feat: entry meta functions

possibility to modify the metadata string of each Entry

Log.cpp

@@ -137,31 +137,33 @@ void log(Level lvl, std::stringbuf* strb) {
 	}
 }
 
-LeveledSink fatal(Level::fatal);
-LeveledSink error(Level::error);
-LeveledSink warn(Level::warn);
-LeveledSink note(Level::note);
-LeveledSink info(Level::info);
-LeveledSink debug(Level::debug);
-LeveledSink trace(Level::trace);
+LeveledSink fatal{Level::fatal};
+LeveledSink error{Level::error};
+LeveledSink warn{Level::warn};
+LeveledSink note{Level::note};
+LeveledSink info{Level::info};
+LeveledSink debug{Level::debug};
+LeveledSink trace{Level::trace};
 
 /*
  * class Entry
  */
 
-Entry::Entry(Level lvl) : lvl(lvl) {
-	addMetadataHeader();
+Entry::Entry(Level lvl) : lvl{lvl} {
+	for(auto metafunc : entryMetaFunctions) {
+		metafunc(ss, *this);
+	}
 }
 
 Entry::~Entry() {
 	log(lvl, ss.rdbuf());
 }
 
-void Entry::addMetadataHeader() {
-	static const char* LevelTag[] = {"",         "[FATAL] ", "[ERROR] ", "[WARN ] ",
-	                                 "[NOTE ] ", "[INFO ] ", "[DEBUG] ", "[TRACE] "};
+std::vector<Entry::MetaFunction> entryMetaFunctions;
+
+std::ostream& defaultEntryMetaTime(std::ostream& os, const Entry& e) {
+	(void) e; // unused
 
-	// datetime
 	using namespace std::chrono;
 	auto now = system_clock::to_time_t(system_clock::now());
 	auto tm = *std::localtime(&now);
@@ -169,16 +171,31 @@ void Entry::addMetadataHeader() {
 	// MinGW doesn't support the ISO8601 formatting characters like "%F" and "%T"
 	// ref: https://sourceforge.net/p/mingw-w64/bugs/793/
 	// Therefore, use a more verbose time string
-	ss << "[" << std::put_time(&tm, "%Y-%m-%d %H:%M:%S") << "]";
+	return os << "[" << std::put_time(&tm, "%Y-%m-%d %H:%M:%S") << "]";
+}
 
-	// log level
-	ss << LevelTag[static_cast<int>(lvl)];
+std::ostream& defaultEntryMetaLevel(std::ostream& os, const Entry& e) {
+	static const char* LevelTag[] = {
+		"", "[FATAL]", "[ERROR]", "[WARN ]", "[NOTE ]", "[INFO ]", "[DEBUG]", "[TRACE]"
+	};
+	return os << LevelTag[static_cast<int>(e.getLevel())];
 }
 
 void init() {
 	// add default console logger
 	if (outputs.empty())
 		outputs.push_back(new ConsoleOutput());
+
+	// set default entry metadata printing functions
+	auto space = [](std::ostream& os, const Entry& e) -> std::ostream& {
+		(void) e;
+		return os << ' ';
+	};
+	entryMetaFunctions = {
+		defaultEntryMetaTime,
+		defaultEntryMetaLevel,
+		space
+	};
 }
 
 void stop() {
@@ -203,6 +220,4 @@ void setColoredOutput(bool enabled) {
 	((ConsoleOutput*) outputs.at(0))->setColoredOutput(enabled); // has to exist
 }
 
-LeveledSink::LeveledSink(Level level) : level(level) {}
-
 } // namespace Log

Log.h

@@ -1,7 +1,9 @@
 #pragma once
 
+#include <functional>
 #include <sstream> // std::stringstream (buffer for log entries)
 #include <string>
+#include <vector>
 
 #ifndef LOG_USEMUTEX
 #define LOG_USEMUTEX 0
@@ -27,7 +29,7 @@ void setColoredOutput(bool enabled);
 class Entry {
 private:
 	std::stringstream ss;
-	Level lvl;
+	const Level lvl;
 
 	void addMetadataHeader();
 
@@ -36,21 +38,32 @@ public:
 	Entry(Entry&&) = default;
 	~Entry();
 
+	Level getLevel() const {
+		return lvl;
+	}
+
+	using MetaFunction = std::function<std::ostream&(std::ostream&, const Entry&)>;
+
 	template <typename T> Entry& operator<<(const T& msg) {
 		ss << msg;
 		return *this;
 	}
 };
 
+extern std::vector<Entry::MetaFunction> entryMetaFunctions;
+
+std::ostream& defaultEntryMetaTime(std::ostream&, const Entry&);
+std::ostream& defaultEntryMetaLevel(std::ostream&, const Entry&);
+
 class LeveledSink {
 private:
 	Level level;
 
 public:
-	LeveledSink(Level level);
+	LeveledSink(Level level) : level{level} {};
 
 	template <typename T> Entry operator<<(const T& msg) {
-		Entry entry(level);
+		Entry entry{level};
 		entry << msg;
 		return entry;
 	}

test.cpp

@@ -44,6 +44,16 @@ int main() {
 	// log streams + colors
 	Log::setColoredOutput(true);
 	Log::setConsoleLogLevel(Log::Level::trace);
+
+	// add entry meta function
+	Log::entryMetaFunctions.insert(
+		Log::entryMetaFunctions.cend() - 1,
+		[](std::ostream& os, const Log::Entry& e) -> std::ostream& {
+			(void) e;
+			return os << "[banana]";
+		}
+	);
+
 	std::cout << "=== Log level debug ===" << std::endl;
 	Log::trace << "trace msg @Lvl trace; " << 42 << "; " << 3.14159;
 	Log::debug << "dbg   msg @Lvl trace; " << 42 << "; " << 3.14159;