std::format

Defined in header `<format>`
template< class... Args > std::string format( /format_string<Args...>/ fmt, Args&&... args );	(1)	(since C++20)
template< class... Args > std::wstring format( /wformat_string<Args...>/ fmt, Args&&... args );	(2)	(since C++20)
template< class... Args > std::string format( const std::locale& loc, /format_string<Args...>/ fmt, Args&&... args );	(3)	(since C++20)
template< class... Args > std::wstring format( const std::locale& loc, /wformat_string<Args...>/ fmt, 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.

1) equivalent to return std::vformat(fmt.str, std::make_format_args(args...));

2) equivalent to return std::vformat(fmt.str, std::make_wformat_args(args...));

3) equivalent to return std::vformat(loc, fmt.str, std::make_format_args(args...));

4) equivalent to return std::vformat(loc, fmt.str, std::make_wformat_args(args...));

The behavior is undefined if std::formatter<Ti, CharT> does not meet the BasicFormatter requirements for any Ti in Args (as required by std::make_format_args and std::make_wformat_args), where CharT is char for overloads (1,3), wchar_t for overloads (2,4).

Parameters

fmt

-

parameter of unspecified type, whose initialization is valid only if the argument is convertible to std::string_view (for (1,3)) or std::wstring_view (for (2,4)), and the result of conversion is a constant expression and a valid format string for Args. 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:


`{` arg-id (optional) `}`	(1)

`{` arg-id (optional) `:` format-spec `}`	(2)

1) replacement field without a format specification

2) replacement field with a format specification

arg-id	-	specifies the index of the argument in `args` whose value is to be used for formatting; if it 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.
format-spec	-	the format specification 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::bad_alloc on allocation failure. 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!"

As of P2216R3, it is an error if the format string is not a constant expression. std::vformat can be used in this case.

std::string f(std::string_view runtime_format_string) {
    // return std::format(runtime_format_string, "foo", "bar"); // error
    return std::vformat(runtime_format_string, std::make_format_args("foo", "bar")); // ok
}

Example

Run this code

#include <format>
#include <iostream>
#include <string>
#include <string_view>
 
template <typename... Args>
std::string dyna_print(std::string_view rt_fmt_str, Args&&... args) {
    return std::vformat(rt_fmt_str, std::make_format_args(args...));
}
 
int main() {
    std::cout << std::format("Hello {}!\n", "world");
 
    std::string fmt;
    for (int i{}; i != 3; ++i) {
        fmt += "{} "; // constructs the formatting string
        std::cout << fmt << " : ";
        std::cout << dyna_print(fmt, "alpha", 'Z', 3.14, "unused");
        std::cout << '\n';
    }
}

Output:

Hello world!
{}  : alpha 
{} {}  : alpha Z 
{} {} {}  : alpha Z 3.14

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P2216R3	C++20	throws std::format_error for invalid format string	invalid format string results in compile-time error
P2418R2	C++20	objects that are neither const-usable nor copyable (such as generator-like objects) are not formattable	allow formatting these objects

Compiler support
Freestanding and hosted
Language
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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_args (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)

format_to (C++20)	writes out formatted representation of its arguments through an output iterator (function template)
format_to_n (C++20)	writes out formatted representation of its arguments through an output iterator, not exceeding specified size (function template)