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fmtlib /fmt

A modern formatting library

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.. image:: :target:

.. image:: :target:

.. image:: :alt: fmt is continuously fuzzed at oss-fuzz :target:\ colspec=ID%20Type%20Component%20Status%20Proj%20Reported%20Owner%20\ Summary&q=proj%3Dlibfmt&can=1

.. image:: :alt: Ask questions at StackOverflow with the tag fmt :target:

{fmt} is an open-source formatting library for C++. It can be used as a safe and fast alternative to (s)printf and iostreams.

Documentation <https:></https:>


Q&A: ask questions on

StackOverflow with the tag fmt<https:></https:>



  • Simple
    format API <https:></https:>
    _ with positional arguments for localization
  • Implementation of
    C++20 std::format<https:></https:>
  • Format string syntax <https:></https:>
    _ similar to Python's
    format <https:></https:>
  • Safe
    printf implementation<https:></https:>
    _ including the POSIX extension for positional arguments
  • Extensibility:
    support for user-defined types<https:></https:>
  • High performance: faster than common standard library implementations of
    , iostreams,
    , see
    Speed tests
    _ and
    Converting a hundred million integers to strings per second<http:></http:>
  • Small code size both in terms of source code with the minimum configuration consisting of just three files,
    , and compiled code; see
    Compile time and code bloat
  • Reliability: the library has an extensive set of
    unit tests<https:></https:>
    _ and is continuously fuzzed
  • Safety: the library is fully type safe, errors in format strings can be reported at compile time, automatic memory management prevents buffer overflow errors
  • Ease of use: small self-contained code base, no external dependencies, permissive MIT
  • Portability <https:></https:>
    _ with consistent output across platforms and support for older compilers
  • Clean warning-free codebase even on high warning levels such as ```
  • Wall -Wextra -pedantic ```
  • Locale-independence by default
  • Optional header-only configuration enabled with the

See the

documentation <https:></https:>

_ for more details.



Hello, world!




.. code:: c++

#include <fmt>

int main() {
  fmt::print("Hello, world!\n");

Format a string:

.. code:: c++

std::string s = fmt::format("The answer is {}.", 42); // s == "The answer is 42."

Format a string using positional arguments:

.. code:: c++

std::string s = fmt::format("I'd rather be {1} than {0}.", "right", "happy"); // s == "I'd rather be happy than right."

Print chrono durations:

.. code:: c++

#include <fmt>

int main() {
  using namespace std::literals::chrono_literals;
  fmt::print("Default format: {} {}\n", 42s, 100ms);
  fmt::print("strftime-like format: {:%H:%M:%S}\n", 3h + 15min + 30s);


Default format: 42s 100ms strftime-like format: 03:15:30

Print a container:

.. code:: c++

#include <vector>
#include <fmt>

int main() {
  std::vector<int> v = {1, 2, 3};
  fmt::print("{}\n", v);


{1, 2, 3}

Check a format string at compile time:

.. code:: c++

std::string s = fmt::format(FMT\_STRING("{:d}"), "don't panic");

This gives a compile-time error because


is an invalid format specifier for a string.

Write a file from a single thread:

.. code:: c++

#include <fmt>

int main() {
  auto out = fmt::output_file("guide.txt");
  out.print("Don't {}", "Panic");

This is up to 6x faster than using



Create your own functions similar to


_ and

print <https:></https:>

_ which take arbitrary arguments (

godbolt <https:></https:>


.. code:: c++

// Prints formatted error message. void vreport\_error(const char\* format, fmt::format\_args args) { fmt::print("Error: "); fmt::vprint(format, args); } template <typename... args>
void report_error(const char* format, const Args &amp; ... args) {
  vreport_error(format, fmt::make_format_args(args...));

report_error("file not found: {}", path);

Note that


is not parameterized on argument types which can improve compile times and reduce code size compared to a fully parameterized version.


Speed tests ~

================= ============= =========== Library Method Run Time, s ================= ============= =========== libc printf 1.04 libc++ std::ostream 3.05 {fmt} 6.1.1 fmt::print 0.75 Boost Format 1.67 boost::format 7.24 Folly Format folly::format 2.23 ================= ============= ===========

{fmt} is the fastest of the benchmarked methods, ~35% faster than



The above results were generated by building


on macOS 10.14.6 with


, and taking the best of three runs. In the test, the format string


or equivalent is filled 2,000,000 times with output sent to


; for further details refer to the



{fmt} is up to 10x faster than




on floating-point formatting (

dtoa-benchmark <https:></https:>

_) and faster than

double-conversion <https:></https:>
```_ :

.. image:: 69767160-cdaca400-112f-11ea-9fc5-347c9f83caad.png :target:

Compile time and code bloat ~~~~~~~~~~~~~~~~~~~~~~~~~~~

The script


\_ from 

format-benchmark https:

\_ tests compile time and code bloat for nontrivial projects. It generates 100 translation units and uses 


 or its alternative five times in each to simulate a medium sized project. The resulting executable size and compile time (Apple LLVM version 8.1.0 (clang-802.0.42), macOS Sierra, best of three) is shown in the following tables.

**Optimized build (-O3)**

============= =============== ==================== ================== Method Compile Time, s Executable size, KiB Stripped size, KiB ============= =============== ==================== ================== printf 2.6 29 26 printf+string 16.4 29 26 iostreams 31.1 59 55 {fmt} 19.0 37 34 Boost Format 91.9 226 203 Folly Format 115.7 101 88 ============= =============== ==================== ==================

As you can see, {fmt} has 60% less overhead in terms of resulting binary code size compared to iostreams and comes pretty close to


. Boost Format and Folly Format have the largest overheads.


 is the same as 


 but with extra 

include to measure the overhead of the latter.

**Non-optimized build**

============= =============== ==================== ================== Method Compile Time, s Executable size, KiB Stripped size, KiB ============= =============== ==================== ================== printf 2.2 33 30 printf+string 16.0 33 30 iostreams 28.3 56 52 {fmt} 18.2 59 50 Boost Format 54.1 365 303 Folly Format 79.9 445 430 ============= =============== ==================== ==================






 are all linked as shared libraries to compare formatting function overhead only. Boost Format is a header-only library so it doesn't provide any linkage options.

Running the tests ~~~~~~~~~~~~~~~~~

Please refer to

Building the library

\_\_ for the instructions on how to build the library and run the unit tests.


Benchmarks reside in a separate repository,

format-benchmarks https:

\_, so to run the benchmarks you first need to clone this repository and generate Makefiles with CMake::

$ git clone --recursive $ cd format-benchmark $ cmake .

Then you can run the speed test::

$ make speed-test

or the bloat test::

$ make bloat-test

## Projects using this library


0 A.D. https:

\_: A free, open-source, cross-platform real-time strategy game

AMPL/MP https:

\_: An open-source library for mathematical programming

Aseprite https:

\_: Animated sprite editor & pixel art tool 

AvioBook https:

\_: A comprehensive aircraft operations suite

Celestia https:

\_: Real-time 3D visualization of space

Ceph https:

\_: A scalable distributed storage system

ccache https:

\_: A compiler cache

ClickHouse https:

\_: analytical database management system

CUAUV http:

\_: Cornell University's autonomous underwater vehicle

Drake https:

\_: A planning, control, and analysis toolbox for nonlinear dynamical systems (MIT)

Envoy https:

\_: C++ L7 proxy and communication bus (Lyft)

FiveM https:

\_: a modification framework for GTA V

Folly https:

\_: Facebook open-source library

HarpyWar/pvpgn https:

\_: Player vs Player Gaming Network with tweaks

KBEngine https:

\_: An open-source MMOG server engine

Keypirinha https:

\_: A semantic launcher for Windows

Kodi https:

\_ (formerly xbmc): Home theater software

Knuth https:

\_: High-performance Bitcoin full-node

Microsoft Verona https:

\_: Research programming language for concurrent ownership

MongoDB https:

\_: Distributed document database

MongoDB Smasher https:

\_: A small tool to generate randomized datasets

OpenSpace https:

\_: An open-source astrovisualization framework

PenUltima Online (POL) https:

\_: An MMO server, compatible with most Ultima Online clients

PyTorch https:

\_: An open-source machine learning library

quasardb https:

\_: A distributed, high-performance, associative database

readpe https:

\_: Read Portable Executable

redis-cerberus https:

\_: A Redis cluster proxy

redpanda https:

\_: A 10x faster Kafka® replacement for mission critical systems written in C++

rpclib http:

\_: A modern C++ msgpack-RPC server and client library

Salesforce Analytics Cloudhttps:

\_: Business intelligence software

Scylla https:

\_: A Cassandra-compatible NoSQL data store that can handle 1 million transactions per second on a single server

Seastar http:

\_: An advanced, open-source C++ framework for high-performance server applications on modern hardware

spdlog https:

\_: Super fast C++ logging library

Stellar https:

\_: Financial platform

Touch Surgery https:

\_: Surgery simulator

TrinityCore https:

\_: Open-source MMORPG framework

Windows Terminal https:

\_: The new Windows Terminal

More... https:


If you are aware of other projects using this library, please let me know by

email <victor.zverovich></victor.zverovich>

\_ or by submitting an

issue https:

## Motivation

So why yet another formatting library?

There are plenty of methods for doing this task, from standard ones like the printf family of function and iostreams to Boost Format and FastFormat libraries. The reason for creating a new library is that every existing solution that I found either had serious issues or didn't provide all the features I needed.

printf ~~~~~~

The good thing about


 is that it is pretty fast and readily available being a part of the C standard library. The main drawback is that it doesn't support user-defined types. 


 also has safety issues although they are somewhat mitigated with 

__attribute__ ((format (printf, ...))https:

\_ in GCC. There is a POSIX extension that adds positional arguments required for

i18n https:

\_ to 


 but it is not a part of C99 and may not be available on some platforms.

iostreams ~~~~~~~~~

The main issue with iostreams is best illustrated with an example:

.. code:: c++

std::cout << std::setprecision(2) << std::fixed << 1.23456 << "\n";

which is a lot of typing compared to printf:

.. code:: c++

printf("%.2f\n", 1.23456);

Matthew Wilson, the author of FastFormat, called this "chevron hell". iostreams don't support positional arguments by design.

The good part is that iostreams support user-defined types and are safe although error handling is awkward.

Boost Format ~~~~~~~~~~~~

This is a very powerful library which supports both


-like format strings and positional arguments. Its main drawback is performance. According to various, benchmarks it is much slower than other methods considered here. Boost Format also has excessive build times and severe code bloat issues (see



FastFormat ~~~~~~~~~~

This is an interesting library which is fast, safe and has positional arguments. However, it has significant limitations, citing its author:

Three features that have no hope of being accommodated within the current design are: * Leading zeros (or any other non-space padding) * Octal/hexadecimal encoding * Runtime width/alignment specification

It is also quite big and has a heavy dependency, STLSoft, which might be too restrictive for using it in some projects.

Boost Spirit.Karma ~~~~~~~~~~~~~~~~~~

This is not really a formatting library but I decided to include it here for completeness. As iostreams, it suffers from the problem of mixing verbatim text with arguments. The library is pretty fast, but slower on integer formatting than


 with format string compilation on Karma's own benchmark, see 

Converting a hundred million integers to strings per secondhttp:

## License

{fmt} is distributed under the MIT


## Documentation License


Format String Syntax https:

\_ section in the documentation is based on the one from Python 

string module documentation https:

_. For this reason the documentation is distributed under the Python Software Foundation license available in 


## Maintainers

The {fmt} library is maintained by Victor Zverovich (


_) and Jonathan Müller (


Contributors https:

\_ and

Releases https:

``` _ for some of the names. Let us know if your contribution is not listed or mentioned incorrectly and we'll make it right.

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