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**fmt** (formerly cppformat) is an open-source formatting library.
It can be used as a safe alternative to printf or as a fast
alternative to C++ IOStreams.
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<div class="panel-heading">What users say:</div>
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Thanks for creating this library. Its been a hole in C++ for a long
time. Ive used both boost::format and loki::SPrintf, and neither felt
like the right answer. This does.
.. _format-api:
Format API
The replacement-based Format API provides a safe alternative to ``printf``,
``sprintf`` and friends with comparable or `better performance
The `format string syntax <syntax.html>`_ is similar to the one used by
`str.format <>`_
in Python:
.. code:: c++
fmt::format("The answer is {}", 42);
The ``fmt::format`` function returns a string "The answer is 42". You can use
``fmt::MemoryWriter`` to avoid constructing ``std::string``:
.. code:: c++
fmt::MemoryWriter w;
w.write("Look, a {} string", 'C');
w.c_str(); // returns a C string (const char*)
The ``fmt::print`` function performs formatting and writes the result to a file:
.. code:: c++
fmt::print(stderr, "System error code = {}\n", errno);
The file argument can be omitted in which case the function prints to
.. code:: c++
fmt::print("Don't {}\n", "panic");
If your compiler supports C++11, then the formatting functions are implemented
with variadic templates. Otherwise variadic functions are emulated by generating
a set of lightweight wrappers. This ensures compatibility with older compilers
while providing a natural API.
The Format API also supports positional arguments useful for localization:
.. code:: c++
fmt::print("I'd rather be {1} than {0}.", "right", "happy");
Named arguments can be created with ``fmt::arg``. This makes it easier to track
what goes where when multiple values are being inserted:
.. code:: c++
fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
fmt::arg("name", "World"), fmt::arg("number", 42));
If your compiler supports C++11 user-defined literals, the suffix ``_a`` offers
an alternative, slightly terser syntax for named arguments:
.. code:: c++
fmt::print("Hello, {name}! The answer is {number}. Goodbye, {name}.",
"name"_a="World", "number"_a=42);
The ``_format`` suffix may be used to format string literals similar to Python:
.. code:: c++
std::string message = "{0}{1}{0}"_format("abra", "cad");
Other than the placement of the format string on the left of the operator,
``_format`` is functionally identical to ``fmt::format``. In order to use the
literal operators, they must be made visible with the directive
``using namespace fmt::literals;``. Note that this brings in only ``_a`` and
``_format`` but nothing else from the ``fmt`` namespace.
.. _write-api:
Write API
The concatenation-based Write API (experimental) provides a `fast
stateless alternative to IOStreams:
.. code:: c++
fmt::MemoryWriter out;
out << "The answer in hexadecimal is " << hex(42);
.. _safety:
The library is fully type safe, automatic memory management prevents buffer
overflow, errors in format strings are reported using exceptions. For example,
the code
.. code:: c++
fmt::format("The answer is {:d}", "forty-two");
throws a ``FormatError`` exception with description
"unknown format code 'd' for string", because the argument
``"forty-two"`` is a string while the format code ``d``
only applies to integers.
Where possible, errors are caught at compile time. For example, the code
.. code:: c++
fmt::format("Cyrillic letter {}", L'\x42e');
produces a compile-time error because wide character ``L'\x42e'`` cannot be
formatted into a narrow string. You can use a wide format string instead:
.. code:: c++
fmt::format(L"Cyrillic letter {}", L'\x42e');
For comparison, writing a wide character to ``std::ostream`` results in
its numeric value being written to the stream (i.e. 1070 instead of letter 'ю'
which is represented by ``L'\x42e'`` if we use Unicode) which is rarely what is
Note that fmt does not use the value of the ``errno`` global to communicate
errors to the user, but it may call system functions which set ``errno``. Since
fmt does not attempt to preserve the value of ``errno``, users should not make
any assumptions about it and always set it to ``0`` before making any system
calls that convey error information via ``errno``.
.. _portability:
The library is highly portable. Here is an incomplete list of operating systems
and compilers where it has been tested and known to work:
* 64-bit (amd64) GNU/Linux with GCC 4.4.3,
`4.6.3 <>`_, 4.7.2, 4.8.1, and Intel C++
Compiler (ICC) 14.0.2
* 32-bit (i386) GNU/Linux with GCC 4.4.3, 4.6.3
* Mac OS X with GCC 4.2.1 and Clang 4.2, 5.1.0
* 64-bit Windows with Visual C++ 2010, 2013 and
`2015 <>`_
* 32-bit Windows with Visual C++ 2010
Although the library uses C++11 features when available, it also works with
older compilers and standard library implementations. The only thing to keep in
mind for C++98 portability:
* Variadic templates: minimum GCC 4.4, Clang 2.9 or VS2013. This feature allows
the Format API to accept an unlimited number of arguments. With older
compilers the maximum is 15.
* User-defined literals: minimum GCC 4.7, Clang 3.1 or VS2015. The suffixes
``_format`` and ``_a`` are functionally equivalent to the functions
``fmt::format`` and ``fmt::arg``.
The output of all formatting functions is consistent across platforms. In
particular, formatting a floating-point infinity always gives ``inf`` while the
output of ``printf`` is platform-dependent in this case. For example,
.. code::
fmt::print("{}", std::numeric_limits<double>::infinity());
always prints ``inf``.
.. _ease-of-use:
Ease of Use
fmt has a small self-contained code base with the core library consisting of
a single header file and a single source file and no external dependencies.
A permissive BSD `license <>`_ allows
using the library both in open-source and commercial projects.
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