#include <iostream>
#include <vector>

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

int main()
{

    std::string s = "hello world";
    // Algorithms like transform, which have output iterators,
    // use the other iterator as an output.
    auto upper = std::string(s.size(), '\0');
    std::transform(s.begin(), s.end(), upper.begin(), to_upper);
}