,n) in(1,8)
       the data after "__DATA__", and load(7,n) in(1,8) any subroutine when it is
       called. The costs are the one-time parsing of the data after
       "__DATA__", and a load(7,n) delay for the _first_ call of any autoloaded
       function. The benefits (hopefully) are a speeded up compilation phase,
       with no need to load(7,n) functions which are never used.

       The SelfLoader will stop reading from "__DATA__" if(3,n) 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(2,3,n) 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(1,n) up to the "__DATA__" token. Subroutines declared
       elsewhere _cannot_ see these types of variables, just as if(3,n) you
       declared subroutines in(1,8) the package but in(1,8) another file(1,n), 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(1,8) 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(3,n) 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(1,3,5) 5.001m or
       later to use this (version(1,3,5) 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(1,8) the 'lib/auto(5,8)' directory.
       There is a maintenance gain in(1,8) not needing to run AutoSplit on the mod-
       ule at installation, and a runtime gain in(1,8) not needing to keep opening
       and closing files to load(7,n) subs. There is a runtime loss in(1,8) needing to
       parse the code after the "__DATA__". Details of the AutoLoader and
       another view of these distinctions can be found in(1,8) that module's docu-
       mentation.

       __DATA__, __END__, and the FOOBAR::DATA filehandle.

       This section is only relevant if(3,n) you want to use the "FOOBAR::DATA"
       together with the SelfLoader.

       Data after the "__DATA__" token in(1,8) a module is read(2,n,1 builtins) using the FOO-
       BAR::DATA filehandle. "__END__" can still be used to denote the end of
       the "__DATA__" section if(3,n) followed by the token DATA - this is sup-
       ported by the SelfLoader. The "FOOBAR::DATA" filehandle is left open(2,3,n) if(3,n)
       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 "FOO-
       BAR::DATA" filehandle is, until the EOF or "__END__". This means that
       if(3,n) you want to use that filehandle (and ONLY if(3,n) 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(2,n,1 builtins) the "FOOBAR::DATA" filehandle first, leaving the
       handle open(2,3,n) and positioned at the first line of subroutine declara-
       tions.

       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(3,n) 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(7,n) the stubs into the module at 'require' time(1,2,n), by
       adding the statement 'SelfLoader->load_stubs();' to the module to do
       this.

       The alternative is to put the stubs in(1,8) before the "__DATA__" token
       BEFORE releasing the module, and for this purpose the "Devel::SelfStub-
       ber" module is available.  However this does require the extra step of
       ensuring that the stubs are in(1,8) 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(1,2,n) in(1,8) general.

Multiple packages and fully qualified subroutine names
       Subroutines in(1,8) multiple packages within the same file(1,n) are supported -
       but you should note that this requires exporting the "Self-
       Loader::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(1,8) the initialization before
       the "__DATA__" by putting a 'use SelfLoader' statement in(1,8) 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.



perl v5.8.5                       2001-09-21                     SelfLoader(3)

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