求大神，帮忙分析下程序问题的原因

#include <iostream>
#include <string>
#include <vector>
#include <cctype>
#include <stdexcept>
using namespace std;
using std::string;
using std::vector;

struct Sales_data {
    Sales_data& combine(const Sales_data&);
    string isbn() const { return bookNo; }
    double avg_price() const;
     
    string bookNo;
    unsigned units_sold = 0;
    double revenue = 0.0;
};
Sales_data add(const Sales_data&, const Sales_data&);
ostream &print(ostream, const Sales_data&);
istream &read(istream, Sales_data&);

int main()
{
    Sales_data total;
   
    if(read(cin, total))
    {
        Sales_data trans;
        while(read(cin, trans)
        {
            if(total.isbn() == trans.isbn())
            {
                (trans);
            }
            else
            {
                print(cout, total) << endl;
                cout << '\n';
                total = trans;
            }
        }
        print(cout, total) << endl;
        
    }
    else
    {
        cerr << "No data?!" << endl;
        return -1;
    }
    return 0;
}

Sales_data& Sales_data::combine(const Sales_data &rhs)
{
    units_sold += rhs.units_sold;
    revenue += rhs.revenue;
    return *this;
}
double Sales_data::avg_price() const
{
    if(units_sold)
        return revenue/units_sold;
    else
        return 0;
}
Sales_data add(const Sales_data &lhs, const Sales_data &rhs)
{
    Sales_data sum = lhs;
    (rhs);
    return sum;
}
ostream &print(ostream &os, const Sales_data &item)
{
    os << item.isbn() << " " << item.units_sold << " " << item.revenue << " " << item.avg_price() << endl;
    return os;
}
istream &read(istream &is, Sales_data &item)
{
    double price = 0;
    is >> item.bookNo >> item.units_sold >> price;
    item.revenue = item.units_sold * price;
    return is;
}