class Logger {
public:
    static Logger& instance() {
        static Logger inst;  // C++11 起：线程安全的延迟初始化
        return inst;
    }

    void log(const std::string& msg) { /* ... */ }

    // 防止拷贝和移动
    Logger(const Logger&) = delete;
    Logger& operator=(const Logger&) = delete;

private:
    Logger() = default;  // 私有构造函数，禁止外部创建
    ~Logger() = default;
};