fmt/doc/api.md

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: printf formatting
• 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.

::: print(format_string<T...>, T&&...)

::: print(FILE*, format_string<T...>, T&&...)

::: println(format_string<T...>, T&&...)

::: println(FILE*, format_string<T...>, T&&...)

::: format_to(OutputIt&&, format_string<T...>, T&&...)

::: format_to_n(OutputIt, size_t, format_string<T...>, T&&...)

::: format_to_n_result

::: formatted_size(format_string<T...>, T&&...)

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.

::: basic_format_parse_context

::: context

::: format_context

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 FMT_STRING macro defined in fmt/format.h instead.

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

::: basic_format_string

::: format_string

::: runtime(string_view)

Named Arguments

::: arg(const Char*, const T&)

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.

::: make_format_args(T&...)

::: basic_format_args

::: format_args

::: basic_format_arg

Compatibility

::: basic_string_view

::: string_view

Format API

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

::: format(format_string<T...>, T&&...)

::: vformat(string_view, format_args)

Utilities

::: ptr(T)

::: underlying(Enum)

::: to_string(const T&)

::: group_digits(T)

::: detail::buffer

::: basic_memory_buffer

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.

::: system_error

::: format_system_error

Literal-Based API

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

::: operator""_a()

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) {
  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.

::: format(const Locale&, format_string<T...>, T&&...)

::: format_to(OutputIt, const Locale&, format_string<T...>, T&&...)

::: formatted_size(const Locale&, format_string<T...>, T&&...)

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.

::: 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.

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

::: join(Range&&, string_view)

::: join(It, Sentinel, string_view)

::: join(std::initializer_list, string_view)

Date and Time Formatting

fmt/chrono.h provides formatters for

• std::chrono::duration
• std::chrono::time_point
• std::tm

The format syntax is described in [Chrono Format Specifications](syntax.md# 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
}

::: localtime(std::time_t)

::: gmtime(std::time_t)

Standard Library Types Formatting

fmt/std.h provides formatters for:

• std::atomic
• std::atomic_flag
• std::bitset
• std::error_code
• std::filesystem::path
• std::monostate
• std::optional
• std::source_location
• std::thread::id
• std::variant

::: ptr(const std::unique_ptr<T, Deleter>&)

::: ptr(const std::shared_ptr&)

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. 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

::: operator""_cf

Terminal Colors and Text Styles

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

::: print(const text_style&, format_string<T...>, T&&...)

::: fg(detail::color_type)

::: bg(detail::color_type)

::: styled(const T&, text_style)

System APIs

::: ostream

::: windows_error

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"

::: streamed(const T&)

::: print(std::ostream&, format_string<T...>, T&&...)

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.

::: dynamic_format_arg_store

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.

::: printf(string_view, const T&...)

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

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

Wide Strings

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

::: is_char

::: wstring_view

::: wformat_context

::: to_wstring(const 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.