练习7.41:

#include <iostream>
#include <string>
using namespace std;class Sales_data {friend std::istream &read(std::istream &is, Sales_data &item);friend std::ostream &print(std::ostream &os, const Sales_data &item);public:Sales_data(const string &book, unsigned num, double sellp, double salep): bookNo(book),units_sold(num), sellingprice(sellp), saleprice(salep) {if (sellingprice)discount = saleprice / sellingprice;cout << "该构造函数接受书号，销售量，原价，实际销售四个信息" << endl;}Sales_data(): Sales_data("", 0, 0, 0) {cout << "该构造函数无须接受任何信息" << endl;}Sales_data(const string &book): Sales_data(book, 0, 0, 0) {cout << "该构造函数接受书号信息" << endl;}Sales_data(std::istream &is): Sales_data() {read(is, *this);cout << "该构造函数接受用户输入的信息" << endl;}private:std::string bookNo;unsigned units_sold = 0;double sellingprice = 0.0;double saleprice = 0.0;double discount = 0.0;
};std::istream &read(std::istream &is, Sales_data &item) {is >> item.bookNo >> item.units_sold >> item.sellingprice >> item.saleprice;return is;
}std::ostream &print(std::ostream &os, const Sales_data &item) {os << item.bookNo << " " << item.units_sold << " " << item.sellingprice << " " << item.saleprice <<" " << item.discount;return os;
}int main() {Sales_data fist("978-7-121-15535-2", 85, 128, 109);Sales_data second;Sales_data third("978-7-121-15535-2");Sales_data last(cin);return 0;
}

练习7.42:

class Book {private:string Name, ISBN, Author, Publisher;double Price = 0;public:
//		Book() = default;Book(const string &n, const string &I, double pr, const string &a, const string &p) {Name = n;ISBN = I;Price = pr;Author = a;Publisher =  p;}Book(): Book("", "", 0, "", "") {}Book(std::istream &is): Book() {is >> *this;}
};