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https://github.com/fmtlib/fmt

A modern formatting library
https://github.com/fmtlib/fmt

c-plus-plus chrono cpp cross-platform floating-point formatting multiplatform output performance printf ranges unicode

Last synced: about 2 months ago
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A modern formatting library

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README 

        
{fmt}



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**{fmt}** is an open-source formatting library providing a fast and safe

alternative to C stdio and C++ iostreams.



If you like this project, please consider donating to one of the funds

that help victims of the war in Ukraine: .



[Documentation](https://fmt.dev)



[Cheat Sheets](https://hackingcpp.com/cpp/libs/fmt.html)



Q&A: ask questions on [StackOverflow with the tag

fmt](https://stackoverflow.com/questions/tagged/fmt).



Try {fmt} in [Compiler Explorer](https://godbolt.org/z/8Mx1EW73v).



# Features



- Simple [format API](https://fmt.dev/latest/api.html) with positional

  arguments for localization

- Implementation of [C++20

  std::format](https://en.cppreference.com/w/cpp/utility/format) and

  [C++23 std::print](https://en.cppreference.com/w/cpp/io/print)

- [Format string syntax](https://fmt.dev/latest/syntax.html) similar

  to Python\'s

  [format](https://docs.python.org/3/library/stdtypes.html#str.format)

- Fast IEEE 754 floating-point formatter with correct rounding,

  shortness and round-trip guarantees using the

  [Dragonbox](https://github.com/jk-jeon/dragonbox) algorithm

- Portable Unicode support

- Safe [printf

  implementation](https://fmt.dev/latest/api.html#printf-formatting)

  including the POSIX extension for positional arguments

- Extensibility: [support for user-defined

  types](https://fmt.dev/latest/api.html#formatting-user-defined-types)

- High performance: faster than common standard library

  implementations of `(s)printf`, iostreams, `to_string` and

  `to_chars`, see [Speed tests](#speed-tests) and [Converting a

  hundred million integers to strings per

  second](http://www.zverovich.net/2020/06/13/fast-int-to-string-revisited.html)

- Small code size both in terms of source code with the minimum

  configuration consisting of just three files, `core.h`, `format.h`

  and `format-inl.h`, and compiled code; see [Compile time and code

  bloat](#compile-time-and-code-bloat)

- Reliability: the library has an extensive set of

  [tests](https://github.com/fmtlib/fmt/tree/master/test) and is

  [continuously fuzzed](https://bugs.chromium.org/p/oss-fuzz/issues/list?colspec=ID%20Type%20Component%20Status%20Proj%20Reported%20Owner%20Summary&q=proj%3Dfmt&can=1)

- 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

  [license](https://github.com/fmtlib/fmt/blob/master/LICENSE.rst)

- [Portability](https://fmt.dev/latest/index.html#portability) 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

  `FMT_HEADER_ONLY` macro



See the [documentation](https://fmt.dev) for more details.



# Examples



**Print to stdout** ([run](https://godbolt.org/z/Tevcjh))



``` c++

#include 



int main() {

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

}

```



**Format a string** ([run](https://godbolt.org/z/oK8h33))



``` c++

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

// s == "The answer is 42."

```



**Format a string using positional arguments**

([run](https://godbolt.org/z/Yn7Txe))



``` c++

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

// s == "I'd rather be happy than right."

```



**Print dates and times** ([run](https://godbolt.org/z/c31ExdY3W))



``` c++

#include 



int main() {

  auto now = std::chrono::system_clock::now();

  fmt::print("Date and time: {}\n", now);

  fmt::print("Time: {:%H:%M}\n", now);

}

```



Output:



    Date and time: 2023-12-26 19:10:31.557195597

    Time: 19:10



**Print a container** ([run](https://godbolt.org/z/MxM1YqjE7))



``` c++

#include 

#include 



int main() {

  std::vector v = {1, 2, 3};

  fmt::print("{}\n", v);

}

```



Output:



    [1, 2, 3]



**Check a format string at compile time**



``` c++

std::string s = fmt::format("{:d}", "I am not a number");

```



This gives a compile-time error in C++20 because `d` is an invalid

format specifier for a string.



**Write a file from a single thread**



``` c++

#include 



int main() {

  auto out = fmt::output_file("guide.txt");

  out.print("Don't {}", "Panic");

}

```



This can be [5 to 9 times faster than

fprintf](http://www.zverovich.net/2020/08/04/optimal-file-buffer-size.html).



**Print with colors and text styles**



``` c++

#include 



int main() {

  fmt::print(fg(fmt::color::crimson) | fmt::emphasis::bold,

             "Hello, {}!\n", "world");

  fmt::print(fg(fmt::color::floral_white) | bg(fmt::color::slate_gray) |

             fmt::emphasis::underline, "Olá, {}!\n", "Mundo");

  fmt::print(fg(fmt::color::steel_blue) | fmt::emphasis::italic,

             "你好{}！\n", "世界");

}

```



Output on a modern terminal with Unicode support:



![image](https://github.com/fmtlib/fmt/assets/%0A576385/2a93c904-d6fa-4aa6-b453-2618e1c327d7)



# Benchmarks



## Speed tests



| Library           | Method        | Run Time, s |

|-------------------|---------------|-------------|

| libc              | printf        |   0.91      |

| libc++            | std::ostream  |   2.49      |

| {fmt} 9.1         | fmt::print    |   0.74      |

| Boost Format 1.80 | boost::format |   6.26      |

| Folly Format      | folly::format |   1.87      |



{fmt} is the fastest of the benchmarked methods, \~20% faster than

`printf`.



The above results were generated by building `tinyformat_test.cpp` on

macOS 12.6.1 with `clang++ -O3 -DNDEBUG -DSPEED_TEST -DHAVE_FORMAT`, and

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

`"%0.10f:%04d:%+g:%s:%p:%c:%%\n"` or equivalent is filled 2,000,000

times with output sent to `/dev/null`; for further details refer to the

[source](https://github.com/fmtlib/format-benchmark/blob/master/src/tinyformat-test.cc).



{fmt} is up to 20-30x faster than `std::ostringstream` and `sprintf` on

IEEE754 `float` and `double` formatting

([dtoa-benchmark](https://github.com/fmtlib/dtoa-benchmark)) and faster

than [double-conversion](https://github.com/google/double-conversion)

and [ryu](https://github.com/ulfjack/ryu):



[![image](https://user-images.githubusercontent.com/576385/95684665-11719600-0ba8-11eb-8e5b-972ff4e49428.png)](https://fmt.dev/unknown_mac64_clang12.0.html)



## Compile time and code bloat



The script [bloat-test.py][test] from [format-benchmark][bench] tests compile

time and code bloat for nontrivial projects. It generates 100 translation units

and uses `printf()` or its alternative five times in each to simulate a

medium-sized project. The resulting executable size and compile time (Apple

clang version 15.0.0 (clang-1500.1.0.2.5), macOS Sonoma, best of three) is shown

in the following tables.



[test]: https://github.com/fmtlib/format-benchmark/blob/master/bloat-test.py

[bench]: https://github.com/fmtlib/format-benchmark



**Optimized build (-O3)**



| Method        | Compile Time, s | Executable size, KiB | Stripped size, KiB |

|---------------|-----------------|----------------------|--------------------|

| printf        |             1.6 |                   54 |                 50 |

| IOStreams     |            25.9 |                   98 |                 84 |

| fmt 83652df   |             4.8 |                   54 |                 50 |

| tinyformat    |            29.1 |                  161 |                136 |

| Boost Format  |            55.0 |                  530 |                317 |



{fmt} is fast to compile and is comparable to `printf` in terms of per-call

binary size (within a rounding error on this system).



**Non-optimized build**



| Method        | Compile Time, s | Executable size, KiB | Stripped size, KiB |

|---------------|-----------------|----------------------|--------------------|

| printf        |             1.4 |                   54 |                 50 |

| IOStreams     |            23.4 |                   92 |                 68 |

| {fmt} 83652df |             4.4 |                   89 |                 85 |

| tinyformat    |            24.5 |                  204 |                161 |

| Boost Format  |            36.4 |                  831 |                462 |



`libc`, `lib(std)c++`, and `libfmt` 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](https://fmt.dev/latest/usage.html#building-the-library) for

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



Benchmarks reside in a separate repository,

[format-benchmarks](https://github.com/fmtlib/format-benchmark), so to

run the benchmarks you first need to clone this repository and generate

Makefiles with CMake:



    $ git clone --recursive https://github.com/fmtlib/format-benchmark.git

    $ cd format-benchmark

    $ cmake .



Then you can run the speed test:



    $ make speed-test



or the bloat test:



    $ make bloat-test



# Migrating code



[clang-tidy](https://clang.llvm.org/extra/clang-tidy/) v17 (not yet

released) provides the

[modernize-use-std-print](https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-std-print.html)

check that is capable of converting occurrences of `printf` and

`fprintf` to `fmt::print` if configured to do so. (By default it

converts to `std::print`.)



# Notable projects using this library



- [0 A.D.](https://play0ad.com/): a free, open-source, cross-platform

  real-time strategy game

- [AMPL/MP](https://github.com/ampl/mp): an open-source library for

  mathematical programming

- [Apple's FoundationDB](https://github.com/apple/foundationdb): an open-source,

  distributed, transactional key-value store

- [Aseprite](https://github.com/aseprite/aseprite): animated sprite

  editor & pixel art tool

- [AvioBook](https://www.aviobook.aero/en): a comprehensive aircraft

  operations suite

- [Blizzard Battle.net](https://battle.net/): an online gaming

  platform

- [Celestia](https://celestia.space/): real-time 3D visualization of

  space

- [Ceph](https://ceph.com/): a scalable distributed storage system

- [ccache](https://ccache.dev/): a compiler cache

- [ClickHouse](https://github.com/ClickHouse/ClickHouse): an

  analytical database management system

- [Contour](https://github.com/contour-terminal/contour/): a modern

  terminal emulator

- [CUAUV](https://cuauv.org/): Cornell University\'s autonomous

  underwater vehicle

- [Drake](https://drake.mit.edu/): a planning, control, and analysis

  toolbox for nonlinear dynamical systems (MIT)

- [Envoy](https://lyft.github.io/envoy/): C++ L7 proxy and

  communication bus (Lyft)

- [FiveM](https://fivem.net/): a modification framework for GTA V

- [fmtlog](https://github.com/MengRao/fmtlog): a performant

  fmtlib-style logging library with latency in nanoseconds

- [Folly](https://github.com/facebook/folly): Facebook open-source

  library

- [GemRB](https://gemrb.org/): a portable open-source implementation

  of Bioware's Infinity Engine

- [Grand Mountain

  Adventure](https://store.steampowered.com/app/1247360/Grand_Mountain_Adventure/):

  a beautiful open-world ski & snowboarding game

- [HarpyWar/pvpgn](https://github.com/pvpgn/pvpgn-server): Player vs

  Player Gaming Network with tweaks

- [KBEngine](https://github.com/kbengine/kbengine): an open-source

  MMOG server engine

- [Keypirinha](https://keypirinha.com/): a semantic launcher for

  Windows

- [Kodi](https://kodi.tv/) (formerly xbmc): home theater software

- [Knuth](https://kth.cash/): high-performance Bitcoin full-node

- [libunicode](https://github.com/contour-terminal/libunicode/): a

  modern C++17 Unicode library

- [MariaDB](https://mariadb.org/): relational database management

  system

- [Microsoft Verona](https://github.com/microsoft/verona): research

  programming language for concurrent ownership

- [MongoDB](https://mongodb.com/): distributed document database

- [MongoDB Smasher](https://github.com/duckie/mongo_smasher): a small

  tool to generate randomized datasets

- [OpenSpace](https://openspaceproject.com/): an open-source

  astrovisualization framework

- [PenUltima Online (POL)](https://www.polserver.com/): an MMO server,

  compatible with most Ultima Online clients

- [PyTorch](https://github.com/pytorch/pytorch): an open-source

  machine learning library

- [quasardb](https://www.quasardb.net/): a distributed,

  high-performance, associative database

- [Quill](https://github.com/odygrd/quill): asynchronous low-latency

  logging library

- [QKW](https://github.com/ravijanjam/qkw): generalizing aliasing to

  simplify navigation, and execute complex multi-line terminal

  command sequences

- [redis-cerberus](https://github.com/HunanTV/redis-cerberus): a Redis

  cluster proxy

- [redpanda](https://vectorized.io/redpanda): a 10x faster Kafka®

  replacement for mission-critical systems written in C++

- [rpclib](http://rpclib.net/): a modern C++ msgpack-RPC server and

  client library

- [Salesforce Analytics

  Cloud](https://www.salesforce.com/analytics-cloud/overview/):

  business intelligence software

- [Scylla](https://www.scylladb.com/): a Cassandra-compatible NoSQL

  data store that can handle 1 million transactions per second on a

  single server

- [Seastar](http://www.seastar-project.org/): an advanced, open-source

  C++ framework for high-performance server applications on modern

  hardware

- [spdlog](https://github.com/gabime/spdlog): super fast C++ logging

  library

- [Stellar](https://www.stellar.org/): financial platform

- [Touch Surgery](https://www.touchsurgery.com/): surgery simulator

- [TrinityCore](https://github.com/TrinityCore/TrinityCore):

  open-source MMORPG framework

- [🐙 userver framework](https://userver.tech/): open-source

  asynchronous framework with a rich set of abstractions and database

  drivers

- [Windows Terminal](https://github.com/microsoft/terminal): the new

  Windows terminal



[More\...](https://github.com/search?q=fmtlib&type=Code)



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

know by [email](mailto:[email protected]) or by submitting an

[issue](https://github.com/fmtlib/fmt/issues).



# 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 `printf` 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. `printf` also has safety

issues although they are somewhat mitigated with [\_\_attribute\_\_

((format (printf,

\...))](https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html) in

GCC. There is a POSIX extension that adds positional arguments required

for

[i18n](https://en.wikipedia.org/wiki/Internationalization_and_localization)

to `printf` 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:



``` c++

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

```



which is a lot of typing compared to printf:



``` 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 that supports both `printf`-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 [Benchmarks](#benchmarks)).



## FastFormat



This is an interesting library that 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, on STLSoft, which might be

too restrictive for use in some projects.



## Boost Spirit.Karma



This is not 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 `fmt::format_to` with format string compilation

on Karma\'s own benchmark, see [Converting a hundred million integers to

strings per

second](http://www.zverovich.net/2020/06/13/fast-int-to-string-revisited.html).



# License



{fmt} is distributed under the MIT

[license](https://github.com/fmtlib/fmt/blob/master/LICENSE).



# Documentation License



The [Format String Syntax](https://fmt.dev/latest/syntax.html) section

in the documentation is based on the one from Python [string module

documentation](https://docs.python.org/3/library/string.html#module-string).

For this reason, the documentation is distributed under the Python

Software Foundation license available in

[doc/python-license.txt](https://raw.github.com/fmtlib/fmt/master/doc/python-license.txt).

It only applies if you distribute the documentation of {fmt}.



# Maintainers



The {fmt} library is maintained by Victor Zverovich

([vitaut](https://github.com/vitaut)) with contributions from many other

people. See

[Contributors](https://github.com/fmtlib/fmt/graphs/contributors) and

[Releases](https://github.com/fmtlib/fmt/releases) for some of the

names. Let us know if your contribution is not listed or mentioned

incorrectly and we\'ll make it right.



# Security Policy



To report a security issue, please disclose it at [security

advisory](https://github.com/fmtlib/fmt/security/advisories/new).



This project is maintained by a team of volunteers on a

reasonable-effort basis. As such, please give us at least *90* days to

work on a fix before public exposure.