WhisperCom/libs/loguru/README.md

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

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## At a glance

![Loguru terminal output](docs/terminal_colors.png)

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

## License
This software is in the public domain. Where that dedication is not recognized, you are granted a perpetual, irrevocable license to copy, modify and distribute it 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).

## Features:
* 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 `#include`s 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](http://yellerapp.com/posts/2015-01-22-upside-down-stacktraces.html):
		* 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](https://github.com/fmtlib/fmt) 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.

## Compiling

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.

## Usage

``` C++
#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:

``` C++
#define LOGURU_WITH_STREAMS 1
#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](https://emilk.github.io/loguru/index.html).

## Grep:able logs
``` bash
# 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!

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

``` C++
int main(int argc, char* argv[])
{
	loguru::init(argc, argv);
	LOG_SCOPE_FUNCTION(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:

```
	     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 **)
```


# `ERROR_CONTEXT`
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:

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

in Glog becomes something along the lines of:

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

Loguru allows you to use whatever style you prefer.