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std::format

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

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

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

4) equivalent to return std::vformat(loc, fmt.get(), 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

an object that represents 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:

`{` 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 std::thread::id, the format specification is interpreted as thread id 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

#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
P2508R1	C++20	there's no user-visible name for this facility	the name `basic_format_string` is exposed

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)