Introduction into Logging with Loguru

This time I would like to give a short introduction into a nice little library I just encountered. It's called loguru and it's a lightweight, Thread-Safe, logging library with an impressive good written documentation and human-readable output.

Hero Image: Logging excerpt

To use it all you need to do is to add a single header and source file to compile with. Unfortunately, that's, in my opinion, also a drawback. Yes, it's easier to install and usable in training, but it can't be exchanged by whoever is operating the system after deployment. In such cases, where the operator wants to define how and what to use for logging, a generic logging interface (facade pattern), such as slf4cxx which is similar to its java pendant slf4j, would be preferable.

Loguru is supporting a various number of features, such as callbacks for logging and fatal errors, verbosity levels, assertions and aborts, and stack traces in case of aborts. It even supports {fmt}. Everyone who ever was used to java/spring log outputs will recognize its similarities.

#include <thread>
#include "loguru.hpp"
#include "loguru.cpp"

void sleep(int ms)
//We can also inject parameters into the logging strings
VLOG_F(0, "Sleeping for %d ms", ms);

void complex()
//LOG_SCOPE_F is indenting the following logs
LOG_SCOPE_F(INFO, "Preparing complex calculation");

//VLOG_F can take a dynamic defined verbosity level
VLOG_F(0, "Heating up CPU");
loguru::set_thread_name("complex lambda");

const bool value = true;
LOG_IF_F(INFO, value, "This log is printed inside another thread");

void crashingFunction(int index)
//ERROR_CONTEXT is logging certain arguments in case of a crash
ERROR_CONTEXT("Computing with index", index);

//Asserts are also possible
std::vector<std::string> list;
CHECK_F(index > 1, "Oh no, wrong index, index is %d!!", index);

int main(int argc, char* argv[])
//Time stamping begin of logging and we can forward cli parameters such as -v (verbosity level) to loguru
loguru::init(argc, argv);

//Additional, if we need we can also put the logs into a defined file
loguru::add_file("important.log", loguru::Truncate, loguru::Verbosity_INFO);

LOG_F(INFO, "We are starting our complex threaded computation!");
LOG_F(INFO, "Complex computation done!");


return 0;
date       time         ( uptime  ) [ thread name/id ]                   file:line     v| 
2019-03-11 21:46:14.591 ( 0.000s) [main thread ] loguru.cpp:587 INFO| arguments: /mnt/c/Develop/LoggingWithLoguru/cmake-build-debug-wsl/LoggingWithLoguru
2019-03-11 21:46:14.591 ( 0.000s) [main thread ] loguru.cpp:590 INFO| Current dir: /mnt/c/Develop/LoggingWithLoguru/cmake-build-debug-wsl
2019-03-11 21:46:14.591 ( 0.000s) [main thread ] loguru.cpp:592 INFO| stderr verbosity: 0
2019-03-11 21:46:14.592 ( 0.000s) [main thread ] loguru.cpp:593 INFO| -----------------------------------
2019-03-11 21:46:14.592 ( 0.001s) [main thread ] loguru.cpp:751 INFO| Logging to 'important.log', mode: 'w', verbosity: 0
2019-03-11 21:46:14.592 ( 0.001s) [main thread ] main.cpp:47 INFO| We are starting our complex threaded computation!
2019-03-11 21:46:14.592 ( 0.001s) [main thread ] main.cpp:15 INFO| { Preparing complex calculation
2019-03-11 21:46:14.592 ( 0.001s) [main thread ] main.cpp:18 INFO| . Heating up CPU
2019-03-11 21:46:14.592 ( 0.001s) [main thread ] main.cpp:8 INFO| . Sleeping for 500 ms
2019-03-11 21:46:15.094 ( 0.502s) [complex lambda ] main.cpp:25 INFO| . This log is printed inside another thread
2019-03-11 21:46:15.094 ( 0.503s) [main thread ] main.cpp:15 INFO| } 0.502 s: Preparing complex calculation
2019-03-11 21:46:15.094 ( 0.503s) [main thread ] main.cpp:49 INFO| Complex computation done!
Stack trace:
3 0x7ff08b806a7a /mnt/c/Develop/LoggingWithLoguru/cmake-build-debug-wsl/LoggingWithLoguru(+0x6a7a) [0x7ff08b806a7a]
2 0x7ff08a641b97 __libc_start_main + 231
1 0x7ff08b80cfb5 /mnt/c/Develop/LoggingWithLoguru/cmake-build-debug-wsl/LoggingWithLoguru(+0xcfb5) [0x7ff08b80cfb5]
0 0x7ff08b80ceb5 /mnt/c/Develop/LoggingWithLoguru/cmake-build-debug-wsl/LoggingWithLoguru(+0xceb5) [0x7ff08b80ceb5]
[ErrorContext] main.cpp:32 Computing with index: -1
2019-03-11 21:46:15.094 ( 0.503s) [main thread ] main.cpp:36 FATL| CHECK FAILED: index > 1 Oh no, wrong index, index is -1!!
bash: line 1: 2324 Aborted (core dumped) env "JETBRAINS_REMOTE_RUN"="1" '/mnt/c/Develop/LoggingWithLoguru/cmake-build-debug-wsl/LoggingWithLoguru'

Process finished with exit code 134

As you can see it's pretty straight forward to use. We are not only logging several messages on INFO verbosity level, but also a message in a named thread called "complex lambda". If we wouldn't have defined the thread name with loguru::set_thread_name("complex lambda"), loguru would state the name of a thread with a hex id. The main thread gets its name by calling loguru::init(...). Because our small tool is crashing, loguru is printing us a stack trace which in my opinion is not as helpful as expected, but with ERROR_CONTEXT we get a little better output.

That's it for now with this post. We have now a short introduction into a, until now, rather unknown, but promising, logging library. Loguru is not only capable of producing Thread-Safe and human readable logging messages but also provides a very simple and handy interface to use.

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