Suppose that class OrderList has a private attribute double…

Suppose that class OrderList has a private attribute double cost which hold the cost of all ordered items, and a private attributes int num_of_items which hold the number of items ordered. For example, if num_of_items is 5, then cost, cost, …, cost hold the cost of these 5 items. Implement the member function named average_cost which returns the average cost of this OrderList.