fmt/doc/index.rst

Overview
========

**fmt** (formerly cppformat) is an open-source formatting library.
It can be used as a fast and safe alternative to printf and IOStreams.

.. raw:: html

   <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.
     </div>
   </div>

.. _format-api-intro:

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::memory_buffer`` to avoid constructing ``std::string``:

.. code:: c++

  fmt::memory_buffer out;
  format_to(out, "For a moment, {} happened.", "nothing");
  out.data(); // returns a pointer to the formatted data

The ``fmt::print`` function performs formatting and writes the result to a stream:

.. 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");

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 or at compile
time. For example, the code

.. code:: c++

  fmt::format("The answer is {:d}", "forty-two");

throws a ``format_error`` 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, while

.. code:: c++

  format(fmt("The answer is {:d}"), "forty-two");

reports a compile-time error for the same reason on compilers that support
relaxed ``constexpr``. See `here <api.html#c.fmt>`_ for how to enable
compile-time checks.

The following 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.

Compact Binary Code
-------------------

The library is designed to produce compact per-call compiled code. For example
(`godbolt <https://godbolt.org/g/TZU4KF>`_),

.. code:: c++

   #include <fmt/core.h>

   int main() {
     fmt::print("The answer is {}.", 42);
   }

compiles to just

.. code:: asm

   main: # @main
     sub rsp, 24
     mov qword ptr [rsp], 42
     mov rcx, rsp
     mov edi, offset .L.str
     mov esi, 17
     mov edx, 2
     call fmt::v5::vprint(fmt::v5::basic_string_view<char>, fmt::v5::format_args)
     xor eax, eax
     add rsp, 24
     ret
   .L.str:
     .asciz "The answer is {}."

.. _portability:

Portability
-----------

The library is highly portable and relies only on a small set of C++11 features:

* variadic templates
* type traits
* rvalue references
* decltype
* trailing return types
* deleted functions

These are available since GCC 4.4, Clang 2.9 and MSVC 18.0 (2013). For older
compilers use fmt `version 4.x
<https://github.com/fmtlib/fmt/releases/tag/4.1.0>`_ which continues to be
maintained and only requires C++98.

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
just three header files 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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