Log/Log.h

#pragma once
#include <chrono>   // date/time
#include <fstream>  // ofstream (logging to file)
#include <iomanip>  // std::put_time
#include <iostream> // std::ostream, std::cout, std::cin
#include <sstream>  // std::stringstream (buffer for log entries)
#include <string>
#include <vector> // list of outputs

class Log {
public:
	enum Level { OFF = 0, FATAL, ERROR, WARN, NOTE, INFO, DEBUG, TRACE };

	// delete ctors as this class is used via static methods only
	Log()           = delete;
	Log(const Log&) = delete;
	Log& operator=(const Log&) = delete;
	//~Log()                     = delete;

	// set up the logger with a ConsoleOutput
	static void init();
	// close all output streams
	static void stop();

private:
	// abstract base class for a log sink
	class Output {
	public:
		Output();
		Output(Log::Level lvl_max);
		virtual ~Output();
		virtual void log(Log::Level lvl, std::stringbuf* sbuf);
		virtual void setLogLevel(Log::Level lvl);

	protected:
		Log::Level lvl_max = INFO;
		// returns the correct ostream for the given log-level
		// or returns nullptr if no ostream is set/enabled for this level
		virtual std::ostream* getOs(Log::Level lvl) = 0; // abstract
		virtual void printLogHeader(std::ostream* os, Log::Level lvl);
	};

	// logging to stdout/stderr
	class ConsoleOutput : public Output {
	public:
		ConsoleOutput();
		virtual bool setColoredOutput(bool enabled);

	private:
		std::ostream* osStd = &std::cout;
		std::ostream* osErr = &std::cerr;
		bool coloredOutput;
		virtual void log(Log::Level lvl, std::stringbuf* sbuf);
		virtual std::ostream* getOs(Log::Level lvl);
	};

	class FileOutput : public Output {
	public:
		FileOutput(const std::string& filename, Log::Level lvl_max);
		FileOutput(const std::string& filename, Log::Level lvl_min, Log::Level lvl_max);

	private:
		std::string filename;
		std::ofstream ofs;
		Log::Level lvl_min = FATAL;
		virtual std::ostream* getOs(Log::Level lvl);
	};

	static std::vector<Output*> outputs;

public:
	static void addLogfile(const std::string& filename, Level max);
	static void addLogfile(const std::string& filename, Level min, Level max);

	static void setConsoleLogLevel(Level lvl);
	static void setColoredOutput(bool enabled);

	// Log entry that can be formed with various mixed data types
	// by concatenation with the << operator
	// Inspired from https://stackoverflow.com/a/8337882
	class Entry {
	private:
		std::stringstream ss;
		Log::Level lvl;

	public:
		Entry(Log::Level lvl) : lvl(lvl) {}
		Entry(Entry&&) = default;
		~Entry() { Log::log(lvl, ss.rdbuf()); }

		template <typename T>
		Entry& operator<<(const T& msg) {
			ss << msg;
			return *this;
		}
	};

	class LeveledSink {
	private:
		Log::Level level;
	public:
		LeveledSink(Log::Level level);

		template <typename T>
		Log::Entry operator<<(const T& msg) {
			Log::Entry entry(level);
			entry << msg;
			return entry;
		}
	};

	// LeveledSinks
	static LeveledSink fatal;
	static LeveledSink error;
	static LeveledSink warn;
	static LeveledSink note;
	static LeveledSink info;
	static LeveledSink debug;
	static LeveledSink trace;

private:
	static void log(Level lvl, std::stringbuf* strb);
};