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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.











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