Online Bookstore Management System

#include<iostream>
using namespace std;
class book
{
  protected:
  string title;
  string author;
  float price;
  string isbn;
  public:
   Book(string title, string author, float price, string isbn)
        : title(title), author(author), price(price), isbn(isbn) {}
  virtual ~Book() {}
    string getTitle() 
    { 
      return title; 
    }
    string getAuthor() 
    { 
      return author; 
    }
    float getPrice() 
    { 
      return price; 
    }
    string getISBN() 
    { 
      return isbn; 
    }

    void setTitle(string title) 
    { 
      title = title; 
    }
    void setAuthor(string author) 
    { 
      author = author; 
    }
    void setPrice(float price) 
    { 
      price = price; 
    }
    void setISBN(string isbn) 
    { 
      isbn = isbn; 
    }
  virtual void displayInfo()
  {
        cout << "Title: " << title << endl;
        cout << "Author: " << author << endl;
        cout << "Price: $" << price << endl;
        cout << "ISBN: " << isbn << endl;
  }
  friend float calculateDiscount(book);
};
class eBook : public Book {
private:
    string format;
    float size; 
public:
    eBook(string title, string author, float price, string isbn,
           string format, float size)
        : book(title, author, price, isbn), format(format), size(size) {}

    
    virtual void displayInfo() 
  {
        book::displayInfo(); 
        cout << "Format: " << format << endl;
        cout << "Size: " << size << " MB" << endl;
    }
};
float calculatediscount(book b)
{
        int bk=b.price;
        if(500<bk<1000)
            {
                cout<<"Discount :"<< bk-(bk*0.3)<<endl;
            }
        else if(bk>1500)
           {
                cout<<"Discount :"<< bk-(bk*0.5)<<endl;
           }
        else
           {
                cout<<"No Discount"<<endl;
    }
    return 0;
}