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std::format_to_n, std::format_to_n_result

Defined in header `<format>`
template< class OutputIt, class... Args > std::format_to_n_result<OutputIt> format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n, std::format_string<Args...> fmt, Args&&... args );	(1)	(since C++20)
template< class OutputIt, class... Args > std::format_to_n_result<OutputIt> format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n, std::wformat_string<Args...> fmt, Args&&... args );	(2)	(since C++20)
template< class OutputIt, class... Args > std::format_to_n_result<OutputIt> format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n, const std::locale& loc, std::format_string<Args...> fmt, Args&&... args );	(3)	(since C++20)
template< class OutputIt, class... Args > std::format_to_n_result<OutputIt> format_to_n( OutputIt out, std::iter_difference_t<OutputIt> n, const std::locale& loc, std::wformat_string<Args...> fmt, Args&&... args );	(4)	(since C++20)
Helper types
template< class OutputIt > struct format_to_n_result { OutputIt out; std::iter_difference_t<OutputIt> size; };	(5)	(since C++20)

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

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 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

#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 constructor
 
    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
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