question_9.cc

      //  $Id$
      
      /*  This program is a solution to Question 9 on final exam:
       *
       *    Write a complete C++ program that uses a dispatch table to
       *    invoke a function named on the command line.  The function is to
       *    be passed an array of pointers to constant strings made up of
       *    the remaining arguments on the command line following the
       *    function name.  Construct a sample dispatch table that allows
       *    the user to invoke either a function named echo() or a function
       *    named reverse_echo().  The echo() function prints its arguments
       *    on standard out, and reverse_echo() prints its argumetns in
       *    reverse order.
       *
       *    $Log$
       */
      #include <stdio.h>
      
      //  Dispatch table setup
      //  ------------------------------------------------------------------
      typedef void func_t( int, const char*[] );
      func_t  do_echo, do_reverse;
      
      struct dispatch_t
      {
        const char  *name;
        func_t      *func;
      } table[] =
      {
        { "echo",         do_echo     },
        { "reverse_echo", do_reverse  },
      };
      static const int num_funcs = sizeof( table ) / sizeof( dispatch_t );
      
      
      //  Dispatchable Functions
      //  ==================================================================
      
      //  do_echo()
      //  ------------------------------------------------------------------
      /*
       *    Print arguments in forward order.
       */
      void
      do_echo( int argc, const char *argv[] )
      {
        for (int i = 0; i < argc; i++ )
          printf( "%s ", argv[i] );
        printf( "\n" );
      }
      
      //  do_reverse()
      //  ------------------------------------------------------------------
      /*
       *    Print arguments in reverse order.
       */
      void
      do_reverse( int argc, const char *argv[] )
      {
        for (int i = argc -1; i >= 0; i-- )
          printf( "%s ", argv[i] );
        printf( "\n" );
      }
      
      
      //  main()
      //  ------------------------------------------------------------------
      /*
       *    Invoke a function named on the command line, passing remaining
       *    command line arguments as function arguments.
       */
      int
      main( int argc, char *argv[], char *envp[] )
      {
        //  Check if there is a function name
        if ( argc < 2 )
        {
          fprintf( stderr, "Usage: question_9 function args\n" );
          exit( 0 );
        }
      
        //  Look up the function in the dispatch table, and invoke it.
        for (int i = 0; i < num_funcs; i++ )
        {
          if ( 0 == strcmp( argv[1], table[i].name ) )
          {
            table[i].func( argc - 2, (const char*[])&argv[2] );
            exit( 0 );
          }
        }
        fprintf( stderr, "%s: unknown function name\n", argv[1] );
        exit( 1 );
      }