（C++入门第5版）练习1.20中的一个问题

Question

我正在阅读 c++ 入门（第 5 版）一书，在练习 1.20 中，问题是使用 Sales_item 类来

编写一个程序，读取一组图书销售交易，编写 每个事务到标准输出。

这是类头文件

/*
 * This file contains code from "C++ Primer, Fifth Edition", by Stanley B.
 * Lippman, Josee Lajoie, and Barbara E. Moo, and is covered under the
 * copyright and warranty notices given in that book:
 *
 * "Copyright (c) 2013 by Objectwrite, Inc., Josee Lajoie, and Barbara E. Moo."
 *
 *
 * "The authors and publisher have taken care in the preparation of this book,
 * but make no expressed or implied warranty of any kind and assume no
 * responsibility for errors or omissions. No liability is assumed for
 * incidental or consequential damages in connection with or arising out of the
 * use of the information or programs contained herein."
 *
 * Permission is granted for this code to be used for educational purposes in
 * association with the book, given proper citation if and when posted or
 * reproduced.Any commercial use of this code requires the explicit written
 * permission of the publisher, Addison-Wesley Professional, a division of
 * Pearson Education, Inc. Send your request for permission, stating clearly
 * what code you would like to use, and in what specific way, to the following
 * address:
 *
 *     Pearson Education, Inc.
 *     Rights and Permissions Department
 *     One Lake Street
 *     Upper Saddle River, NJ  07458
 *     Fax: (201) 236-3290
*/

/* This file defines the Sales_item class used in chapter 1.
 * The code used in this file will be explained in
 * Chapter 7 (Classes) and Chapter 14 (Overloaded Operators)
 * Readers shouldn't try to understand the code in this file
 * until they have read those chapters.
*/

#ifndef SALESITEM_H
// we're here only if SALESITEM_H has not yet been defined 
#define SALESITEM_H

// Definition of Sales_item class and related functions goes here
#include <iostream>
#include <string>

class Sales_item {
    // these declarations are explained section 7.2.1, p. 270 
    // and in chapter 14, pages 557, 558, 561
    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:
    // constructors are explained in section 7.1.4, pages 262 - 265
    // default constructor needed to initialize members of built-in type
    Sales_item() = default;
    Sales_item(const std::string& book) : bookNo(book) { }
    Sales_item(std::istream& is) { is >> *this; }
public:
    // operations on Sales_item objects
    // member binary operator: left-hand operand bound to implicit this pointer
    Sales_item& operator+=(const Sales_item&);

    // operations on Sales_item objects
    std::string isbn() const { return bookNo; }
    double avg_price() const;
    // private members as before
private:
    std::string bookNo;      // implicitly initialized to the empty string
    unsigned units_sold = 0; // explicitly initialized
    double revenue = 0.0;
};

// used in chapter 10
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

这是我的程序：

#include <iostream>
#include "Sales_item.h"
int main() {
    Sales_item book;
    while (std::cin >> book)
    {
        std::cout << book << std::endl;

    }
}

这是错误列表：

我是编程新手，如果这是一个愚蠢的问题，我很抱歉。

Answer 1

错误消息告诉您链接器 (LNK…) 抱怨 std::istream& operator>>(std::istream& in, Sales_item& s)（以及其他提到的）在 boostoreproblem 和 Sales_item 中定义。

原因是定义在 Sales_item 的标头中，它包含在两个编译项中（或者您甚至可以编译 .h 文件本身）。到目前为止，这些函数的定义在两个编译单元中。

要解决该问题，您需要在 Sales_item 标头 inline 中进行这些定义（就像您为 inline bool operator!=(const Sales_item& lhs, const Sales_item& rhs) 所做的那样，或者仅在标头中声明它们并将定义移动到专用的编译单元（一个自己的 cpp文件）。