写一个C++饭卡管理系统，要求面向对象，数据成员私有，文件输入输出，循环显示菜单

以下是一个简单的饭卡管理系统，使用 C++ 实现，要求面向对象，数据成员私有，文件输入输出，循环显示菜单。

#include <iostream>
#include <fstream>
#include <vector>
#include <string>

using namespace std;

class Card {
private:
    int id;
    string name;
    double balance;
public:
    Card(int id, string name, double balance) : id(id), name(name), balance(balance) {}
    int getId() const { return id; }
    string getName() const { return name; }
    double getBalance() const { return balance; }
    void setBalance(double new_balance) { balance = new_balance; }
};

class CardManager {
private:
    vector<Card> cards;
public:
    void loadFromFile(string filename) {
        ifstream infile(filename);
        int id;
        string name;
        double balance;
        while (infile >> id >> name >> balance) {
            cards.push_back(Card(id, name, balance));
        }
        infile.close();
    }
    void saveToFile(string filename) {
        ofstream outfile(filename);
        for (auto& card : cards) {
            outfile << card.getId() << " " << card.getName() << " " << card.getBalance() << endl;
        }
        outfile.close();
    }
    void addCard(Card card) {
        cards.push_back(card);
    }
    void removeCard(int id) {
        for (auto it = cards.begin(); it != cards.end(); ++it) {
            if (it->getId() == id) {
                cards.erase(it);
                break;
            }
        }
    }
    Card* findCard(int id) {
        for (auto& card : cards) {
            if (card.getId() == id) {
                return &card;
            }
        }
        return nullptr;
    }
    void showMenu() {
        int choice;
        do {
            cout << "1. Add card" << endl;
            cout << "2. Remove card" << endl;
            cout << "3. Check balance" << endl;
            cout << "4. Add balance" << endl;
            cout << "5. Exit" << endl;
            cout << "Enter your choice: ";
            cin >> choice;
            switch (choice) {
            case 1:
                addCard(getNewCard());
                break;
            case 2:
                removeCard(getCardId());
                break;
            case 3:
                checkBalance(getCardId());
                break;
            case 4:
                addBalance(getCardId());
                break;
            case 5:
                break;
            default:
                cout << "Invalid choice" << endl;
            }
        } while (choice != 5);
    }
private:
    Card getNewCard() {
        int id;
        string name;
        double balance;
        cout << "Enter card id: ";
        cin >> id;
        cout << "Enter card name: ";
        cin >> name;
        cout << "Enter initial balance: ";
        cin >> balance;
        return Card(id, name, balance);
    }
    int getCardId() {
        int id;
        cout << "Enter card id: ";
        cin >> id;
        return id;
    }
    void checkBalance(int id) {
        Card* card = findCard(id);
        if (card == nullptr) {
            cout << "Card not found" << endl;
        } else {
            cout << "Balance: " << card->getBalance() << endl;
        }
    }
    void addBalance(int id) {
        Card* card = findCard(id);
        if (card == nullptr) {
            cout << "Card not found" << endl;
        } else {
            double amount;
            cout << "Enter amount to add: ";
            cin >> amount;
            card->setBalance(card->getBalance() + amount);
            cout << "New balance: " << card->getBalance() << endl;
        }
    }
};

int main() {
    CardManager manager;
    manager.loadFromFile("cards.txt");
    manager.showMenu();
    manager.saveToFile("cards.txt");
    return 0;
}

在这个程序中，`Card` 类表示一张饭卡，包含 id、name 和 balance 三个数据成员，分别表示卡号、姓名和余额。`CardManager` 类是饭卡管理器，包含了添加、删除、查看余额和充值等操作。它内部维护了一个 vector，存储所有的饭卡。`loadFromFile` 和 `saveToFile` 方法分别用于从文件中读取饭卡信息和将饭卡信息写入文件。`showMenu` 方法是程序入口，循环显示菜单，根据用户的选择调用不同的方法。

你可以根据自己的需求修改这个程序，例如添加更多的操作，修改文件格式等。