单例模式：c++代码懒加载无法通过

[Cupcc]

确保多线程下输出顺序和输入顺序保持一致，因为输入和输出并不同时进行，所以不用加锁。进行记录输入顺序。

#include <iostream>
#include <string>
#include <unordered_map>
#include <vector>

class ShoppingCartManager {
private:
    // Stores the order of item insertion
    std::vector<std::string> order;

    // Maps item names to their quantities
    std::unordered_map<std::string, int> cart;

    // Private constructor for Singleton
    ShoppingCartManager() {}

    // Deleted copy constructor and assignment operator for Singleton
    ShoppingCartManager(const ShoppingCartManager&) = delete;
    ShoppingCartManager& operator=(const ShoppingCartManager&) = delete;

public:
    // Method to get the Singleton instance
    static ShoppingCartManager& getInstance() {
        static ShoppingCartManager instance;
        return instance;
    }

    // Method to add items to the cart
    void addToCart(const std::string& itemName, int quantity) {
       if (cart.find(itemName) == cart.end()){
           order.emplace_back(itemName);
       }
       cart[itemName] += quantity; 
    }

    // Method to view the cart
    void viewCart() {
        for (const auto& itemName : order) {
            std::cout << itemName << " " << cart[itemName] << std::endl;
        }
    }
};

int main() {
    ShoppingCartManager& cart = ShoppingCartManager::getInstance();
    std::string itemName;
    int quantity;

    // Reading items until end-of-file or error
    while (std::cin >> itemName >> quantity) {
        cart.addToCart(itemName, quantity);
    }

    // Output cart contents
    cart.viewCart();

    return 0;
}