#include<iostream>
#include<vector>

using namespace std;
class Book;

class Visitor
{
    public:
        virtual void visit(Book* book) = 0;
        //其他物品的visit方法
};

class Item
{
    public:
        virtual void accept(Visitor* visitor) = 0;

        virtual double getPrice() = 0;
};

class Book : public Item  //(1)
{
    private:
        double price;

    public:
        Book(double price)
        {
            //访问本元素
            this->price = price; //(2)
        }

        void accept(Visitor* visitor)
        {
            visitor->visit(this); //(3)
        }
        double getPrice()
        {
            return price;
        }
};

class Cashier : public Visitor  //(4)
{
    private:
        double totalForCart;
    
    public:
        //访问BooK类型对象的价格并累加
        void visit(Book* book)    //(5)
        {
            if(book->getPrice()<10.0) //假设Book类型的物品价格超过10元打8折
            {
                totalForCart += book->getPrice();
            }
            else
            {
                totalForCart += book->getPrice() * 0.8;
            }
        }

        double getTotal()
        {
            return totalForCart;
        }
};


class ShoppingCart
{
    private:
        vector<Item*> items;

    public:
        double calculatePrice()
        {
            Cashier* visitor = new Cashier();
            for(int i=0;i<items.size();i++)
            {
                items.at(i)->accept(visitor);  //(6)
            }
            double total = visitor->getTotal();
            return total;
        }

        void add(Item* e)
        {
            items.push_back(e);
        }
};

int main()
{
    return 0;
}

答案：
(1) : public Item

(2) this->price = price

(3) visitor->visit(this); //易错

(4) : public Visitor

(5) void visit(Book* book)

(6) items.at(i)->accept(visitor);