C++ :: Overloading Addition And Comparing Expressions In A While Loop

I have two questions that are related to each other. The first one is about overloading the addition operator.

I have defined a struct as the following:

Code:
#include <iostream>
#include <string>
struct Sales_data {

[Code] ....

I then overloaded the I/O operators so I could print to the screen information related Sales_data.

Code:
// overload ostream in order for cout to work
std::ostream& operator << (std::ostream & out,
const Sales_data & cSales_data) {
out << cSales_data.bookNo << ", " << cSales_data.units_sold << ", "

[Code] ....

My first issue is with overloading the addition operator. Everyone works correctly except for std::cout << item << std:endl; will no not output the ISBN number only the units_sold and revenue when added together.

Code:
// addition operator rules
Sales_data Sales_data::operator + (const Sales_data & data2) {
units_sold += data2.units_sold;
revenue += data2.revenue;
return *this;
}

Now here is the code in its entirety

Code:
#include <iostream>
#include <string>
// Sales_data structure
struct Sales_data {
std:: string bookNo;
unsigned units_sold = 0;

[code] .....

After total = total + item;, I would like to print the total for this particular ISBN. However, I only get: blank, total units, total revenue where blank is where the ISBN would go but doesn't print after addition. My second question has to do with comparing the ISBN's of the books entered during the while loop. I would like to do something like

Code:
if (item_i.bookNo == item_i+1.bookNo) {
total = total + item;
} else {
std::cerr << "Books entered must have the same ISBNs" << std::endl;
}

Unfortunately, I cannot figure out how to set up a comparison of the bookNos. If I used #include <casset> in the overload + rule, it will immediately exit since I have no way to compare the ISBNs.