API Reference

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

fmt/base.h: the base API providing main formatting functions for char/UTF-8 with C++20 compile-time checks and minimal dependencies
fmt/format.h: fmt::format and other formatting functions as well as locale support
fmt/ranges.h: formatting of ranges and tuples
fmt/chrono.h: date and time formatting
fmt/std.h: formatters for standard library types
fmt/compile.h: format string compilation
fmt/color.h: terminal colors and text styles
fmt/os.h: system APIs
fmt/ostream.h: std::ostream support
fmt/args.h: dynamic argument lists
fmt/printf.h: safe printf
fmt/xchar.h: optional wchar_t support

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

Base API

fmt/base.h defines the base 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 the following types:

int, long long,
unsigned, unsigned long long
float, double, long double
bool
char
const char*, fmt::string_view
const void*

The following functions use format string syntax similar to that of str.format in Python. 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.

I/O errors are reported as std::system_error exceptions unless specified otherwise.

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

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

Example:

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

template <typename... T>
void print(FILE* f, format_string<T...> fmt, T&&... args);

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

Example:

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

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

Formats args according to specifications in fmt and writes the output to stdout followed by a newline.

template <typename... T>
void println(FILE* f, format_string<T...> fmt, T&&... args);

Formats args according to specifications in fmt and writes the output to the file f followed by a newline.

template <typename OutputIt, typename... T>
auto format_to(OutputIt&& out, format_string<T...> fmt, T&&... args) -⁠> remove_cvref_t<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. format_to does not append a terminating null character.

Example:

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

template <typename OutputIt, typename... T>
auto format_to_n(OutputIt out, size_t n, format_string<T...> fmt, 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. format_to_n does not append a terminating null character.

template <typename OutputIt>
struct format_to_n_result;


OutputIt out;

Iterator past the end of the output range.


size_t size;

Total (not truncated) output size.

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

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

Formatting User-Defined Types

The {fmt} library provides formatters for many standard C++ types. See fmt/ranges.h for ranges and tuples including standard containers such as std::vector, fmt/chrono.h for date and time formatting and fmt/std.h 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 (run):

#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); // Output: 7
}

Using specialization 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.

The recommended way of defining a formatter is by reusing an existing one via inheritance or composition. This way you can support standard format specifiers without implementing them yourself. For example:

// color.h:
#include <fmt/base.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
    -> format_context::iterator;
};

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

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

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

will return " blue".

In general the formatter has the following form:

template <> struct fmt::formatter<T> {
  // Parses format specifiers and stores them in the formatter.
  //
  // [ctx.begin(), ctx.end()) is a, possibly empty, character range that
  // contains a part of the format string starting from the format
  // specifications to be parsed, e.g. in
  //
  //   fmt::format("{:f} continued", ...);
  //
  // the range will contain "f} continued". 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 '}'.
  constexpr auto parse(format_parse_context& ctx)
    -> format_parse_context::iterator;

  // Formats value using the parsed format specification stored in this
  // formatter and writes the output to ctx.out().
  auto format(const T& value, format_context& ctx) const
    -> format_context::iterator;
};

It is recommended to at least support fill, align and width that apply to the whole object and have the same semantics as in standard formatters.

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_v<A, T>, char>> :
    fmt::formatter<std::string> {
  auto format(const A& a, format_context& ctx) const {
    return 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); // Output: B
}

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

template <typename Char>
using basic_format_parse_context = parse_context<Char>;


class context;


context(iterator out, format_args args, detail::locale_ref loc);

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


using format_context = context;

Compile-Time Checks

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

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

template <typename Char, typename... T>
using basic_format_string = basic_fstring<Char, T...>;

template <typename... T>
using format_string = typename fstring<T...>::t;


auto runtime(string_view s) -⁠> runtime_format_string<>;

Creates a runtime format string.

Example:

// Check format string at runtime instead of compile-time.
fmt::print(fmt::runtime("{:d}"), "I am not a number");

Named Arguments

template <typename Char, typename T>
auto 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.

Example:

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

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

Type Erasure

You can create your own formatting function with compile-time checks and small binary footprint, for example (run):

#include <fmt/format.h>

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

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

#define MY_LOG(fmt, ...) log(__FILE__, __LINE__, fmt, __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 Context, typename... T, int NUM_ARGS, int NUM_NAMED_ARGS, unsigned long long DESC>
constexpr auto make_format_args(T&... args) -⁠> detail::format_arg_store<Context, NUM_ARGS, NUM_NAMED_ARGS, DESC>;

Constructs an object that stores 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>
class 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(fmt::string_view fmt, fmt::format_args args);  // OK
fmt::format_args args = fmt::make_format_args();  // Dangling reference


constexpr basic_format_args(const store<NUM_ARGS, NUM_NAMED_ARGS, DESC>& s);

Constructs a basic_format_args object from format_arg_store.


constexpr basic_format_args(const format_arg* args, int count, bool has_named);

Constructs a basic_format_args object from a dynamic list of arguments.


auto get(int id) -⁠> format_arg;

Returns the argument with the specified id.


using format_args = basic_format_args<context>;

template <typename Context>
class basic_format_arg;


auto visit(Visitor&& vis) -⁠> decltype(vis(0));

Visits an argument dispatching to the appropriate visit method based on the argument type. For example, if the argument type is double then vis(value) will be called with the value of type double.

Compatibility

template <typename Char>
class 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::basic_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).


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.


basic_string_view(const S& s);

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


constexpr auto data() -⁠> const Char*;

Returns a pointer to the string data.


constexpr auto size() -⁠> size_t;

Returns the string size.


using string_view = basic_string_view<char>;

Format API

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

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

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

Example:

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


auto vformat(string_view fmt, format_args args) -⁠> std::string;

template <detail::fixed_string S>
constexpr auto operator""_a();

Utilities

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

Converts p to const void* for pointer formatting.

Example:

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

template <typename Enum>
constexpr auto underlying(Enum e) -⁠> underlying_t<Enum>;

Converts e to the underlying type.

Example:

enum class color { red, green, blue };
auto s = fmt::format("{}", fmt::underlying(color::red));  // s == "0"

template <typename T>
auto to_string(const T& value) -⁠> std::string;

template <typename T>
auto group_digits(T value) -⁠> group_digits_view<T>;

Returns a view that formats an integer value using ',' as a locale-independent thousands separator.

Example:

fmt::print("{}", fmt::group_digits(12345));
// Output: "12,345"

template <typename T>
class detail::buffer;

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


constexpr auto size() -⁠> size_t;

Returns the size of this buffer.


constexpr auto capacity() -⁠> size_t;

Returns the capacity of this buffer.


auto data() -⁠> T*;

Returns a pointer to the buffer data (not null-terminated).


void clear();

Clears this buffer.


void append(const U* begin, const U* end);

Appends data to the end of the buffer.

template <typename T, size_t SIZE, typename Allocator>
class basic_memory_buffer;

A dynamically growing memory buffer for trivially copyable/constructible types with the first SIZE elements stored in the object itself. Most commonly used via the memory_buffer alias for char.

Example:

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

This will append "The answer is 42." to out. The buffer content can be converted to std::string with to_string(out).


basic_memory_buffer(basic_memory_buffer&& other);

Constructs a 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.

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 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.

Example:

// 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";
FILE* file = fopen(filename, "r");
if (!file)
  throw fmt::system_error(errno, "cannot open file '{}'", filename);


void 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>;

auto vformat(custom_allocator alloc, fmt::string_view fmt,
             fmt::format_args args) -> custom_string {
  auto buf = custom_memory_buffer(alloc);
  fmt::vformat_to(std::back_inserter(buf), fmt, args);
  return custom_string(buf.data(), buf.size(), alloc);
}

template <typename ...Args>
auto format(custom_allocator alloc, fmt::string_view fmt,
            const Args& ... args) -> custom_string {
  return vformat(alloc, fmt, 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.

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

template <typename OutputIt, typename... T>
auto format_to(OutputIt out, detail::locale_ref loc, format_string<T...> fmt, T&&... args) -⁠> OutputIt;

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

Legacy Compile-Time Checks

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


FMT_STRING(s)

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

Example:

// 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 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.

Range and Tuple Formatting

fmt/ranges.h provides formatting support for ranges and tuples:

#include <fmt/ranges.h>

fmt::print("{}", std::tuple<char, int>{'a', 42});
// Output: ('a', 42)

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

#include <fmt/ranges.h>

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

template <typename Range>
auto join(Range&& r, string_view sep) -⁠> join_view<decltype(detail::range_begin(r)), decltype(detail::range_end(r))>;

Returns a view that formats range with elements separated by sep.

Example:

auto v = std::vector<int>{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 join(It begin, Sentinel end, string_view sep) -⁠> join_view<It, Sentinel>;

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

template <typename T>
auto join(std::initializer_list<T> list, string_view sep) -⁠> join_view<const T*, const T*>;

Returns an object that formats std::initializer_list with elements separated by sep.

Example:

fmt::print("{}", fmt::join({1, 2, 3}, ", "));
// Output: "1, 2, 3"

Date and Time Formatting

fmt/chrono.h provides formatters for

The format syntax is described in Chrono Format Specifications.

Example:

#include <fmt/chrono.h>

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

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

  using namespace std::literals::chrono_literals;

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

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


auto localtime(std::time_t time) -⁠> std::tm;

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.


auto gmtime(std::time_t time) -⁠> std::tm;

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.

Standard Library Types Formatting

fmt/std.h provides formatters for:

template <typename T, typename Deleter>
auto ptr(const std::unique_ptr<T, Deleter>& p) -⁠> const void*;

template <typename T>
auto ptr(const std::shared_ptr<T>& p) -⁠> const void*;

Formatting Variants

A std::variant is only formattable if every variant alternative is formattable, and requires the __cpp_lib_variant library feature.

Example:

#include <fmt/std.h>

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

fmt::print("{}", std::variant<std::monostate, char>());
// Output: variant(monostate)

Format String Compilation

fmt/compile.h provides format string compilation enabled via the FMT_COMPILE macro or the _cf user-defined literal defined in namespace fmt::literals. 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.


FMT_COMPILE(s)

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.

Example:

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

template <detail::fixed_string Str>
constexpr auto operator""_cf();

Terminal Colors and Text Styles

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

template <typename... T>
void print(const text_style& ts, format_string<T...> fmt, T&&... args);

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

Example:

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


auto fg(detail::color_type foreground) -⁠> text_style;

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


auto bg(detail::color_type background) -⁠> text_style;

Creates a text style from the background color.

template <typename T>
auto styled(const T& value, text_style ts) -⁠> detail::styled_arg<remove_cvref_t<T>>;

Returns an argument that will be formatted using ANSI escape sequences, to be used in a formatting function.

Example:

fmt::print("Elapsed time: {0:.2f} seconds",
           fmt::styled(1.23, fmt::fg(fmt::color::green) |
                             fmt::bg(fmt::color::blue)));

System APIs


class ostream;

A fast buffered output stream for writing from a single thread. Writing from multiple threads without external synchronization may result in a data race.


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>
auto windows_error(int error_code, string_view message, const T&... args) -⁠> std::system_error;

Constructs a std::system_error object with the description of the form

<message>: <system-message>

where <message> is the formatted message and <system-message> is the system message corresponding to the error code. error_code is a Windows error code as given by GetLastError. If error_code is not a valid error code such as -1, the system message will look like "error -1".

Example:

// This throws a system_error with the description
//   cannot open file 'madeup': The system cannot find the file

specified. // or similar (system message may vary). const char *filename = "madeup"; LPOFSTRUCT of = LPOFSTRUCT(); HFILE file = OpenFile(filename, &of, OF_READ); if (file == HFILE_ERROR) { throw fmt::windows_error(GetLastError(), "cannot open file '{}'", filename); }

`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"

template <typename T>
constexpr auto streamed(const T& value) -⁠> detail::streamed_view<T>;

Returns a view that formats value via an ostream operator<<.

Example:

fmt::print("Current thread id: {}\n",
           fmt::streamed(std::this_thread::get_id()));

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

Prints formatted data to the stream os.

Example:

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

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.

template <typename Context>
class dynamic_format_arg_store;

A dynamic list of formatting arguments with storage.

It can be implicitly converted into fmt::basic_format_args for passing into type-erased formatting functions such as fmt::vformat.


void push_back(const T& arg);

Adds an argument into the dynamic store for later passing to a formatting function.

Note that custom types and string types (but not string views) are copied into the store dynamically allocating memory if necessary.

Example:

fmt::dynamic_format_arg_store<fmt::format_context> store;
store.push_back(42);
store.push_back("abc");
store.push_back(1.5f);
std::string result = fmt::vformat("{} and {} and {}", store);


void push_back(std::reference_wrapper<T> arg);

Adds a reference to the argument into the dynamic store for later passing to a formatting function.

Example:

fmt::dynamic_format_arg_store<fmt::format_context> store;
char band[] = "Rolling Stones";
store.push_back(std::cref(band));
band[9] = 'c'; // Changing str affects the output.
std::string result = fmt::vformat("{}", store);
// result == "Rolling Scones"


void push_back(const detail::named_arg<char_type, T>& arg);

Adds named argument into the dynamic store for later passing to a formatting function. std::reference_wrapper is supported to avoid copying of the argument. The name is always copied into the store.


void clear();

Erase all elements from the store.


void reserve(size_t new_cap, size_t new_cap_named);

Reserves space to store at least new_cap arguments including new_cap_named named arguments.

Safe `printf`

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... T>
auto printf(string_view fmt, const T&... args) -⁠> int;

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

Example:

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

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

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

Example:

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

template <typename S, typename... T, typename Char>
auto sprintf(const S& fmt, const T&... args) -⁠> std::basic_string<Char>;

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

Example:

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

Wide Strings

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

template <typename T>
struct is_char;


using wstring_view = basic_string_view<wchar_t>;


using wformat_context = buffered_context<wchar_t>;

template <typename T>
auto 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.

API Reference

Base API

Formatting User-Defined Types

Compile-Time Checks

Named Arguments

Type Erasure

Compatibility

Format API

Utilities

System Errors

Custom Allocators

Locale

Legacy Compile-Time Checks

Range and Tuple Formatting

Date and Time Formatting

Standard Library Types Formatting

Formatting Variants

Format String Compilation

Terminal Colors and Text Styles

System APIs

std::ostream Support

Dynamic Argument Lists

Safe printf

Wide Strings

Compatibility with C++20 std::format

`std::ostream` Support

Safe `printf`

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