练习14.6:

class Sales_data
{friend ostream&operator<<(sotream&os,const Salse_data &item);
};ostream&operator<<(ostream &os,const Sales_data &item)
{const char *sep = ' ';os<<item.isbn()<<sep<<item.units_sold<<sep<<item.revenue<<sep<<item.avg_price();return os;
}

练习14.7:

class String {public:string();string(const char *str);friend ostream &operator<<(ostream &os, const String &str)private:char *str;
};ostream &operator<<(ostream &os, const String &str) {os << str;return os;
}

练习14.8:
略