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226 lines
6.8 KiB
ReStructuredText
226 lines
6.8 KiB
ReStructuredText
Overview
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========
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**fmt** (formerly cppformat) is an open-source formatting library.
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It can be used as a safe alternative to printf or as a fast
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alternative to 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 a long
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time. I’ve used both boost::format and loki::SPrintf, and neither felt
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like the right answer. 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 replacement-based Format API provides a safe alternative to ``printf``,
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``sprintf`` and friends with comparable or `better performance
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<http://zverovich.net/2013/09/07/integer-to-string-conversion-in-cplusplus.html>`_.
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The `format string syntax <syntax.html>`_ is similar to the one used by
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`str.format <http://docs.python.org/3/library/stdtypes.html#str.format>`_
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in Python:
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.. code:: c++
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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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fmt::memory_buffer out;
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format_to(out, "For a moment, {} happened.", "nothing");
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out.data(); // returns a pointer to the formatted data
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The ``fmt::print`` function performs formatting and writes the result to a file:
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.. code:: c++
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fmt::print(stderr, "System error code = {}\n", errno);
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The file argument can be omitted in which case the function prints to
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``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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Named arguments can be created with ``fmt::arg``. This makes it easier to track
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what goes where when multiple values are being inserted:
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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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fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
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"name"_a="World", "number"_a=42);
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The ``_format`` suffix may be used to format string literals similar to Python:
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.. code:: c++
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std::string message = "{0}{1}{0}"_format("abra", "cad");
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Other than the placement of the format string on the left of the operator,
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``_format`` is functionally identical to ``fmt::format``. In order to use the
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literal operators, they must be made visible with the directive
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``using namespace fmt::literals;``. Note that this brings in only ``_a`` and
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``_format`` but nothing else from the ``fmt`` namespace.
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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. For example,
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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 a ``format_error`` exception with description
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"unknown format code 'd' for string", because the argument
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``"forty-two"`` is a string while the format code ``d``
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only applies to integers.
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Where possible, errors are caught at compile time. For example, the 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. You can use a wide format string instead:
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.. code:: c++
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fmt::format(L"Cyrillic letter {}", L'\x42e');
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For comparison, writing a wide character to ``std::ostream`` results in
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its numeric value being written to the stream (i.e. 1070 instead of letter 'ю'
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which is represented by ``L'\x42e'`` if we use Unicode) which is rarely what is
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needed.
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Note that fmt does not use the value of the ``errno`` global to communicate
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errors to the user, but it may call system functions which set ``errno``. Since
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fmt does not attempt to preserve the value of ``errno``, users should not make
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any assumptions about it and always set it to ``0`` before making any system
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calls that convey error information via ``errno``.
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Compact binary code
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-------------------
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Each call to a formatting function results in a compact binary 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, 2
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call fmt::v5::vprint(fmt::v5::basic_string_view<char>, fmt::v5::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. Here is an incomplete list of operating systems
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and compilers where it has been tested and known to work:
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* 64-bit (amd64) GNU/Linux with GCC 4.4.3,
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`4.6.3 <https://travis-ci.org/fmtlib/fmt>`_, 4.7.2, 4.8.1, and Intel C++
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Compiler (ICC) 14.0.2
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* 32-bit (i386) GNU/Linux with GCC 4.4.3, 4.6.3
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* Mac OS X with GCC 4.2.1 and Clang 4.2, 5.1.0
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* 64-bit Windows with Visual C++ 2010, 2013 and
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`2015 <https://ci.appveyor.com/project/vitaut/fmt>`_
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* 32-bit Windows with Visual C++ 2010
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Although the library uses C++11 features when available, it also works with
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older compilers and standard library implementations. The only thing to keep in
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mind for C++98 portability:
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* Variadic templates: minimum GCC 4.4, Clang 2.9 or VS2013. This feature allows
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the Format API to accept an unlimited number of arguments. With older
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compilers the maximum is 15.
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* User-defined literals: minimum GCC 4.7, Clang 3.1 or VS2015. The suffixes
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``_format`` and ``_a`` are functionally equivalent to the functions
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``fmt::format`` and ``fmt::arg``.
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The output of all formatting functions is consistent across platforms. In
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particular, formatting a floating-point infinity always gives ``inf`` while the
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output of ``printf`` is platform-dependent in this case. 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``.
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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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a single header file and a single source file and no external dependencies.
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A permissive BSD `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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.. 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="http://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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