single instance

单例模式是什么

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示例题目的代码

cpp:

# include <iostream>
# include <map>

using namespace std;

class shopping_cart_manager
{
private:
    map<string, int> cart;

    shopping_cart_manager() {};

public:
    static shopping_cart_manager& get_instance()
    {
        static shopping_cart_manager instance;
        return instance;
    }

    void add_to_cart(const string& item_name, int item_num)
    {
        cart[item_name] += item_num;
    }

    void display_cart() const
    {
        for (const auto& item : cart)
        {
            cout << item.first << " " << item.second << endl;
        }
    }
};

int main()
{
    string item_name;
    int item_num;

    shopping_cart_manager& cart = shopping_cart_manager::get_instance();
    
    while (cin >> item_name >> item_num)
    {
        cart.add_to_cart(item_name, item_num);
    }

    cart.display_cart();

    return 0;
}