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{fmt}
=====
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.. image:: https://github.com/fmtlib/fmt/workflows/linux/badge.svg
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:target: https://github.com/fmtlib/fmt/actions?query=workflow%3Alinux
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.. image:: https://github.com/fmtlib/fmt/workflows/macos/badge.svg
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:target: https://github.com/fmtlib/fmt/actions?query=workflow%3Amacos
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.. image:: https://github.com/fmtlib/fmt/workflows/windows/badge.svg
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:target: https://github.com/fmtlib/fmt/actions?query=workflow%3Awindows
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.. image:: https://ci.appveyor.com/api/projects/status/ehjkiefde6gucy1v?svg=true
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:target: https://ci.appveyor.com/project/vitaut/fmt
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.. image:: https://oss-fuzz-build-logs.storage.googleapis.com/badges/fmt.svg
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:alt: fmt is continuously fuzzed at oss-fuzz
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:target: https://bugs.chromium.org/p/oss-fuzz/issues/list?\
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colspec=ID%20Type%20Component%20Status%20Proj%20Reported%20Owner%20\
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Summary&q=proj%3Dfmt&can=1
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.. image:: https://img.shields.io/badge/stackoverflow-fmt-blue.svg
:alt: Ask questions at StackOverflow with the tag fmt
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:target: https://stackoverflow.com/questions/tagged/fmt
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**{fmt}** is an open-source formatting library providing a fast and safe
alternative to C stdio and C++ iostreams.
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If you like this project, please consider donating to the BYSOL
Foundation that helps victims of political repressions in Belarus:
https://bysol.org/en/bs/general/.
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`Documentation <https://fmt.dev>`__
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Q&A: ask questions on `StackOverflow with the tag 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 arguments
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for localization
* Implementation of `C++20 std::format
<https://en.cppreference.com/w/cpp/utility/format>`__
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* `Format string syntax <https://fmt.dev/latest/syntax.html>`_ similar 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, shortness and
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round-trip guarantees
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* Safe `printf implementation
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<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>`_
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* High performance: faster than common standard library implementations of
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``(s)printf``, iostreams, ``to_string`` and ``to_chars``, see `Speed tests`_
and `Converting a hundred million integers to strings per second
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<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 configuration
consisting of just three files, ``core.h``, ``format.h`` and ``format-inl.h``,
and compiled code; see `Compile time and code bloat`_
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* Reliability: the library has an extensive set of `tests
<https://github.com/fmtlib/fmt/tree/master/test>`_ and is `continuously fuzzed
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<https://bugs.chromium.org/p/oss-fuzz/issues/list?colspec=ID%20Type%20
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Component%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 be
reported at compile time, automatic memory management prevents buffer overflow
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errors
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* Ease of use: small self-contained code base, no external dependencies,
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permissive MIT `license
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<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
``-Wall -Wextra -pedantic``
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* Locale-independence by default
* Optional header-only configuration enabled with the ``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>`_)
.. code:: c++
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#include <fmt/core.h>
int main() {
fmt::print("Hello, world!\n");
}
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**Format a string** (`run <https://godbolt.org/z/oK8h33>`_)
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.. code:: c++
std::string s = fmt::format("The answer is {}.", 42);
// s == "The answer is 42."
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**Format a string using positional arguments** (`run <https://godbolt.org/z/Yn7Txe>`_)
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.. code:: c++
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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**Print chrono durations** (`run <https://godbolt.org/z/K8s4Mc>`_)
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.. code:: c++
#include <fmt/chrono.h>
int main() {
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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);
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}
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Output::
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Default format: 42s 100ms
strftime-like format: 03:15:30
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**Print a container** (`run <https://godbolt.org/z/MjsY7c>`_)
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.. code:: c++
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#include <vector>
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#include <fmt/ranges.h>
int main() {
std::vector<int> v = {1, 2, 3};
fmt::print("{}\n", v);
}
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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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.. code:: c++
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std::string s = fmt::format("{:d}", "I am not a number");
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This gives a compile-time error in C++20 because ``d`` is an invalid format
specifier for a string.
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**Write a file from a single thread**
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.. code:: c++
#include <fmt/os.h>
int main() {
auto out = fmt::output_file("guide.txt");
out.print("Don't {}", "Panic");
}
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This can be `5 to 9 times faster than 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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.. 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, "Hello, {}!\n", "мир");
fmt::print(fg(fmt::color::steel_blue) | fmt::emphasis::italic,
"Hello, {}!\n", "世界");
}
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Output on a modern terminal:
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.. image:: https://user-images.githubusercontent.com/
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576385/88485597-d312f600-cf2b-11ea-9cbe-61f535a86e28.png
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Benchmarks
----------
Speed tests
~~~~~~~~~~~
================= ============= ===========
Library Method Run Time, s
================= ============= ===========
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libc printf 1.04
libc++ std::ostream 3.05
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{fmt} 6.1.1 fmt::print 0.75
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Boost Format 1.67 boost::format 7.24
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Folly Format folly::format 2.23
================= ============= ===========
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{fmt} is the fastest of the benchmarked methods, ~35% faster than ``printf``.
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The above results were generated by building ``tinyformat_test.cpp`` on macOS
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10.14.6 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"``
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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/tinyformat_test.cpp>`_.
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{fmt} is up to 20-30x faster than ``std::ostringstream`` and ``sprintf`` on
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floating-point formatting (`dtoa-benchmark <https://github.com/fmtlib/dtoa-benchmark>`_)
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and faster than `double-conversion <https://github.com/google/double-conversion>`_ and
`ryu <https://github.com/ulfjack/ryu>`_:
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.. image:: https://user-images.githubusercontent.com/576385/
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95684665-11719600-0ba8-11eb-8e5b-972ff4e49428.png
:target: https://fmt.dev/unknown_mac64_clang12.0.html
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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 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
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compare formatting function overhead only. Boost Format is a
header-only library so it doesn't provide any linkage options.
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Running the tests
~~~~~~~~~~~~~~~~~
Please refer to `Building the library`__ for the instructions on how to build
the library and run the unit tests.
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__ https://fmt.dev/latest/usage.html#building-the-library
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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
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Migrating code
--------------
`clang-tidy-fmt <https://github.com/mikecrowe/clang-tidy-fmt>`_ provides clang
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tidy checks for converting occurrences of ``printf`` and ``fprintf`` to
``fmt::print``.
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Projects using this library
---------------------------
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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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* `2GIS <https://2gis.ru/>`_: free business listings with a city map
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* `AMPL/MP <https://github.com/ampl/mp>`_:
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an open-source library for mathematical programming
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* `Aseprite <https://github.com/aseprite/aseprite>`_:
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animated sprite editor & pixel art tool
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* `AvioBook <https://www.aviobook.aero/en>`_: a comprehensive aircraft
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operations suite
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* `Blizzard Battle.net <https://battle.net/>`_: an online gaming platform
* `Celestia <https://celestia.space/>`_: real-time 3D visualization of 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>`_: analytical database
management system
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* `CUAUV <https://cuauv.org/>`_: Cornell University's autonomous underwater
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vehicle
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* `Drake <https://drake.mit.edu/>`_: a planning, control, and analysis toolbox
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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
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* `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
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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>`_:
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Player vs Player Gaming Network with tweaks
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* `KBEngine <https://github.com/kbengine/kbengine>`_: an open-source MMOG server
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engine
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* `Keypirinha <https://keypirinha.com/>`_: a semantic launcher for Windows
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* `Kodi <https://kodi.tv/>`_ (formerly xbmc): home theater software
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* `Knuth <https://kth.cash/>`_: high-performance Bitcoin full-node
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* `Microsoft Verona <https://github.com/microsoft/verona>`_:
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research programming language for concurrent ownership
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* `MongoDB <https://mongodb.com/>`_: distributed document database
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* `MongoDB Smasher <https://github.com/duckie/mongo_smasher>`_: a small tool to
generate randomized datasets
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* `OpenSpace <https://openspaceproject.com/>`_: an open-source
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astrovisualization framework
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* `PenUltima Online (POL) <https://www.polserver.com/>`_:
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an MMO server, compatible with most Ultima Online clients
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* `PyTorch <https://github.com/pytorch/pytorch>`_: an open-source machine
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learning library
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* `quasardb <https://www.quasardb.net/>`_: a distributed, high-performance,
associative database
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* `Quill <https://github.com/odygrd/quill>`_: asynchronous low-latency logging library
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* `QKW <https://github.com/ravijanjam/qkw>`_: generalizing aliasing to simplify
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navigation, and executing complex multi-line terminal command sequences
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* `redis-cerberus <https://github.com/HunanTV/redis-cerberus>`_: a Redis cluster
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proxy
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* `redpanda <https://vectorized.io/redpanda>`_: a 10x faster Kafka® replacement
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for mission critical systems written in C++
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* `rpclib <http://rpclib.net/>`_: a modern C++ msgpack-RPC server and client
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library
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* `Salesforce Analytics Cloud
<https://www.salesforce.com/analytics-cloud/overview/>`_:
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business intelligence software
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* `Scylla <https://www.scylladb.com/>`_: a Cassandra-compatible NoSQL data store
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that can handle 1 million transactions per second on a single server
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* `Seastar <http://www.seastar-project.org/>`_: an advanced, open-source C++
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framework for high-performance server applications on modern hardware
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* `spdlog <https://github.com/gabime/spdlog>`_: super fast C++ logging library
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* `Stellar <https://www.stellar.org/>`_: financial platform
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* `Touch Surgery <https://www.touchsurgery.com/>`_: surgery simulator
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* `TrinityCore <https://github.com/TrinityCore/TrinityCore>`_: open-source
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MMORPG framework
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* `Windows Terminal <https://github.com/microsoft/terminal>`_: the new Windows
terminal
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`More... <https://github.com/search?q=fmtlib&type=Code>`_
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If you are aware of other projects using this library, please let me know
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by `email <mailto:victor.zverovich@gmail.com>`_ or by submitting an
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`issue <https://github.com/fmtlib/fmt/issues>`_.
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Motivation
----------
So why yet another formatting library?
There are plenty of methods for doing this task, from standard ones like
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the printf family of function and iostreams to Boost Format and FastFormat
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libraries. The reason for creating a new library is that every existing
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solution that I found either had serious issues or didn't provide
all the features I needed.
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printf
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~~~~~~
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The good thing about ``printf`` is that it is pretty fast and readily available
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being a part of the C standard library. The main drawback is that it
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doesn't support user-defined types. ``printf`` also has safety issues although
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they are somewhat mitigated with `__attribute__ ((format (printf, ...))
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<https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>`_ in GCC.
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There is a POSIX extension that adds positional arguments required for
`i18n <https://en.wikipedia.org/wiki/Internationalization_and_localization>`_
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to ``printf`` but it is not a part of C99 and may not be available on some
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platforms.
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iostreams
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~~~~~~~~~
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The main issue with iostreams is best illustrated with an example:
.. code:: c++
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std::cout << std::setprecision(2) << std::fixed << 1.23456 << "\n";
which is a lot of typing compared to printf:
.. code:: c++
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printf("%.2f\n", 1.23456);
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Matthew Wilson, the author of FastFormat, called this "chevron hell". iostreams
don't support positional arguments by design.
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The good part is that iostreams support user-defined types and are safe although
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error handling is awkward.
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Boost Format
~~~~~~~~~~~~
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This is a very powerful library which supports both ``printf``-like format
strings and positional arguments. Its main drawback is performance. According to
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various, benchmarks it is much slower than other methods considered here. Boost
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Format also has excessive build times and severe code bloat issues (see
`Benchmarks`_).
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FastFormat
~~~~~~~~~~
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This is an interesting library which is fast, safe and has positional arguments.
However, it has significant limitations, citing its author:
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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
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It is also quite big and has a heavy dependency, STLSoft, which might be too
restrictive for using it in some projects.
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2012-12-25 05:31:05 +00:00
Boost Spirit.Karma
~~~~~~~~~~~~~~~~~~
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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
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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>`_.
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License
-------
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{fmt} is distributed under the MIT `license
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<https://github.com/fmtlib/fmt/blob/master/LICENSE.rst>`_.
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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
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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
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<https://raw.github.com/fmtlib/fmt/master/doc/python-license.txt>`_.
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It only applies if you distribute the documentation of {fmt}.
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Maintainers
-----------
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The {fmt} library is maintained by Victor Zverovich (`vitaut
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<https://github.com/vitaut>`_) and Jonathan Müller (`foonathan
<https://github.com/foonathan>`_) 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.