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Clutchlog — versatile (de)clutchable logging

Clutchlog is a logging system that targets versatile debugging. It allows to (de)clutch messages for a given: log level, source code location or call stack depth.



Clutchlog allows to select which log messages will be displayed, based on their locations:

  • Classical log levels: each message has a given detail level and it is displayed if you ask for a at least the same one.
  • Call stack depth: you can ask to display messages within functions that are called up to a given stack depth.
  • Source code location: you can ask to display messages called from given files, functions and line number, all based on regular expressions.

Additionally, Clutchlog will do its best to allow the compiler to optimize out calls, for instance debug messages in "Release" builds.

Additional features:

  • Templated log format, to easily design your own format.
  • Colored log. By default only important ones are colored (critical and error in red, warning in magenta).
  • Macro to dump the content of a container in a file with automatic naming (yes, it is useful for fast debugging).
  • Generic clutching wrapper, to wrap any function call. Useful to (de)clutch asserts for example.


Adding a message is a simple as calling a macro (which is declutched in Debug build type, when NDEBUG is not defined):

CLUTCHLOG(info, "matrix size: " << m << "x" << n);

To configure the display, you indicate the three types of locations, for example in your main function:

    auto& log = clutchlog::logger();
    log.depth(2);                          // Log functions called from "main" but not below.
    log.threshold("Info"); // Log only "info", "warning", "error" or "critical" messages.
    log.file("algebra/.*");                // Will match any file in the "algebra" directory.
    log.func("(mul|add|sub|div)");         // Will match "multiply", for instance.

For more detailled examples, see the "Usage" sections below and the tests directory.


Most of existing logging systems targets service events storage, like fast queuing of transactions in a round-robin database. Their aim is to provide a simple interface to efficiently store messages somewhere, which is appropriated when you have a well known service running and you want to be able to trace complex users interactions across its states.

Clutchlog, however, targets the debugging of a (typically single-run) program. While you develop your software, it's common practice to output several detailled informations on the internal states around the feature you are currently programming. However, once the feature is up and running, those detailled informations are only useful if you encounter a bug traversing this specific part.

While tracing a bug, it is tedious to uncomment old debugging code (and go on the build-test cycle) or to set up a full debugger session that displays all appropriate data (with ad-hoc fancy hooks).

To solve this problem, Clutchlog allows to disengage at runtime your debug log messages in various parts of the program, allowing for the fast tracking of a bug across the execution.

Basic Usage


The main entrypoint is the CLUTCHLOG macro, which takes the desired log level and message. The message can be anything that can be output in an ostringstream.

// Simple string:
CLUTCHLOG(info, "hello world");

// Serialisable variable:
double value = 0;
CLUTCHLOG(error, value);

// passed using inline output stream operators:
CLUTCHLOG(debug, "hello " << value << " world");

There is also a macro to dump the content of an iterable within a separate file: CLUTCHDUMP. This function takes care of incrementing a numeric suffix in the file name, if an existing file with this name exists.

std::vector<int> v(10);
std::generate(v.begin(), v.end(), std::rand);
CLUTCHDUMP(debug, vec, "test_{n}.dat");
/* Will output in cat "rand_0.dat"
* # [t-dump] Info in main (at depth 5) @ /home/nojhan/code/clutchlog/tests/t-dump.cpp:22
* 1804289383
* 846930886
* 1681692777

Note that if you pass a file name without the {n} tag, the file will be overwritten as is.

Log level semantics

Log levels use a classical semantics for a human skilled in the art, in decreasing order of importance:

  • Critical: an error that cannot be recovered. For instance, something which will make a server stop right here.
  • Error: an error that invalidates a function, but may still be recovered. For example, a bad user input that will make a server reset its state, but not crash.
  • Warning: something that is strange, but is probably legit. For example a default parameter is set because the user forgot to indicate its preference.
  • Progress: the state at which computation currently is.
  • Note: some state worth noting to understand what's going on.
  • Info: any information that would help ensuring that everything is going well.
  • Debug: data that would help debugging the program if there was a bug later on.
  • XDebug: debugging information that would be heavy to read.

Note: the log levels constants are lower case (for example: clutchlog::level::xdebug), but their string representation is not (e.g. "XDebug", this should be taken into account when using threshold or level_of).

Location filtering

To configure the global behaviour of the logger, you must first get a reference on its (singleton) instance:

auto& log = clutchlog::logger();

One can configure the location(s) at which messages should actually be logged:

log.depth(3); // Depth of the call stack, defaults to the maximum possible value.
log.threshold(clutchlog::level::error); // Log level, defaults to error.

Current levels are defined in an enumeration as clutchlog::level:

enum level {critical=0, error=1, warning=2, progress=3, note=4, info=5, debug=6, xdebug=7};

File, function and line filters are indicated using (ECMAScript) regular expressions:

log.file(".*"); // File location, defaults to any.
log.func(".*"); // Function location, defaults to any.
log.line(".*"); // Line location, defaults to any.

A shortcut function can be used to filter all at once:

log.location(file, func, line); // Defaults to any, second and last parameters being optional.

Strings may be used to set up the threshold:

log.threshold("Error"); // You have to know the exact —case sensitive— string.

Note that the case of the log levels strings matters (see below).

Output Configuration

The output stream can be configured using the out method:

log.out(std::clog); // Defaults to clog.

The format of the messages can be defined with the format method, passing a string with standardized tags surrounded by {}:


Available tags are:

  • {msg}: the logged message,
  • {level}: the current log level (i.e. Critical, Error, Warning, Progress, Note, Info, Debug or XDebug),
  • {level_letter}: the first letter of the current log level,
  • {file}: the current file (absolute path),
  • {func}: the current function,
  • {line}: the current line number.

Some tags are only available on POSIX operating systems as of now:

  • {name}: the name of the current binary,
  • {depth}: the current depth of the call stack,
  • {depth_marks}: as many chevrons > as there is calls in the stack,
  • {hfill}: Inserts a sequence of characters that will stretch to fill the space available in the current terminal, between the rightmost and leftmost part of the log message.

Log Format

The default log format is "[{name}] {level_letter}:{depth_marks} {msg} {hfill} {func} @ {file}:{line}\n", it can be overriden at compile time by defining the CLUTCHLOG_DEFAULT_FORMAT macro.

By default, and if CLUTCHLOG_DEFAULT_FORMAT is not defined, clutchlog will not put the location-related tags in the message formats (i.e. {name}, {func}, and {line}) when not in Debug builds.

Output style

The output can be colored differently depending on the log level.

// Print error messages in bold red:, // First, the log level.
    clutchlog::fmt::fg::red,       // Then the styles, in any order...

Or, if you want to declare some semantics beforehand:

// Print warning messages in bold magenta:
using fmt = clutchlog::fmt;
fmt warn(fmt::fg::magenta, fmt::typo::bold);, warn);

Using the clutchlog::fmt class, you can style:

  • the foreground color, passing a clutchlog::fmt::fg,
  • the background color, passing a clutchlog::fmt::bg,
  • some typographic style, passing a clutchlog::fmt::typo.

Any of the three arguments may be passed, in any order, if an argument is omitted, it defaults to no color/style.

Available colors are:

  • black,
  • red,
  • green,
  • yellow,
  • blue,
  • magenta,
  • cyan,
  • white,
  • none.

Available typographies:

  • reset (remove any style),
  • bold,
  • underline,
  • inverse,
  • none.

You may use styling within the format message template itself, to add even more colors:

using fmt = clutchlog::fmt;
std::ostringstream format;
fmt discreet(fmt::fg::blue);
format << "{level}: "
    << discreet("{file}:") // Used as a function (inserts a reset at the end).
    << fmt(fmt::fg::yellow) << "{line}" // Used as a tag (no reset inserted).
    << fmt(fmt::typo::reset) << " {msg}" << std::endl; // This is a reset.

Note: messages at the "critical", "error" and "warning" log levels are colored by default. You may want to set their style to none if you want to stay in control of inserted colors in the format template.

The horizontal filling line (the {hfill} tag) can be configured separately with hfill_style, for example:


Advanced Usage

More Output Configuration

Dump Format

The default format of the first line of comment added with the dump macro is "# [{name}] {level} in {func} (at depth {depth}) @ {file}:{line}". It can be edited with the format_comment method. If it is set to an empty string, then no comment line is added. The default can be modified at compile time with CLUTCHDUMP_DEFAULT_FORMAT.

By default, the separator between items in the container is a new line. To change this behaviour, you can change CLUTCHDUMP_DEFAULT_SEP or call the low-level dump method.

By default, and if CLUTCHDUMP_DEFAULT_FORMAT is not defined, clutchlog will not put the location-related tags in the message formats (i.e. {file} and {line}) when not in Debug builds.


The mark used with the {depth_marks} tag can be configured with the depth_mark method, and its default with the CLUTCHLOG_DEFAULT_DEPTH_MARK macro:

log.depth_mark(CLUTCHLOG_DEFAULT_DEPTH_MARK); // Defaults to ">".

The character used with the {hfill} tag can be configured wth the hfill_mark method, and its default with the CLUTCHLOG_DEFAULT_HFILL_MARK macro:

log.hfill_mark(CLUTCHLOG_DEFAULT_HFILL_MARK); // Defaults to '.'.

Clutchlog measures the width of the standard error channel. If it is redirected, it may be measured as very large. Thus, the hfill_max accessors allow to set a maximum width (in number of characters).

log.hfill_max(CLUTCHLOG_DEFAULT_HFILL_MAX); // Defaults to 300.

Note: clutchlog will select the minimum between hfill_max and the measured number of columns in the terminal, so that you may use hfill_max as a way to constraint the output width in any cases.

Stack Depth

By default, clutchlog removes 5 levels of the calls stack, so that your main entrypoint corresponds to a depth of zero. You can change this behaviour by defining the CLUTCHLOG_STRIP_CALLS macro, or calling strip_calls.

log.strip_calls(CLUTCHLOG_STRIP_CALLS); // Defaults to 5.

Disabled calls

By default, clutchlog is always enabled if the NDEBUG preprocessor variable is not defined (this variable is set by CMake in build types that differs from Debug).

You can however force clutchlog to be enabled in any build type by setting the WITH_CLUTCHLOG preprocessor variable.

When the NDEBUG preprocessor variable is set (e.g. in Release build), clutchlog will do its best to allow the compiler to optimize out any calls for log levels that are under progress.

You can change this behavior at compile time by setting the CLUTCHLOG_DEFAULT_DEPTH_BUILT_NODEBUG preprocessor variable to the desired maximum log level, for example:

// Will always allow to log everything even in Release mode.
#define CLUTCHLOG_DEFAULT_DEPTH_BUILT_NODEBUG clutchlog::level::xdebug

Note that allowing a log level does not mean that it will actually output something. If the configured log level at runtime is lower than the log level of the message, it will still not be printed.

This behavior intend to remove as many conditional statements as possible when not debugging, without having to use preprocessor guards around calls to clutchlog, thus saving run time at no readability cost.

Low-level API

All configuration setters have a getters counterpart, with the same name but taking no parameter, for example:

std::string mark = log.depth_mark();

To control more precisely the logging, one can use the low-level log method:

log.log(clutchlog::level::xdebug, "hello world", "main.cpp", "main", 122);

A helper macro can helps to fill in the location with the actual one, as seen by the compiler:

log.log(clutchlog::level::xdebug, "hello world", CLUTCHLOC);

A similar dump method exists:

log.dump(clutchlog::level::xdebug, cont.begin(), cont.end(), CLUTCHLOC, "dumped_{n}.dat", "\n");
log.dump(clutchlog::level::xdebug, cont.begin(), cont.end(), "main.cpp", "main", 122, "dumped.dat", "\n\n");

You can access the identifier of log levels with level_of:

log.threshold( log.level_of("XDebug") ); // You have to know the exact string.

(De)clutch any function call

The CLUTHFUNC macro allows to wrap any function within the current logger.

For instance, this can be useful if you want to (de)clutch calls to asserts. To do that, just declare your own macro:

#define ASSERT(...) { CLUTCHFUNC(error, assert, __VA_ARGS__) }

Thus, any call like ASSERT(x > 3); will be declutchable with the same configuration than a call to CLUTCHLOG.

(De)clutch any code section

The CLUTCHCODE macro allows to wrap any code within the current logger.

For instance:

    std::clog << "We are clutched!\n";


Here what you would do to setup clutchlog with the default configuration:

    auto& log = clutchlog::logger();
    // Location filtering.
    // Colors of the 3 firsts levels., clutchlog::fmt(
            clutchlog::fmt::typo::underline);, clutchlog::fmt(
            clutchlog::fmt::typo::bold);, clutchlog::fmt(
    // Assuming you are on a POSIX system.
    log.format("[{name}] {level_letter}:{depth_marks} {msg} {hfill} {func} @ {file}:{line}\n");

And here are all the functions you may call to log something:

    // Basic message.
    CLUTCHLOG(debug, "x = " << x);

    // Any code section.
        if(x < 0) std::cerr << "WTF?" << std::endl;

    // Container to a file.
    CLUTCHDUMP(note, my_vector, "my_vector.dat");

    // Container to a numbered file.
    CLUTCHDUMP(note, my_list, "my_list_{n}.dat");

    // Function call.
    CLUTCHFUNC(warning, my_check, x, y); // Calls `my_check(x,y);`

    // Declutchable asserts.
    #define ASSERT(...) { CLUTCHFUNC(critical, assert, __VA_ARGS__) }


System-dependent stack depth

Because access to the call stack depth and program name are system-dependent, the features relying on the depth of the call stack and the display of the program name are only available for operating systems having the following headers: execinfo.h, stdlib.h and libgen.h (so far, tested with Linux).

Clutchlog sets the CLUTCHLOG_HAVE_UNIX_SYSINFO to 1 if the headers are available, and to 0 if they are not. You can make portable code using something like:

    log.depth( x );

System-dependent horizontal fill

Because access to the current terminal width is system-dependent, the {hfill} format tag feature is only available for operating systems having the following headers: sys/ioctl.h, stdio.h and unistd.h (so far, tested with Linux).

Clutchlog sets the CLUTCHLOG_HAVE_UNIX_SYSIOCTL to 1 if the headers are available, and to 0 if they are not. You can make portable code using something like:

    log.hfill_mark( '_' );


Some colors/styles may not be supported by some exotic terminal emulators.

Clutchlog needs C++-17 with the filesystem feature. You may need to indicate -std=c++17 -lstdc++fs to some compilers.

Variable names within the CLUTCHLOG macro

Calling the CLUTCHLOG macro with a message using a variable named clutchlog__msg will end in an error.


What Clutchlog do not provide at the moment (but may in a near future):

  • Super fast log writing.
  • Thread safety.

What Clutchlog will most certainly never provide:

  • Round-robin log managers.
  • Duplicated messages management.
  • External output systems (only allow output stream, you can still do the proxy yourself).
  • External error handlers (not my job, come on).
  • Automatic argument parser (please, use a dedicated lib).
  • Signal handling (WTF would you do that, anyway?).

Build and tests

To use clutchlog, just include its header in your code and either ensure that the NDEBUG preprocessor variable is not set, either define the WITH_CLUTCHLOG preprocessor variable.

If you're using CMake (or another modern build system), it will unset NDEBUG —and thus enable clutchlog— only for the "Debug" build type, which is usually what you want if you use clutchlog, anyway.

To build and run the tests, just use a classical CMake workflow:

mkdir build
cd build

There's a script that tests all the build types combinations: ./


C++ logging system which targets versatile, (de)clutchable, _debugging_ instead of service event storage, in a single header.








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