学习C++。使用入门书

Question

我有练习1.20

它说下载这个头文件Sales_item.h

/*********************************************************** 
* filename: sales_item.h 
**********************************************************/ 
#ifndef __SALES_ITEM_H__ 
#define __SALES_ITEM_H__ 

#include <iostream> 
#include <string> 
#include <stdexcept> 
//using std::istream; using std::ostream; 

class Sales_item 
{ 
    /* operations on sales_item object */ 
public: 

    double avg_price(void) const; 

    bool same_isbn(const Sales_item &rhs) const /* inline function */ 
    { 
     return isbn == rhs.isbn; /* return this->isbn == rhs.isbn*/ 
    } 

    std::istream &read_item(std::istream &in); 
    std::ostream &write_item(std::ostream &out) const; 

    //  bool add_item(const sales_item &rhs); 
    Sales_item add_item(const Sales_item &other); 
    /* default constructor --- constructor without parameters  */ 
    /* default constructor needed to initialize members of built-in type */ 
    Sales_item(): units_sold(0), revenue(0.0) {} 

    /* private members as before  */ 
private: 
    std::string isbn;  /* ISBN     */ 
    unsigned units_sold;  /* the number of book sold  */ 
    double revenue;   /* the total revenue from that book */ 
}; 

#if 0 
inline 
double sales_item::avg_price(void) const 
{ 
    if (units_sold) 
     return (revenue/units_sold); /* return (this->revenue/this->units_sold); */ 

    else 
     return 0; 
} 
#endif 

inline 
double Sales_item::avg_price(void) const 
{ 
    using std::runtime_error; 

    try 
    { 
     if (units_sold == 0) 
      throw runtime_error("when units_sold equal 0, error..."); 

     return (revenue/units_sold); /* return (this->revenue/this->units_sold); */ 
    } 

    catch (runtime_error err) 
    { 
     std::cout << err.what() << std::endl; 
     return 0; 
    } 
} 

#endif /* __SALES_ITEM_H__ */ 

，然后用这个代码中使用了一个问题：

#include <iostream> 
#include "Sales_item.h" 

int main(int argc, const char * argv[]) 
{ 
    Sales_item book; 
    int gne; 
    // read ISBN, number of copies sold, and sales price 
    std::cin >> book; 
    // write ISBN, number of copies sold, total revenue, and average price 
    std::cout << book << std::endl; 
    return 0; 
} 

我在std :: cin和std :: cout行收到错误。两个错误是：

无效的操作数的二进制表达式（ '的IStream'（又名 'basic_istream'）和 '的Sales_item'）

我使用的Xcode 5.0。 它可能是什么？

Answer 1

您的课程不会超载operator <<和operator >>因此您无法将其与cout <<和cin >>配合使用。你的类确实有

std::istream &read_item(std::istream &in); 
std::ostream &write_item(std::ostream &out) const; 

，您可以像使用

Sales_item book; 
int gne; 
// read ISBN, number of copies sold, and sales price 
book.read_item(std::cin); 
// write ISBN, number of copies sold, total revenue, and average price 
book.write_item(std::cout); 

Answer 2

没有运营商< <并没有运营商>>您Sales_item类型定义，所以编译器不知道该怎么办与陈述“cin >>书”和“cout < <书”。 您必须为basic_istream和basic_ostream以及Sales_item编写运算符重载函数。

对于您的情况：

std::ostream& operator<<(std::ostream& os, const Sales_item& item) 
{ 
    return item.write_item(os); 
} 

std::istream& operator>>(std::istream& is, Sales_item& item) 
{ 
    return item.read_item(is); 
}