6.7 Autoloading

If you invoke a function called Test::func() , for example, and if func() has not been defined in module Test, Perl automatically looks for a subroutine called Test::AUTOLOAD() . If such a subroutine exists, Perl calls it with the arguments that were passed to func() . In addition, a variable called $AUTOLOAD is set to the full name of the function that was called (" Test::func "). Objective-C programmers will recognize this as being similar to the ":forward" declaration, in which an object uses this statement to trap all procedure calls it doesn't handle, in order to forward it to a "delegate."

The AUTOLOAD subroutine can do just about anything it wants. For example, it can do one of the following:

• Handle the call itself. The caller of Test::func does not know that AUTOLOAD really handled the call.

• Automatically create a subroutine on the fly (using eval ) to do the right thing, and then call that new subroutine. In fact, instead of calling that subroutine, you can simply go to it, like this:

      sub AUTOLOAD {         ... create subroutine ...         goto &$AUTOLOAD;  # jump to it     }

  This is a special form of goto that erases the stack-frame of the AUTOLOAD routine so that Test::func will not know it has been being called from AUTOLOAD .

• Dynamically load an object file (or a DLL in Microsoft Windows), using the standard Dynaloader module, and then execute the appropriate call. This is one of the more popular uses of AUTOLOAD .

• Use the system function to launch another program by the same name. Shell.pm in your Perl library is the really fancy version of this facility. Here is a simplified version:

  #------------------------------------------------------------------ package Shell; #------------------------------------------------------------------ sub AUTOLOAD {     my($program) = $AUTOLOAD;     # We are just interested in the command name, not in      # the package name     $program =~ s/^.*:://;      system ("$program @_"); } #------------------------------------------------------------------ use Shell; ls ('-lR'); # Triggers a call to AUTOLOAD since sub ls() doesn't exist mail ('-s "This is a test" [email protected] < letter.txt');

Autoloading can also be used to delay the loading of subroutines until they are absolutely necessary. A module called Autosplit (in the standard distribution) is used to split a module into multiple modules, each with one subroutine from the original, and the Autoloader module can subsequently be used to subsequently load only the file corresponding to the called subroutine.