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 [email protected].

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).

• Header only

  • No linking woes! Just include and enjoy.

• Small, simple library.

  • Small header with no #includes for fast compile times (see separate heading).
  • No dependencies.
  • Cross-platform (but not tested on Windows yet...)

• 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:

  • Around 3us per call with no log file (just stderr).
  • Around 10us extra per log file.
  • About 25%-75% faster than GLOG on my MacBook Pro (Clang).
  • About the same as GLOG on my Linux Disktop (GCC).

• Drop-in replacement for most of GLOG (except for setup code).

• Choose between using printf-style formatting or streams.

• Compile-time checked printf-formating (on supported compilers).

• 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.

• 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.
  • File and line.
  • Log level.
  • Indentation (see Scopes).

• 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/loguru.hpp> where you want to use Loguru. Then, in one .cpp file:

	#define LOGURU_IMPLEMENTATION
	#include <loguru/loguru.hpp>

Make sure you compile with -std=c++11 -lpthread -ldl

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

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

// Only log INFO, WARNING, ERROR and FATAL to "latest_readable.log":
loguru::add_file("latest_readable.log", loguru::Truncate, loguru::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("Only written in debug-builds");

// Turn off writing to stderr:
loguru::g_alsologtostderr = false;

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

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

If you prefer logging with streams:

#define LOGURU_WITH_STREAMS 1
#include <loguru/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;

# 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 "\[main thread     \]"

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 Louru 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/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_SCOPE_FUNCTION_F(INFO);
	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:

date       time         ( uptime  ) [ thread name/id ]                   file:line     v|
2015-10-04 15:28:30.547 (   0.000s) [main thread     ]             loguru.cpp:184      0| arguments:       ./loguru_test test -v1
2015-10-04 15:28:30.548 (   0.000s) [main thread     ]             loguru.cpp:185      0| Verbosity level: 1
2015-10-04 15:28:30.548 (   0.000s) [main thread     ]             loguru.cpp:186      0| -----------------------------------
2015-10-04 15:28:30.548 (   0.000s) [main thread     ]        loguru_test.cpp:108      0| { int main_test(int, char **)
2015-10-04 15:28:30.548 (   0.000s) [main thread     ]        loguru_test.cpp:109      0| .   Doing some stuff...
2015-10-04 15:28:30.548 (   0.000s) [main thread     ]        loguru_test.cpp:111      1| .   { Iteration 0
2015-10-04 15:28:30.681 (   0.133s) [main thread     ]        loguru_test.cpp:111      1| .   } 0.133 s: Iteration 0
2015-10-04 15:28:30.681 (   0.133s) [main thread     ]        loguru_test.cpp:111      1| .   { Iteration 1
2015-10-04 15:28:30.815 (   0.267s) [main thread     ]        loguru_test.cpp:113      0| .   .   Bad result
2015-10-04 15:28:30.815 (   0.267s) [main thread     ]        loguru_test.cpp:111      1| .   } 0.134 s: Iteration 1
2015-10-04 15:28:30.815 (   0.267s) [main thread     ]        loguru_test.cpp:115      0| .   Time to go!
2015-10-04 15:28:30.815 (   0.267s) [main thread     ]        loguru_test.cpp:108      0| } 0.267 s: int main_test(int, char **)

Which looks like this in the terminal:

(Notice how verbosity levels higher than 0 are slightly gray).

Scopes affects logging on all threads.

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.

• Test on Windows.
• Rename ERROR to avoid conflict with windows.h macro?
• File-only logging: LOG_F(FILE, "Always written to file, never to stderr")