fmt

A modern formatting library

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Quick Overview

fmt is a modern formatting library for C++ that provides a fast and safe alternative to C stdio and C++ iostreams. It offers simple and efficient string formatting with a Python-like syntax, type-safe format string compilation, and support for user-defined types.

Pros

Fast and efficient performance, often outperforming traditional C++ I/O methods
Type-safe and compile-time checked format strings
Easy-to-use API with Python-like syntax
Extensive support for various data types and customization options

Cons

Requires C++11 or later, which may not be available in all environments
Adds an external dependency to projects
Learning curve for developers accustomed to traditional C++ I/O methods

Code Examples

Basic string formatting:

#include <fmt/core.h>

std::string name = "Alice";
int age = 30;
std::string result = fmt::format("My name is {} and I'm {} years old.", name, age);

Positional arguments and named arguments:

#include <fmt/core.h>

std::string result = fmt::format("The {1} in the {0} says {2}.", "hat", "cat", "meow");
result = fmt::format("I am {age} years old and my name is {name}.", fmt::arg("name", "Bob"), fmt::arg("age", 25));

Custom type formatting:

#include <fmt/format.h>

struct Point {
    int x, y;
};

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

    template <typename FormatContext>
    auto format(const Point& p, FormatContext& ctx) -> decltype(ctx.out()) {
        return fmt::format_to(ctx.out(), "({}, {})", p.x, p.y);
    }
};

Point p{1, 2};
std::string result = fmt::format("The point is: {}", p);

Getting Started

To use fmt in your project:

Install fmt using a package manager or download it from GitHub.
Include the necessary headers in your C++ file:

#include <fmt/core.h>
// For extended functionality:
// #include <fmt/format.h>

Link against the fmt library when compiling.
Use the fmt::format function or other fmt features in your code:

#include <fmt/core.h>
#include <iostream>

int main() {
    std::string message = fmt::format("Hello, {}!", "World");
    std::cout << message << std::endl;
    return 0;
}

Competitor Comparisons

spdlog

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Fast C++ logging library.

Pros of spdlog

Comprehensive logging framework with built-in asynchronous logging and file rotation
Supports multiple sinks (console, file, syslog, etc.) out of the box
Includes a thread-safe logger registry for easy management of multiple loggers

Cons of spdlog

Slightly higher memory footprint due to additional features
May have a steeper learning curve for users who only need basic formatting

Code Comparison

spdlog:

#include "spdlog/spdlog.h"

spdlog::info("Welcome to spdlog!");
spdlog::error("An error message with arg: {}", 1);

fmt:

#include <fmt/core.h>

fmt::print("Welcome to fmt!\n");
fmt::print(stderr, "An error message with arg: {}\n", 1);

Both libraries offer similar formatting syntax, but spdlog provides additional logging-specific features. fmt focuses on string formatting and I/O, while spdlog builds upon fmt's functionality to offer a complete logging solution.

spdlog is ideal for projects requiring a full-featured logging system, whereas fmt is better suited for general string formatting and I/O operations. The choice between the two depends on the specific needs of your project.

abseil-cpp

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Abseil Common Libraries (C++)

Pros of abseil-cpp

Broader scope: Offers a wide range of utilities beyond string formatting
Google-backed: Developed and maintained by Google, ensuring high quality and long-term support
Seamless integration: Designed to work well with other Google libraries and projects

Cons of abseil-cpp

Steeper learning curve: More complex due to its broader scope
Larger footprint: Includes many components, which may increase binary size
Less specialized: String formatting is just one of many features, potentially less optimized than fmt

Code Comparison

fmt:

#include <fmt/core.h>
std::string s = fmt::format("Hello, {}!", "world");

abseil-cpp:

#include "absl/strings/str_format.h"
std::string s = absl::StrFormat("Hello, %s!", "world");

Both libraries provide similar functionality for string formatting, but fmt uses a more modern syntax with curly braces, while abseil-cpp uses printf-style format specifiers. fmt is generally considered more type-safe and easier to use for string formatting specifically, while abseil-cpp offers a broader set of utilities beyond just formatting.

folly

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An open-source C++ library developed and used at Facebook.

Pros of Folly

Broader scope: Folly is a comprehensive C++ library with a wide range of utilities and components
Performance-oriented: Designed for high-performance applications at scale
Active development: Regularly updated with new features and optimizations

Cons of Folly

Steeper learning curve: More complex due to its extensive feature set
Larger footprint: Heavier library compared to fmt's focused approach
Potential overkill: May be excessive for projects only needing formatting capabilities

Code Comparison

fmt:

#include <fmt/core.h>
std::string s = fmt::format("Hello, {}!", "world");

Folly:

#include <folly/Format.h>
std::string s = folly::format("Hello, {}!", "world").str();

Both libraries offer similar formatting functionality, but Folly provides a broader set of utilities beyond just formatting. fmt is more focused and lightweight, making it easier to integrate for specific formatting needs. Folly, on the other hand, offers a comprehensive suite of tools for large-scale C++ development, which may be beneficial for more complex projects but could be overwhelming for simpler applications.

glog

7,028

C++ implementation of the Google logging module

Pros of glog

More comprehensive logging system with severity levels and conditional logging
Includes additional features like stack trace logging and crash handling
Better integration with Google's ecosystem of tools and libraries

Cons of glog

Heavier and more complex than fmt, which may be overkill for simpler projects
Less focus on formatting capabilities compared to fmt
Slower compilation times due to its larger codebase

Code Comparison

glog:

#include <glog/logging.h>

int main(int argc, char* argv[]) {
  google::InitGoogleLogging(argv[0]);
  LOG(INFO) << "Hello, World!";
  return 0;
}

fmt:

#include <fmt/core.h>

int main() {
  fmt::print("Hello, {}!\n", "World");
  return 0;
}

glog provides a more structured logging approach with severity levels, while fmt focuses on efficient and flexible string formatting. glog is better suited for larger projects requiring comprehensive logging, whereas fmt is ideal for projects needing fast and simple formatting capabilities.

json

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JSON for Modern C++

Pros of json

Specialized for JSON parsing and manipulation
Extensive JSON-specific features (e.g., serialization, deserialization)
Intuitive API for working with JSON data structures

Cons of json

Limited to JSON-related operations
Larger library size compared to fmt
May have slower performance for simple string operations

Code Comparison

json:

#include <nlohmann/json.hpp>
using json = nlohmann::json;

json j = {{"name", "John"}, {"age", 30}};
std::string serialized = j.dump();

fmt:

#include <fmt/core.h>

std::string name = "John";
int age = 30;
std::string formatted = fmt::format("Name: {}, Age: {}", name, age);

Summary

While json excels in JSON-specific tasks, fmt is a more general-purpose formatting library. json offers robust JSON handling but may be overkill for simple string operations. fmt provides efficient and type-safe formatting for various data types but lacks JSON-specific features. Choose json for JSON-centric projects and fmt for general string formatting and output needs.

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README

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

If you like this project, please consider donating to one of the funds that help victims of the war in Ukraine: https://www.stopputin.net/.

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, "OlÃ¡, {}!\n", "Mundo");
  fmt::print(fg(fmt::color::steel_blue) | fmt::emphasis::italic,
             "ä½ å¥½{}ï¼\n", "ä¸ç");
}

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 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	25.9	98	84
fmt 83652df	4.8	54	50
tinyformat	29.1	161	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	23.4	92	68
{fmt} 83652df	4.4	89	85
tinyformat	24.5	204	161
Boost Format	36.4	831	462

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
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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 execute complex multi-line terminal command sequences
redis-cerberus: a Redis cluster proxy
redpanda: a 10x faster KafkaÂ® 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
ð 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.

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