NAME¶

std::format - std::format

Synopsis¶

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&&... (2) (since C++20)
args );
template< class... Args >

std::string format( const std::locale& loc, (3) (since C++20)

/*format_string<Args...>*/ fmt, Args&&... args );
template< class... Args >

std::wstring format( const std::locale& loc, (4) (since C++20)

/*wformat_string<Args...>*/ fmt, Args&&... args );

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¶

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)

fmt - 1) replacement field without a format specification
2) replacement field with a format specification

specifies the index of the argument in args whose value is
to be used for formatting; if it is omitted, the arguments
arg-id - 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
throws std::format_error for invalid invalid format string
P2216R3 C++20 format string results in compile-time
error
objects that are neither const-usable nor
P2418R2 C++20 copyable allow formatting these
(such as generator-like objects) are not objects
formattable

See also¶

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