Our revised class will have the same data members as the version we defined in § 2.6.1 (p. 72):
string representing the ISBN;
unsigned that says how many copies of the book were sold; and
double representing the total revenue for those sales.
As we’ve seen, our class will also have two member functions,
isbn. In addition, we’ll give
Sales_data another member function to return the average price at which the books were sold. This function, which we’ll name
avg_price, isn’t intended for general use. It will be part of the implementation, not part of the ...