#include <algorithm>
#include <string>
#include <iostream>

bool ist_Vokal(char c)
{
  const char vokale[] = "aeiou";
  return std::binary_search(vokale, vokale+5, std::tolower(c));
}

int main()
{
  std::string s = "Hallo Welt";
  std::cout << std::count_if(begin(s), end(s), ist_Vokal) << '\n';
}