27. What can we do with pointers?

• Pointer is very similar to a normal integer variable:

  
  #include <iostream>
  using namespace std;
  
  // function declarations
  void process_zipcodes( int * zip_codes, int size );
  
  int main()
  {
      //-------------------------------------------------------------
      // (a) we can find out how far apart our objects are in memory
      //-------------------------------------------------------------
      double car_price = 17000.00;
      double milk_price = 3.76;
  
      double * ptr1 = &car_price;
      double * ptr2 = &milk_price;
  
      // Units of measurement - one double - not one byte!
      int distance = ptr1 - ptr2;
  
      cout << "Distance between two double variables in memory is " << distance << '\n';
  
      //--------------------------------------------------------------------
      // (b) we can use simple arithmetic operations to move pointer around
      //--------------------------------------------------------------------
      int * zip_codes = new int[ 5 ];
  
      /*
      |  zip_codes[0]    zip_codes[1]    zip_codes[2]    zip_codes[3]    zip_codes[4]
      |  ?????_______    ?????_______    ?????_______    ?????_______    ?????_______
      */
  
      zip_codes[ 0 ] = 2184;
      zip_codes[ 1 ] = 90210;
      zip_codes[ 2 ] = 12345;
      zip_codes[ 3 ] = 54321;
      zip_codes[ 4 ] = 77889;
  
      /* *zip_codes
      |  zip_codes[0]    zip_codes[1]    zip_codes[2]    zip_codes[3]    zip_codes[4]
      |  02184_______    90210_______    12345_______    54321_______    77889_______
      */
  
      ++zip_codes;
      cout << *zip_codes << '\n';
  
      /*                 *zip_codes
      |  zip_codes[-1]   zip_codes[0]    zip_codes[1]    zip_codes[2]    zip_codes[3]
      |  02184_______    90210_______    12345_______    54321_______    77889_______
      */
  
      zip_codes = zip_codes + 2;
  
      /*                                                 *zip_codes
      |  zip_codes[-3]   zip_codes[-2]   zip_codes[-1]   zip_codes[0]    zip_codes[1]
      |  02184_______    90210_______    12345_______    54321_______    77889_______
      */
  
      cout << *zip_codes << '\n';
      cout << zip_codes[ -3 ] << '\n';
  
  
      //--------------------------------------------------------------------
      // (c) we can pass information to a function via pointer
      //--------------------------------------------------------------------
      process_zipcodes( zip_codes, 2 );
  
      /*                                                 *zip_codes
      |  zip_codes[-3]   zip_codes[-2]   zip_codes[-1]   zip_codes[0]    zip_codes[1]
      |  02184_______    90210_______    12345_______    77889_______    77889_______
      */
  
      cout << zip_codes[ 0 ] << '\n';
      cout << zip_codes[ 1 ] << '\n';
  
      process_zipcodes( zip_codes - 3, 5 );
  
      return 0;
  }
  
  void process_zipcodes( int * zip_codes, int size )
  {
      cout << "\t Inside process_zipcodes()\n";
      cout << *zip_codes << '\n';
      cout << zip_codes[ -3 ] << '\n';
      *zip_codes = zip_codes[ 1 ];
      cout << "\t Exiting process_zipcodes()\n";
  }