{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, "Ol, {}!\n", "Mundo");
  fmt::print(fg(fmt::color::steel_blue) | fmt::emphasis::italic,
             "{}\n", "");
}

Output on a modern terminal with Unicode support:

Benchmarks

Speed tests

{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 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)

{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

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 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.

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, 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.

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.