c++的类系芥么顶一地，卧槽
摘自c＋＋primer
#ifndef SALESITEM_H#define SALESITEM_H// Definition of Sales_item class and related functions goes here#include <iostream>#include <string>class Sales_item {friend bool operator==(const Sales_item&, const Sales_item&);// other members as beforepublic: // added constructors to initialize from a string or an istream 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: // operations on Sales_item objects // member binary operator: left-hand operand bound to implicit this pointer Sales_item& operator+=(const Sales_item&); // other members as before public: // operations on Sales_item objects double avg_price() const; bool same_isbn(const Sales_item &rhs) const { return isbn == rhs.isbn; } // default constructor needed to initialize members of built-in type Sales_item(): units_sold(0), revenue(0.0) { }// private members as beforeprivate: std::string isbn; unsigned units_sold; double revenue;};// nonmember binary operator: must declare a parameter for each operandSales_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.same_isbn(rhs);}inline bool operator!=(const Sales_item &lhs, const Sales_item &rhs){ return !(lhs == rhs); // != defined in terms of operator==}using std::istream; using std::ostream;// assumes that both objects refer to the same isbninlineSales_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 isbninlineSales_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}inlineistream& operator>>(istream& in, Sales_item& s){ double price; in >> s.isbn >> 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;}inlineostream& operator<<(ostream& out, const Sales_item& s){ out << s.isbn << "\t" << s.units_sold << "\t" << s.revenue << "\t" << s.avg_price(); return out;}inlinedouble Sales_item::avg_price() const{ if (units_sold) return revenue/units_sold; else return 0;}#endif