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)

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 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

{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)

{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.

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.