水平太菜，最近捣鼓这个 log，折腾了好一会。由于之前都是用 std::cout，不能满足同时输出到屏与文件的目的，故经过一番搜索，在stackoverflow 找到了答案，现总结如下：

1. 头文件 logger.hpp, 代码如下：

   #ifndef __LOGGER
   #define __LOGGER

   #include
   #include
   #include
   #include
   #include

   #include

   typedef enum
   {
   EMERG = LOG_EMERG, /* system is unusable / ALERT = LOG_ALERT, / action must be taken immediately / CRIT = LOG_CRIT, / critical conditions / ERROR = LOG_ERR, / error conditions / WARNING = LOG_WARNING, / warning conditions / NOTICE = LOG_NOTICE, / normal but significant condition / INFO = LOG_INFO, / informational / DEBUG = LOG_DEBUG / debug-level messages */
   } Severity;

   static std::string severity_str[] = {
   [EMERG] = "EMERG", /* system is unusable / [ALERT] = "ALERT", / action must be taken immediately / [CRIT] = "CRIT", / critical conditions / [ERROR] = "ERROR", / error conditions / [WARNING] = "WARNING", / warning conditions / [NOTICE] = "NOTICE,", / normal but significant condition / [INFO] = "INFO", / informational / [DEBUG] = "DEBUG", / debug-level messages */
   };

   class Logger
   {
   private:
   Logger() {}

   ~Logger() {}
   
   /* forbide copy */
   Logger &operator=(const Logger &) = delete;
   Logger(const Logger &) = delete;

   private:
   static Severity m_log_level; /* only show log that with level
   not higher than m_log_level / static bool m_sync_syslog; / sync log to syslog ? / static bool m_sync_stdout; / sync log to stdout / static bool m_sync_file; / sync log to file / static std::string m_logfile; / file to save log */
   static Severity m_severity;
   static std::string m_filename;
   static int m_line;
   static std::string m_funcname;
   std::stringstream ss;
   std::mutex m_lk;

   public:
   static Logger &Instance(Severity s = Severity::INFO,
   const char *f = "", int l = 0,
   const char *fc = "")
   {
   static Logger instance_;

       instance_.m_severity = s;
       instance_.m_filename = f;
       instance_.m_line = l;
       instance_.m_funcname = fc;
       return instance_;
   }
   
   static void Init(bool sync_flag, const char *logfile)
   {
       m_sync_file = sync_flag;
       m_logfile = logfile;
   }
   
   static void Init(bool sync_stdout, bool sync_file, bool sync_syslog)
   {
       m_sync_file = sync_file;
       m_sync_stdout = sync_stdout;
       m_sync_syslog = sync_syslog;
   }
   
   static void Init(const char *label = "LOGGER",
                    Severity s = Severity::INFO,
                    bool sync_syslog = false)
   {
       m_log_level = s;
       if (sync_syslog)
       {
           m_sync_syslog = true;
           openlog(label, LOG_CONS | LOG_NDELAY, LOG_USER | LOG_LOCAL1);
       }
   }
   
   Logger &operator<<(const char *t)
   {
       std::lock_guard<std::mutex> _lk(m_lk);
       ss << t;
       return *this;
   }
   
   template <typename T>
   Logger &operator<<(const T &t)
   {
       std::lock_guard<std::mutex> _lk(m_lk);
       ss << t;
       return *this;
   }
   
   Logger &operator<<(std::ostream &(*os)(std::ostream &))
   {
       dump_line();
       return *this;
   }
   
   void dump_line()
   {
       std::lock_guard<std::mutex> _lk(m_lk);
       time_t rawtime;
       char now[80];
   time(&amp;rawtime);
   struct tm *tt = localtime(&amp;rawtime);
   
   strftime(now, 80, "%Y-%m-%d %H:%M:%S", tt);
   
   std::string obuf = "";
   if (!m_filename.empty())
   {
       obuf += "[" + m_filename + ":" +
               std::to_string(m_line) + " " +
               m_funcname + "]";
   }
   
   obuf += "[" + std::string(now) + "][" +
           severity_str[static_cast&lt;int&gt;(m_severity)] + "] ";
   
   obuf += ss.str();
   
   if (m_log_level &gt;= m_severity)
   {
       /* syslog */
       if (m_sync_syslog)
           syslog(LOG_USER | m_severity, "%s", obuf.c_str());
   
       /* stdout/stderr */
       if (m_sync_stdout)
           std::cerr &lt;&lt; obuf &lt;&lt; std::endl;
   
       if (m_sync_file)
       {
           std::ofstream fout(m_logfile, std::ios::app);
           fout.write(obuf.c_str(), obuf.length());
           fout.close();
       }
   }
   
   ss.str("");
   m_funcname = "";
   m_filename = "";
   m_line = 0;
   }
   
   template <typename T>
   void operator()(T t)
   {
       ss << t;
       dump_line();
   }
   
   template <typename T, typename... Args>
   void operator()(const T &t, const Args &... args)
   {
       ss << t;
       (*this)(args...);
   }

   };

   #define ENDL std::endl

   #ifndef NDEBUG
   #define LOG(v) Logger::Instance(v, FILE, LINE, func)
   #define LOGI Logger::Instance(Severity::INFO, FILE, LINE, func)
   #define LOGV Logger::Instance(Severity::NOTICE, FILE, LINE, func)
   #define LOGD Logger::Instance(Severity::DEBUG, FILE, LINE, func)
   #define LOGW Logger::Instance(Severity::WARNING, FILE, LINE, func)
   #define LOGE Logger::Instance(Severity::ERROR, FILE, LINE, func)

   #define LOGFI(a, v…) Logger::Instance(Severity::INFO, FILE, LINE, func)(a, ##v)
   #define LOGFV(a, v…) Logger::Instance(Severity::NOTICE, FILE, LINE, func)(a, ##v)
   #define LOGFD(a, v…) Logger::Instance(Severity::DEBUG, FILE, LINE, func)(a, ##v)
   #define LOGFW(a, v…) Logger::Instance(Severity::WARNING, FILE, LINE, func)(a, ##v)
   #define LOGFE(a, v…) Logger::Instance(Severity::ERROR, FILE, LINE, func)(a, ##v)

   #else
   #define LOG(v) Logger::Instance(v)
   #define LOGI Logger::Instance()
   #define LOGV Logger::Instance()
   #define LOGD Logger::Instance()
   #define LOGW Logger::Instance()
   #define LOGE Logger::Instance()

   #define LOGFI(a, v…) Logger::Instance()(a, ##v)
   #define LOGFV(a, v…) Logger::Instance()(a, ##v)
   #define LOGFD(a, v…) Logger::Instance()(a, ##v)
   #define LOGFW(a, v…) Logger::Instance()(a, ##v)
   #define LOGFE(a, v…) Logger::Instance()(a, ##v)
   #endif // NDEBUG

   #define LINE LOGI << FILE << LINE << ENDL
   #endif //__LOGGER

cpp 源码文件

#include "logger.h"

Severity Logger::m_log_level = Severity::INFO; /* only show log that with level
                                    not higher than m_log_level */
bool Logger::m_sync_syslog = false;            /* sync log to syslog ? */
bool Logger::m_sync_stdout = true;             /* sync log to stdout */
bool Logger::m_sync_file = false;              /* sync log to file */
std::string Logger::m_logfile;
Severity Logger::m_severity = Severity::INFO;
std::string Logger::m_filename;
int Logger::m_line;
std::string Logger::m_funcname;

头文件提供了5个宏，实现了5种log级别。目前没有加入色彩显示功能。如果定义了 DEBUG_MODE 宏，会有文件，行号等信息。

1. 使用

   #include "logger.h"

   using namespace LOGGER;

   int main()
   {
   Logger::Init(Severity::DEBUG, "./log.log");
   LOG(Severity::ERROR) << "Severity error" << std::endl;
   LOGI << "INFO LOG" << std::endl;
   LOGV << "VERBOSE LOG" << std::endl;
   LOGD << "DEBUG LOG" << std::endl;
   LOGW << "WARNNING LOG" << std::endl;
   LOGE << "ERROR LOG" << std::endl;

   LOGFI ("INFO LOG");
   LOGFV ("VERBOSE LOG");
   LOGFD ("DEBUG LOG");
   LOGFW ("WARNNING LOG");
   LOGFE ("ERROR LOG");
   
   return 0;

   }