14.2.1

14.6

friend std::ostream& operator<<(std::ostream&, const Sales_data&);

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

14.7

friend std::ostream& operator<<(std::ostream&, const String&);

std::ostream& operator<<(std::ostream &os, const String &s){    char *c = const_cast<char*>(s.c_str());    while (*c)        os << *c++;    return os;}

14.8

std::ostream& operator<<(std::ostream &out, const Book &book){    out << book.no_ << " " << book.name_ << " " << book.author_ << " " << book.pubdate_;    return out;}