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{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, 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)

``` c++

include

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

Format a string (run)

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

Format a string using positional arguments
(run)

c++ 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)

c++

include

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)

c++

include

include

int main() { std::vector v = {1, 2, 3}; fmt::print("{}\n", v); }

Output:

[1, 2, 3]
Check a format string at compile time

c++ 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

c++

include

int main() { auto out = fmt::output_file("guide.txt"); out.print("Don't {}", "Panic"); }

This can be [up to 9 times faster than fprintf](
https://vitaut.net/posts/2020/optimal-file-buffer-size/).

Print with colors and text styles

c++

include

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:

Performance

{fmt} can be tens of percent to 20–30 times faster than sprintf and iostreams, especially for numeric formatting. It minimizes dynamic memory allocations and can optionally [compile format strings]( https://fmt.dev/latest/api/#compile-api) into efficient formatting code.

See format-benchmark and dtoa-benchmark for benchmarks and methodology.

Time per double (smaller is better):

[]( https://github.com/fmtlib/dtoa-benchmark)

ostringstream and sprintf are omitted because they are an order of magnitude slower than the other methods.

Compile time and code bloat

The script [bloat-test.py][test] from [format-benchmark][bench] measures the compile-time and code-size overhead each formatting method adds to application code. It generates 100 translation units and uses printf or its alternative five times in each to simulate a medium-sized project. Library and module build costs are excluded. Results on an Apple M5 Max running macOS 26.6.2 with Apple Clang 21.0.0 (clang-2100.1.1.101), taking the best of three runs, are shown in the following tables.

[test]: https://github.com/fmtlib/format-benchmark/blob/master/bloat-test.py [bench]: https://github.com/fmtlib/format-benchmark

Optimized build (-O3)

| Method | Compile time, s | Binary size, KiB | Stripped size, KiB | |--------------------|----------------:|-----------------:|-------------------:| | printf | 1.6 | 54 | 50 | | IOStreams | 25.5 | 98 | 84 | | fmt 12.2 (headers) | 5.1 | 54 | 50 | | fmt 12.2 (module) | 3.7 | 59 | 50 | | Boost Format 1.92 | 49.1 | 517 | 317 |

Using modular {fmt} reduces optimized application-code compile time by 27% without changing the reported stripped binary size.

Non-optimized build

| Method | Compile time, s | Binary size, KiB | Stripped size, KiB | |--------------------|----------------:|-----------------:|-------------------:| | printf | 1.6 | 54 | 50 | | IOStreams | 26.0 | 88 | 68 | | fmt 12.2 (headers) | 4.9 | 87 | 84 | | fmt 12.2 (module) | 3.2 | 77 | 68 | | Boost Format 1.92 | 35.7 | 741 | 431 |

libc, libc++, libfmt, and libfmt-module were linked as shared libraries to compare formatting function overhead only. Boost Format is header-only.

Projects using {fmt}

Notable users include:

• Apple's FoundationDB
• Blizzard Battle.net
• Ceph
• ClickHouse
• Envoy
• Folly
• MariaDB
• MongoDB
• PyTorch
• Quill
• Seastar
• spdlog
• Windows Terminal

[Find more projects using {fmt} on GitHub]( https://github.com/search?q=fmtlib&type=Code).