C++ Primer_1 书店程序

C++ Primer_1 书店程序

一个简单的书单程序，能够实现书本的编码isbn、销售册数、单价的输入；
输出：如果两次输入的isbn相同，则输出isbn、累加销售册数、销售总额和平均单价；
不同则输出上一次输入的书本销售相关信息。

Sales_item.h （新手可跳过这一段代码）

//条件编译，防止重命名
#ifndef SALESITEM_H

#define SALESITEM_H

#include 
#include 

//using namespace std;

class Sales_item {
// 7.2.1, p270 
// 第14章, p557, 558, 561
//重载运算符>> / << / == / <
friend std::istream& operator>>(std::istream&, Sales_item&);
friend std::ostream& operator<<(std::ostream&, const Sales_item&);
friend bool operator<(const Sales_item&, const Sales_item&);
friend bool 
operator==(const Sales_item&, const Sales_item&);
public:
    // 7.1.4, p262 - 265
#if defined(IN_CLASS_INITS) && defined(DEFAULT_FCNS)
    Sales_item() = default;
#else
    Sales_item(): units_sold(0), revenue(0.0) { }
#endif
    Sales_item(const std::string &book):
              bookNo(book), units_sold(0), revenue(0.0) { }
    Sales_item(std::istream &is) { is >> *this; }
public:
    Sales_item& operator+=(const Sales_item&);
    std::string isbn() const { return bookNo; }
    double avg_price() const;

private:
    std::string bookNo;      // implicitly initialized to the empty string
#ifdef IN_CLASS_INITS
    unsigned units_sold = 0; // explicitly initialized
    double revenue = 0.0;
#else
    unsigned units_sold;  
    double revenue;       
#endif
};

inline long lround(double d)
{
	//实现小数的四舍五入
	return (d >= 0) ? long(d + 0.5) : long(d - 0.5);
}


inline
bool compareIsbn(const Sales_item &lhs, const Sales_item &rhs) 
{ return lhs.isbn() == rhs.isbn(); }

// nonmember binary operator: must declare a parameter for each operand
Sales_item operator+(const Sales_item&, const Sales_item&);

inline bool 
operator==(const Sales_item &lhs, const Sales_item &rhs)
{
    // must be made a friend of Sales_item
    return lhs.units_sold == rhs.units_sold &&
           lhs.revenue == rhs.revenue &&
           lhs.isbn() == rhs.isbn();
}

inline bool 
operator!=(const Sales_item &lhs, const Sales_item &rhs)
{
    return !(lhs == rhs); // != defined in terms of operator==
}

// assumes that both objects refer to the same ISBN
Sales_item& Sales_item::operator+=(const Sales_item& rhs) 
{
    units_sold += rhs.units_sold; 
    revenue += rhs.revenue; 
    return *this;
}

// assumes that both objects refer to the same ISBN
Sales_item 
operator+(const Sales_item& lhs, const Sales_item& rhs) 
{
    Sales_item ret(lhs);  // copy (|lhs|) into a local object that we'll return
    ret += rhs;           // add in the contents of (|rhs|) 
    return ret;           // return (|ret|) by value
}

std::istream& 
operator>>(std::istream& in, Sales_item& s)
{
    double price;
    in >> s.bookNo >> s.units_sold >> price;
    // check that the inputs succeeded
    if (in)
        s.revenue = s.units_sold * price;
    else 
        s = Sales_item();  // input failed: reset object to default state
    return in;
}

std::ostream& 
operator<<(std::ostream& out, const Sales_item& s)
{
    out << s.isbn() << " " << s.units_sold << " "
        << s.revenue << " " << s.avg_price();
    return out;
}

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

Book_sales.cpp（这一段是本章重点）

#include 
#include "Sales_item.h"

//using namespace std;
//防止命名空间名称·冲突
using std::cin;
using std::cout;
using std::endl;

int main()
{
	Sales_item total;

	if (cin >> total)
	{
		Sales_item trans;
		while (cin >> trans)
		{
			if (total.isbn() == trans.isbn()) 
			{
				total += trans;
			}
			else
			{
				cout << total << endl;
				total = trans;
			}
		}
		cout << total << endl;
	}
	else {
		//发出警告
		std::cerr << "No data!" << endl;
		return -1;
	}
	return 0;
}

主要是关于std命名空间的使用细节，为防止变量名称冲突，尽量少用using namespace std;