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" joe@foo.com < 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.