# std::format_to_n, std::format_to_n_result

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< cpp‎ | utility‎ | format

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. 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 thread id format specification and stacktrace entry format specification.
- For `std::basic_stacktrace`, no format specifier is allowed.

(since C++23)

- For `std::filesystem::path`, see path format specification.

(since C++26)

- 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.

### Notes

The libstdc++ implementation prior to GCC-13.3 had a bug in reporting the correct format_to_n_result::out value.

### Example

At Godbolt's Compiler Explorer: clang (trunk) + libc++, GCC (trunk) + libstdc++.

Run this code

    #include <format>
    #include <initializer_list>
    #include <iomanip>
    #include <iostream>
    #include <string_view>

    int main()
    {
        char buffer[64];

        for (std::size_t max_chars_to_write : {std::size(buffer) - 1, 23uz, 21uz})
        {
            const std::format_to_n_result result =
                std::format_to_n(
                    buffer, max_chars_to_write,
                    "Hubble's H{2} {3} {0}{4}{1} km/sec/Mpc.", // 24 bytes w/o formatters
                    71,       // {0}, occupies 2 bytes
                    8,        // {1}, occupies 1 byte
                    "\u2080", // {2}, occupies 3 bytes, '₀' (SUBSCRIPT ZERO)
                    "\u2245", // {3}, occupies 3 bytes, '≅' (APPROXIMATELY EQUAL TO)
                    "\u00B1"  // {4}, occupies 2 bytes, '±' (PLUS-MINUS SIGN)
                    ); // 24 + 2 + 1 + 3 + 3 + 2 == 35, no trailing '\0'

            *result.out = '\0'; // adds terminator to buffer

            const std::string_view str(buffer, result.out);

            std::cout << "Buffer until '\\0': " << std::quoted(str) << '\n'
                      << "Max chars to write: " << max_chars_to_write << '\n'
                      << "result.out offset: " << result.out - buffer << '\n'
                      << "Untruncated output size: " << result.size << "\n\n";
        }
    }

Output:

    Buffer until '\0': "Hubble's H₀ ≅ 71±8 km/sec/Mpc."
    Max chars to write: 63
    result.out offset: 35
    Untruncated output size: 35

    Buffer until '\0': "Hubble's H₀ ≅ 71±8"
    Max chars to write: 23
    result.out offset: 23
    Untruncated output size: 35

    Buffer until '\0': "Hubble's H₀ ≅ 71�"
    Max chars to write: 21
    result.out offset: 21
    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

### See also

format (C++20) | stores formatted representation of the arguments in a new string (function template)
format_to (C++20) | writes out formatted representation of its arguments through an output iterator (function template)
formatted_size (C++20) | determines the number of characters necessary to store the formatted representation of its arguments (function template)

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