NAME

std::format_to_n - std::format_to_n

Synopsis

Defined in header <format>
template< class OutputIt, class... Args >

std::format_to_n_result<OutputIt> (1) (since C++20)
format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n,

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

std::format_to_n_result<OutputIt> (2) (since C++20)
format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n,

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

std::format_to_n_result<OutputIt>
format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n, (3) (since C++20)

const std::locale& loc, /*format_string<Args...>*/ fmt, Args&&...
args );
template< class OutputIt, class... Args >

std::format_to_n_result<OutputIt>
format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n, (4) (since C++20)

const std::locale& loc, /*wformat_string<Args...>*/ fmt, Args&&...
args );
template< class OutputIt >

struct format_to_n_result {
OutputIt out; (5) (since C++20)
std::iter_difference_t<OutputIt> size;

};

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

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<std::remove_cvref_t<Ti>, CharT> does not meet the BasicFormatter
requirements for any Ti in Args.

5) std::format_to_n_result has no base classes, or members other than out, size and
implicitly declared special member functions.

Parameters

out - iterator to the output buffer
n - maximum number of characters to be written to the buffer
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 format_to_n_result such that the out member is an iterator past the end of the
output range, and the size member is the total (not truncated) output size.

Exceptions

Propagates any exception thrown by formatter or iterator operations.

Example

// Run this code

#include <format>
#include <iostream>
#include <string_view>

int main()
{
char buffer[64];

const auto result =
std::format_to_n(buffer, std::size(buffer) - 1,
"Hubble's H{2} {3} {0}{4}{1} km/sec/Mpc.",
71, // {0}, occupies 2 bytes
8, // {1}, occupies 1 byte
"\u2080", // {2}, occupies 3 bytes
"\u2245", // {3}, occupies 3 bytes
"\u00B1" // {4}, occupies 2 bytes
);
*result.out = '\0';

const std::string_view str{buffer, result.out}; // uses C++20 ctor

std::cout << "Buffer: \"" << str << "\"\n"
<< "Buffer size = " << std::size(buffer) << '\n'
<< "Untruncated output size = " << result.size << '\n';
}

Output:

Buffer: "Hubble's H₀ ≅ 71±8 km/sec/Mpc."
Buffer size = 64
Untruncated output size = 35

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