[C++知识库]单例模式学习Logger

//final表示不可被继承
class Logger final
{
public:
	enum class LogLevel {
		Error,
		Info,
		Debug
	};
	// 所有方法只能通过instance返回的引用来访问
	static Logger& instance();
	// 显式删除所有复制构造，
	Logger(const Logger&) = delete;
	Logger(Logger&&) = delete;
	// 显式删除所有赋值运算符
	Logger& operator=(const Logger&) = delete;
	Logger& operator=(Logger&&) = delete;

	// Sets the log level.
	void setLogLevel(LogLevel level);
	// Logs a single message at the given log level.
	void log(std::string_view message, LogLevel logLevel);
	// Logs a vector of messages at the given log level.
	void log(const std::vector<std::string>& messages,
		LogLevel logLevel);
private:
	// 构造和析构函数使用private限制防止被外部调用
	Logger();
	~Logger();
	// Converts a log level to a human readable string.
	std::string_view getLogLevelString(LogLevel level) const;
	static const char* const kLogFileName;
 	//static inline const char* const kLogFileName ="log.out";//可以定义为内联变量直接初始化
	std::ofstream mOutputStream;
	LogLevel mLogLevel = LogLevel::Error;
};
/*以下例举一些关键定义和使用的代码*/
/*静态成员变量初始化必须在外部*/
const char* const Logger::kLogFileName = "log.out";
//注意返回的是引用
Logger& Logger::instance()
{
    //使用静态变量可以保证全局只有一份
	static Logger instance;
	return instance;
}
/*单例模式的使用*/
	Logger::instance().setLogLevel(Logger::LogLevel::Debug);
	// Log some messages.
    Logger::instance().log("test message", Logger::LogLevel::Error);