How do I ... Convert ASCII to hex C/C++ ?

#include <iostream>
#include <sstream>
using namespace std;


std::string to_hex(std::string s )
{
  std::ostringstream oss;

  for (auto c: s)
  {
    oss.width(2);
    oss << std::hex << (unsigned int) c;
  }
  return oss.str();
}

int main()
{
  std::string s{"foo"};
  std::cout << s << ", " << to_hex(s) << std::endl;
}