fmt/doc/api.md

# API Reference

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

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

## C++ Module API

With the C++ module API, the headers listed above don't need to be included.
You can use the `import fmt;` statement instead. All other functionality,
listed below, remains the same.

## 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`](#basic_string_view)
- `const void*`

The following functions use [format string syntax](syntax.md) similar to that
of [str.format](https://docs.python.org/3/library/stdtypes.html#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`](#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`](#runtime).

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

I/O errors are reported as [`std::system_error`](
https://en.cppreference.com/w/cpp/error/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&&...)

<a id="udt"></a>
### Formatting User-Defined Types

The {fmt} library provides formatters for many standard C++ types.
See [`fmt/ranges.h`](#ranges-api) for ranges and tuples including standard
containers such as `std::vector`, [`fmt/chrono.h`](#chrono-api) for date and
time formatting and [`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.
Formatting of non-void pointer types is intentionally disallowed and they
cannot be made formattable via either extension API.

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.
`format_as` cannot be used when a type also matches another `formatter`
specialization, such as the range `formatter`, because the specializations
would be ambiguous. Disable the conflicting specialization, if possible,
or provide an explicit `formatter` specialization instead.

Example ([run](https://godbolt.org/z/nvME4arz8)):

    #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 a 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:

```c++
// 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;
};
```

```c++
// 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

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

will return `"      blue"`.

<!-- The experimental `nested_formatter` provides an easy way of applying a
formatter to one or more subobjects.

For example:

    #include <fmt/format.h>

    struct point {
      double x, y;
    };

    template <>
    struct fmt::formatter<point> : nested_formatter<double> {
      auto format(point p, format_context& ctx) const {
        return write_padded(ctx, [=](auto out) {
          return format_to(out, "({}, {})", this->nested(p.x),
                           this->nested(p.y));
        });
      }
    };

    int main() {
      fmt::print("[{:>20.2f}]", point{1, 2});
    }

prints:

    [          (1.00, 2.00)]

Notice that fill, align and width are applied to the whole object which
is the recommended behavior while the remaining specifiers apply to
elements. -->

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:

```c++
// demo.h:
#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_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);
  }
};
```

```c++
// 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 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](#legacy-checks) macro defined in `fmt/format.h` instead.

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

See [Type Erasure](#type-erasure) for an example of how to enable compile-time
checks in your own functions with `fmt::format_string` while avoiding template
bloat.

::: fstring

::: format_string

::: runtime(string_view)

### Type Erasure

You can create your own formatting function with compile-time checks and
small binary footprint, for example ([run](https://godbolt.org/z/b9Pbasvzc)):

```c++
#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

### Named Arguments

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

### Compatibility

::: basic_string_view

::: string_view

## Format API

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

<a id="format"></a>
::: format(format_string<T...>, T&&...)

::: vformat(string_view, format_args)

::: operator""_a()

### 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

### 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`](#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/format.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(locale_ref, format_string<T...>, T&&...)

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

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

<a id="legacy-checks"></a>
### 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

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.

<a id="ranges-api"></a>
## 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<T>, string_view)

<a id="chrono-api"></a>
## 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 [Chrono Format Specifications](syntax.md#
chrono-format-specifications).

**Example**:

    #include <fmt/chrono.h>

    int main() {
      auto now = std::chrono::system_clock::now();

      fmt::print("The date is {:%Y-%m-%d}.\n", now);
      // 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
    }

::: gmtime(std::time_t)

<a id="std-api"></a>
## Standard Library Types Formatting

`fmt/std.h` provides formatters for:

- [`std::atomic`](https://en.cppreference.com/w/cpp/atomic/atomic)
- [`std::atomic_flag`](https://en.cppreference.com/w/cpp/atomic/atomic_flag)
- [`std::bitset`](https://en.cppreference.com/w/cpp/utility/bitset)
- [`std::error_code`](https://en.cppreference.com/w/cpp/error/error_code)
- [`std::exception`](https://en.cppreference.com/w/cpp/error/exception)
- [`std::filesystem::path`](https://en.cppreference.com/w/cpp/filesystem/path)
- [`std::monostate`](
  https://en.cppreference.com/w/cpp/utility/variant/monostate)
- [`std::optional`](https://en.cppreference.com/w/cpp/utility/optional)
- [`std::source_location`](
  https://en.cppreference.com/w/cpp/utility/source_location)
- [`std::thread::id`](https://en.cppreference.com/w/cpp/thread/thread/id)
- [`std::variant`](https://en.cppreference.com/w/cpp/utility/variant/variant)

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

::: ptr(const std::shared_ptr<T>&)

### Variants

A `std::variant` can be formatted only if every alternative is
formattable, and requires the `__cpp_lib_variant` [library
feature](https://en.cppreference.com/w/cpp/feature_test).

**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)

## Bit-Fields and Packed Structs

To format a bit-field or a field of a struct with `__attribute__((packed))`
applied to it, you need to convert it to the underlying or compatible type via
a cast or a unary `+` ([godbolt](https://www.godbolt.org/z/3qKKs6T5Y)):

```c++
struct smol {
  int bit : 1;
};

auto s = smol();
fmt::print("{}", +s.bit);
```

This is a known limitation of "perfect" forwarding in C++.

<a id="enum-api"></a>
## Enum Formatting

`fmt/enum.h` provides formatting of enums annotated with
`fmt::as_identifiers`. Such an enum is formatted as the identifier of the
enumerator matching the formatted value:

    #include <fmt/enum.h>

    enum class [[=fmt::as_identifiers]] color { red, green, blue };

    fmt::print("{}", color::green);
    // Output: green

Such enums are formatted using the string [Format Specification](
syntax.md#format-specification), for example:

    fmt::print("[{:>7}]", color::red);
    // Output: [    red]

Identifiers are only available as `char` strings so annotated enums are not
formattable with other character types.

If several enumerators have the same value, the first one in the order of
declaration is used. A value that doesn't match any enumerator is represented
as its underlying value in decimal before applying string formatting:

    fmt::print("{}", static_cast<color>(42));
    // Output: 42

Enums without the annotation are not affected and are formatted as before, i.e.
scoped enums require `format_as` or a `formatter` specialization, see
[Formatting User-Defined Types](#udt).

Identifiers are retrieved with C++26 reflection ([P2996](
https://wg21.link/p2996)) and the annotation with [P3394](
https://wg21.link/p3394), so this requires a compiler with reflection support,
which may need an extra flag such as `-freflection` in GCC. The macro
`FMT_USE_REFLECTION` is set to 1 if reflection is available and to 0 otherwise.
It can also be defined by the user to disable the use of reflection, in which
case `fmt/enum.h` is empty.

When {fmt} is built as a module, reflection support is detected when the module
itself is compiled, so this API is only available to importers if the module was
built with reflection enabled. An importing translation unit may also have to
include `<meta>` itself: some compilers, such as GCC 16, fail to look up
implementation details of `std::define_static_string` when instantiating the
formatter otherwise.

<a id="compile-api"></a>
## Compile-Time Support

`fmt/compile.h` provides format string compilation and compile-time
(`constexpr`) formatting 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` methods taking
the format context type as a template parameter in their `formatter`
specializations. For example ([run](https://www.godbolt.org/z/3c13erEoq)):

    struct point {
      double x;
      double y;
    };

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

      template <typename FormatContext>
      auto format(const point& p, FormatContext& ctx) const {
        return format_to(ctx.out(), "({}, {})"_cf, p.x, p.y);
      }
    };

    using namespace fmt::literals;
    std::string s = fmt::format("{}"_cf, point(4, 2));

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.

The same APIs support formatting at compile time e.g. in `constexpr`
and `consteval` functions. Additionally there is an experimental
`FMT_STATIC_FORMAT` that allows formatting into a string of the exact
required size at compile time. Compile-time formatting works with built-in
and user-defined formatters that have `constexpr` `format` methods.
Example:

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

      template <typename FormatContext>
      constexpr auto format(const point& p, FormatContext& ctx) const {
        return format_to(ctx.out(), "({}, {})"_cf, p.x, p.y);
      }
    };

    constexpr auto s = FMT_STATIC_FORMAT("{}", point(4, 2));
    const char* cstr = s.c_str(); // Points the static string "(4, 2)".

::: operator""_cf

::: FMT_COMPILE

::: FMT_STATIC_FORMAT

<a id="color-api"></a>
## Terminal Colors and Text Styles

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

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

::: fg(detail::color_type)

::: bg(detail::color_type)

::: styled(const T&, text_style)

<a id="os-api"></a>
## System APIs

::: ostream

::: output_file(cstring_view, T...)

::: windows_error

<a id="ostream-api"></a>
## `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&&...)

<a id="args-api"></a>
## 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

<a id="printf-api"></a>
## Safe `printf`

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.

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

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

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

<a id="xchar-api"></a>
## Wide Strings

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

::: wstring_view

::: wformat_context

::: to_wstring(const T&)

## 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.

- The default floating-point representation in {fmt} uses the smallest
  precision that provides round-trip guarantees similarly to other languages
  like Java and Python. `std::format` is currently specified in terms of
  `std::to_chars` which tries to generate the smallest number of characters
  (ignoring redundant digits and sign in exponent) and may produce more
  decimal digits than necessary.

## Configuration Options

{fmt} provides configuration via CMake options and preprocessor macros to
enable or disable features and to optimize for binary size. For example, you
can disable OS-specific APIs defined in `fmt/os.h` with `-DFMT_OS=OFF` when
configuring CMake.

### CMake Options

- **`FMT_OS`**: When set to `OFF`, disables OS-specific APIs (`fmt/os.h`).
- **`FMT_UNICODE`**: When set to `OFF`, disables Unicode support on
  Windows/MSVC. Unicode support is always enabled on other platforms.

### Macros

- **`FMT_HEADER_ONLY`**: Enables the header-only mode when defined. It is an
  alternative to using the `fmt::fmt-header-only` CMake target.
  Default: not defined.

- **`FMT_USE_EXCEPTIONS`**: Disables the use of exceptions when set to `0`.
  Default: `1` (`0` if compiled with `-fno-exceptions`).

- **`FMT_USE_LOCALE`**: When set to `0`, disables locale support.
  Default: `1` (`0` when `FMT_OPTIMIZE_SIZE > 1`).

- **`FMT_CUSTOM_ASSERT_FAIL`**: When set to `1`, allows users to provide a
  custom `fmt::assert_fail` function which is called on assertion failures and,
  if exceptions are disabled, on runtime errors. Default: `0`.

- **`FMT_BUILTIN_TYPES`**: When set to `0`, disables built-in handling of
  arithmetic and string types other than `int`. This reduces library size at
  the cost of per-call overhead. Default: `1`.

- **`FMT_OPTIMIZE_SIZE`**: Controls binary size optimizations:
    - `0` - off (default)
    - `1` - disables locale support and applies some optimizations
    - `2` - disables some Unicode features, named arguments and applies more
      aggressive optimizations

### Binary Size Optimization

To minimize the binary footprint of {fmt} as much as possible at the cost of
some features, you can use the following configuration:

- CMake options:
    - `FMT_OS=OFF`
- Macros:
    - `FMT_BUILTIN_TYPES=0`
    - `FMT_OPTIMIZE_SIZE=2`