Fmt – A Modern Formatting Library


{fmt}is an open-source formatting library for C++. It can be used as a safe and fast alternative to (s)printf and iostreams.


Q&A: ask questions on StackOverflow with the tag fmt .


  • Replacement-based format API with positional arguments for localization.
  • Format string syntax similar to the one of str.format in Python.
  • Safe printf implementation including the POSIX extension for positional arguments.
  • Implementation of C++20 std::format .
  • Support for user-defined types.
  • High performance: faster than common standard library implementations of printf and iostreams. Seeand Fast integer to string conversion in C++ .
  • Small code size both in terms of source code (the minimum configuration consists of just three header files, core.h , format.h and format-inl.h ) and compiled code. SeeCompile time and code bloat.
  • Reliability: the library has an extensive set of unit 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 ( -Wall -Wextra -pedantic ).
  • Support for wide strings.
  • Optional header-only configuration enabled with the FMT_HEADER_ONLY macro.

See the documentation for more details.


Print Hello, world! to stdout :

fmt::print("Hello, {}!", "world");  // Python-like format string syntax
fmt::printf("Hello, %s!", "world"); // printf format string syntax

Format a string and use positional arguments:

std::string s = fmt::format("I'd rather be {1} than {0}.", "right", "happy");
// s == "I'd rather be happy than right."

Check a format string at compile time:

#include <fmt/format.h>
std::string s = format(FMT_STRING("{2}"), 42);
$ c++ -Iinclude -std=c++14
... note: in instantiation of function template specialization 'fmt::v5::format<S, int>' requested here
std::string s = format(FMT_STRING("{2}"), 42);
include/fmt/core.h:778:19: note: non-constexpr function 'on_error' cannot be used in a constant expression
include/fmt/format.h:2226:16: note: in call to '&checker.context_->on_error(&"argument index out of range"[0])'
      context_.on_error("argument index out of range");

Use {fmt} as a safe portable replacement for itoa ( godbolt ):

fmt::memory_buffer buf;
format_to(buf, "{}", 42);    // replaces itoa(42, buffer, 10)
format_to(buf, "{:x}", 42);  // replaces itoa(42, buffer, 16)
// access the string with to_string(buf) or

Format objects of user-defined types via a simple extension API :

#include "fmt/format.h"

struct date {
  int year, month, day;

template <>
struct fmt::formatter<date> {
  constexpr auto parse(format_parse_context& ctx) { return ctx.begin(); }

  template <typename FormatContext>
  auto format(const date& d, FormatContext& ctx) {
    return format_to(ctx.out(), "{}-{}-{}", d.year, d.month,;

std::string s = fmt::format("The date is {}", date{2012, 12, 9});
// s == "The date is 2012-12-9"

Create your own functions similar to format and print which take arbitrary arguments ( godbolt ):

// Prints formatted error message.
void vreport_error(const char* format, fmt::format_args args) {
  fmt::print("Error: ");
  fmt::vprint(format, args);
template <typename... Args>
void report_error(const char* format, const Args & ... args) {
  vreport_error(format, fmt::make_format_args(args...));

report_error("file not found: {}", path);

Note that vreport_error is not parameterized on argument types which can improve compile times and reduce code size compared to a fully parameterized version.


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 -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 10x faster than std::ostringstream and sprintf on floating-point formatting ( dtoa-benchmark ) and as fast as double-conversion :

Compile time and code bloat

The script 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 <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 the 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
$ 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
  • AMPL/MP : An open-source library for mathematical programming
  • AvioBook : A comprehensive aircraft operations suite
  • Celestia : Real-time 3D visualization of space
  • Ceph : A scalable distributed storage system
  • ccache : A compiler cache
  • 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
  • readpe : Read Portable Executable
  • 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


If you are aware of other projects using this library, please let me know by email or by submitting an issue .


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.


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.


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 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).


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_int on Karma's own benchmark, see Fast integer to string conversion in C++ .


Q: how can I capture formatting arguments and format them later?

A: use std::tuple :

template <typename... Args>
auto capture(const Args&... args) {
  return std::make_tuple(args...);

auto print_message = [](const auto&... args) {

// Capture and store arguments:
auto args = capture("{} {}", 42, "foo");
// Do formatting:
std::apply(print_message, args);


{fmt} is distributed under the MIT license .

The Format String Syntax section in the documentation is based on the one from Python string module documentation adapted for the current library. 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.


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.

The benchmark section of this readme file and the performance tests are taken from the excellent tinyformat library written by Chris Foster. Boost Format library is acknowledged transitively since it had some influence on tinyformat. Some ideas used in the implementation are borrowed from Loki SafeFormat and Diagnostic API in Clang . Format string syntax and the documentation are based on Python's str.format . Thanks Doug Turnbull for his valuable comments and contribution to the design of the type-safe API and Gregory Czajkowski for implementing binary formatting. Thanks Ruslan Baratov for comprehensive comparison of integer formatting algorithms and useful comments regarding performance, Boris Kaul for C++ counting digits benchmark . Thanks to CarterLi for contributing various improvements to the code.