class exercise

e.cpp

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

int main()
{
    Book b1("lippman", "c++primer", "blahblah", "2211");

    display(b1);

    b1.modify_author("kryssel").modify_title("blah blah").modify_date("nov 28 1998").modify_isbn("2220-222");

    display(b1);


    return 0;
}

e.h

#ifndef EXERCISE_H
#define EXERCISE_H

class Book
{
    public:

    friend void display (Book &);

        Book (std::string a,std::string i,std::string d, std::string t) :
            author (a), isbn (i) , date (d), title (t) { }

    Book &modify_author (const std::string &);
    Book &modify_isbn   (const std::string &);
    Book &modify_date   (const std::string &);
    Book &modify_title  (const std::string &);

    private:

    std::string author, isbn, date, title;

};

void display (Book &o)
{
    std::cout << o.author << std::endl
              << o.isbn << std::endl
              << o.date << std::endl
              << o.title << std::endl;
}

Book &Book::modify_author (const std::string &p)
{
    this->author = p;
    return *this;
}

Book &Book::modify_isbn (const std::string &p)
{
    this->isbn = p;
    return *this;
}

Book &Book::modify_date (const std::string &p)
{
    this->date = p;
    return *this;
}

Book &Book::modify_title (const std::string &p)
{
    this->title = p;
    return *this;
}

#endif // EXERCISE_H