API Reference — fmt 6.2.1 documentation
Author: Victor Zverovich
Description: Small, safe and fast formatting library
Keywords: C++, formatting, printf, string, library

# API Reference

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

- fmt/core.h: the core API providing argument handling facilities and a lightweight subset of formatting functions
- fmt/format.h: the full format API providing compile-time format string checks, output iterator and user-defined type support
- fmt/ranges.h: additional formatting support for ranges and tuples
- fmt/chrono.h: date and time formatting
- fmt/ostream.h: `std::ostream` support
- fmt/printf.h: `printf` formatting

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 argument handling facilities and a lightweight subset of formatting functions. In the header-only mode include `fmt/format.h` instead of `fmt/core.h`.

The following functions use format string syntax similar to that of Python’s str.format. They take format_str and args as arguments.

format_str 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 format_str 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 S, typename... Args, typename Char = char_t<S>>
std::basic_string<Char> `fmt::``format`(const S &format_str, Args&&... args)

  Formats arguments and returns the result as a string.

  Example:

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

Warning

doxygenfunction: Unable to resolve multiple matches for function “vformat” with arguments (const S&, basic_format_args<buffer_context<Char>>) in doxygen xml output for project “format” from directory: /build/fmt/src/build/doc/doxyxml. Potential matches:

    - template <typename Char>
      std::basic_string<Char> fmt::internal::vformat(basic_string_view<Char>, basic_format_args<buffer_context<type_identity_t<Char>>>)
    - template <typename S, typename Char = char_t<S>>
      std::basic_string<Char> fmt::vformat(const S&, basic_format_args<buffer_context<type_identity_t<Char>>>)

template <typename S, typename... Args, typename Char = char_t<S>>
void `fmt::``print`(const S &format_str, Args&&... args)

  Formats `args` according to specifications in `format_str` and writes the output to `stdout`. Strings are assumed to be Unicode-encoded unless the `FMT_UNICODE` macro is set to 0.

  Example:

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

void `fmt::buffered_file::``vprint`(string_view format_str, format_args args)

template <typename S, typename... Args, typename Char = char_t<S>>
void `fmt::``print`(std::FILE *f, const S &format_str, Args&&... args)

  Formats `args` according to specifications in `format_str` and writes the output to the file `f`. Strings are assumed to be Unicode-encoded unless the `FMT_UNICODE` macro is set to 0.

  Example:

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

void `fmt::``vprint`(std::FILE *, string_view, format_args)

### Named Arguments

template <typename S, typename T, typename Char = char_t<S>>
internal::named_arg<T, Char> `fmt::``arg`(const S &name, const T &arg)

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

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

### Argument Lists

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

  Constructs an `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 `fmt::``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 `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>
  `basic_format_args`(const format_arg_store<Context, Args...> &store)

    Constructs a `basic_format_args` object from `format_arg_store`.

  `basic_format_args`(const dynamic_format_arg_store<Context> &store)

    Constructs a `basic_format_args` object from `dynamic_format_arg_store`.

  `basic_format_args`(const format_arg *args, int count)

    Constructs a `basic_format_args` object from a dynamic set of arguments.

  format_arg `get`(int index) const

    Returns the argument at specified index.

struct `fmt::``format_args`

  An alias to `basic_format_args<context>`.

  Inherits from fmt::basic_format_args< format_context >

template <typename Context>
class `fmt::``basic_format_arg`

### Compatibility

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

  `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.

  const Char *`data`() const

    Returns a pointer to the string data.

  size_t `size`() const

    Returns the string size.

using `fmt::string_view = typedef basic_string_view<char>`

using `fmt::wstring_view = typedef basic_string_view<wchar_t>`

### Locale

All formatting is locale-independent by default. Use the `'n'` 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("{:n}", 1000000);  // s == "1,000,000"

## Format API

`fmt/format.h` defines the full format API providing compile-time format string checks, output iterator and user-defined type support.

### Compile-time Format String Checks

Compile-time checks are supported for built-in and string types as well as user-defined types with `constexpr` `parse` functions in their `formatter` specializations.

`FMT_STRING`(s)

  Constructs a 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 = format(FMT_STRING("{:d}"), "foo");

### 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) {
        // [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) {
        // ctx.out() is an output iterator to write to.
        return format_to(
            ctx.out(),
            presentation == 'f' ? "({:.1f}, {:.1f})" : "({:.1e}, {:.1e})",
            p.x, p.y);
      }
    };

Then you can pass objects of type `point` to any formatting function:

    point p = {1, 2};
    std::string s = fmt::format("{:f}", p);
    // s == "(1.0, 2.0)"

You can also reuse existing formatters via inheritance or composition, for example:

    enum class color {red, green, blue};

    template <>
    struct fmt::formatter<color>: formatter<string_view> {
      // parse is inherited from formatter<string_view>.
      template <typename FormatContext>
      auto format(color c, FormatContext& ctx) {
        string_view name = "unknown";
        switch (c) {
        case color::red:   name = "red"; break;
        case color::green: name = "green"; break;
        case color::blue:  name = "blue"; break;
        }
        return formatter<string_view>::format(name, ctx);
      }
    };

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"
    }

template <typename Char, typename ErrorHandler = internal::error_handler>
class `fmt::``basic_format_parse_context`

  Parsing context consisting of a format string range being parsed and an argument counter for automatic indexing.

  You can use one of the following type aliases for common character types:

  Type | Definition
  --- | ---
  format_parse_context | basic_format_parse_context<char>
  wformat_parse_context | basic_format_parse_context<wchar_t>

  Inherits from fmt::internal::error_handler

  Public Functions

  iterator `begin`() const

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

  iterator `end`() const

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

  void `advance_to`(iterator it)

    Advances the begin iterator to `it`.

  int `next_arg_id`()

    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.

### Output Iterator Support

template <typename OutputIt, typename S, typename... Args>
OutputIt `fmt::``format_to`(OutputIt out, const S &format_str, Args&&... args)

  Formats arguments, writes the result to the output iterator `out` and returns the iterator past the end of the output range.

  Example:

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

template <typename OutputIt, typename S, typename... Args>
format_to_n_result<OutputIt> `fmt::``format_to_n`(OutputIt out, std::size_t n, const S &format_str, const Args&... args)

  Formats arguments, writes up to `n` characters of the result to the output iterator `out` and returns the total output size and the iterator past the end of the output range.

template <typename OutputIt>
struct `fmt::``format_to_n_result`

  Public Members

  OutputIt `out`

    Iterator past the end of the output range.

  std::size_t `size`

    Total (not truncated) output size.

### Literal-based API

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

internal::udl_formatter<char> `fmt::literals::``operator""_format`(const char *s, std::size_t n)

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

  Example:

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

internal::udl_arg<char> `fmt::literals::``operator""_a`(const char *s, std::size_t n)

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

  Example:

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

### Utilities

template <typename T>
struct `fmt::``is_char`

  Specifies if `T` is a character type.

  Can be specialized by users.

  Inherits from false_type

using `fmt::char_t = typedef typename internal::char_t_impl<S>::type`

  String’s character type.

template <typename... Args>
std::size_t `fmt::``formatted_size`(string_view format_str, const Args&... args)

  Returns the number of characters in the output of `format(format_str, args...)`.

template <typename T>
std::string `fmt::``to_string`(const T &value)

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

  Example:

      #include <fmt/format.h>

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

template <typename T>
std::wstring `fmt::``to_wstring`(const T &value)

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

template <typename Char>
basic_string_view<Char> `fmt::``to_string_view`(const Char *s)

  Returns a string view of `s`. In order to add custom string type support to {fmt} provide an overload of `to_string_view` for it in the same namespace as the type for the argument-dependent lookup to work.

  Example:

      namespace my_ns {
      inline string_view to_string_view(const my_string& s) {
        return {s.data(), s.length()};
      }
      }
      std::string message = fmt::format(my_string("The answer is {}"), 42);

template <typename Range>
arg_join<internal::iterator_t<const Range>, char> `fmt::``join`(const Range &range, string_view sep)

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

  Example:

      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>
arg_join<It, char> `fmt::``join`(It begin, It end, string_view sep)

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

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

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

  You can use one of the following type aliases for common character types:

  Type | Definition
  --- | ---
  memory_buffer | basic_memory_buffer<char>
  wmemory_buffer | basic_memory_buffer<wchar_t>

  Example:

      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)`.

  Inherits from std::allocator< T >, fmt::internal::buffer< T >

  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.

  basic_memory_buffer &`operator=`(basic_memory_buffer &&other)

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

### 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.

class `fmt::``system_error`

  An error returned by an operating system or a language runtime, for example a file opening error.

  Inherits from runtime_error

  Subclassed by fmt::windows_error

  Public Functions

  template <typename... Args>
  `system_error`(int error_code, string_view message, const Args&... args)

    Constructs a `fmt::system_error` object with a description formatted with `fmt::format_system_error()`. message and additional arguments passed into the constructor are formatted similarly to `fmt::format()`.

    Example:

        // This throws a 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`(internal::buffer<char> &out, int error_code, string_view message)

  Formats an error returned by an operating system or a language runtime, for example a file opening error, and writes it to out in the following form:

      <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`. If error_code is not a valid error code such as -1, the system message may look like “Unknown error -1” and is platform-dependent.

class `fmt::``windows_error`

  A Windows error.

  Inherits from fmt::system_error

  Public Functions

  template <typename... Args>
  `windows_error`(int error_code, string_view message, const Args&... args)

    Constructs a `fmt::windows_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 windows_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);
        }

### 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. If you are using named arguments, the container that stores pointers to them will be allocated using the default allocator. Also floating-point formatting falls back on `sprintf` which may do allocations.

### Custom Formatting of Built-in Types

It is possible to change the way arguments are formatted by providing a custom argument formatter class:

    using arg_formatter = fmt::arg_formatter<fmt::buffer_range<char>>;

    // A custom argument formatter that formats negative integers as unsigned
    // with the ``x`` format specifier.
    class custom_arg_formatter : public arg_formatter {
     public:
      custom_arg_formatter(fmt::format_context& ctx,
                           fmt::format_parse_context* parse_ctx = nullptr,
                           fmt::format_specs* spec = nullptr)
        : arg_formatter(ctx, parse_ctx, spec) {}

      using arg_formatter::operator();

      auto operator()(int value) {
        if (specs() && specs()->type == 'x')
          return (*this)(static_cast<unsigned>(value)); // convert to unsigned and format
        return arg_formatter::operator()(value);
      }
    };

    std::string custom_vformat(fmt::string_view format_str, fmt::format_args args) {
      fmt::memory_buffer buffer;
      // Pass custom argument formatter as a template arg to vformat_to.
      fmt::vformat_to<custom_arg_formatter>(buffer, format_str, args);
      return fmt::to_string(buffer);
    }

    template <typename ...Args>
    inline std::string custom_format(
        fmt::string_view format_str, const Args&... args) {
      return custom_vformat(format_str, fmt::make_format_args(args...));
    }

    std::string s = custom_format("{:x}", -42); // s == "ffffffd6"

template <typename Range>
class `fmt::``arg_formatter`

  The default argument formatter.

  Inherits from fmt::internal::arg_formatter_base< Range >

  Public Functions

  `arg_formatter`(context_type &ctx, basic_format_parse_context<char_type> *parse_ctx = nullptr, format_specs *specs = nullptr)

    Constructs an argument formatter object. ctx is a reference to the formatting context, specs contains format specifier information for standard argument types.

  iterator `operator()`(typename basic_format_arg<context_type>::handle handle)

    Formats an argument of a user-defined type.

## 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

The library supports strftime-like date and time formatting:

    #include <fmt/chrono.h>

    std::time_t t = std::time(nullptr);
    // Prints "The date is 2016-04-29." (with the current date)
    fmt::print("The date is {:%Y-%m-%d}.", *std::localtime(&t));

The format string syntax is described in the documentation of strftime.

## `std::ostream` Support

`fmt/ostream.h` provides `std::ostream` support including formatting of user-defined types that have overloaded `operator<<`:

    #include <fmt/ostream.h>

    class date {
      int year_, month_, day_;
    public:
      date(int year, int month, int day): year_(year), month_(month), day_(day) {}

      friend std::ostream& operator<<(std::ostream& os, const date& d) {
        return os << d.year_ << '-' << d.month_ << '-' << d.day_;
      }
    };

    std::string s = fmt::format("The date is {}", date(2012, 12, 9));
    // s == "The date is 2012-12-9"

template <typename S, typename... Args, typename Char = enable_if_t<internal::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.

  Example:

      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... Args>
int `fmt::``printf`(const S &format_str, const Args&... args)

  Prints formatted data to `stdout`.

  Example:

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

template <typename S, typename... Args, typename Char = enable_if_t<internal::is_string<S>::value, char_t<S>>>
int `fmt::``fprintf`(std::FILE *f, const S &format, const Args&... args)

  Prints formatted data to the file f.

  Example:

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

template <typename S, typename... Args, typename Char = char_t<S>>
int `fmt::``fprintf`(std::basic_ostream<Char> &os, const S &format_str, const Args&... args)

  Prints formatted data to the stream os.

  Example:

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

template <typename S, typename... Args, typename Char = enable_if_t<internal::is_string<S>::value, char_t<S>>>
std::basic_string<Char> `fmt::``sprintf`(const S &format, const Args&... args)

  Formats arguments and returns the result as a string.

  Example:

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