习题14-21 Sales_data& Sales_data::operator+=(const Sales_data &rhs) { Sales_data temp = *this; *this = temp + rhs; return *this; } Sales_data& operator+ (const Sales_data& lhs, const Sales_data& rhs) { Sales_data ret(lhs); lhs.units_sold += rhs.units_sold; lhs.revenue += rhs.revenue;