NAME¶

std::format - std::format

Synopsis¶

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

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

std::format_string<Args...> fmt, Args&&...
args );
template< class... Args >

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

std::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.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...));.

Since P2216R3, std::format does a compile-time check on the format string (via the
helper type std::format_string or std::wformat_string). If it is found to be invalid
for the types of the arguments to be formatted, a compilation error will be emitted.
If the format string cannot be a compile-time constant, or the compile-time check
needs to be avoided, use std::vformat
or std::runtime_format on fmt
(since C++26) instead.

The following requirements apply to each type T in Args, where CharT is char for
overloads (1,3), wchar_t for overloads (2,4):

* std::formatter<T, CharT> must satisfy BasicFormatter
* std::formatter<T, CharT>::parse() must be constexpr since P2216R3 (std::vformat
does not have this requirement)

Parameters¶

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

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.
arg-id -
The arg-id s in a format string must all be present or all
be omitted. Mixing manual and automatic indexing is an
error.
fmt - the format specification defined by the std::formatter
format-spec - specialization for the corresponding argument. Cannot start
with }.

* 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 range types, the format specification is interpreted
as range format specification.
* For std::pair and std::tuple, the format specification
is interpreted as tuple format specification.
* For std::thread::id and std::stacktrace_entry, see (since C++23)
thread id format specification and stacktrace entry
format specification.
* For std::basic_stacktrace, no format specifier is
allowed.

* For std::filesystem::path, see path format (since C++26)
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
}

std::runtime_format can be used directly on std::format instead of std::vformat
which requires std::basic_format_args as an argument.
(since
std::string f(std::string_view runtime_format_string) C++26)
{
return std::format(std::runtime_format(runtime_format_string), "foo", "bar");
}

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 results in
invalid format string compile-time error
objects that are neither
P2418R2 C++20 const-usable nor copyable allow formatting these objects
(such as generator-like objects)
are not formattable
P2508R1 C++20 there's no user-visible name for the name basic_format_string is
this facility exposed

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)