Need help with fmt?
Click the “chat” button below for chat support from the developer who created it, or find similar developers for support.

About the developer

fmtlib
9.8K Stars 1.2K Forks Other 4.8K Commits 13 Opened issues

Description

A modern formatting library

Services available

!
?

Need anything else?

Contributors list

{fmt}

.. image:: https://github.com/fmtlib/fmt/workflows/linux/badge.svg :target: https://github.com/fmtlib/fmt/actions?query=workflow%3Alinux

.. image:: https://github.com/fmtlib/fmt/workflows/macos/badge.svg :target: https://github.com/fmtlib/fmt/actions?query=workflow%3Amacos

.. image:: https://github.com/fmtlib/fmt/workflows/windows/badge.svg :target: https://github.com/fmtlib/fmt/actions?query=workflow%3Awindows

.. image:: https://ci.appveyor.com/api/projects/status/ehjkiefde6gucy1v :target: https://ci.appveyor.com/project/vitaut/fmt

.. image:: https://oss-fuzz-build-logs.storage.googleapis.com/badges/fmt.svg :alt: fmt is continuously fuzzed at oss-fuzz :target: https://bugs.chromium.org/p/oss-fuzz/issues/list?\ colspec=ID%20Type%20Component%20Status%20Proj%20Reported%20Owner%20\ Summary&q=proj%3Dfmt&can=1

.. image:: https://img.shields.io/badge/stackoverflow-fmt-blue.svg :alt: Ask questions at StackOverflow with the tag fmt :target: https://stackoverflow.com/questions/tagged/fmt

{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 BYSOL, an initiative to help victims of political repressions in Belarus: https://www.facebook.com/donate/988051675049849/.

Documentation 
__

Q&A: ask questions on

StackOverflow with the tag fmt
_.

Try {fmt} in

Compiler Explorer 
_.

Features

  • Simple
    format API 
    _ with positional arguments for localization
  • Implementation of
    C++20 std::format
    
    __
  • Format string syntax 
    _ similar to Python's
    format 
    _
  • Fast IEEE 754 floating-point formatter with correct rounding, shortness and round-trip guarantees
  • Safe
    printf implementation
    
    _ including the POSIX extension for positional arguments
  • Extensibility:
    support for user-defined types
    
    _
  • High performance: faster than common standard library implementations of
    (s)printf
    , iostreams,
    to_string
    and
    to_chars
    , see
    Speed tests
    _ and
    Converting a hundred million integers to strings per second
    
    _
  • 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
    _
  • Reliability: the library has an extensive set of
    tests
    
    _ 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
    license
    
    _
  • 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 
_ for more details.

Examples

Print to stdout (

run 
_)

.. code:: c++

#include 

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

Format a string (

run 
_)

.. code:: c++

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

Format a string using positional arguments (

run 
_)

.. 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 (

run 
_)

.. code:: c++

#include 

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); }

Output::

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

Print a container (

run 
_)

.. code:: 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

.. code:: c++

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

This gives a compile-time error because

d
is an invalid format specifier for a string.

Write a file from a single thread

.. code:: 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
_.

Print with colors and text styles

.. code:: 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, "Hello, {}!\n", "мир"); fmt::print(fg(fmt::color::steel_blue) | fmt::emphasis::italic, "Hello, {}!\n", "世界"); }

Output on a modern terminal:

.. image:: https://user-images.githubusercontent.com/ 576385/88485597-d312f600-cf2b-11ea-9cbe-61f535a86e28.png

Benchmarks

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

printf
.

The above results were generated by building

tinyformat_test.cpp
on macOS 10.14.6 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
_.

{fmt} is up to 20-30x faster than

std::ostringstream
and
sprintf
on floating-point formatting (
dtoa-benchmark 
) and faster than
double-conversion 
and
ryu 
_:

.. image:: https://user-images.githubusercontent.com/576385/ 95684665-11719600-0ba8-11eb-8e5b-972ff4e49428.png :target: https://fmt.dev/unknownmac64clang12.0.html

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

The script

bloat-test.py
_ from
format-benchmark 
_ 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 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

printf
. Boost Format and Folly Format have the largest overheads.

printf+string
is the same as
printf
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 ============= =============== ==================== ==================

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

__ https://fmt.dev/latest/usage.html#building-the-library

Benchmarks reside in a separate repository,

format-benchmarks 
_, 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

Projects using this library

  • 0 A.D. 
    _: a free, open-source, cross-platform real-time strategy game
  • 2GIS 
    _: free business listings with a city map
  • AMPL/MP 
    _: an open-source library for mathematical programming
  • Aseprite 
    _: animated sprite editor & pixel art tool
  • AvioBook 
    _: a comprehensive aircraft operations suite
  • Blizzard Battle.net 
    _: an online gaming platform
  • Celestia 
    _: real-time 3D visualization of space
  • Ceph 
    _: a scalable distributed storage system
  • ccache 
    _: a compiler cache
  • ClickHouse 
    _: analytical database management system
  • CUAUV 
    _: Cornell University's autonomous underwater vehicle
  • Drake 
    _: a planning, control, and analysis toolbox for nonlinear dynamical systems (MIT)
  • Envoy 
    _: C++ L7 proxy and communication bus (Lyft)
  • FiveM 
    _: a modification framework for GTA V
  • Folly 
    _: Facebook open-source library
  • HarpyWar/pvpgn 
    _: Player vs Player Gaming Network with tweaks
  • KBEngine 
    _: an open-source MMOG server engine
  • Keypirinha 
    _: a semantic launcher for Windows
  • Kodi 
    _ (formerly xbmc): home theater software
  • Knuth 
    _: high-performance Bitcoin full-node
  • Microsoft Verona 
    _: research programming language for concurrent ownership
  • MongoDB 
    _: distributed document database
  • MongoDB Smasher 
    _: a small tool to generate randomized datasets
  • OpenSpace 
    _: an open-source astrovisualization framework
  • PenUltima Online (POL) 
    _: an MMO server, compatible with most Ultima Online clients
  • PyTorch 
    _: an open-source machine learning library
  • quasardb 
    _: a distributed, high-performance, associative database
  • Quill 
    _: asynchronous low-latency logging library
  • QKW 
    _: generalizing aliasing to simplify navigation, and executing complex multi-line terminal command sequences
  • redis-cerberus 
    _: a Redis cluster proxy
  • redpanda 
    _: a 10x faster Kafka® replacement for mission critical systems written in C++
  • rpclib 
    _: a modern C++ msgpack-RPC server and client library
  • Salesforce Analytics Cloud
    
    _: business intelligence software
  • Scylla 
    _: a Cassandra-compatible NoSQL data store that can handle 1 million transactions per second on a single server
  • Seastar 
    _: an advanced, open-source C++ framework for high-performance server applications on modern hardware
  • spdlog 
    _: super fast C++ logging library
  • Stellar 
    _: financial platform
  • Touch Surgery 
    _: surgery simulator
  • TrinityCore 
    _: open-source MMORPG framework
  • Windows Terminal 
    _: the new Windows terminal

More... 
_

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

email 
_ or by submitting an
issue 
_.

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, ...))
_ in GCC. There is a POSIX extension that adds positional arguments required for
i18n 
_ 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:

.. 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

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
_).

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

fmt::format_to
with format string compilation on Karma's own benchmark, see
Converting a hundred million integers to strings per second
_.

License

{fmt} is distributed under the MIT

license
_.

Documentation License

The

Format String Syntax 
_ section in the documentation is based on the one from Python
string module
documentation 
. For this reason the documentation is distributed under the Python Software Foundation license available in
doc/python-license.txt
. It only applies if you distribute the documentation of {fmt}.

Maintainers

The {fmt} library is maintained by Victor Zverovich (

vitaut
) and Jonathan Müller (
foonathan
) with contributions from many other people. See
Contributors 
_ and
Releases 
_ for some of the names. Let us know if your contribution is not listed or mentioned incorrectly and we'll make it right.

We use cookies. If you continue to browse the site, you agree to the use of cookies. For more information on our use of cookies please see our Privacy Policy.