fmt/doc/api.rst

.. _string-formatting-api:

*************
API Reference
*************

The {fmt} library API consists of the following parts:

* :ref:`fmt/core.h <core-api>`: the core API providing argument handling
  facilities and a lightweight subset of formatting functions
* :ref:`fmt/format.h <format-api>`: the full format API providing compile-time
  format string checks, wide string, output iterator and user-defined type
  support
* :ref:`fmt/ranges.h <ranges-api>`: additional formatting support for ranges
  and tuples
* :ref:`fmt/chrono.h <chrono-api>`: date and time formatting
* :ref:`fmt/compile.h <compile-api>`: format string compilation
* :ref:`fmt/ostream.h <ostream-api>`: ``std::ostream`` support
* :ref:`fmt/printf.h <printf-api>`: ``printf`` formatting

All functions and types provided by the library reside in namespace ``fmt`` and
macros have prefix ``FMT_``.

.. _core-api:

Core API
========

``fmt/core.h`` defines the core API which provides argument handling facilities
and a lightweight subset of formatting functions. In the header-only mode
include ``fmt/format.h`` instead of ``fmt/core.h``.

The following functions use :ref:`format string syntax <syntax>`
similar to that of Python's `str.format
<http://docs.python.org/3/library/stdtypes.html#str.format>`_.
They take *format_str* and *args* as arguments.

*format_str* is a format string that contains literal text and replacement
fields surrounded by braces ``{}``. The fields are replaced with formatted
arguments in the resulting string. A function taking *format_str* doesn't
participate in an overload resolution if the latter is not a string.

*args* is an argument list representing objects to be formatted.

.. _format:

.. doxygenfunction:: format(const S&, Args&&...)
.. doxygenfunction:: vformat(const S&, basic_format_args<buffer_context<type_identity_t<Char>>>)

.. _print:

.. doxygenfunction:: print(const S&, Args&&...)
.. doxygenfunction:: vprint(string_view, format_args)

.. doxygenfunction:: print(std::FILE *, const S&, Args&&...)
.. doxygenfunction:: vprint(std::FILE *, string_view, format_args)

Named Arguments
---------------

.. doxygenfunction:: fmt::arg(const S&, const T&)

Named arguments are not supported in compile-time checks at the moment.

Argument Lists
--------------

.. doxygenfunction:: fmt::make_format_args(const Args&...)

.. doxygenclass:: fmt::format_arg_store
   :members:

.. doxygenclass:: fmt::dynamic_format_arg_store
   :members:

.. doxygenclass:: fmt::basic_format_args
   :members:

.. doxygenstruct:: fmt::format_args

.. doxygenclass:: fmt::basic_format_arg
   :members:

Compatibility
-------------

.. doxygenclass:: fmt::basic_string_view
   :members:

.. doxygentypedef:: fmt::string_view
.. doxygentypedef:: fmt::wstring_view

Locale
------

All formatting is locale-independent by default. Use the ``'n'`` format
specifier to insert the appropriate number separator characters from the
locale::

  #include <fmt/core.h>
  #include <locale>

  std::locale::global(std::locale("en_US.UTF-8"));
  auto s = fmt::format("{:L}", 1000000);  // s == "1,000,000"

.. _format-api:

Format API
==========

``fmt/format.h`` defines the full format API providing compile-time format
string checks, wide string, output iterator and user-defined type support.

Compile-time Format String Checks
---------------------------------

Compile-time checks are supported for built-in and string types as well as
user-defined types with ``constexpr`` ``parse`` functions in their ``formatter``
specializations.

.. doxygendefine:: FMT_STRING

Formatting User-defined Types
-----------------------------

To make a user-defined type formattable, specialize the ``formatter<T>`` struct
template and implement ``parse`` and ``format`` methods::

  #include <fmt/format.h>

  struct point { double x, y; };

  template <>
  struct fmt::formatter<point> {
    // Presentation format: 'f' - fixed, 'e' - exponential.
    char presentation = 'f';

    // Parses format specifications of the form ['f' | 'e'].
    constexpr auto parse(format_parse_context& ctx) {
    // auto parse(format_parse_context &ctx) -> decltype(ctx.begin()) // c++11
      // [ctx.begin(), ctx.end()) is a character range that contains a part of
      // the format string starting from the format specifications to be parsed,
      // e.g. in
      //
      //   fmt::format("{:f} - point of interest", point{1, 2});
      //
      // the range will contain "f} - point of interest". The formatter should
      // parse specifiers until '}' or the end of the range. In this example
      // the formatter should parse the 'f' specifier and return an iterator
      // pointing to '}'.

      // Parse the presentation format and store it in the formatter:
      auto it = ctx.begin(), end = ctx.end();
      if (it != end && (*it == 'f' || *it == 'e')) presentation = *it++;

      // Check if reached the end of the range:
      if (it != end && *it != '}')
        throw format_error("invalid format");

      // Return an iterator past the end of the parsed range:
      return it;
    }

    // Formats the point p using the parsed format specification (presentation)
    // stored in this formatter.
    template <typename FormatContext>
    auto format(const point& p, FormatContext& ctx) {
    // auto format(const point &p, FormatContext &ctx) -> decltype(ctx.out()) // c++11
      // ctx.out() is an output iterator to write to.
      return format_to(
          ctx.out(),
          presentation == 'f' ? "({:.1f}, {:.1f})" : "({:.1e}, {:.1e})",
          p.x, p.y);
    }
  };

Then you can pass objects of type ``point`` to any formatting function::

  point p = {1, 2};
  std::string s = fmt::format("{:f}", p);
  // s == "(1.0, 2.0)"

You can also reuse existing formatters via inheritance or composition, for
example::

  enum class color {red, green, blue};

  template <> struct fmt::formatter<color>: formatter<string_view> {
    // parse is inherited from formatter<string_view>.
    template <typename FormatContext>
    auto format(color c, FormatContext& ctx) {
      string_view name = "unknown";
      switch (c) {
      case color::red:   name = "red"; break;
      case color::green: name = "green"; break;
      case color::blue:  name = "blue"; break;
      }
      return formatter<string_view>::format(name, ctx);
    }
  };

Since ``parse`` is inherited from ``formatter<string_view>`` it will recognize
all string format specifications, for example

.. code-block:: c++

   fmt::format("{:>10}", color::blue)

will return ``"      blue"``.

You can also write a formatter for a hierarchy of classes::

  #include <type_traits>
  #include <fmt/format.h>

  struct A {
    virtual ~A() {}
    virtual std::string name() const { return "A"; }
  };

  struct B : A {
    virtual std::string name() const { return "B"; }
  };

  template <typename T>
  struct fmt::formatter<T, std::enable_if_t<std::is_base_of<A, T>::value, char>> :
      fmt::formatter<std::string> {
    template <typename FormatCtx>
    auto format(const A& a, FormatCtx& ctx) {
      return fmt::formatter<std::string>::format(a.name(), ctx);
    }
  };

  int main() {
    B b;
    A& a = b;
    fmt::print("{}", a); // prints "B"
  }

If a type provides both a ``formatter`` specialization and an implicit
conversion to a formattable type, the specialization takes precedence over the
conversion.

.. doxygenclass:: fmt::basic_format_parse_context
   :members:

Output Iterator Support
-----------------------

.. doxygenfunction:: fmt::format_to(OutputIt, const S&, Args&&...)
.. doxygenfunction:: fmt::format_to_n(OutputIt, size_t, const S&, const Args&...)
.. doxygenstruct:: fmt::format_to_n_result
   :members:

Literal-based API
-----------------

The following user-defined literals are defined in ``fmt/format.h``.

.. doxygenfunction:: operator""_format(const char *, size_t)

.. doxygenfunction:: operator""_a(const char *, size_t)

Utilities
---------

.. doxygenstruct:: fmt::is_char

.. doxygentypedef:: fmt::char_t

.. doxygenfunction:: fmt::formatted_size(string_view, const Args&...)

.. doxygenfunction:: fmt::to_string(const T&)

.. doxygenfunction:: fmt::to_wstring(const T&)

.. doxygenfunction:: fmt::to_string_view(const Char *)

.. doxygenfunction:: fmt::join(const Range&, string_view)

.. doxygenfunction:: fmt::join(It, Sentinel, string_view)

.. doxygenclass:: fmt::detail::buffer
   :members:

.. doxygenclass:: fmt::basic_memory_buffer
   :protected-members:
   :members:

System Errors
-------------

fmt does not use ``errno`` to communicate errors to the user, but it may call
system functions which set ``errno``. Users should not make any assumptions about
the value of ``errno`` being preserved by library functions.

.. doxygenclass:: fmt::system_error
   :members:

.. doxygenfunction:: fmt::format_system_error

.. doxygenclass:: fmt::windows_error
   :members:

.. _formatstrings:

Custom Allocators
-----------------

The {fmt} library supports custom dynamic memory allocators.
A custom allocator class can be specified as a template argument to
:class:`fmt::basic_memory_buffer`::

    using custom_memory_buffer = 
      fmt::basic_memory_buffer<char, fmt::inline_buffer_size, custom_allocator>;

It is also possible to write a formatting function that uses a custom
allocator::

    using custom_string =
      std::basic_string<char, std::char_traits<char>, custom_allocator>;

    custom_string vformat(custom_allocator alloc, fmt::string_view format_str,
                          fmt::format_args args) {
      custom_memory_buffer buf(alloc);
      fmt::vformat_to(buf, format_str, args);
      return custom_string(buf.data(), buf.size(), alloc);
    }

    template <typename ...Args>
    inline custom_string format(custom_allocator alloc,
                                fmt::string_view format_str,
                                const Args& ... args) {
      return vformat(alloc, format_str, fmt::make_format_args(args...));
    }

The allocator will be used for the output container only. If you are using named
arguments, the container that stores pointers to them will be allocated using
the default allocator. Also floating-point formatting falls back on ``sprintf``
which may do allocations.

.. _ranges-api:

Ranges and Tuple Formatting
===========================

The library also supports convenient formatting of ranges and tuples::

  #include <fmt/ranges.h>

  std::tuple<char, int, float> t{'a', 1, 2.0f};
  // Prints "('a', 1, 2.0)"
  fmt::print("{}", t);


NOTE: currently, the overload of ``fmt::join`` for iterables exists in the main
``format.h`` header, but expect this to change in the future.

Using ``fmt::join``, you can separate tuple elements with a custom separator::

  #include <fmt/ranges.h>

  std::tuple<int, char> t = {1, 'a'};
  // Prints "1, a"
  fmt::print("{}", fmt::join(t, ", "));

.. _chrono-api:

Date and Time Formatting
========================

The library supports `strftime
<http://en.cppreference.com/w/cpp/chrono/c/strftime>`_-like date and time
formatting::

  #include <fmt/chrono.h>

  std::time_t t = std::time(nullptr);
  // Prints "The date is 2016-04-29." (with the current date)
  fmt::print("The date is {:%Y-%m-%d}.", fmt::localtime(t));

The format string syntax is described in the documentation of
`strftime <http://en.cppreference.com/w/cpp/chrono/c/strftime>`_.

.. _compile-api:

Format string compilation
=========================

``fmt/compile.h`` provides format string compilation support. Format strings
are parsed at compile time and converted into efficient formatting code. This
supports arguments of built-in and string types as well as user-defined types
with ``constexpr`` ``parse`` functions in their ``formatter`` specializations.
Format string compilation can generate more binary code compared to the default
API and is only recommended in places where formatting is a performance
bottleneck.

.. doxygendefine:: FMT_COMPILE

.. _ostream-api:

``std::ostream`` Support
========================

``fmt/ostream.h`` provides ``std::ostream`` support including formatting of
user-defined types that have overloaded ``operator<<``::

  #include <fmt/ostream.h>

  class date {
    int year_, month_, day_;
  public:
    date(int year, int month, int day): year_(year), month_(month), day_(day) {}

    friend std::ostream& operator<<(std::ostream& os, const date& d) {
      return os << d.year_ << '-' << d.month_ << '-' << d.day_;
    }
  };

  std::string s = fmt::format("The date is {}", date(2012, 12, 9));
  // s == "The date is 2012-12-9"

.. doxygenfunction:: print(std::basic_ostream<Char>&, const S&, Args&&...)

.. _printf-api:

``printf`` Formatting
=====================

The header ``fmt/printf.h`` provides ``printf``-like formatting functionality.
The following functions use `printf format string syntax
<http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html>`_ with
the POSIX extension for positional arguments. Unlike their standard
counterparts, the ``fmt`` functions are type-safe and throw an exception if an
argument type doesn't match its format specification.

.. doxygenfunction:: printf(const S&, const Args&...)

.. doxygenfunction:: fprintf(std::FILE *, const S&, const Args&...)

.. doxygenfunction:: fprintf(std::basic_ostream<Char>&, const S&, const Args&...)

.. doxygenfunction:: sprintf(const S&, const Args&...)

Compatibility with C++20 ``std::format``
========================================

{fmt} implements nearly all of the `C++20 formatting library
<https://en.cppreference.com/w/cpp/utility/format>`_ with the following
differences:

* Names are defined in the ``fmt`` namespace instead of ``std`` to avoid
  collisions with standard library implementations.
* The ``'L'`` format specifier cannot be combined with presentation specifiers
  yet.
* Width calculation doesn't use grapheme clusterization. The latter has been
  implemented in a separate branch but hasn't been integrated yet.
* Chrono formatting doesn't support C++20 date types since they are not provided
  by standard library implementations.