198 lines
5.3 KiB
ReStructuredText
198 lines
5.3 KiB
ReStructuredText
Overview
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========
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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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.. raw:: html
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<div class="panel panel-default">
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<div class="panel-heading">What users say:</div>
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<div class="panel-body">
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Thanks for creating this library. It’s been a hole in C++ for
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a long time. I’ve used both <code>boost::format</code> and
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<code>loki::SPrintf</code>, and neither felt like the right answer.
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This does.
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</div>
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</div>
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.. _format-api-intro:
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Format API
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----------
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The format API is similar in spirit to the C ``printf`` family of function but
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is safer, simpler and several times `faster
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<https://www.zverovich.net/2020/06/13/fast-int-to-string-revisited.html>`_
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than common standard library implementations.
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The `format string syntax <syntax.html>`_ is similar to the one used by
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`str.format <https://docs.python.org/3/library/stdtypes.html#str.format>`_ in
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Python:
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.. code:: c++
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std::string s = fmt::format("The answer is {}.", 42);
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The ``fmt::format`` function returns a string "The answer is 42.". You can use
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``fmt::memory_buffer`` to avoid constructing ``std::string``:
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.. code:: c++
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auto out = fmt::memory_buffer();
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fmt::format_to(std::back_inserter(out),
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"For a moment, {} happened.", "nothing");
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auto data = out.data(); // pointer to the formatted data
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auto size = out.size(); // size of the formatted data
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The ``fmt::print`` function performs formatting and writes the result to a stream:
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.. code:: c++
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fmt::print(stderr, "System error code = {}\n", errno);
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If you omit the file argument the function will print to ``stdout``:
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.. code:: c++
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fmt::print("Don't {}\n", "panic");
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The format API also supports positional arguments useful for localization:
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.. code:: c++
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fmt::print("I'd rather be {1} than {0}.", "right", "happy");
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You can pass named arguments with ``fmt::arg``:
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.. code:: c++
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fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
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fmt::arg("name", "World"), fmt::arg("number", 42));
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If your compiler supports C++11 user-defined literals, the suffix ``_a`` offers
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an alternative, slightly terser syntax for named arguments:
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.. code:: c++
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using namespace fmt::literals;
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fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
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"name"_a="World", "number"_a=42);
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.. _safety:
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Safety
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------
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The library is fully type safe, automatic memory management prevents buffer
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overflow, errors in format strings are reported using exceptions or at compile
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time. For example, the code
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.. code:: c++
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fmt::format("The answer is {:d}", "forty-two");
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throws the ``format_error`` exception because the argument ``"forty-two"`` is a
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string while the format code ``d`` only applies to integers.
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The code
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.. code:: c++
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format(FMT_STRING("The answer is {:d}"), "forty-two");
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reports a compile-time error on compilers that support relaxed ``constexpr``.
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See `here <api.html#compile-time-format-string-checks>`_ for details.
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The following code
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.. code:: c++
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fmt::format("Cyrillic letter {}", L'\x42e');
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produces a compile-time error because wide character ``L'\x42e'`` cannot be
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formatted into a narrow string. For comparison, writing a wide character to
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``std::ostream`` results in its numeric value being written to the stream
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(i.e. 1070 instead of letter 'ю' which is represented by ``L'\x42e'`` if we
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use Unicode) which is rarely desirable.
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Compact Binary Code
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-------------------
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The library produces compact per-call compiled code. For example
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(`godbolt <https://godbolt.org/g/TZU4KF>`_),
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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("The answer is {}.", 42);
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}
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compiles to just
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.. code:: asm
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main: # @main
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sub rsp, 24
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mov qword ptr [rsp], 42
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mov rcx, rsp
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mov edi, offset .L.str
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mov esi, 17
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mov edx, 1
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call fmt::v7::vprint(fmt::v7::basic_string_view<char>, fmt::v7::format_args)
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xor eax, eax
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add rsp, 24
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ret
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.L.str:
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.asciz "The answer is {}."
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.. _portability:
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Portability
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-----------
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The library is highly portable and relies only on a small set of C++11 features:
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* variadic templates
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* type traits
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* rvalue references
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* decltype
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* trailing return types
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* deleted functions
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* alias templates
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These are available in GCC 4.8, Clang 3.4, MSVC 19.0 (2015) and more recent
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compiler version. For older compilers use {fmt} `version 4.x
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<https://github.com/fmtlib/fmt/releases/tag/4.1.0>`_ which is maintained and
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only requires C++98.
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The output of all formatting functions is consistent across platforms.
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For example,
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.. code::
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fmt::print("{}", std::numeric_limits<double>::infinity());
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always prints ``inf`` while the output of ``printf`` is platform-dependent.
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.. _ease-of-use:
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Ease of Use
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-----------
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{fmt} has a small self-contained code base with the core library consisting of
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just three header files and no external dependencies.
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A permissive MIT `license <https://github.com/fmtlib/fmt#license>`_ allows
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using the library both in open-source and commercial projects.
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`Learn more... <contents.html>`_
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.. raw:: html
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<a class="btn btn-success" href="https://github.com/fmtlib/fmt">GitHub Repository</a>
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<div class="section footer">
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<iframe src="https://ghbtns.com/github-btn.html?user=fmtlib&repo=fmt&type=watch&count=true"
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class="github-btn" width="100" height="20"></iframe>
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</div>
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