Question

我对练习有问题1.20

它说下载此头文件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__ */

然后将其与此代码一起使用：

#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。这两个错误都是：

二进制表达式的无效操作数（'istream'（又名'basic_istream'）和'Sales_item'）

我正在使用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 >>一起使用。你的班级确实有

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

你可以使用

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＆lt;＆lt;并且没有运算符＆gt;＆gt;为您的Sales_item类型定义，因此编译器不知道如何处理语句＆＃34; cin＆gt;＆gt;书＆＃34;和＆＃34; cout＆lt;＆lt;本书＆＃34 ;. 您必须为basic_istream和basic_ostream以及Sales_item编写运算符重载函数。

对于你的情况：

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);
}

学习C ++。使用Primer书

2 个答案: