556: Mainline

There are two other important reasons to use call-by-reference parameters.

• First, argument copying may take time that cannot be spared, particularly if the argument is a large class object.
• Second, you may wish to write a function that modifies one of its arguments, which it cannot do if it has only a copy with which to work.

Suppose that you have an array of box_car objects, rather than railroad_car objects. Then, you could define floor_space_function as follows:

double floor_space_function (box_car b) { 
  return b.width * b.length; 
} 

Equipped with the floor_space_function, you could incorporate it into a program that uses your array of box_car objects:

box_car *box_car_array[100]; 
... 
main ( ) { 
... 
  for (n = 0; n < car_count; ++n) { 
    // Display floor space and terminate the line:  
    cout << floor_space_function (*box_car_array[n]) 
         << endl; 
  } 
} 

This program will run faster if you define floor_space_function with a call-by-reference parameter to avoid copying the box_car object:

double floor_space_function (box_car& b) { 
  return b.width * b.length; 
}