#include <iostream>
#include <vector>

char to_upper(char value)
{
    return static_cast<char>(std::toupper(static_cast<unsigned char>(value)));
}

int main()
{

    std::string s = "hello world";
    // std::for_each modifies each element
    std::for_each(s.begin(), s.end(), toupper);

    std::string upper;
    // std::transform adds to third iterator.
    std::transform(s.begin(), s.end(), std::back_inserter(upper), to_upper);
}