class Item_base {
public:
	Item_base(const std::string &book = "",
		double sales_price = 0.0):
		isbn(book), price(sales_price) {}
	std::string book() const
	{
		return isbn;
	}

	virtual double net_price(size_t n) const
	{
		return n * price;
	}
	virtual ~Item_base() {}
private:
	std::string isbn;
protected:
	double price;
};
