logger.h

#include <iostream>
#include <fstream>
#include <ctime>
#include <string>

#define LOG_DEBUG \
    if(Logger::getLogger().getLoggingLevel() < DEBUG); \
	else Logger::getLogger().log("DEBUG")
#define LOG_WARNING \
	if(Logger::getLogger().getLoggingLevel() < WARNING); \
	else Logger::getLogger().log("WARNING")
#define LOG_ERROR \
	if(Logger::getLogger().getLoggingLevel() < ERROR); \
	else Logger::getLogger().log("ERROR")

#define LOG_SET_LEVEL(value) Logger::getLogger().setLoggingLevel(value);
#define LOG_SCOPE LogScope logscope(__FUNCTION__);

using namespace std;

enum LEVEL  {ERROR,WARNING,DEBUG};

class Logger{
public:
	static Logger& getLogger(){// Meyers Singleton.
		static Logger Instance;// Will be delete at the end.
		return Instance;
	}
    ofstream& log(string s){// return ofstream, so cascading <<..<<..<< is possible.
   		logfile_<<time(NULL)<<" "<<s<<" ";// time ticks in seconds. Check OS for more accurate timer.
    	return logfile_;
    }
    void setLoggingLevel(LEVEL level){
    	logfile_<<"setting output level to "<<level<<endl;
    	logginglevel_ = level;}
    LEVEL getLoggingLevel(){return logginglevel_;}
private:
	Logger()// make these private because of singleton.
	: logginglevel_(DEBUG)// set default level
	{		logfile_.open("logging.txt",ios::trunc);
			logfile_<<"LOGFILE. Build: "<<__DATE__<<", "<<__TIME__<<endl;
			}
	~Logger(){		logfile_<<"END OF LOGFILE"<<endl; logfile_.close();}

	LEVEL logginglevel_;
	ofstream logfile_;
};

class LogScope{
public:
	LogScope(const string& s) : logger_(Logger::getLogger()) , functionname_(s){
		logger_.log("SCOPE")<<"entering "<<functionname_<<" \n";
	}
	~LogScope(){
		logger_.log("SCOPE")<<"exiting "<<functionname_<<" \n";
	}
private:
	Logger& logger_;
	string functionname_;
};

main.cpp

int main()
{
	LOG_SCOPE;
	LOG_SET_LEVEL(DEBUG);
	LOG_DEBUG<<"hello world \n";
}