< cpp‎ | utility‎ | format
Formatting library
Formatting functions
Formatting arguments
Format error
Defined in header <format>
template<class... Args>
std::string format(std::string_view fmt, const Args&... args);
(1) (since C++20)
template<class... Args>
std::wstring format(std::wstring_view fmt, const Args&... args);
(2) (since C++20)
template<class... Args>
std::string format(const std::locale& loc, std::string_view fmt, const Args&... args);
(3) (since C++20)
template<class... Args>
std::wstring format(const std::locale& loc, std::wstring_view fmt, const Args&... args);
(4) (since C++20)

Format args according to the format string fmt, and return the result as a string. If present, loc is used for locale-specific formatting.

The behavior is undefined if std::formatter<Ti, CharT> does not meet the Formatter requirements for any Ti in Args, where CharT is decltype(fmt)::char_type (char for overloads (1,3), wchar_t for overloads (2,4)).


[edit] Parameters

fmt - string view representing the format string. The format string consists of
  • ordinary characters (except { and }), which are copied unchanged to the output,
  • escape sequences {{ and }}, which are replaced with { and } respectively in the output, and
  • replacement fields.

Each replacement field has the following format:

  • introductory { character;
  • (optional) arg-id, a non-negative number;
  • (optional) a colon (:) followed by a format specification;
  • final } character.

arg-id specifies the index of the argument in args whose value is to be used for formatting; if arg-id is omitted, the arguments are used in order. The arg-ids in a format string must all be present or all be omitted. Mixing manual and automatic indexing is an error.

The format specification is defined by the std::formatter specialization for the corresponding argument.

  • For basic types and standard string types, the format specification is interpreted as standard format specification.
  • For chrono types, the format specification is interpreted as chrono format specification.
  • For other formattable types, the format specification is determined by user-defined formatter specializations.

args... - arguments to be formatted
loc - std::locale used for locale-specific formatting

[edit] Return value

A string object holding the formatted result.

[edit] Exceptions

Throws std::format_error if fmt is not a valid format string for the provided arguments. Also propagates exception thrown by any formatter.

[edit] Notes

It is not an error to provide more arguments than the format string requires:

std::format("{} {}!", "Hello", "world", "something"); // OK, produces "Hello world!"

[edit] Example

#include <iostream>
#include <format>
int main() {
    std::cout << std::format("Hello {}!\n", "world");


Hello world!

[edit] See also