7.1.4 构造函数

练习7.11
头文件：

#ifndef SALES_DATA_H
#define SALES_DATA_H
#include <string>
using namespace std;

struct Sales_data {

	Sales_data() = default;
	Sales_data(const string &s):bookNo(s){}
	Sales_data(const string &s, unsigned n, double p):bookNo(s),units_sold(n),revenue(p*n){}
	Sales_data(std::istream &);

	//定义isbn成员函数
	std::string isbn() const { return bookNo; }

	//声明combine成员函数
	Sales_data& combine(const Sales_data&);

	//声明avg_price成员函数
	double avg_price() const;

	//其他数据成员
	std::string bookNo;
	unsigned units_sold = 0;
	double revenue = 0;
};
istream& read(istream& is, Sales_data& item);
ostream& print(ostream& os, const Sales_data& item);
Sales_data add(const Sales_data& lhs, const Sales_data& rhs);

#endif

保存函数定义的源文件：

#include <iostream>
#include <vector>
#include <string>
#include "Sales_data.h"
using namespace std;

double Sales_data::avg_price() const {
    if (units_sold)
        return revenue / units_sold;
    else
        return 0;
}

Sales_data& Sales_data::combine(const Sales_data& rhs)
{
    units_sold += rhs.units_sold;
    revenue += rhs.revenue;
    return *this;

}

istream& read(istream& is, Sales_data& item) {
    double price = 0;
    is >> item.bookNo >> item.units_sold >> price;
    item.revenue = price * item.units_sold;
    return is;
}

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

Sales_data add(const Sales_data& lhs, const Sales_data& rhs)
{
    Sales_data sum = lhs;
    sum.combine(rhs);
    return sum;
}

Sales_data::Sales_data(istream& is) {
    read(is, *this);
}

调用构造函数的源文件：

int main() {

    Sales_data a;
    print(cout,a) << endl; //空字符串 0 0 0

    Sales_data b("PORT", 4, 6);
    print(cout, b) << endl; //PORT 4 24 6

    Sales_data c(cin);
    print(cout, c);
}

练习7.12

struct Sales_data 
{
	Sales_data(istream& is) 
	{ read(is, *this); }
}

练习7.13

#include <iostream>
#include <vector>
#include <string>
#include "Sales_data.h"
using namespace std;

int main() {

    Sales_data total(cin);//保存下一条交易记录的变量
   
    //读入第一条交易记录，并确保有数据可以处理
    if (cin)
    {
        Sales_data trans(cin);
        do {
            //如果我们在处理相同的书
            if (total.isbn() == trans.isbn()) {
                total.combine(trans);
            }
            else {
                //打印前一笔交易的次数
                print(cout, total) << endl;
                total = trans;
           }
        } while (read(cin, trans));
        print(cout, total) << endl;
    }
    else {
        //没有输入
        cerr << "No data?" << endl;
        return -1;
    }
    return 0;

    Sales_data a;
    print(cout,a) << endl; //空字符串 0 0 0

    Sales_data b("PORT", 4, 6);
    print(cout, b) << endl; //PORT 4 24 6

    Sales_data c(cin);
    print(cout, c);
}