这是 C++ Primer 4th Edition 的一个问题。我正在使用这个版本，因为它是我唯一可以访问的版本。
问题：
该网站 ( http://www.awprofessional.com/cpp_primer ) 在第 1 章代码目录中包含一份 Sales_item.h 的副本。将该文件复制到您的工作目录。编写一个程序，循环遍历一组图书销售交易，读取每笔交易并将该交易写入标准输出。
它提供的链接不起作用，所以我从另一个网站下载了 Sales_item.h。
我在底部粘贴了头文件的内容。
Proposed Solution
#include <iostream>
#include "Sales_item.h"
using namespace std;
int main()
{
Sales_item item1;
cout<<"Enter sales item one"<<endl;
cin>>item1;
cout<<"Item one is " <<item1;
return 0;
}
好吧，我可能不明白这个模棱两可的问题，但这不是问题所在。当我用命令编译文件时：
g++ salesitem1.cpp -o salesitem1
我得到这个错误
In file included from salesitem1.cpp:2:0:
c:\mingw\bin\../lib/gcc/mingw32/4.6.2/../../../../include/Sales_item.h:70:27: er
ror: ISO C++ forbids initialization of member 'units_sold' [-fpermissive]
c:\mingw\bin\../lib/gcc/mingw32/4.6.2/../../../../include/Sales_item.h:70:27: er
ror: making 'units_sold' static [-fpermissive]
c:\mingw\bin\../lib/gcc/mingw32/4.6.2/../../../../include/Sales_item.h:70:27: er
ror: ISO C++ forbids in-class initialization of non-const static member 'units_s
old'
c:\mingw\bin\../lib/gcc/mingw32/4.6.2/../../../../include/Sales_item.h:71:22: er
ror: ISO C++ forbids initialization of member 'revenue' [-fpermissive]
c:\mingw\bin\../lib/gcc/mingw32/4.6.2/../../../../include/Sales_item.h:71:22: er
ror: making 'revenue' static [-fpermissive]
c:\mingw\bin\../lib/gcc/mingw32/4.6.2/../../../../include/Sales_item.h:71:22: er
ror: ISO C++ forbids in-class initialization of non-const static member 'revenue
'
所以我的问题是：为什么会发生这种情况？这本书有问题吗？如果有，我该如何继续练习？在旁注中，我尝试编译书中的示例并收到类似的错误消息。
感谢您耐心阅读此问题。任何好的答案将不胜感激。
Sales_item.h 源代码
/*
* 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