Sales_item.h不是C++自带的，需要自己安装，这个库作者已经写好了,
Sales_item.h 头文件代码如下：
#ifndef SALESITEM_H
#define SALESITEM_H
#include <iostream>
#include <string>class Sales_item{
public:Sales_item(const std::string &book):isbn(book),units_sold(0),revenue(0.0){}Sales_item(std::istream &is){ is >> *this;}friend std::istream& operator>>(std::istream &,Sales_item &);friend std::ostream& operator<<(std::ostream &,const Sales_item &);
public:Sales_item & operator+=(const Sales_item&);
public:double avg_price() const;bool same_isbn(const Sales_item &rhs)const{return isbn == rhs.isbn;}Sales_item():units_sold(0),revenue(0.0){}
public:std::string isbn;unsigned units_sold;double revenue;
};using std::istream;
using std::ostream;
Sales_item operator+(const Sales_item &,const Sales_item &);inline bool operator==(const Sales_item &lhs,const Sales_item &rhs){return lhs.units_sold == rhs.units_sold && lhs.revenue == rhs.revenue && lhs.same_isbn(rhs);
}inline bool operator!=(const Sales_item &lhs,const Sales_item &rhs){return !(lhs == rhs);
}inline Sales_item & Sales_item::operator +=(const Sales_item &rhs){units_sold += rhs.units_sold;revenue += rhs.revenue;return *this;
}inline Sales_item operator+(const Sales_item &lhs,const Sales_item &rhs){Sales_item ret(lhs);ret += rhs;return ret;
}inline istream& operator>>(istream &in,Sales_item &s){double price;in >> s.isbn >> s.units_sold >> price;if(in)s.revenue = s.units_sold * price;elses = Sales_item();return in;
}inline ostream& operator<<(ostream &out,const Sales_item &s){out << s.isbn << "\t" <<s.units_sold << "\t" << s.revenue << "\t" << s.avg_price();return out;
}inline double Sales_item::avg_price() const{if(units_sold)return revenue/units_sold;elsereturn 0;
}
#endif
注意，Sales_item.h的储存路径要和源代码的储存路径一致。这样，源代码的头文件才可以访问这个类。
如何安装头文件？
- DevC++如何安装自定义头文件并使用
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