Calling it from C

// Declare our C++ function's returning result type. Only needs to be done once.
// This declares an `status_result<size_t, system_code>` which is an alias to
// `basic_result<size_t, status_code<erased<intptr_t>>>`.
//
// The first parameter is some unique identifier for this type which will be used
// whenever we reference this type in the future.
CXX_DECLARE_RESULT_SYSTEM(to_string_rettype, size_t)

// Tell C about our extern C++ function `to_string()`
extern CXX_RESULT_SYSTEM(to_string_rettype) _Z9to_stringPcmi(char *buffer, size_t bufferlen, int v);

void print(int v)
{
  char buffer[4];
  CXX_RESULT_SYSTEM(to_string_rettype) res;

  res = _Z9to_stringPcmi(buffer, sizeof(buffer), v);
  if(CXX_RESULT_HAS_VALUE(res))
  {
    printf("to_string(%d) fills buffer with '%s' of %zu characters\n", v, buffer, res.value);
    return;
  }
  // Is the error returned in the POSIX domain and thus an errno?
  if(CXX_RESULT_ERROR_IS_ERRNO(res))
  {
    fprintf(stderr, "to_string(%d) failed with error code %d (%s)\n", v, (int) res.error.value, strerror((int) res.error.value));
    return;
  }
  fprintf(stderr, "to_string(%d) failed with unknown error code %lld\n", v, (long long) res.error.value);
}

int main(void)
{
  print(9);
  print(99);
  print(999);
  print(9999);
  return 0;
}