Sales_data

#include <iostream>
#define NUS
#ifndef NUSE
using namespace std;

class Sales_data {
    friend istream &read(istream &is, Sales_data &item);
    friend ostream &print(ostream &os, const Sales_data &item);
    friend Sales_data add(const Sales_data &lhs, const Sales_data &rhs);
public:
    Sales_data(const string &bn, const unsigned unSold, const double p):
        bookNo(bn), units_sold(unSold), revenue(p*unSold) { cout <<"3 arg ctor" << endl; }
    Sales_data(): Sales_data("", 0, 0) { cout <<"Empty ctor" << endl; }
    Sales_data(string s): Sales_data(s, 0, 0) { cout <<"String ctor" << endl; }
    Sales_data(istream &is): Sales_data() {
        read(is, *this);
        cout <<"Stream ctor" << endl;
    }

    string isbn() const {
        return bookNo;
    }
    Sales_data& combine(const Sales_data&);
   // Sales_data(istream &is);
private:
    inline double avg_price() const {
        return units_sold ? revenue/units_sold : 0;
    }
    string bookNo;
    unsigned units_sold = 0;
    double revenue = 0.0;
};

istream &read(istream &is, Sales_data &item);
ostream &print(ostream &os, const Sales_data &item);
Sales_data add(const Sales_data &lhs, const Sales_data &rhs);
void total(const Sales_data &item);

istream &read(istream &is, Sales_data &item) {
    double price = 0;
    is >> item.bookNo >> item.units_sold >> price;
    item.revenue = price * item.units_sold;
    return is;
}

ostream &print(ostream &os, const Sales_data &item) {
    os << item.isbn() <<" " << item.units_sold <<" " << item.revenue <<" " << item.avg_price();
    return os;
}

Sales_data add(const Sales_data &lhs, const Sales_data &rhs) {
    Sales_data sum = lhs;
    sum.combine(rhs);
    return sum;
}
Sales_data& Sales_data::combine(const Sales_data &item) {
        this->revenue += item.revenue;
        this->units_sold += item.units_sold;
        return *this;
}

void total(const Sales_data &item) {
    Sales_data total;
    if(read(cin, total)) {
        Sales_data trans;
    while(read(cin, trans)) {
        if(total.isbn() == trans.isbn())
            total.combine(trans);
        else {
            print(cout, total) << endl;
            total = trans;
        }
    }
    print(cout, total) << endl;
    }
    else
        cerr <<"No data?" << endl;
}
int main() {
    Sales_data c;
    return 0;
}
#endif