std::format...to_n_result(3)

💖 std::format...to_n_result(3) 💖

okay so this is the C++ Standard Libary manual page for std::format_to_n and std::format_to_n_result, and bestie it is literally giving “write formatted output into a buffer, but cap it at n characters” main character energy. no cap. 💅

💅 NAME 💅

std::format_to_n,std::format_to_n_result - std::format_to_n,std::format_to_n_result

✨ Synopsis ✨

Defined in header <format>

Overload (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::format_string<Args...> fmt, Args&&... args
               );

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

Overload (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::format_string<Args...> fmt, Args&&... args
               );

Overload (4) (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
               );

🎀 Helper types 🎀

Helper type (5) (since C++20):

template< class OutputIt >

struct format_to_n_result {
    OutputIt out;
    std::iter_difference_t<OutputIt> size;

};

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

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.

🌸 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-id s 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. (since C++23)
  • For std::basic_stacktrace, no format specifier is allowed.
  • 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.

🔥 Example 🔥

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

// Run this code

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

int main()
{
    char buffer[64];

    for (std::size_t max_chars_to_write : {std::size(buffer) - 1, 23uz})
    {
        const auto 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}; // uses C++20 constructor

        std::cout << "Buffer until '\\0': \"" << 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

🦋 Defect reports 🦋

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• P2216R3 — applied to C++20; behavior as published: throws std::format_error for invalid format string; correct behavior: invalid format string results in compile-time error.
• P2418R2 — applied to C++20; behavior as published: objects that are neither const-usable nor copyable (such as generator-like objects) are not formattable; correct behavior: allow formatting these objects.
• P2508R1 — applied to C++20; behavior as published: there's no user-visible name for this facility; correct behavior: 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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okay bestie that's the whole page, and it ate 💅 source/version: http://cppreference.com; date: 2024.06.10. periodt. 🫶