fmt/doc/index.rst

Overview
========

**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 panel-default">
     <div class="panel-heading">What users say:</div>
     <div class="panel-body">
       Thanks for creating this library. It’s been a hole in C++ for a long
       time. I’ve used both boost::format and loki::SPrintf, and neither felt
       like the right answer. This does.
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.. _format-api:

Format API
----------

The replacement-based Format API provides a safe alternative to ``printf``,
``sprintf`` and friends with comparable or `better performance
<http://zverovich.net/2013/09/07/integer-to-string-conversion-in-cplusplus.html>`_.
The `format string syntax <syntax.html>`_ is similar to the one used by
`str.format <http://docs.python.org/3/library/stdtypes.html#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
``stdout``:

.. 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.

.. _safety:

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
needed.

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:

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 <https://travis-ci.org/fmtlib/fmt>`_, 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 <https://ci.appveyor.com/project/vitaut/fmt>`_

* 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 <https://github.com/fmtlib/fmt#license>`_ allows
using the library both in open-source and commercial projects.

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