// .h
class Logger {
public:
    void Log(const std::string &message);

    Logger &operator<<(const std::string& message);

    static Logger* Get();

    Logger(const Logger&) = delete;
    Logger(const Logger&&) = delete;
    Logger & operator=(const Logger&) = delete;
    Logger & operator=(const Logger&&) = delete;
private:
    Logger() = default;
    ~Logger() = default;

    static std::mutex mtx;

    static std::string fileName;

    static Logger *pThis;

    static std::ofstream logFile;
};

// .cpp
Logger *Logger::Get() {
    if (pThis == nullptr) {
        pThis = new Logger();
        logFile.open(fileName, std::ios::out | std::ios::app);
        logFile << "Start log session : " << Utils::getCurrentDateTime()  << std::endl;
    }
    return pThis;
}

void Logger::Log(const std::string &message) {
    std::lock_guard<std::mutex> lock(mtx);
    logFile << Utils::getCurrentDateTime() << " : " << message << std::endl;
}

Logger &Logger::operator<<(const std::string &message) {
    Log(message);
    return *pThis;
}

std::string Logger::fileName = "log.txt";
std::ofstream Logger::logFile;
Logger* Logger::pThis = nullptr;
std::mutex Logger::mtx;