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 main formatting functions
  for ``char``/UTF-8 with C++20 compile-time checks and minimal dependencies
* :ref:`fmt/format.h <format-api>`: the full format API providing additional
  formatting functions and locale support
* :ref:`fmt/ranges.h <ranges-api>`: formatting of ranges and tuples
* :ref:`fmt/chrono.h <chrono-api>`: date and time formatting
* :ref:`fmt/std.h <std-api>`: formatters for standard library types
* :ref:`fmt/compile.h <compile-api>`: format string compilation
* :ref:`fmt/color.h <color-api>`: terminal color and text style
* :ref:`fmt/os.h <os-api>`: system APIs
* :ref:`fmt/ostream.h <ostream-api>`: ``std::ostream`` support
* :ref:`fmt/args.h <args-api>`: dynamic argument lists
* :ref:`fmt/printf.h <printf-api>`: ``printf`` formatting
* :ref:`fmt/xchar.h <xchar-api>`: optional ``wchar_t`` support 

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 main formatting functions
for ``char``/UTF-8 with C++20 compile-time checks. It has minimal include
dependencies for better compile times. This header is only beneficial when
using {fmt} as a library (the default) and not in the header-only mode.
It also provides ``formatter`` specializations for built-in and string types.

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

*fmt* 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. `~fmt::format_string` is a format string which can be
implicitly constructed from a string literal or a ``constexpr`` string and is
checked at compile time in C++20. To pass a runtime format string wrap it in
`fmt::runtime`.

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

.. _format:

.. doxygenfunction:: format(format_string<T...> fmt, T&&... args) -> std::string
.. doxygenfunction:: vformat(string_view fmt, format_args args) -> std::string

.. doxygenfunction:: format_to(OutputIt out, format_string<T...> fmt, T&&... args) -> OutputIt
.. doxygenfunction:: format_to_n(OutputIt out, size_t n, format_string<T...> fmt, T&&... args) -> format_to_n_result<OutputIt>
.. doxygenfunction:: formatted_size(format_string<T...> fmt, T&&... args) -> size_t

.. doxygenstruct:: fmt::format_to_n_result
   :members:

.. _print:

.. doxygenfunction:: fmt::print(format_string<T...> fmt, T&&... args)
.. doxygenfunction:: fmt::vprint(string_view fmt, format_args args)

.. doxygenfunction:: print(std::FILE *f, format_string<T...> fmt, T&&... args)
.. doxygenfunction:: vprint(std::FILE *f, string_view fmt, format_args args)

Compile-Time Format String Checks
---------------------------------

Compile-time format string checks are enabled by default on compilers
that support C++20 ``consteval``. On older compilers you can use the
:ref:`FMT_STRING <legacy-checks>`: macro defined in ``fmt/format.h`` instead.

Unused arguments are allowed as in Python's `str.format` and ordinary functions.

.. doxygenclass:: fmt::basic_format_string
   :members:

.. doxygentypedef:: fmt::format_string

.. doxygenfunction:: fmt::runtime(string_view) -> runtime_format_string<>

.. _udt:

Formatting User-Defined Types
-----------------------------

The {fmt} library provides formatters for many standard C++ types.
See :ref:`fmt/ranges.h <ranges-api>` for ranges and tuples including standard
containers such as ``std::vector``, :ref:`fmt/chrono.h <chrono-api>` for date
and time formatting and :ref:`fmt/std.h <std-api>` for other standard library
types.

There are two ways to make a user-defined type formattable: providing a
``format_as`` function or specializing the ``formatter`` struct template.

Use ``format_as`` if you want to make your type formattable as some other type
with the same format specifiers. The ``format_as`` function should take an
object of your type and return an object of a formattable type. It should be
defined in the same namespace as your type.

Example (https://godbolt.org/z/r7vvGE1v7)::

  #include <fmt/format.h>

  namespace kevin_namespacy {
  enum class film {
    house_of_cards, american_beauty, se7en = 7
  };
  auto format_as(film f) { return fmt::underlying(f); }
  }

  int main() {
    fmt::print("{}\n", kevin_namespacy::film::se7en); // prints "7"
  }

Using the specialization API is more complex but gives you full control over
parsing and formatting. To use this method specialize the ``formatter`` struct
template for your type and implement ``parse`` and ``format`` methods.
For example::

  #include <fmt/core.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) -> format_parse_context::iterator {
      // [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 '}'.
      
      // Please also note that this character range may be empty, in case of
      // the "{}" format string, so therefore you should check ctx.begin()
      // for equality with ctx.end().

      // 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.
    auto format(const point& p, format_context& ctx) const -> format_context::iterator {
      // ctx.out() is an output iterator to write to.
      return presentation == 'f'
                ? fmt::format_to(ctx.out(), "({:.1f}, {:.1f})", p.x, p.y)
                : fmt::format_to(ctx.out(), "({:.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::

  // color.h:
  #include <fmt/core.h>

  enum class color {red, green, blue};

  template <> struct fmt::formatter<color>: formatter<string_view> {
    // parse is inherited from formatter<string_view>.

    auto format(color c, format_context& ctx) const;
  };

  // color.cc:
  #include "color.h"
  #include <fmt/format.h>

  auto fmt::formatter<color>::format(color c, format_context& ctx) const {
    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);
  }

Note that ``formatter<string_view>::format`` is defined in ``fmt/format.h`` so
it has to be included in the source file.
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::

  // demo.h:
  #include <type_traits>
  #include <fmt/core.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> {
    auto format(const A& a, format_context& ctx) const {
      return fmt::formatter<std::string>::format(a.name(), ctx);
    }
  };

  // demo.cc:
  #include "demo.h"
  #include <fmt/format.h>

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

Providing both a ``formatter`` specialization and a ``format_as`` overload is
disallowed.

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

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

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

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

You can create your own formatting function with compile-time checks and small
binary footprint, for example (https://godbolt.org/z/vajfWEG4b):

.. code:: c++

    #include <fmt/core.h>

    void vlog(const char* file, int line, fmt::string_view format,
              fmt::format_args args) {
      fmt::print("{}: {}: ", file, line);
      fmt::vprint(format, args);
    }

    template <typename... T>
    void log(const char* file, int line, fmt::format_string<T...> format, T&&... args) {
      vlog(file, line, format, fmt::make_format_args(args...));
    }

    #define MY_LOG(format, ...) log(__FILE__, __LINE__, format, __VA_ARGS__)

    MY_LOG("invalid squishiness: {}", 42);

Note that ``vlog`` is not parameterized on argument types which improves compile
times and reduces binary code size compared to a fully parameterized version.

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

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

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

.. doxygentypedef:: fmt::format_args

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

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

.. doxygenclass:: fmt::basic_format_context
   :members:

.. doxygentypedef:: fmt::format_context

.. _args-api:

Dynamic Argument Lists
----------------------

The header ``fmt/args.h`` provides ``dynamic_format_arg_store``, a builder-like
API that can be used to construct format argument lists dynamically.

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

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

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

.. doxygentypedef:: fmt::string_view

.. _format-api:

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

``fmt/format.h`` defines the full format API providing additional formatting
functions and locale support.

Literal-Based API
-----------------

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

.. doxygenfunction:: operator""_a()

Utilities
---------

.. doxygenfunction:: fmt::ptr(T p) -> const void*
.. doxygenfunction:: fmt::ptr(const std::unique_ptr<T, Deleter> &p) -> const void*
.. doxygenfunction:: fmt::ptr(const std::shared_ptr<T> &p) -> const void*

.. doxygenfunction:: fmt::underlying(Enum e) -> typename std::underlying_type<Enum>::type

.. doxygenfunction:: fmt::to_string(const T &value) -> std::string

.. doxygenfunction:: fmt::join(Range &&range, string_view sep) -> join_view<detail::iterator_t<Range>, detail::sentinel_t<Range>>

.. doxygenfunction:: fmt::join(It begin, Sentinel end, string_view sep) -> join_view<It, Sentinel>

.. doxygenfunction:: fmt::group_digits(T value) -> group_digits_view<T>

.. 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.

.. doxygenfunction:: fmt::system_error

.. doxygenfunction:: fmt::format_system_error

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) {
      auto buf = custom_memory_buffer(alloc);
      fmt::vformat_to(std::back_inserter(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. Formatting functions
normally don't do any allocations for built-in and string types except for
non-default floating-point formatting that occasionally falls back on
``sprintf``.

Locale
------

All formatting is locale-independent by default. Use the ``'L'`` 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"

``fmt/format.h`` provides the following overloads of formatting functions that
take ``std::locale`` as a parameter. The locale type is a template parameter to
avoid the expensive ``<locale>`` include.

.. doxygenfunction:: format(const Locale& loc, format_string<T...> fmt, T&&... args) -> std::string
.. doxygenfunction:: format_to(OutputIt out, const Locale& loc, format_string<T...> fmt, T&&... args) -> OutputIt
.. doxygenfunction:: formatted_size(const Locale& loc, format_string<T...> fmt, T&&... args) -> size_t

.. _legacy-checks:

Legacy Compile-Time Format String Checks
----------------------------------------

``FMT_STRING`` enables compile-time checks on older compilers. It requires C++14
or later and is a no-op in C++11.

.. doxygendefine:: FMT_STRING

To force the use of legacy compile-time checks, define the preprocessor variable
``FMT_ENFORCE_COMPILE_STRING``. When set, functions accepting ``FMT_STRING``
will fail to compile with regular strings.

.. _ranges-api:

Range 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
========================

``fmt/chrono.h`` provides formatters for

* `std::chrono::duration <https://en.cppreference.com/w/cpp/chrono/duration>`_
* `std::chrono::time_point
  <https://en.cppreference.com/w/cpp/chrono/time_point>`_
* `std::tm <https://en.cppreference.com/w/cpp/chrono/c/tm>`_

The format syntax is described in :ref:`chrono-specs`.

**Example**::

  #include <fmt/chrono.h>

  int main() {
    std::time_t t = std::time(nullptr);

    // Prints "The date is 2020-11-07." (with the current date):
    fmt::print("The date is {:%Y-%m-%d}.", fmt::localtime(t));

    using namespace std::literals::chrono_literals;

    // Prints "Default format: 42s 100ms":
    fmt::print("Default format: {} {}\n", 42s, 100ms);

    // Prints "strftime-like format: 03:15:30":
    fmt::print("strftime-like format: {:%H:%M:%S}\n", 3h + 15min + 30s);
  }

.. doxygenfunction:: localtime(std::time_t time)

.. doxygenfunction:: gmtime(std::time_t time)

.. _std-api:

Standard Library Types Formatting
=================================

``fmt/std.h`` provides formatters for:

* `std::filesystem::path <https://en.cppreference.com/w/cpp/filesystem/path>`_
* `std::thread::id <https://en.cppreference.com/w/cpp/thread/thread/id>`_
* `std::monostate <https://en.cppreference.com/w/cpp/utility/variant/monostate>`_
* `std::variant <https://en.cppreference.com/w/cpp/utility/variant/variant>`_
* `std::optional <https://en.cppreference.com/w/cpp/utility/optional>`_

Formatting Variants
-------------------

A ``std::variant`` is only formattable if every variant alternative is formattable, and requires the
``__cpp_lib_variant`` `library feature <https://en.cppreference.com/w/cpp/feature_test>`_.
  
**Example**::

  #include <fmt/std.h>

  std::variant<char, float> v0{'x'};
  // Prints "variant('x')"
  fmt::print("{}", v0);

  std::variant<std::monostate, char> v1;
  // Prints "variant(monostate)"

.. _compile-api:

Format String Compilation
=========================

``fmt/compile.h`` provides format string compilation enabled via the
``FMT_COMPILE`` macro or the ``_cf`` user-defined literal. Format strings
marked with ``FMT_COMPILE`` or ``_cf`` are parsed, checked and converted into
efficient formatting code at compile-time. This supports arguments of built-in
and string types as well as user-defined types with ``format`` functions taking
the format context type as a template parameter in their ``formatter``
specializations. For example::

  template <> struct fmt::formatter<point> {
    constexpr auto parse(format_parse_context& ctx);

    template <typename FormatContext>
    auto format(const point& p, FormatContext& ctx) const;
  };

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

.. doxygenfunction:: operator""_cf()

.. _color-api:

Terminal Color and Text Style
=============================

``fmt/color.h`` provides support for terminal color and text style output.

.. doxygenfunction:: print(const text_style &ts, const S &format_str, const Args&... args)

.. doxygenfunction:: fg(detail::color_type)

.. doxygenfunction:: bg(detail::color_type)

.. doxygenfunction:: styled(const T& value, text_style ts)

.. _os-api:

System APIs
===========

.. doxygenclass:: fmt::ostream
   :members:

.. doxygenfunction:: fmt::windows_error
   :members:

.. _ostream-api:

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

``fmt/ostream.h`` provides ``std::ostream`` support including formatting of
user-defined types that have an overloaded insertion operator (``operator<<``).
In order to make a type formattable via ``std::ostream`` you should provide a
``formatter`` specialization inherited from ``ostream_formatter``::

  #include <fmt/ostream.h>

  struct date {
    int year, month, day;

    friend std::ostream& operator<<(std::ostream& os, const date& d) {
      return os << d.year << '-' << d.month << '-' << d.day;
    }
  };

  template <> struct fmt::formatter<date> : ostream_formatter {};

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

.. doxygenfunction:: streamed(const T &)

.. doxygenfunction:: print(std::ostream &os, format_string<T...> fmt, T&&... args)

.. _printf-api:

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

The header ``fmt/printf.h`` provides ``printf``-like formatting functionality.
The following functions use `printf format string syntax
<https://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(string_view fmt, const T&... args) -> int

.. doxygenfunction:: fprintf(std::FILE *f, const S &fmt, const T&... args) -> int

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

.. _xchar-api:

``wchar_t`` Support
===================

The optional header ``fmt/xchar.h`` provides support for ``wchar_t`` and exotic
character types.

.. doxygenstruct:: fmt::is_char

.. doxygentypedef:: fmt::wstring_view

.. doxygentypedef:: fmt::wformat_context

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

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.
* Width calculation doesn't use grapheme clusterization. The latter has been
  implemented in a separate branch but hasn't been integrated yet.
* Most C++20 chrono types are not supported yet.