API Reference¶

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 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

Subclassed by fmt::format_args, fmt::wformat_args

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 format_args : public fmt::basic_format_args<format_context>¶

An alias to basic_format_args<context>.

template<typename Context>
class 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).

Subclassed by fmt::u8string_view

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 = basic_string_view<char>¶

using fmt::wstring_view = 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"

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 : private fmt::internal::error_handler¶

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>

Subclassed by fmt::basic_printf_parse_context< Char >

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::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::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 is_char : public std::false_type¶

Specifies if T is a character type.

Can be specialized by users.

template<typename S>
using fmt::char_t = 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 : private std::allocator<T>, public fmt::internal::buffer<T>¶

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).

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.

Protected Functions

void grow(std::size_t size)¶

Increases the buffer capacity to hold at least capacity elements.

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 : public std::runtime_error¶

An error returned by an operating system or a language runtime, for example a file opening 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 : public fmt::system_error¶

A Windows 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 : public fmt::internal::arg_formatter_base<Range>¶

The default argument formatter.

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.