494 lines
18 KiB
ReStructuredText
494 lines
18 KiB
ReStructuredText
{fmt}
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=====
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.. image:: https://travis-ci.org/fmtlib/fmt.png?branch=master
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:target: https://travis-ci.org/fmtlib/fmt
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.. image:: https://ci.appveyor.com/api/projects/status/ehjkiefde6gucy1v
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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/libfmt.svg
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:alt: fmt is continuously fuzzed att 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%3Dlibfmt&can=1
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.. image:: https://img.shields.io/badge/stackoverflow-fmt-blue.svg
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: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 for C++.
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It can be used as a safe and fast alternative to (s)printf and iostreams.
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`Documentation <https://fmt.dev/latest/>`__
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Q&A: ask questions on `StackOverflow with the tag fmt
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<https://stackoverflow.com/questions/tagged/fmt>`_.
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Features
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--------
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* Simple `format API <https://fmt.dev/dev/api.html>`_ with positional arguments
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for localization
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* Implementation of `C++20 std::format
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<https://en.cppreference.com/w/cpp/utility/format>`__
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* `Format string syntax <https://fmt.dev/dev/syntax.html>`_ similar to the one
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of Python's
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`format <https://docs.python.org/3/library/stdtypes.html#str.format>`_
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* Safe `printf implementation
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<https://fmt.dev/latest/api.html#printf-formatting>`_ including
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the POSIX extension for positional arguments
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* Extensibility: support for user-defined types
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* High performance: faster than common standard library implementations of
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`printf <https://en.cppreference.com/w/cpp/io/c/fprintf>`_,
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iostreams, ``to_string`` and ``to_chars``, see `Speed tests`_ and
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`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 (the minimum configuration
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consists of just three header files, ``core.h``, ``format.h`` and
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``format-inl.h``) and compiled code. See `Compile time and code bloat`_
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* Reliability: the library has an extensive set of `unit tests
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<https://github.com/fmtlib/fmt/tree/master/test>`_ and is continuously fuzzed
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* Safety: the library is fully type safe, errors in format strings can be
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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
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(``-Wall -Wextra -pedantic``)
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* Locale-independence by default
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* Support for wide strings
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* Optional header-only configuration enabled with the ``FMT_HEADER_ONLY`` macro
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See the `documentation <https://fmt.dev/latest/>`_ for more details.
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Examples
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--------
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Print ``Hello, world!`` to ``stdout``:
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.. code:: 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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Format a string:
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.. code:: 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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Format a string using positional arguments:
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.. code:: 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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Print a chrono duration:
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.. code:: c++
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#include <fmt/chrono.h>
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int main() {
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using namespace std::chrono_literals;
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fmt::print("Elapsed time: {}", 42ms);
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}
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prints "Elapsed time: 42ms".
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Check a format string at compile time:
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.. code:: c++
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// test.cc
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#include <fmt/format.h>
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std::string s = format(FMT_STRING("{:d}"), "hello");
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gives a compile-time error because ``d`` is an invalid format specifier for a
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string.
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Use {fmt} as a safe portable replacement for ``itoa``
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(`godbolt <https://godbolt.org/g/NXmpU4>`_):
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.. code:: c++
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fmt::memory_buffer buf;
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format_to(buf, "{}", 42); // replaces itoa(42, buffer, 10)
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format_to(buf, "{:x}", 42); // replaces itoa(42, buffer, 16)
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// access the string with to_string(buf) or buf.data()
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Format objects of user-defined types via a simple `extension API
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<https://fmt.dev/latest/api.html#formatting-user-defined-types>`_:
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.. code:: c++
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#include <fmt/format.h>
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struct date {
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int year, month, day;
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};
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template <>
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struct fmt::formatter<date> {
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constexpr auto parse(format_parse_context& ctx) { return ctx.begin(); }
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template <typename FormatContext>
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auto format(const date& d, FormatContext& ctx) {
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return format_to(ctx.out(), "{}-{}-{}", d.year, d.month, d.day);
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}
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};
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std::string s = fmt::format("The date is {}", date{2012, 12, 9});
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// s == "The date is 2012-12-9"
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Create your own functions similar to `format
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<https://fmt.dev/latest/api.html#format>`_ and
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`print <https://fmt.dev/latest/api.html#print>`_
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which take arbitrary arguments (`godbolt <https://godbolt.org/g/MHjHVf>`_):
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.. code:: c++
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// Prints formatted error message.
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void vreport_error(const char* format, fmt::format_args args) {
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fmt::print("Error: ");
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fmt::vprint(format, args);
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}
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template <typename... Args>
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void report_error(const char* format, const Args & ... args) {
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vreport_error(format, fmt::make_format_args(args...));
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}
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report_error("file not found: {}", path);
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Note that ``vreport_error`` is not parameterized on argument types which can
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improve compile times and reduce code size compared to a fully parameterized
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version.
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Benchmarks
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----------
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Speed tests
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~~~~~~~~~~~
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================= ============= ===========
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Library Method Run Time, s
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================= ============= ===========
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libc printf 1.04
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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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================= ============= ===========
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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
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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
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further details refer to the `source
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<https://github.com/fmtlib/format-benchmark/blob/master/tinyformat_test.cpp>`_.
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{fmt} is up to 10x 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>`_:
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.. image:: https://user-images.githubusercontent.com/576385/69767160-cdaca400-112f-11ea-9fc5-347c9f83caad.png
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:target: https://fmt.dev/unknown_mac64_clang10.0.html
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Compile time and code bloat
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The script `bloat-test.py
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<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.
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It generates 100 translation units and uses ``printf()`` or its alternative
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five times in each to simulate a medium sized project. The resulting
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executable size and compile time (Apple LLVM version 8.1.0 (clang-802.0.42),
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macOS Sierra, best of three) is shown in the following tables.
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**Optimized build (-O3)**
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============= =============== ==================== ==================
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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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============= =============== ==================== ==================
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As you can see, {fmt} has 60% less overhead in terms of resulting binary code
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size compared to iostreams and comes pretty close to ``printf``. Boost Format
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and Folly Format have the largest overheads.
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``printf+string`` is the same as ``printf`` but with 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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============= =============== ==================== ==================
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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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============= =============== ==================== ==================
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``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
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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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~~~~~~~~~~~~~~~~~
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Please refer to `Building the library`__ for the instructions on how to build
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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,
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`format-benchmarks <https://github.com/fmtlib/format-benchmark>`_,
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so to run the benchmarks you first need to clone this repository and
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generate 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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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>`_:
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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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* `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 <http://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)
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* `Envoy <https://lyft.github.io/envoy/>`_: C++ L7 proxy and communication bus
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(Lyft)
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* `FiveM <https://fivem.net/>`_: a modification framework for GTA V
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* `Folly <https://github.com/facebook/folly>`_: Facebook open-source library
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* `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://kbengine.org/>`_: An open-source MMOG server 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
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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,
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associative database
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* `readpe <https://bitbucket.org/sys_dev/readpe>`_: Read Portable Executable
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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
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<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
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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
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----------
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So why yet another formatting library?
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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
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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
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`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:
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.. code:: c++
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std::cout << std::setprecision(2) << std::fixed << 1.23456 << "\n";
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which is a lot of typing compared to printf:
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.. 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
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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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~~~~~~~~~~~~
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This is a very powerful library which supports both ``printf``-like format
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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
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`Benchmarks`_).
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FastFormat
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~~~~~~~~~~
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This is an interesting library which is fast, safe and has positional arguments.
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However, it has significant limitations, citing its author:
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Three features that have no hope of being accommodated within the
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current design are:
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* Leading zeros (or any other non-space padding)
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* Octal/hexadecimal encoding
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* Runtime width/alignment specification
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It is also quite big and has a heavy dependency, STLSoft, which might be too
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restrictive for using it in some projects.
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Boost Spirit.Karma
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~~~~~~~~~~~~~~~~~~
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This is not really a formatting library but I decided to include it here for
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completeness. As iostreams, it suffers from the problem of mixing verbatim text
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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,
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see `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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License
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-------
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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
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---------------------
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The `Format String Syntax <https://fmt.dev/latest/syntax.html>`_
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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>`_.
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For this reason the documentation is distributed under the Python Software
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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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-----------
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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
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<https://github.com/foonathan>`_) with contributions from many other people.
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See `Contributors <https://github.com/fmtlib/fmt/graphs/contributors>`_ and
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`Releases <https://github.com/fmtlib/fmt/releases>`_ for some of the names.
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Let us know if your contribution is not listed or mentioned incorrectly and
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we'll make it right.
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