std::format_to_n(3)

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

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