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# 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/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);
      }
    };

    // 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.

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

    template <typename... T>
    struct fstring;

A compile-time format string. Use `format_string` in the public API to prevent type deduction.

    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");

### 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`.

### 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.

### 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 view object from a C string and a size.

    basic_string_view(const Char* s);

Constructs a string view object from a C string.

    basic_string_view(const S& s);

Constructs a string view 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/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.

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

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

    template <typename Locale, typename... T>
    auto formatted_size(const Locale& 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

- `std::chrono::duration`
- `std::chrono::time_point`
- `std::tm`

The format syntax is described in 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
    }

    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:

- `std::atomic`
- `std::atomic_flag`
- `std::bitset`
- `std::error_code`
- `std::exception`
- `std::filesystem::path`
- `std::monostate`
- `std::optional`
- `std::source_location`
- `std::thread::id`
- `std::variant`

    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*;

### 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)

## 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):

    struct smol {
      int bit : 1;
    };

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

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

## Format String Compilation

`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` 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(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.

    size_t size();

Returns the number of elements in the store.

## 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.

- 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 procude more decimal digits than necessary.