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fmt
fmtlib
fmtlib/fmt
A modern formatting library
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<img src="https://user-images.githubusercontent.com/576385/156254208-f5b743a9-88cf-439d-b0c0-923d53e8d551.png" alt="{fmt}" width="25%"/> []( https://github.com/fmtlib/fmt/actions?query=workflow%3Alinux) []( https://github.com/fmtlib/fmt/actions?query=workflow%3Amacos) []( https://github.com/fmtlib/fmt/actions?query=workflow%3Awindows) []( https://issues.oss-fuzz.com/issues?q=title:fmt%20cc:victor.zverovich@gmail.com) []( https://www.bestpractices.dev/projects/8880) []( https://securityscorecards.dev/viewer/?uri=github.com/fmtlib/fmt) [](https://stackoverflow.com/questions/tagged/fmt) [](https://novaukraine.org/) **{fmt}** is an open-source formatting library providing a fast and safe alternative to C stdio and C++ iostreams. [Documentation](https://fmt.dev) [Cheat Sheets](https://hackingcpp.com/cpp/libs/fmt.html) Q&A: ask questions on [StackOverflow with the tag fmt](https://stackoverflow.com/questions/tagged/fmt). Try {fmt} in [Compiler Explorer](https://godbolt.org/z/8Mx1EW73v). [](https://build.demoshell.com/launch?snapshot=demoshell%2Ftools%3Afmt) # Features - Simple [format API](https://fmt.dev/latest/api/) with positional arguments for localization - Implementation of [C++20 std::format](https://en.cppreference.com/w/cpp/utility/format) and [C++23 std::print](https://en.cppreference.com/w/cpp/io/print) - [Format string syntax](https://fmt.dev/latest/syntax/) similar to Python\'s [format](https://docs.python.org/3/library/stdtypes.html#str.format) - Fast IEEE 754 floating-point formatter with correct rounding, shortness and round-trip guarantees using the [Dragonbox](https://github.com/jk-jeon/dragonbox) algorithm - Portable Unicode support - Safe [printf implementation](https://fmt.dev/latest/api/#printf-api) including the POSIX extension for positional arguments - Extensibility: [support for user-defined types](https://fmt.dev/latest/api/#formatting-user-defined-types) - High performance: faster than common standard library implementations of `(s)printf`, iostreams, `to_string` and `to_chars`, see [Speed tests](#speed-tests) and [Converting a hundred million integers to strings per second](https://vitaut.net/posts/2020/fast-int-to-string-revisited/) - 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](#compile-time-and-code-bloat) - Reliability: the library has an extensive set of [tests](https://github.com/fmtlib/fmt/tree/master/test) and is [continuously fuzzed](https://bugs.chromium.org/p/oss-fuzz/issues/list?colspec=ID%20Type%20Component%20Status%20Proj%20Reported%20Owner%20Summary&q=proj%3Dfmt&can=1) - 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](https://github.com/fmtlib/fmt/blob/master/LICENSE) - [Portability](https://fmt.dev/latest/#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](https://fmt.dev) for more details. # Examples **Print to stdout** ([run](https://godbolt.org/z/Tevcjh)) ``` c++ #include <fmt/core.h> int main() { fmt::print("Hello, world!\n"); } ``` **Format a string** ([run](https://godbolt.org/z/oK8h33)) ``` c++ std::string s = fmt::format("The answer is {}.", 42); // s == "The answer is 42." ``` **Format a string using positional arguments** ([run](https://godbolt.org/z/Yn7Txe)) ``` 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](https://godbolt.org/z/c31ExdY3W)) ``` c++ #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](https://godbolt.org/z/MxM1YqjE7)) ``` c++ #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** ``` 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 <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`]( https://vitaut.net/posts/2020/optimal-file-buffer-size/). **Print with colors and text styles** ``` c++ #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:  # 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](https://github.com/fmtlib/format-benchmark) and [dtoa-benchmark](https://github.com/fmtlib/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](https://github.com/apple/foundationdb) - [Blizzard Battle.net](https://battle.net/) - [Ceph](https://ceph.com/) - [ClickHouse](https://github.com/ClickHouse/ClickHouse) - [Envoy](https://github.com/envoyproxy/envoy) - [Folly](https://github.com/facebook/folly) - [MariaDB](https://mariadb.org/) - [MongoDB](https://mongodb.com/) - [PyTorch](https://github.com/pytorch/pytorch) - [Quill](https://github.com/odygrd/quill) - [Seastar](https://seastar.io/) - [spdlog](https://github.com/gabime/spdlog) - [Windows Terminal](https://github.com/microsoft/terminal) [Find more projects using {fmt} on GitHub]( https://github.com/search?q=fmtlib&type=Code).
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