log.h

#ifndef LIGHTGBM_UTILS_LOG_H_
#define LIGHTGBM_UTILS_LOG_H_

#include <cstdio>
#include <cstdlib>
#include <cstdarg>
#include <cstring>
#include <fstream>

namespace LightGBM {


#ifndef CHECK
#define CHECK(condition)                                   \
  if (!(condition)) Log::Fatal("Check failed: " #condition \
     " at %s, line %d .\n", __FILE__,  __LINE__);
#endif

#ifndef CHECK_NOTNULL
#define CHECK_NOTNULL(pointer)                             \
  if ((pointer) == nullptr) LightGBM::Log::Fatal(#pointer " Can't be NULL\n");
#endif

// A enumeration type of log message levels. The values are ordered:
// Debug < Info < Error < Fatal.
enum class LogLevel: int {
  Fatal = -1,
  Error = 0,
  Info = 1,
  Debug = 2,
};


/*!
* \brief The Log class is a static wrapper of a global Logger instance in
*        the scope of a process. Users can write logging messages easily
*        with the static methods.
*/
class Log {
public:
  /*!
  * \brief Resets the minimal log level. It is INFO by default.
  * \param level The new minimal log level.
  */
  static void ResetLogLevel(LogLevel level) {
    GetLevel() = level;
  }

  static void Debug(const char *format, ...) {
    va_list val;
    va_start(val, format);
    Write(LogLevel::Debug, "Debug", format, val);
    va_end(val);
  }
  static void Info(const char *format, ...) {
    va_list val;
    va_start(val, format);
    Write(LogLevel::Info, "Info", format, val);
    va_end(val);
  }
  static void Error(const char *format, ...) {
    va_list val;
    va_start(val, format);
    Write(LogLevel::Error, "Error", format, val);
    va_end(val);
  }
  static void Fatal(const char *format, ...) {
    va_list val;
    va_start(val, format);
    Write(LogLevel::Fatal, "Fatal", format, val);
    va_end(val);
  }

private:

  static void Write(LogLevel level, const char* level_str, const char *format, va_list val) {
    if (level <= GetLevel()) {  // omit the message with low level
      // write to STDOUT
      printf("[LightGBM] [%s] ", level_str);
      vprintf(format, val);
      fflush(stdout);

      if (level == LogLevel::Fatal) {
        exit(1);
      }
    }
  }

  // a trick to use static variable in header file. 
  // May be not good, but avoid to use an additional cpp file
  static LogLevel& GetLevel() {
    static LogLevel level;
    return level;
  };

};

}  // namespace LightGBM
#endif   // LightGBM_UTILS_LOG_H_