std::format

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)).

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

Return value

A string object holding the formatted result.

Exceptions

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

Notes

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

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

Example

Run this code

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

Output:

Hello world!

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Formatting functions
format (C++20)
format_to (C++20)
format_to_n (C++20)
formatted_size (C++20)
vformat (C++20)
vformat_to (C++20)
Formatter
formatter (C++20)
basic_format_parse_contextformat_parse_contextwformat_parse_context (C++20)(C++20)(C++20)
basic_format_contextformat_contextwformat_context (C++20)(C++20)(C++20)
Formatting arguments
basic_format_arg (C++20)
basic_format_argsformat_argswformat_argsformat_args_t (C++20)(C++20)(C++20)(C++20)
visit_format_arg (C++20)
make_format_argsmake_wformat_args (C++20)(C++20)
Format error
format_error (C++20)