Sales_data.h

#ifndef SALES_DATA_H
#define SALES_DATA_H

#include <iostream>
#include<string>

struct Sales_data;

Sales_data operator+(const Sales_data & sa, const Sales_data & sb);

std::istream & operator>>(std::istream & is, Sales_data & sa); 

std::ostream & operator<<(std::ostream & os, const Sales_data & sa);

bool operator==(const Sales_data &lhs, const Sales_data &rhs); 
bool operator!=(const Sales_data &lhs, const Sales_data &rhs); 

class Sales_data
{
	
friend Sales_data operator+(const Sales_data & sa, const Sales_data & sb);
friend std::istream & operator>>(std::istream & is, Sales_data & sa); 
friend std::ostream & operator<<(std::ostream & os, const Sales_data & sa);
friend bool operator==(const Sales_data &lhs, const Sales_data &rhs);
public:
	
	Sales_data() = default;
	Sales_data(const std::string &s) : ISBN(s) { }
	Sales_data(const std::string &s, int a, double t) : ISBN(s), amount(a), totalPrice(t * a) { } 
	Sales_data(std::istream &is) { is >> *this; }
	
	std::string isbn() const { return ISBN; }
	Sales_data & operator+=(const Sales_data & sa);
	Sales_data & operator=(const std::string & s);
	
	explicit operator double() const { return totalPrice; }
	explicit operator std::string() const { return isbn(); }
	
private:
	
	std::string ISBN;
	double totalPrice = 0.0;
	int amount = 0;
	
	inline double avg_price() const; 
};

#endif