主程式（eCash.cpp）：

#include <iostream>
#include <fstream>
#include <map>
#include <string>

using namespace std;

template<class K, class V>
class eCash{
    public:map<K,V> data;

    public:void load(map<string, int> &data) {
        ifstream file;

        file.open("eCash.txt");

        if(file.is_open()) {
            while(file.good()) {
                string account;
                int money;

                file >> account;
                file >> money;

                data[account] = money;
            }
            file.close();
        }
        else {
            cout << "[!]系統文件遺失: eCash.txt" << endl;
            exit(EXIT_FAILURE);
        }
    }

    public:void refresh(map<string, int> &data) {
        ofstream file;

        file.open("eCash.txt");

        if(file.is_open()) {
            for(const auto& x : data) {
                file << x.first << " " << x.second << " ";
            }
            file.close();
        }
        else {
            cout << "[!]系統文件遺失: eCash.txt" << endl;
            exit(EXIT_FAILURE);
        }
    }
};

int main() {
    eCash<string, int> database;
    map<string, int>::iterator iter;

    string account;

    database.load(database.data);

    cout << "=== 歡迎使用eCash ===" << endl;

    while(true) {
        cout << "eCash: 請輸入你的帳號: ";
        cin >> account;

        iter = database.data.find(account);

        if(iter == database.data.end()) {
            cout << "查無資料!" << endl;
            cout << "正在建立..." << endl;
            database.data[account] = 0;
            cout << "建立完成!" << endl << endl;
        }
        else {
            cout << "帳號開啟完成!";
            int flag = 1;
            while(flag) {
                char select;
                cout << account << "您好，請選擇項目:" << endl;
                cout << "s: 儲值" << endl;
                cout << "p: 消費" << endl;
                cout << "d: 查詢餘額" << endl;
                cout << "q: 離開" << endl;
                cout << "> ";
                cin >> select;

                switch(select) {
                    int money;
                    case 's':
                        {
                            cin >> money;
                            if(money < 0)
                                cout << "eCash: 請輸入大餘0的金額" << endl << endl;
                            else {
                                database.data[account] += money;
                                cout << "eCash: 您存了" << money << "元" << endl << endl;
                            }
                            break;
                        }
                    case 'p':
                        {
                            cin >> money;
                            if(money < 0){
                                cout << "eCash: 請輸入大餘0的金額" << endl << endl;
                            } else {
                                if(money > database.data[account]){
                                    cout << "eCash: 您的錢不夠" << endl << endl;
                                }
                                else {
                                    database.data[account] -= money;
                                    cout << "eCash: 您花了" << money << "元" << endl << endl;
                                }
                            }
                            break;
                        }
                    case 'd':
                        cout << "eCash: 您尚有: " << iter->second << "元" << endl << endl;
                        break;
                    default:
                        {
                            flag = 0;
                            cout << endl;
                            break;
                        }
                }
                database.refresh(database.data);
            }
        }
    }
}

額外檔案（eCash.txt）：

admin -1

附註：eCash.txt 一定得先有一筆資料。

可以參考 std::fstream - C++ Reference 其中的用法範例

std::fstream fs("filename", std::fstream::in | std::fstream::out);

/* 對 fs 操作 */

fs.close();

請問樓主大大 最後有得到解答嗎