A lightweight C++ logging library
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Loguru: a lightweight and flexible C++ logging library.

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At a glance

Loguru terminal output


Documentation can be found at https://emilk.github.io/loguru/index.html.


This software is in the public domain. Where that dedication is not recognized, you are granted a perpetual, irrevocable license to copy and modify this file as you see fit.

That being said, I would appreciate credit! If you find Loguru useful, tweet me at @ernerfeldt mail me at emil.ernerfeldt@gmail.com.

Why another logging library?

I have yet to come across a nice, light-weight logging library for C++ that does everything I want. So I made one!

In particular, I want logging that produces logs that are both human-readable and easily grep:ed. I also want to be able to hook into the logging process to print some of the more severe messages on-screen in my app (for dev-purposes).


  • Simple integration
    • Just two files: loguru.hpp and loguru.cpp.
    • Either build and link loguru.cpp or just #include <loguru.cpp> in one of your own .cpp files.
  • Small, simple library.
    • Small header with no #includes for fast compile times (see separate heading).
    • No dependencies.
    • Cross-platform
  • Flexible:
    • User can install callbacks for logging (e.g. to draw log messages on screen in a game).
    • User can install callbacks for fatal error (e.g. to pause an attached debugger or throw an exception).
  • Support multiple file outputs, either trunc or append:
    • e.g. a logfile with just the latest run at low verbosity (high readability).
    • e.g. a full logfile at highest verbosity which is appended to on every run.
  • Full featured:
    • Verbosity levels.
    • Supports assertions: CHECK_F(fp != nullptr, "Failed to open '%s'", filename)
    • Supports abort: ABORT_F("Something went wrong, debug value is %d", value).
  • Stack traces printed on abort.
    • Stack traces are cleaned up somewhat.
      • Before cleanup: some_function_name(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&)
      • After cleanup: some_function_name(std::vector<std::string> const&)
    • Stack traces are printed the right way:
      • Chronological order with the most relevant at the end.
  • (most) signals writes stack traces.
  • Fast:
    • When configured in unbuffered mode (loguru::g_flush_interval_ms = 0):
      • 6-8 us when logging to stderr + file (rMBP + SSD + Clang).
      • About 25%-75% faster than GLOG on my MacBook Pro (Clang).
      • About the same as GLOG on my Linux Desktop (GCC).
    • With loguru::g_flush_interval_ms set to ~100 ms:
      • 3-5 us when logging to stderr + file (rMBP + SSD + Clang).
      • About twice as fast as GLOG.
  • Drop-in replacement for most of GLOG (except for setup code).
  • Choose between using printf-style or std::cout-style formatting.
  • Compile-time checked printf-formating (on supported compilers).
  • Support for fmtlib formatting.
    • Add #define LOGURU_USE_FMTLIB 1, before including loguru.hpp
    • You also need to set up the fmtlib include directory for building as well as linking against fmtlib, alternatively use the FMT_HEADER_ONLY preprocessor definition.
  • Assertion failures are marked with noreturn for the benefit of the static analyzer and optimizer.
  • All logging also written to stderr.
    • With colors on supported terminals.
  • Thread-safe.
  • Can be configured to either:
    • Flush every loguru::g_flush_interval_ms in a background thread
    • Flushes output on each call so you won't miss anything even on hard crashes (and still faster than buffered GLOG!).
  • Prefixes each log line with:
    • Date and time to millisecond precision.
    • Application uptime to millisecond precision.
    • Thread name or id (you can set the name with loguru::set_thread_name).
    • File and line.
    • Log level.
    • Indentation (see Scopes).
  • Error context:
    • Catch the values of local variables and print them only on a crash (see Error context).
  • Scopes (see Scopes).
  • grep:able logs:
    • Each line has all the info you need (e.g. date).
    • You can easily filter out high verbosity levels after the fact.


Just include <loguru.hpp> where you want to use Loguru. Then either compile and link with loguru.cpp or in one .cpp file: #include <loguru.cpp> Make sure you compile with -std=c++11 -lpthread -ldl on relevant environments.


#include <loguru.hpp>// Optional, but useful to time-stamp the start of the log.
// Will also detect verbosity level on command line as -v.
loguru::init(argc, argv);

// Put every log message in "everything.log":
loguru::add_file("everything.log", loguru::Append, loguru::Verbosity_MAX);

// Only log INFO, WARNING, ERROR and FATAL to "latest_readable.log":
loguru::add_file("latest_readable.log", loguru::Truncate, loguru::Verbosity_INFO);

// Only show most relevant things on stderr:
loguru::g_stderr_verbosity = 1;

LOG_SCOPE_F(INFO, "Will indent all log messages within this scope.");
LOG_F(INFO, "I'm hungry for some %.3f!", 3.14159);
LOG_F(2, "Will only show if verbosity is 2 or higher");
VLOG_F(get_log_level(), "Use vlog for dynamic log level (integer in the range 0-9, inclusive)");
LOG_IF_F(ERROR, badness, "Will only show if badness happens");
auto fp = fopen(filename, "r");
CHECK_F(fp != nullptr, "Failed to open file '%s'", filename);
CHECK_GT_F(length, 0); // Will print the value of `length` on failure.
CHECK_EQ_F(a, b, "You can also supply a custom message, like to print something: %d", a + b);

// Each function also comes with a version prefixed with D for Debug:
DCHECK_F(expensive_check(x)); // Only checked #if !NDEBUG
DLOG_F(INFO, "Only written in debug-builds");

// Turn off writing to stderr:
loguru::g_stderr_verbosity = loguru::Verbosity_OFF;

// Turn off writing err/warn in red:
loguru::g_colorlogtostderr = false;

// Throw exceptions instead of aborting on CHECK fails:
loguru::set_fatal_handler([](const loguru::Message& message){
	throw std::runtime_error(std::string(message.prefix) + message.message);

If you prefer logging with streams:

#include <loguru.hpp>
LOG_S(INFO) << "Look at my custom object: " << a.cross(b);
CHECK_EQ_S(pi, 3.14) << "Maybe it is closer to " << M_PI;

For more info, see the official documentation.

Grep:able logs

# Only show warnings, errors and fatal messages:
cat logfile.txt | egrep "[^0-9]\|"

# Ignore verbosity-levels 4 and above:
cat logfile.txt | egrep "[^4-9]\|"

# Only show verbosity-level 6:
cat logfile.txt | egrep "6\|"

# Only show messages from the main thread:
cat logfile.txt | egrep "\[main thread     \]"

No includes in loguru.hpp

I abhor logging libraries that #include's everything from iostream to windows.h into every compilation unit in your project. Logging should be frequent in your source code, and thus as lightweight as possible. Loguru's header has no #includes. This means it will not slow down the compilation of your project.

In a test of a medium-sized project, including loguru.hpp instead of glog/logging.hpp everywhere gave about 10% speedup in compilation times.

Note, however, that this gives you the bare-bones version of Loguru with printf-style logging. If you want std::ostream style logging (or GLOG functionality) you need to #define LOGURU_WITH_STREAMS 1 before #include <loguru.hpp>, and that will make loguru.hpp include <sstream>. No away around it!


The library supports scopes for indenting the log-file. Here's an example:

int main(int argc, char* argv[])
	loguru::init(argc, argv);
	LOG_F(INFO, "Doing some stuff...");
	for (int i=0; i<2; ++i) {
		VLOG_SCOPE_F(1, "Iteration %d", i);
		auto result = some_expensive_operation();
		LOG_IF_F(WARNING, result == BAD, "Bad result");
	LOG_F(INFO, "Time to go!");
	return 0;

This will output:

	     loguru.cpp:184      0| arguments:       ./loguru_test test -v1
	     loguru.cpp:185      0| Verbosity level: 1
	     loguru.cpp:186      0| -----------------------------------
	loguru_test.cpp:108      0| { int main_test(int, char **)
	loguru_test.cpp:109      0| .   Doing some stuff...
	loguru_test.cpp:111      1| .   { Iteration 0
	loguru_test.cpp:111      1| .   } 0.133 s: Iteration 0
	loguru_test.cpp:111      1| .   { Iteration 1
	loguru_test.cpp:113      0| .   .   Bad result
	loguru_test.cpp:111      1| .   } 0.134 s: Iteration 1
	loguru_test.cpp:115      0| .   Time to go!
	loguru_test.cpp:108      0| } 0.267 s: int main_test(int, char **)


You can also optionally log things ONLY if there is a crash. This is a very useful feature:

	void process_file(const char* filename)
		ERROR_CONTEXT("filename", filename);
		parse_file(filename); // Only if this crashes will filename be logged.

Streams vs printf

Some logging libraries only supports stream style logging, not printf-style. This means that what in Loguru is:

LOG_F(INFO, "Some float: %+05.3f", number);

in Glog becomes something along the lines of:

LOG(INFO) << "Some float: " << std::setfill('0') << std::setw(5) << std::setprecision(3) << number;

Loguru allows you to use whatever style you prefer.