struct sales_data
{
	sales_data() = default;
	sales_data(string&s) :isbn(s){}
	sales_data(string&s, double&b, int&c) :isbn(s), price(b), num(c) { revenue1 = price * num; }
	string isbn = "";
	double price = 0;
	int num = 0;
	double revenue1;
	sales_data&operator+=(const sales_data&rhs)
	{
		//cout<<price<<rhs.price;
		cout << "its revenue1= " << revenue1 << "rhs revenue1= " << rhs.revenue1 << endl;
		if (isbn != rhs.isbn)
			throw isbn_mismatch("wrong isbns", isbn, rhs.isbn);
		num += rhs.num;
		revenue1 += rhs.revenue1;
		return *this;
	}
};