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Victor Zverovich
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<img src="https://user-images.githubusercontent.com/576385/156254208-f5b743a9-88cf-439d-b0c0-923d53e8d551.png" alt="{fmt}" width="25%"/>
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[](https://github.com/fmtlib/fmt/actions?query=workflow%3Alinux)
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[](https://github.com/fmtlib/fmt/actions?query=workflow%3Amacos)
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[](https://github.com/fmtlib/fmt/actions?query=workflow%3Awindows)
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[](https://bugs.chromium.org/p/oss-fuzz/issues/list?\%0Acolspec=ID%20Type%20Component%20Status%20Proj%20Reported%20Owner%20\%0ASummary&q=proj%3Dfmt&can=1)
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[](https://stackoverflow.com/questions/tagged/fmt)
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[](https://securityscorecards.dev/viewer/?uri=github.com/fmtlib/fmt)
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**{fmt}** is an open-source formatting library providing a fast and safe
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alternative to C stdio and C++ iostreams.
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If you like this project, please consider donating to one of the funds
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that help victims of the war in Ukraine: <https://www.stopputin.net/>.
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[Documentation](https://fmt.dev)
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[Cheat Sheets](https://hackingcpp.com/cpp/libs/fmt.html)
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Q&A: ask questions on [StackOverflow with the tag
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fmt](https://stackoverflow.com/questions/tagged/fmt).
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Try {fmt} in [Compiler Explorer](https://godbolt.org/z/Eq5763).
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# Features
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- Simple [format API](https://fmt.dev/latest/api.html) with positional
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arguments for localization
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- Implementation of [C++20
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std::format](https://en.cppreference.com/w/cpp/utility/format) and
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[C++23 std::print](https://en.cppreference.com/w/cpp/io/print)
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- [Format string syntax](https://fmt.dev/latest/syntax.html) similar
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to Python\'s
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[format](https://docs.python.org/3/library/stdtypes.html#str.format)
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- Fast IEEE 754 floating-point formatter with correct rounding,
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shortness and round-trip guarantees using the
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[Dragonbox](https://github.com/jk-jeon/dragonbox) algorithm
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- Portable Unicode support
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- Safe [printf
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implementation](https://fmt.dev/latest/api.html#printf-formatting)
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including the POSIX extension for positional arguments
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- Extensibility: [support for user-defined
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types](https://fmt.dev/latest/api.html#formatting-user-defined-types)
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- High performance: faster than common standard library
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implementations of `(s)printf`, iostreams, `to_string` and
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`to_chars`, see [Speed tests](#speed-tests) and [Converting a
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hundred million integers to strings per
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second](http://www.zverovich.net/2020/06/13/fast-int-to-string-revisited.html)
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- Small code size both in terms of source code with the minimum
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configuration consisting of just three files, `core.h`, `format.h`
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and `format-inl.h`, and compiled code; see [Compile time and code
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bloat](#compile-time-and-code-bloat)
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- Reliability: the library has an extensive set of
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[tests](https://github.com/fmtlib/fmt/tree/master/test) and is
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[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)
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- Safety: the library is fully type-safe, errors in format strings can
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be reported at compile time, automatic memory management prevents
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buffer overflow errors
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- Ease of use: small self-contained code base, no external
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dependencies, permissive MIT
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[license](https://github.com/fmtlib/fmt/blob/master/LICENSE.rst)
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- [Portability](https://fmt.dev/latest/index.html#portability) with
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consistent output across platforms and support for older compilers
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- Clean warning-free codebase even on high warning levels such as
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`-Wall -Wextra -pedantic`
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- Locale independence by default
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- Optional header-only configuration enabled with the
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`FMT_HEADER_ONLY` macro
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See the [documentation](https://fmt.dev) for more details.
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# Examples
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**Print to stdout** ([run](https://godbolt.org/z/Tevcjh))
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``` c++
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#include <fmt/core.h>
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int main() {
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fmt::print("Hello, world!\n");
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}
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```
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**Format a string** ([run](https://godbolt.org/z/oK8h33))
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``` c++
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std::string s = fmt::format("The answer is {}.", 42);
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// s == "The answer is 42."
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```
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**Format a string using positional arguments**
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([run](https://godbolt.org/z/Yn7Txe))
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``` c++
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std::string s = fmt::format("I'd rather be {1} than {0}.", "right", "happy");
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// s == "I'd rather be happy than right."
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```
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**Print chrono durations** ([run](https://godbolt.org/z/K8s4Mc))
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``` c++
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#include <fmt/chrono.h>
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int main() {
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using namespace std::literals::chrono_literals;
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fmt::print("Default format: {} {}\n", 42s, 100ms);
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fmt::print("strftime-like format: {:%H:%M:%S}\n", 3h + 15min + 30s);
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}
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```
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Output:
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Default format: 42s 100ms
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strftime-like format: 03:15:30
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**Print a container** ([run](https://godbolt.org/z/MxM1YqjE7))
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``` c++
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#include <vector>
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#include <fmt/ranges.h>
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int main() {
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std::vector<int> v = {1, 2, 3};
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fmt::print("{}\n", v);
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}
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```
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Output:
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[1, 2, 3]
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**Check a format string at compile time**
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``` c++
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std::string s = fmt::format("{:d}", "I am not a number");
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```
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This gives a compile-time error in C++20 because `d` is an invalid
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format specifier for a string.
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**Write a file from a single thread**
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``` c++
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#include <fmt/os.h>
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int main() {
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auto out = fmt::output_file("guide.txt");
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out.print("Don't {}", "Panic");
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}
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```
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This can be [5 to 9 times faster than
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fprintf](http://www.zverovich.net/2020/08/04/optimal-file-buffer-size.html).
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**Print with colors and text styles**
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``` c++
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#include <fmt/color.h>
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int main() {
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fmt::print(fg(fmt::color::crimson) | fmt::emphasis::bold,
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"Hello, {}!\n", "world");
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fmt::print(fg(fmt::color::floral_white) | bg(fmt::color::slate_gray) |
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fmt::emphasis::underline, "Olá, {}!\n", "Mundo");
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fmt::print(fg(fmt::color::steel_blue) | fmt::emphasis::italic,
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"你好{}！\n", "世界");
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}
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```
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Output on a modern terminal with Unicode support:
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# Benchmarks
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## Speed tests
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+-------------------+---------------+-------------+
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| Library | Method | Run Time, s |
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+===================+===============+=============+
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| libc | printf | > 0.91 |
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+-------------------+---------------+-------------+
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| libc++ | std::ostream | > 2.49 |
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+-------------------+---------------+-------------+
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| {fmt} 9.1 | fmt::print | > 0.74 |
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+-------------------+---------------+-------------+
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| Boost Format 1.80 | boost::format | > 6.26 |
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+-------------------+---------------+-------------+
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| Folly Format | folly::format | > 1.87 |
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+-------------------+---------------+-------------+
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{fmt} is the fastest of the benchmarked methods, \~20% faster than
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`printf`.
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The above results were generated by building `tinyformat_test.cpp` on
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macOS 12.6.1 with `clang++ -O3 -DNDEBUG -DSPEED_TEST -DHAVE_FORMAT`, and
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taking the best of three runs. In the test, the format string
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`"%0.10f:%04d:%+g:%s:%p:%c:%%\n"` or equivalent is filled 2,000,000
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times with output sent to `/dev/null`; for further details refer to the
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[source](https://github.com/fmtlib/format-benchmark/blob/master/src/tinyformat-test.cc).
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{fmt} is up to 20-30x faster than `std::ostringstream` and `sprintf` on
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IEEE754 `float` and `double` formatting
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([dtoa-benchmark](https://github.com/fmtlib/dtoa-benchmark)) and faster
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than [double-conversion](https://github.com/google/double-conversion)
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and [ryu](https://github.com/ulfjack/ryu):
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[](https://fmt.dev/unknown_mac64_clang12.0.html)
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## Compile time and code bloat
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The script
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[bloat-test.py](https://github.com/fmtlib/format-benchmark/blob/master/bloat-test.py)
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from [format-benchmark](https://github.com/fmtlib/format-benchmark)
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tests compile time and code bloat for nontrivial projects. It generates
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100 translation units and uses `printf()` or its alternative five times
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in each to simulate a medium-sized project. The resulting executable
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size and compile time (Apple LLVM version 8.1.0 (clang-802.0.42), macOS
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Sierra, best of three) is shown in the following tables.
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**Optimized build (-O3)**
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| Method | Compile Time, s | Executable size, KiB | Stripped size, KiB |
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| ------------- | --------------- | -------------------- | ------------------ |
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| printf | > 2.6 | > 29 | > 26 |
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| printf+string | > 16.4 | > 29 | > 26 |
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| iostreams | > 31.1 | > 59 | > 55 |
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| {fmt} | > 19.0 | > 37 | > 34 |
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| Boost Format | > 91.9 | > 226 | > 203 |
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| Folly Format | > 115.7 | > 101 | > 88 |
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As you can see, {fmt} has 60% less overhead in terms of resulting binary
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code size compared to iostreams and comes pretty close to `printf`.
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Boost Format and Folly Format have the largest overheads.
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`printf+string` is the same as `printf` but with an extra `<string>`
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include to measure the overhead of the latter.
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**Non-optimized build**
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| Method | Compile Time, s | Executable size, KiB | Stripped size, KiB |
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| ------------- | --------------- | -------------------- | ------------------ |
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| printf | > 2.2 | > 33 | > 30 |
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| printf+string | > 16.0 | > 33 | > 30 |
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| iostreams | > 28.3 | > 56 | > 52 |
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| {fmt} | > 18.2 | > 59 | > 50 |
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| Boost Format | > 54.1 | > 365 | > 303 |
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| Folly Format | > 79.9 | > 445 | > 430 |
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`libc`, `lib(std)c++`, and `libfmt` are all linked as shared libraries
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to compare formatting function overhead only. Boost Format is a
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header-only library so it doesn\'t provide any linkage options.
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## Running the tests
|
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Please refer to [Building the
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library](https://fmt.dev/latest/usage.html#building-the-library) for
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instructions on how to build the library and run the unit tests.
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Benchmarks reside in a separate repository,
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[format-benchmarks](https://github.com/fmtlib/format-benchmark), so to
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run the benchmarks you first need to clone this repository and generate
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Makefiles with CMake:
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$ git clone --recursive https://github.com/fmtlib/format-benchmark.git
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$ cd format-benchmark
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$ cmake .
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Then you can run the speed test:
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$ make speed-test
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or the bloat test:
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$ make bloat-test
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# Migrating code
|
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|
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[clang-tidy](https://clang.llvm.org/extra/clang-tidy/) v17 (not yet
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released) provides the
|
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[modernize-use-std-print](https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-std-print.html)
|
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check that is capable of converting occurrences of `printf` and
|
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`fprintf` to `fmt::print` if configured to do so. (By default it
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converts to `std::print`.)
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# Projects using this library
|
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|
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- [0 A.D.](https://play0ad.com/): a free, open-source, cross-platform
|
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real-time strategy game
|
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- [AMPL/MP](https://github.com/ampl/mp): an open-source library for
|
||||
mathematical programming
|
||||
- [Aseprite](https://github.com/aseprite/aseprite): animated sprite
|
||||
editor & pixel art tool
|
||||
- [AvioBook](https://www.aviobook.aero/en): a comprehensive aircraft
|
||||
operations suite
|
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- [Blizzard Battle.net](https://battle.net/): an online gaming
|
||||
platform
|
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- [Celestia](https://celestia.space/): real-time 3D visualization of
|
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space
|
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- [Ceph](https://ceph.com/): a scalable distributed storage system
|
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- [ccache](https://ccache.dev/): a compiler cache
|
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- [ClickHouse](https://github.com/ClickHouse/ClickHouse): an
|
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analytical database management system
|
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- [Contour](https://github.com/contour-terminal/contour/): a modern
|
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terminal emulator
|
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- [CUAUV](https://cuauv.org/): Cornell University\'s autonomous
|
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underwater vehicle
|
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- [Drake](https://drake.mit.edu/): a planning, control, and analysis
|
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toolbox for nonlinear dynamical systems (MIT)
|
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- [Envoy](https://lyft.github.io/envoy/): C++ L7 proxy and
|
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communication bus (Lyft)
|
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- [FiveM](https://fivem.net/): a modification framework for GTA V
|
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- [fmtlog](https://github.com/MengRao/fmtlog): a performant
|
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fmtlib-style logging library with latency in nanoseconds
|
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- [Folly](https://github.com/facebook/folly): Facebook open-source
|
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library
|
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- [GemRB](https://gemrb.org/): a portable open-source implementation
|
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of Bioware's Infinity Engine
|
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- [Grand Mountain
|
||||
Adventure](https://store.steampowered.com/app/1247360/Grand_Mountain_Adventure/):
|
||||
a beautiful open-world ski & snowboarding game
|
||||
- [HarpyWar/pvpgn](https://github.com/pvpgn/pvpgn-server): Player vs
|
||||
Player Gaming Network with tweaks
|
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- [KBEngine](https://github.com/kbengine/kbengine): an open-source
|
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MMOG server engine
|
||||
- [Keypirinha](https://keypirinha.com/): a semantic launcher for
|
||||
Windows
|
||||
- [Kodi](https://kodi.tv/) (formerly xbmc): home theater software
|
||||
- [Knuth](https://kth.cash/): high-performance Bitcoin full-node
|
||||
- [libunicode](https://github.com/contour-terminal/libunicode/): a
|
||||
modern C++17 Unicode library
|
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- [MariaDB](https://mariadb.org/): relational database management
|
||||
system
|
||||
- [Microsoft Verona](https://github.com/microsoft/verona): research
|
||||
programming language for concurrent ownership
|
||||
- [MongoDB](https://mongodb.com/): distributed document database
|
||||
- [MongoDB Smasher](https://github.com/duckie/mongo_smasher): a small
|
||||
tool to generate randomized datasets
|
||||
- [OpenSpace](https://openspaceproject.com/): an open-source
|
||||
astrovisualization framework
|
||||
- [PenUltima Online (POL)](https://www.polserver.com/): an MMO server,
|
||||
compatible with most Ultima Online clients
|
||||
- [PyTorch](https://github.com/pytorch/pytorch): an open-source
|
||||
machine learning library
|
||||
- [quasardb](https://www.quasardb.net/): a distributed,
|
||||
high-performance, associative database
|
||||
- [Quill](https://github.com/odygrd/quill): asynchronous low-latency
|
||||
logging library
|
||||
- [QKW](https://github.com/ravijanjam/qkw): generalizing aliasing to
|
||||
simplify navigation, and executing complex multi-line terminal
|
||||
command sequences
|
||||
- [redis-cerberus](https://github.com/HunanTV/redis-cerberus): a Redis
|
||||
cluster proxy
|
||||
- [redpanda](https://vectorized.io/redpanda): a 10x faster Kafka®
|
||||
replacement for mission-critical systems written in C++
|
||||
- [rpclib](http://rpclib.net/): a modern C++ msgpack-RPC server and
|
||||
client library
|
||||
- [Salesforce Analytics
|
||||
Cloud](https://www.salesforce.com/analytics-cloud/overview/):
|
||||
business intelligence software
|
||||
- [Scylla](https://www.scylladb.com/): a Cassandra-compatible NoSQL
|
||||
data store that can handle 1 million transactions per second on a
|
||||
single server
|
||||
- [Seastar](http://www.seastar-project.org/): an advanced, open-source
|
||||
C++ framework for high-performance server applications on modern
|
||||
hardware
|
||||
- [spdlog](https://github.com/gabime/spdlog): super fast C++ logging
|
||||
library
|
||||
- [Stellar](https://www.stellar.org/): financial platform
|
||||
- [Touch Surgery](https://www.touchsurgery.com/): surgery simulator
|
||||
- [TrinityCore](https://github.com/TrinityCore/TrinityCore):
|
||||
open-source MMORPG framework
|
||||
- [🐙 userver framework](https://userver.tech/): open-source
|
||||
asynchronous framework with a rich set of abstractions and database
|
||||
drivers
|
||||
- [Windows Terminal](https://github.com/microsoft/terminal): the new
|
||||
Windows terminal
|
||||
|
||||
[More\...](https://github.com/search?q=fmtlib&type=Code)
|
||||
|
||||
If you are aware of other projects using this library, please let me
|
||||
know by [email](mailto:victor.zverovich@gmail.com) or by submitting an
|
||||
[issue](https://github.com/fmtlib/fmt/issues).
|
||||
|
||||
# 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,
|
||||
\...))](https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html) in
|
||||
GCC. There is a POSIX extension that adds positional arguments required
|
||||
for
|
||||
[i18n](https://en.wikipedia.org/wiki/Internationalization_and_localization)
|
||||
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:
|
||||
|
||||
``` c++
|
||||
std::cout << std::setprecision(2) << std::fixed << 1.23456 << "\n";
|
||||
```
|
||||
|
||||
which is a lot of typing compared to printf:
|
||||
|
||||
``` c++
|
||||
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](#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, STLSoft, which might be
|
||||
too restrictive for using it 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](http://www.zverovich.net/2020/06/13/fast-int-to-string-revisited.html).
|
||||
|
||||
# License
|
||||
|
||||
{fmt} is distributed under the MIT
|
||||
[license](https://github.com/fmtlib/fmt/blob/master/LICENSE).
|
||||
|
||||
# Documentation License
|
||||
|
||||
The [Format String Syntax](https://fmt.dev/latest/syntax.html) section
|
||||
in the documentation is based on the one from Python [string module
|
||||
documentation](https://docs.python.org/3/library/string.html#module-string).
|
||||
For this reason, the documentation is distributed under the Python
|
||||
Software Foundation license available in
|
||||
[doc/python-license.txt](https://raw.github.com/fmtlib/fmt/master/doc/python-license.txt).
|
||||
It only applies if you distribute the documentation of {fmt}.
|
||||
|
||||
# Maintainers
|
||||
|
||||
The {fmt} library is maintained by Victor Zverovich
|
||||
([vitaut](https://github.com/vitaut)) with contributions from many other
|
||||
people. See
|
||||
[Contributors](https://github.com/fmtlib/fmt/graphs/contributors) and
|
||||
[Releases](https://github.com/fmtlib/fmt/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](https://github.com/fmtlib/fmt/security/advisories/new).
|
||||
|
||||
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.
|
||||
553
README.rst
553
README.rst
@ -1,553 +0,0 @@
|
||||
.. image:: https://user-images.githubusercontent.com/
|
||||
576385/156254208-f5b743a9-88cf-439d-b0c0-923d53e8d551.png
|
||||
:width: 25%
|
||||
:alt: {fmt}
|
||||
|
||||
.. image:: https://github.com/fmtlib/fmt/workflows/linux/badge.svg
|
||||
:target: https://github.com/fmtlib/fmt/actions?query=workflow%3Alinux
|
||||
|
||||
.. image:: https://github.com/fmtlib/fmt/workflows/macos/badge.svg
|
||||
:target: https://github.com/fmtlib/fmt/actions?query=workflow%3Amacos
|
||||
|
||||
.. image:: https://github.com/fmtlib/fmt/workflows/windows/badge.svg
|
||||
:target: https://github.com/fmtlib/fmt/actions?query=workflow%3Awindows
|
||||
|
||||
.. image:: https://oss-fuzz-build-logs.storage.googleapis.com/badges/fmt.svg
|
||||
:alt: fmt is continuously fuzzed at oss-fuzz
|
||||
:target: https://bugs.chromium.org/p/oss-fuzz/issues/list?\
|
||||
colspec=ID%20Type%20Component%20Status%20Proj%20Reported%20Owner%20\
|
||||
Summary&q=proj%3Dfmt&can=1
|
||||
|
||||
.. image:: https://img.shields.io/badge/stackoverflow-fmt-blue.svg
|
||||
:alt: Ask questions at StackOverflow with the tag fmt
|
||||
:target: https://stackoverflow.com/questions/tagged/fmt
|
||||
|
||||
.. image:: https://api.securityscorecards.dev/projects/github.com/fmtlib/fmt/badge
|
||||
:target: https://securityscorecards.dev/viewer/?uri=github.com/fmtlib/fmt
|
||||
|
||||
**{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 <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/Eq5763>`_.
|
||||
|
||||
Features
|
||||
--------
|
||||
|
||||
* Simple `format API <https://fmt.dev/latest/api.html>`_ 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.html>`_ 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.html#printf-formatting>`_ including the POSIX
|
||||
extension for positional arguments
|
||||
* Extensibility: `support for user-defined types
|
||||
<https://fmt.dev/latest/api.html#formatting-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
|
||||
<http://www.zverovich.net/2020/06/13/fast-int-to-string-revisited.html>`_
|
||||
* 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
|
||||
<https://github.com/fmtlib/fmt/tree/master/test>`_ and is `continuously fuzzed
|
||||
<https://bugs.chromium.org/p/oss-fuzz/issues/list?colspec=ID%20Type%20
|
||||
Component%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.rst>`_
|
||||
* `Portability <https://fmt.dev/latest/index.html#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>`_)
|
||||
|
||||
.. code:: c++
|
||||
|
||||
#include <fmt/core.h>
|
||||
|
||||
int main() {
|
||||
fmt::print("Hello, world!\n");
|
||||
}
|
||||
|
||||
**Format a string** (`run <https://godbolt.org/z/oK8h33>`_)
|
||||
|
||||
.. code:: 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>`_)
|
||||
|
||||
.. code:: c++
|
||||
|
||||
std::string s = fmt::format("I'd rather be {1} than {0}.", "right", "happy");
|
||||
// s == "I'd rather be happy than right."
|
||||
|
||||
**Print chrono durations** (`run <https://godbolt.org/z/K8s4Mc>`_)
|
||||
|
||||
.. code:: c++
|
||||
|
||||
#include <fmt/chrono.h>
|
||||
|
||||
int main() {
|
||||
using namespace std::literals::chrono_literals;
|
||||
fmt::print("Default format: {} {}\n", 42s, 100ms);
|
||||
fmt::print("strftime-like format: {:%H:%M:%S}\n", 3h + 15min + 30s);
|
||||
}
|
||||
|
||||
Output::
|
||||
|
||||
Default format: 42s 100ms
|
||||
strftime-like format: 03:15:30
|
||||
|
||||
**Print a container** (`run <https://godbolt.org/z/MxM1YqjE7>`_)
|
||||
|
||||
.. code:: 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**
|
||||
|
||||
.. code:: 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**
|
||||
|
||||
.. code:: c++
|
||||
|
||||
#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
|
||||
<http://www.zverovich.net/2020/08/04/optimal-file-buffer-size.html>`_.
|
||||
|
||||
**Print with colors and text styles**
|
||||
|
||||
.. code:: 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:
|
||||
|
||||
.. image:: https://github.com/fmtlib/fmt/assets/
|
||||
576385/2a93c904-d6fa-4aa6-b453-2618e1c327d7
|
||||
|
||||
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
|
||||
<https://github.com/fmtlib/format-benchmark/blob/master/src/tinyformat-test.cc>`_.
|
||||
|
||||
{fmt} is up to 20-30x faster than ``std::ostringstream`` and ``sprintf`` on
|
||||
IEEE754 ``float`` and ``double`` formatting (`dtoa-benchmark <https://github.com/fmtlib/dtoa-benchmark>`_)
|
||||
and faster than `double-conversion <https://github.com/google/double-conversion>`_ and
|
||||
`ryu <https://github.com/ulfjack/ryu>`_:
|
||||
|
||||
.. image:: https://user-images.githubusercontent.com/576385/
|
||||
95684665-11719600-0ba8-11eb-8e5b-972ff4e49428.png
|
||||
:target: https://fmt.dev/unknown_mac64_clang12.0.html
|
||||
|
||||
Compile time and code bloat
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
The script `bloat-test.py
|
||||
<https://github.com/fmtlib/format-benchmark/blob/master/bloat-test.py>`_
|
||||
from `format-benchmark <https://github.com/fmtlib/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 an 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 instructions on how to build
|
||||
the library and run the unit tests.
|
||||
|
||||
__ https://fmt.dev/latest/usage.html#building-the-library
|
||||
|
||||
Benchmarks reside in a separate repository,
|
||||
`format-benchmarks <https://github.com/fmtlib/format-benchmark>`_,
|
||||
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 <https://clang.llvm.org/extra/clang-tidy/>`_ v17 (not yet
|
||||
released) provides the `modernize-use-std-print
|
||||
<https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-std-print.html>`_
|
||||
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``.)
|
||||
|
||||
Projects using this library
|
||||
---------------------------
|
||||
|
||||
* `0 A.D. <https://play0ad.com/>`_: a free, open-source, cross-platform
|
||||
real-time strategy game
|
||||
|
||||
* `AMPL/MP <https://github.com/ampl/mp>`_:
|
||||
an open-source library for mathematical programming
|
||||
|
||||
* `Aseprite <https://github.com/aseprite/aseprite>`_:
|
||||
animated sprite editor & pixel art tool
|
||||
|
||||
* `AvioBook <https://www.aviobook.aero/en>`_: a comprehensive aircraft
|
||||
operations suite
|
||||
|
||||
* `Blizzard Battle.net <https://battle.net/>`_: an online gaming platform
|
||||
|
||||
* `Celestia <https://celestiaproject.space/>`_: real-time 3D visualization of space
|
||||
|
||||
* `Ceph <https://ceph.com/>`_: a scalable distributed storage system
|
||||
|
||||
* `ccache <https://ccache.dev/>`_: a compiler cache
|
||||
|
||||
* `ClickHouse <https://github.com/ClickHouse/ClickHouse>`_: an analytical database
|
||||
management system
|
||||
|
||||
* `Contour <https://github.com/contour-terminal/contour/>`_: a modern terminal emulator
|
||||
|
||||
* `CUAUV <https://cuauv.org/>`_: Cornell University's autonomous underwater
|
||||
vehicle
|
||||
|
||||
* `Drake <https://drake.mit.edu/>`_: a planning, control, and analysis toolbox
|
||||
for nonlinear dynamical systems (MIT)
|
||||
|
||||
* `Envoy <https://lyft.github.io/envoy/>`_: C++ L7 proxy and communication bus
|
||||
(Lyft)
|
||||
|
||||
* `FiveM <https://fivem.net/>`_: a modification framework for GTA V
|
||||
|
||||
* `fmtlog <https://github.com/MengRao/fmtlog>`_: a performant fmtlib-style
|
||||
logging library with latency in nanoseconds
|
||||
|
||||
* `Folly <https://github.com/facebook/folly>`_: Facebook open-source library
|
||||
|
||||
* `GemRB <https://gemrb.org/>`_: a portable open-source implementation of
|
||||
Bioware’s Infinity Engine
|
||||
|
||||
* `Grand Mountain Adventure
|
||||
<https://store.steampowered.com/app/1247360/Grand_Mountain_Adventure/>`_:
|
||||
a beautiful open-world ski & snowboarding game
|
||||
|
||||
* `HarpyWar/pvpgn <https://github.com/pvpgn/pvpgn-server>`_:
|
||||
Player vs Player Gaming Network with tweaks
|
||||
|
||||
* `KBEngine <https://github.com/kbengine/kbengine>`_: an open-source MMOG server
|
||||
engine
|
||||
|
||||
* `Keypirinha <https://keypirinha.com/>`_: a semantic launcher for Windows
|
||||
|
||||
* `Kodi <https://kodi.tv/>`_ (formerly xbmc): home theater software
|
||||
|
||||
* `Knuth <https://kth.cash/>`_: high-performance Bitcoin full-node
|
||||
|
||||
* `libunicode <https://github.com/contour-terminal/libunicode/>`_: a modern C++17 Unicode library
|
||||
|
||||
* `MariaDB <https://mariadb.org/>`_: relational database management system
|
||||
|
||||
* `Microsoft Verona <https://github.com/microsoft/verona>`_:
|
||||
research programming language for concurrent ownership
|
||||
|
||||
* `MongoDB <https://mongodb.com/>`_: distributed document database
|
||||
|
||||
* `MongoDB Smasher <https://github.com/duckie/mongo_smasher>`_: a small tool to
|
||||
generate randomized datasets
|
||||
|
||||
* `OpenSpace <https://openspaceproject.com/>`_: an open-source
|
||||
astrovisualization framework
|
||||
|
||||
* `PenUltima Online (POL) <https://www.polserver.com/>`_:
|
||||
an MMO server, compatible with most Ultima Online clients
|
||||
|
||||
* `PyTorch <https://github.com/pytorch/pytorch>`_: an open-source machine
|
||||
learning library
|
||||
|
||||
* `quasardb <https://www.quasardb.net/>`_: a distributed, high-performance,
|
||||
associative database
|
||||
|
||||
* `Quill <https://github.com/odygrd/quill>`_: asynchronous low-latency logging library
|
||||
|
||||
* `QKW <https://github.com/ravijanjam/qkw>`_: generalizing aliasing to simplify
|
||||
navigation, and executing complex multi-line terminal command sequences
|
||||
|
||||
* `redis-cerberus <https://github.com/HunanTV/redis-cerberus>`_: a Redis cluster
|
||||
proxy
|
||||
|
||||
* `redpanda <https://vectorized.io/redpanda>`_: a 10x faster Kafka® replacement
|
||||
for mission-critical systems written in C++
|
||||
|
||||
* `rpclib <http://rpclib.net/>`_: a modern C++ msgpack-RPC server and client
|
||||
library
|
||||
|
||||
* `Salesforce Analytics Cloud
|
||||
<https://www.salesforce.com/analytics-cloud/overview/>`_:
|
||||
business intelligence software
|
||||
|
||||
* `Scylla <https://www.scylladb.com/>`_: a Cassandra-compatible NoSQL data store
|
||||
that can handle 1 million transactions per second on a single server
|
||||
|
||||
* `Seastar <http://www.seastar-project.org/>`_: an advanced, open-source C++
|
||||
framework for high-performance server applications on modern hardware
|
||||
|
||||
* `spdlog <https://github.com/gabime/spdlog>`_: super fast C++ logging library
|
||||
|
||||
* `Stellar <https://www.stellar.org/>`_: financial platform
|
||||
|
||||
* `Touch Surgery <https://www.touchsurgery.com/>`_: surgery simulator
|
||||
|
||||
* `TrinityCore <https://github.com/TrinityCore/TrinityCore>`_: open-source
|
||||
MMORPG framework
|
||||
|
||||
* `🐙 userver framework <https://userver.tech/>`_: open-source asynchronous
|
||||
framework with a rich set of abstractions and database drivers
|
||||
|
||||
* `Windows Terminal <https://github.com/microsoft/terminal>`_: the new Windows
|
||||
terminal
|
||||
|
||||
`More... <https://github.com/search?q=fmtlib&type=Code>`_
|
||||
|
||||
If you are aware of other projects using this library, please let me know
|
||||
by `email <mailto:victor.zverovich@gmail.com>`_ or by submitting an
|
||||
`issue <https://github.com/fmtlib/fmt/issues>`_.
|
||||
|
||||
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, ...))
|
||||
<https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_ in GCC.
|
||||
There is a POSIX extension that adds positional arguments required for
|
||||
`i18n <https://en.wikipedia.org/wiki/Internationalization_and_localization>`_
|
||||
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:
|
||||
|
||||
.. code:: c++
|
||||
|
||||
std::cout << std::setprecision(2) << std::fixed << 1.23456 << "\n";
|
||||
|
||||
which is a lot of typing compared to printf:
|
||||
|
||||
.. code:: c++
|
||||
|
||||
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, STLSoft, which might be too
|
||||
restrictive for using it 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
|
||||
<http://www.zverovich.net/2020/06/13/fast-int-to-string-revisited.html>`_.
|
||||
|
||||
License
|
||||
-------
|
||||
|
||||
{fmt} is distributed under the MIT `license
|
||||
<https://github.com/fmtlib/fmt/blob/master/LICENSE>`_.
|
||||
|
||||
Documentation License
|
||||
---------------------
|
||||
|
||||
The `Format String Syntax <https://fmt.dev/latest/syntax.html>`_
|
||||
section in the documentation is based on the one from Python `string module
|
||||
documentation <https://docs.python.org/3/library/string.html#module-string>`_.
|
||||
For this reason, the documentation is distributed under the Python Software
|
||||
Foundation license available in `doc/python-license.txt
|
||||
<https://raw.github.com/fmtlib/fmt/master/doc/python-license.txt>`_.
|
||||
It only applies if you distribute the documentation of {fmt}.
|
||||
|
||||
Maintainers
|
||||
-----------
|
||||
|
||||
The {fmt} library is maintained by Victor Zverovich (`vitaut
|
||||
<https://github.com/vitaut>`_) with contributions from many other people.
|
||||
See `Contributors <https://github.com/fmtlib/fmt/graphs/contributors>`_ and
|
||||
`Releases <https://github.com/fmtlib/fmt/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 <https://github.com/fmtlib/fmt/security/advisories/new>`_.
|
||||
|
||||
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.
|
||||
Loading…
Reference in New Issue
Block a user