blame: source/misc/embedded_libs/fmt-8.1.1/README.rst

{fmt}

=====

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**{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 the BYSOL

Foundation that helps victims of political repressions in Belarus:

https://bysol.org/en/bs/general/.

`Documentation <https://fmt.dev>`__

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>`__

* `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

* 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/MjsY7c>`_)

.. 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, "Hello, {}!\n", "мир");

      fmt::print(fg(fmt::color::steel_blue) | fmt::emphasis::italic,

                 "Hello, {}!\n", "世界");

    }

Output on a modern terminal:

.. image:: https://user-images.githubusercontent.com/

           576385/88485597-d312f600-cf2b-11ea-9cbe-61f535a86e28.png

Benchmarks

----------

Speed tests

~~~~~~~~~~~

================= ============= ===========

Library           Method        Run Time, s

================= ============= ===========

libc              printf          1.04

libc++            std::ostream    3.05

{fmt} 6.1.1       fmt::print      0.75

Boost Format 1.67 boost::format   7.24

Folly Format      folly::format   2.23

================= ============= ===========

{fmt} is the fastest of the benchmarked methods, ~35% faster than ``printf``.

The above results were generated by building ``tinyformat_test.cpp`` on macOS

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"``

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

floating-point 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 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 the 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-fmt <https://github.com/mikecrowe/clang-tidy-fmt>`_ provides clang

tidy checks for converting occurrences of ``printf`` and ``fprintf`` to

``fmt::print``.

Projects using this library

---------------------------

* `0 A.D. <https://play0ad.com/>`_: a free, open-source, cross-platform

  real-time strategy game

* `2GIS <https://2gis.ru/>`_: free business listings with a city map

* `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://celestia.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>`_: analytical database

  management system

* `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

* `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

* `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

* `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 which 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 which 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 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

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.rst>`_.

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>`_) 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.