C++ Primer doubt

[jub4jive]

Hi, I am a beginner c++ programmer. I just started c++ primer and am working on the exercises in the first chapter. I am stuck on the first exercise problem related to classes. Here is the question:
Exercise 1.20: http://www.informit.com/title/0321714113 contains a copy of Sales_item.h in the Chapter 1 code directory. Copy that file to your working directory. Use it to write a program that reads a set of book sales transactions, writing each transaction to the standard output.

I went to the site but didn't find it, so I googled it. But, I don't know what a 'working directory' is. I do have the code, it's 3 pages long. I don't understand how I am supposed to use that to write the program as well. Here's the sales_item.h file

/*
 * 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: [phone]
*/ 

/* 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  

Any help would be appreciated. I'm stuck on this for a while now. Please don't tell me the answer. Please tell me what to do, giving an outline of how I'm supposed to do it.
Thanks in advance.

[Yanson]

Do yourself a favor and get a different book. C++ primer is a good book, but I don't recommend it for a beginner. Try the tutorials on this site.


The webpage that you linked to does have the source code download. Scroll to the bottom and click the button that says download. Its next to the description of the book.

How you would go about including/using the Sales_item class depends on the IDE you are using. You didn't provide that information so I can't give you any details.

[JLBorges]

> I don't know what a 'working directory'

For this purpose, the 'working directory' is the directory which contains your program.

"Copy that file to your working directory" means: copy that file to the same directory that will contain your main.cpp.

Then include the header in main.cpp.

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

int main()
{
      Sales_item item ;
      
      // take it up from there
}

[jub4jive]

@Yanson:
c++ primer has good ratings and is recommended for beginners. I have gone through some basic python books and a few c++ books, but I had given up. I really don't want to switch to another book now. (It's the 11th programming book I've picked up).

I use the gnu compiler.

@JLBorges:

Do I simply save the code as sales_item.h in the directory; and then just include the header :
#include <Sales_item.h> in the program?

Thanks for the help. I'll try it out and let you guys know.

[dhayden]

Do I simply save the code as sales_item.h in the directory; and then just include the header :
#include <Sales_item.h> in the program?

Yes.

Your program should read book transactions from cin and write them to cout.

There is one very odd thing about this class: The << and >> operators are not symmetrical. In other words, if you write a Sales_item with << then you can't read it with >>