fmt

Wed, 02 Sep 2026 20:12:48 +0800

fmtlib/fmt

{fmt} is an open-source formatting library providing a fast and safe alternative to C stdio and C++ iostreams.

Documentation

Cheat Sheets

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 and C++23 std::print
Format string syntax similar to Python's format
Fast IEEE 754 floating-point formatter with correct rounding, shortness and round-trip guarantees using the Dragonbox algorithm
Portable Unicode support
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, base.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)

#include <fmt/base.h>

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

Format a string (run)

1
2

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

Format a string using positional arguments (run)

1
2

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)

#include <fmt/chrono.h>

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)

#include <vector>
#include <fmt/ranges.h>

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

Output:

[1, 2, 3]

Check a format string at compile time

`1`	`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

#include <fmt/os.h>

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

This can be up to 9 times faster than fprintf.

Print with colors and text styles

#include <fmt/color.h>

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:

Benchmarks

Speed tests

Library	Method	Run Time, s
libc	printf	0.66
libc++	std::ostream	1.63
{fmt} 12.1	fmt::print	0.44
Boost Format 1.88	boost::format	3.89
Folly Format	folly::format	1.28

{fmt} is the fastest of the benchmarked methods, ~50% faster than printf.

The above results were generated by building tinyformat_test.cpp on macOS 15.6.1 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.

{fmt} is up to 20-30x faster than std::ostringstream and sprintf on IEEE754 float and double formatting (dtoa-benchmark) and faster than double-conversion and ryu:

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 clang version 15.0.0 (clang-1500.1.0.2.5), macOS Sonoma, best of three) is shown in the following tables.

Optimized build (-O3)

Method	Compile Time, s	Executable size, KiB	Stripped size, KiB
printf	1.6	54	50
IOStreams	28.4	98	84
{fmt} `1122268`	5.0	54	50
tinyformat	32.6	164	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	27.0	88	68
{fmt} `1122268`	4.7	87	84
tinyformat	28.1	185	145
Boost Format	38.9	678	381

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

Benchmarks reside in a separate repository, 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 v18 provides the modernize-use-std-print 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.: a free, open-source, cross-platform real-time strategy game
AMPL/MP: an open-source library for mathematical programming
Apple’s FoundationDB: an open-source, distributed, transactional key-value store
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: an analytical database management system
ContextVision: medical imaging software
Contour: a modern terminal emulator
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
fmtlog: a performant fmtlib-style logging library with latency in nanoseconds
Folly: Facebook open-source library
GemRB: a portable open-source implementation of Bioware’s Infinity Engine
Grand Mountain Adventure: a beautiful open-world ski & snowboarding game
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
libunicode: a modern C++17 Unicode library
MariaDB: relational database management system
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 execute 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
🐙 userver framework: open-source asynchronous framework with a rich set of abstractions and database drivers
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.

Maintainers

The {fmt} library is maintained by Victor Zverovich (vitaut) 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.

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