NAME¶

std::format_to - std::format_to

Synopsis¶

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

OutputIt format_to( OutputIt out, const std::locale& loc, (3) (since C++20)

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

OutputIt format_to( OutputIt out, const std::locale& loc, (4) (since C++20)

std::wformat_string<Args...> fmt, Args&&...
args );

Format args according to the format string fmt, and write the result to the output
iterator out. If present, loc is used for locale-specific formatting.

1) equivalent to return std::vformat_to(out, fmt.str,
std::make_format_args(args...));
2) equivalent to return std::vformat_to(std::move(out), fmt.str,
std::make_wformat_args(args...));
3) equivalent to return std::vformat_to(out, loc, fmt.str,
std::make_format_args(args...));
4) equivalent to return std::vformat_to(std::move(out), loc, fmt.str,
std::make_wformat_args(args...));

Let CharT be char for overloads (1,3), wchar_t for overloads (2,4).

These overloads participate in overload resolution only if OutputIt satisfies the
concept std::output_iterator<const CharT&>.

The behavior is undefined if OutputIt does not model (meet the semantic requirements
of) the concept std::output_iterator<const CharT&>, or 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).

Parameters¶

out - iterator to the output buffer

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¶

Iterator past the end of the output range.

Exceptions¶

Propagates any exception thrown by formatter or iterator operations.

Notes¶

As of P2216R3, it is an error if the format string is not a constant expression.
std::vformat_to
or std::runtime_format
(since C++26) can be used in this case.

Example¶

// Run this code

#include <format>
#include <iostream>
#include <iterator>
#include <string>

auto main() -> int
{
std::string buffer;

std::format_to(
std::back_inserter(buffer), //< OutputIt
"Hello, C++{}!\n", //< fmt
"20"); //< arg
std::cout << buffer;
buffer.clear();

std::format_to(
std::back_inserter(buffer), //< OutputIt
"Hello, {0}::{1}!{2}", //< fmt
"std", //< arg {0}
"format_to()", //< arg {1}
"\n", //< arg {2}
"extra param(s)..."); //< unused
std::cout << buffer;

std::wstring wbuffer;
std::format_to(
std::back_inserter(wbuffer),//< OutputIt
L"Hello, {2}::{1}!{0}", //< fmt
L"\n", //< arg {0}
L"format_to()", //< arg {1}
L"std", //< arg {2}
L"...is not..." //< unused
L"...an error!"); //< unused
std::wcout << wbuffer;
}

Output:¶

Hello, C++20!
Hello, std::format_to()!
Hello, std::format_to()!

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 stores formatted representation of the arguments in a new string
(C++20) (function template)
format_to_n writes out formatted representation of its arguments through an output
(C++20) iterator, not exceeding specified size
(function template)