.. _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 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/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/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 compile-time checks. It has minimal include dependencies for
better compile times. This header is only beneficial when using {fmt} as a
library and not in the header-only mode.
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. A function taking *fmt* 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(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, const 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:: 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 checks are enabled when using ``FMT_STRING``. They support built-in
and string types as well as user-defined types with ``constexpr`` ``parse``
functions in their ``formatter`` specializations.
.. doxygendefine:: FMT_STRING
To force the use of compile-time checks, define the preprocessor variable
``FMT_ENFORCE_COMPILE_STRING``. When set, functions accepting ``FMT_STRING``
will fail to compile with regular strings. Runtime-checked
formatting is still possible using ``fmt::vformat``, ``fmt::vprint``, etc.
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/oba4Mc):
.. code:: c++
#include <fmt/format.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 S, typename... Args>
void log(const char* file, int line, const S& format, Args&&... args) {
vlog(file, line, format,
fmt::make_args_checked<Args...>(format, args...));
}
#define MY_LOG(format, ...) \
log(__FILE__, __LINE__, FMT_STRING(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_args_checked(const S&, const remove_reference_t<Args>&...)
.. 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:
.. doxygentypedef:: fmt::format_args
.. doxygenclass:: fmt::basic_format_arg
:members:
.. doxygenclass:: fmt::basic_format_context
:members:
.. doxygentypedef:: fmt::format_context
Compatibility
-------------
.. doxygenclass:: fmt::basic_string_view
:members:
.. doxygentypedef:: fmt::string_view
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"
.. _format-api:
Format API
==========
``fmt/format.h`` defines the full format API providing additional formatting
functions and locale support.
.. _udt:
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) -> decltype(ctx.begin()) {
// [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) -> decltype(ctx.out()) {
// 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:
Literal-based API
-----------------
The following user-defined literals are defined in ``fmt/format.h``.
.. doxygenfunction:: operator""_format(const char *s, size_t n) -> detail::udl_formatter<char>
.. doxygenfunction:: operator""_a(const char *s, size_t) -> detail::udl_arg<char>
Utilities
---------
.. doxygenfunction:: fmt::ptr(T p) -> const void*
.. doxygenfunction:: fmt::ptr(const std::unique_ptr<T> &p) -> const void*
.. doxygenfunction:: fmt::ptr(const std::shared_ptr<T> &p) -> const void*
.. doxygenfunction:: fmt::to_string(const T &value) -> std::string
.. doxygenfunction:: fmt::to_string_view(const Char *s) -> basic_string_view<Char>
.. 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>
.. 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``.
.. _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
========================
``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)
.. _compile-api:
Format string compilation
=========================
``fmt/compile.h`` provides format string compilation support when using
``FMT_COMPILE``. Format strings 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 ``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
.. _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)
.. _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<<``)::
#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"
{fmt} only supports insertion operators that are defined in the same namespaces
as the types they format and can be found with the argument-dependent lookup.
.. doxygenfunction:: print(std::basic_ostream<Char> &os, const S &format_str, Args&&... 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(const S &format_str, const T&... args)
.. 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/wchar_t.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.