std::format_to_n, std::format_to_n_result

#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 +たす 2 +たす 1 +たす 3 +たす 3 +たす 2 =わ=わ 35, no trailing '0円'
 
 *result.out = '0円'; // 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";
 }
}

Buffer until '0円': "Hubble's H0 ≅ 71±8 km/sec/Mpc."
Max chars to write: 63
result.out offset: 35
Untruncated output size: 35
 
Buffer until '0円': "Hubble's H0 ≅ 71±8"
Max chars to write: 23
result.out offset: 23
Untruncated output size: 35
 
Buffer until '0円': "Hubble's H0 ≅ 71�"
Max chars to write: 21
result.out offset: 21
Untruncated output size: 35