API Reference

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

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

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 format string syntax similar to that of Python’s 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.

template<typename ...T>
auto fmt::format(format_string<T...> fmt, T&&... args) -> std::string

Formats args according to specifications in fmt and returns the result as a string.


#include <fmt/core.h>
std::string message = fmt::format("The answer is {}", 42);

auto fmt::vformat(string_view fmt, format_args args) -> std::string
template<typename OutputIt, typename ...T>
auto fmt::format_to(OutputIt out, format_string<T...> fmt, T&&... args) -> OutputIt

Formats args according to specifications in fmt, writes the result to the output iterator out and returns the iterator past the end of the output range.


auto out = std::vector<char>();
fmt::format_to(std::back_inserter(out), "{}", 42);

template<typename OutputIt, typename ...T>
auto fmt::format_to_n(OutputIt out, size_t n, format_string<T...> fmt, const T&... args) -> format_to_n_result<OutputIt>

Formats args according to specifications in fmt, writes up to n characters of the result to the output iterator out and returns the total (not truncated) output size and the iterator past the end of the output range.

template<typename ...T>
auto fmt::formatted_size(format_string<T...> fmt, T&&... args) -> size_t

Returns the number of chars in the output of format(fmt, args...).

template<typename OutputIt>
struct fmt::format_to_n_result

Public Members

OutputIt out

Iterator past the end of the output range.

size_t size

Total (not truncated) output size.

template<typename ...T>
void fmt::print(format_string<T...> fmt, T&&... args)

Formats args according to specifications in fmt and writes the output to stdout.


fmt::print("Elapsed time: {0:.2f} seconds", 1.23);

void fmt::buffered_file::vprint(string_view format_str, format_args args)
template<typename ...T>
void fmt::print(std::FILE *f, format_string<T...> fmt, T&&... args)

Formats args according to specifications in fmt and writes the output to the file f.


fmt::print(stderr, "Don't {}!", "panic");

void fmt::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.


Constructs a compile-time format string from a string literal s.


// A compile-time error because 'd' is an invalid specifier for strings.
std::string s = fmt::format(FMT_STRING("{:d}"), "foo");

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

template<typename Char, typename T>
auto fmt::arg(const Char *name, const T &arg) -> detail::named_arg<Char, T>

Returns a named argument to be used in a formatting function. It should only be used in a call to a formatting function or dynamic_format_arg_store::push_back.


fmt::print("Elapsed time: {s:.2f} seconds", fmt::arg("s", 1.23));

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):

#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.

template<typename ...Args, typename S, typename Char = char_t<S>>
auto fmt::make_args_checked(const S &fmt, const remove_reference_t<Args>&... args) -> format_arg_store<buffer_context<Char>, remove_reference_t<Args>...>

Constructs a format_arg_store object that contains references to arguments and can be implicitly converted to format_args. If fmt is a compile-time string then make_args_checked() checks its validity at compile time.

template<typename Context = format_context, typename ...Args>
constexpr auto fmt::make_format_args(const Args&... args) -> format_arg_store<Context, Args...>

Constructs a format_arg_store object that contains references to arguments and can be implicitly converted to format_args. Context can be omitted in which case it defaults to context. See arg() for lifetime considerations.

template<typename Context, typename ...Args>
class format_arg_store

An array of references to arguments. It can be implicitly converted into basic_format_args for passing into type-erased formatting functions such as vformat().

template<typename Context>
class dynamic_format_arg_store
template<typename Context>
class fmt::basic_format_args

A view of a collection of formatting arguments. To avoid lifetime issues it should only be used as a parameter type in type-erased functions such as vformat:

void vlog(string_view format_str, format_args args);  // OK
format_args args = make_format_args(42);  // Error: dangling reference

Public Functions

template<typename ...Args>
constexpr basic_format_args(const format_arg_store<Context, Args...> &store)

Constructs a basic_format_args() object from format_arg_store.

constexpr basic_format_args(const dynamic_format_arg_store<Context> &store)

Constructs a basic_format_args() object from dynamic_format_arg_store.

constexpr basic_format_args(const format_arg *args, int count)

Constructs a basic_format_args() object from a dynamic set of arguments.

auto get(int id) const -> format_arg

Returns the argument with the specified id.

using fmt::format_args = basic_format_args<format_context>

An alias to basic_format_args<format_context>.

template<typename Context>
class basic_format_arg
template<typename OutputIt, typename Char>
class fmt::basic_format_context

Public Types

using char_type = Char

The character type for the output.

Public Functions

constexpr basic_format_context(OutputIt out, basic_format_args<basic_format_context> ctx_args, detail::locale_ref loc = detail::locale_ref())

Constructs a basic_format_context object.

References to the arguments are stored in the object so make sure they have appropriate lifetimes.

using fmt::format_context = buffer_context<char>


template<typename Char>
class fmt::basic_string_view

An implementation of std::basic_string_view for pre-C++17.

It provides a subset of the API. fmt::basic_string_view is used for format strings even if std::string_view is available to prevent issues when a library is compiled with a different -std option than the client code (which is not recommended).

Public Functions

constexpr basic_string_view(const Char *s, size_t count)

Constructs a string reference object from a C string and a size.

basic_string_view(const Char *s)

Constructs a string reference object from a C string computing the size with std::char_traits<Char>::length.

template<typename Traits, typename Alloc>
basic_string_view(const std::basic_string<Char, Traits, Alloc> &s)

Constructs a string reference from a std::basic_string object.

constexpr auto data() const -> const Char*

Returns a pointer to the string data.

constexpr auto size() const -> size_t

Returns the string size.

using fmt::string_view = basic_string_view<char>


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>

auto s = fmt::format("{:L}", 1000000);  // s == "1,000,000"

Format API

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

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(
        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

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.

template<typename Char, typename ErrorHandler = detail::error_handler>
class fmt::basic_format_parse_context : private fmt::detail::error_handler

Parsing context consisting of a format string range being parsed and an argument counter for automatic indexing. You can use the format_parse_context type alias for char instead.

Subclassed by fmt::detail::compile_parse_context< Char, ErrorHandler >, fmt::basic_printf_parse_context< Char >

Public Functions

constexpr auto begin() const -> iterator

Returns an iterator to the beginning of the format string range being parsed.

constexpr auto end() const -> iterator

Returns an iterator past the end of the format string range being parsed.

void advance_to(iterator it)

Advances the begin iterator to it.

auto next_arg_id() -> int

Reports an error if using the manual argument indexing; otherwise returns the next argument index and switches to the automatic indexing.

void check_arg_id(int)

Reports an error if using the automatic argument indexing; otherwise switches to the manual indexing.

Literal-based API

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

constexpr auto fmt::operator""_format(const char *s, size_t n) -> detail::udl_formatter<char>

User-defined literal equivalent of fmt::format().


using namespace fmt::literals;
std::string message = "The answer is {}"_format(42);

constexpr auto fmt::operator""_a(const char *s, size_t) -> detail::udl_arg<char>

User-defined literal equivalent of fmt::arg().


using namespace fmt::literals;
fmt::print("Elapsed time: {s:.2f} seconds", "s"_a=1.23);


template<typename T>
auto fmt::ptr(T p) -> const void*

Converts p to const void* for pointer formatting.


auto s = fmt::format("{}", fmt::ptr(p));

template<typename T>
auto fmt::ptr(const std::unique_ptr<T> &p) -> const void*
template<typename T>
auto fmt::ptr(const std::shared_ptr<T> &p) -> const void*
template<typename T>
auto fmt::to_string(const T &value) -> std::string

Converts value to std::string using the default format for type T.


#include <fmt/format.h>

std::string answer = fmt::to_string(42);

template<typename Char>
auto fmt::to_string_view(const Char *s) -> basic_string_view<Char>
template<typename Range>
auto fmt::join(Range &&range, string_view sep) -> join_view<detail::iterator_t<Range>, detail::sentinel_t<Range>>

Returns an object that formats range with elements separated by sep.


std::vector<int> v = {1, 2, 3};
fmt::print("{}", fmt::join(v, ", "));
// Output: "1, 2, 3"

fmt::join applies passed format specifiers to the range elements:

fmt::print("{:02}", fmt::join(v, ", "));
// Output: "01, 02, 03"

template<typename It, typename Sentinel>
auto fmt::join(It begin, Sentinel end, string_view sep) -> join_view<It, Sentinel>

Returns an object that formats the iterator range [begin, end) with elements separated by sep.

template<typename T>
class fmt::detail::buffer

A contiguous memory buffer with an optional growing ability. It is an internal class and shouldn’t be used directly, only via basic_memory_buffer.

Subclassed by fmt::basic_memory_buffer< wchar_t >, fmt::basic_memory_buffer< T, SIZE, Allocator >, fmt::detail::iterator_buffer< OutputIt, T, Traits >, fmt::detail::iterator_buffer< T *, T >

Public Functions

auto size() const -> size_t

Returns the size of this buffer.

auto capacity() const -> size_t

Returns the capacity of this buffer.

auto data() -> T*

Returns a pointer to the buffer data.

auto data() const -> const T*

Returns a pointer to the buffer data.

void clear()

Clears this buffer.

template<typename U>
void append(const U *begin, const U *end)

Appends data to the end of the buffer.

template<typename T, size_t SIZE = inline_buffer_size, typename Allocator = std::allocator<T>>
class fmt::basic_memory_buffer : public fmt::detail::buffer<T>

A dynamically growing memory buffer for trivially copyable/constructible types with the first SIZE elements stored in the object itself.

You can use the memory_buffer type alias for char instead.


fmt::memory_buffer out;
format_to(out, "The answer is {}.", 42);

This will append the following output to the out object:

The answer is 42.

The output can be converted to an std::string with to_string(out).

Public Functions

basic_memory_buffer(basic_memory_buffer &&other)

Constructs a fmt::basic_memory_buffer object moving the content of the other object to it.

auto operator=(basic_memory_buffer &&other) -> basic_memory_buffer&

Moves the content of the other basic_memory_buffer object to this one.

void resize(size_t count)

Resizes the buffer to contain count elements.

If T is a POD type new elements may not be initialized.

void reserve(size_t new_capacity)

Increases the buffer capacity to new_capacity.

Protected Functions

void grow(size_t size) final

Increases the buffer capacity to hold at least capacity elements.

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.

template<typename ...T>
auto fmt::system_error(int error_code, format_string<T...> fmt, T&&... args) -> std::system_error

Constructs std::system_error with a message formatted with fmt::format(fmt, args...).

error_code is a system error code as given by errno.


// This throws std::system_error with the description
//   cannot open file 'madeup': No such file or directory
// or similar (system message may vary).
const char* filename = "madeup";
std::FILE* file = std::fopen(filename, "r");
if (!file)
  throw fmt::system_error(errno, "cannot open file '{}'", filename);

void fmt::format_system_error(detail::buffer<char> &out, int error_code, const char *message)

Formats an error message for an error returned by an operating system or a language runtime, for example a file opening error, and writes it to out. The format is the same as the one used by std::system_error(ec, message) where ec is std::error_code(error_code, std::generic_category()}). It is implementation-defined but normally looks like:

<message>: <system-message>

where <message> is the passed message and <system-message> is the system message corresponding to the error code. error_code is a system error code as given by errno.

Custom Allocators

The {fmt} library supports custom dynamic memory allocators. A custom allocator class can be specified as a template argument to 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. 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 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, ", "));

Date and Time Formatting

fmt/chrono.h provides formatters for

The format syntax is described in Chrono Format Specifications.


#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);
std::tm fmt::localtime(std::time_t time)

Converts given time since epoch as std::time_t value into calendar time, expressed in local time.

Unlike std::localtime, this function is thread-safe on most platforms.

std::tm fmt::gmtime(std::time_t time)

Converts given time since epoch as std::time_t value into calendar time, expressed in Coordinated Universal Time (UTC).

Unlike std::gmtime, this function is thread-safe on most platforms.

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.


Converts a string literal s into a format string that will be parsed at compile time and converted into efficient formatting code. Requires C++17 constexpr if compiler support.


// Converts 42 into std::string using the most efficient method and no
// runtime format string processing.
std::string s = fmt::format(FMT_COMPILE("{}"), 42);

Terminal color and text style

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

template<typename S, typename ...Args>
void fmt::print(const text_style &ts, const S &format_str, const Args&... args)

Formats a string and prints it to stdout using ANSI escape sequences to specify text formatting.


fmt::print(fmt::emphasis::bold | fg(fmt::color::red),
           "Elapsed time: {0:.2f} seconds", 1.23);

text_style fmt::fg(detail::color_type foreground)

Creates a text style from the foreground (text) color.

text_style fmt::bg(detail::color_type background)

Creates a text style from the background color.

System APIs

class fmt::ostream : private fmt::detail::buffer<char>

A fast output stream which is not thread-safe.

Public Functions

template<typename ...T>
void print(format_string<T...> fmt, T&&... args)

Formats args according to specifications in fmt and writes the output to the file.


template<typename ...T>
ostream output_file(cstring_view path, T... params)

Opens a file for writing. Supported parameters passed in params:

  • <integer>: Flags passed to open (file::WRONLY | file::CREATE by default)

  • buffer_size=<integer>: Output buffer size


auto out = fmt::output_file("guide.txt");
out.print("Don't {}", "Panic");

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_;
  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.

template<typename S, typename ...Args, typename Char = enable_if_t<detail::is_string<S>::value, char_t<S>>>
void fmt::print(std::basic_ostream<Char> &os, const S &format_str, Args&&... args)

Prints formatted data to the stream os.


fmt::print(cerr, "Don't {}!", "panic");

printf Formatting

The header fmt/printf.h provides printf-like formatting functionality. The following functions use printf format string syntax 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.

template<typename S, typename ...T>
auto fmt::printf(const S &fmt, const T&... args) -> int

Prints formatted data to stdout.


fmt::printf("Elapsed time: %.2f seconds", 1.23);

template<typename S, typename ...T, typename Char = char_t<S>>
auto fmt::fprintf(std::FILE *f, const S &fmt, const T&... args) -> int

Prints formatted data to the file f.


fmt::fprintf(stderr, "Don't %s!", "panic");

template<typename S, typename ...T, typename Char = enable_if_t<detail::is_string<S>::value, char_t<S>>>
auto fmt::sprintf(const S &fmt, const T&... args) -> std::basic_string<Char>

Formats arguments and returns the result as a string.


std::string message = fmt::sprintf("The answer is %d", 42);

wchar_t Support

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

template<typename T>
struct is_char : public std::false_type

Specifies if T is a character type.

Can be specialized by users.

using fmt::wstring_view = basic_string_view<wchar_t>
using fmt::wformat_context = buffer_context<wchar_t>
template<typename T>
auto fmt::to_wstring(const T &value) -> std::wstring

Converts value to std::wstring using the default format for type T.

Compatibility with C++20 std::format

{fmt} implements nearly all of the C++20 formatting library 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.