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

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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, 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)

#include <fmt/core.h>

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

Format a string (run)

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

Format a string using positional arguments (run)

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

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 5 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	0.91
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.

{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 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 an extra <string> 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 instructions on how to build the library and run the unit tests.

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

Migrating code

clang-tidy v17 (not yet released) 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
• 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 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
• 🐙 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.

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:

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

which is a lot of typing compared to printf:

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).

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, STLSoft, which might be too restrictive for using it 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.

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) 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.

Security Policy

To report a security issue, please disclose it at security advisory.

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.