Question

Could someone explain why those calls are not returning the same expected result?

unsigned int GetDigit(const string& s, unsigned int pos)
{
      // Works as intended
      char c = s[pos];
      return atoi(&c);

      // doesn't give expected results
      return atoi(&s[pos]);
      return atoi(&static_cast<char>(s[pos]));
      return atoi(&char(s[pos]));
}

Remark: I’m not looking for the best way to convert a char to an int.

Answer 1

None of your attempts are correct, including the “works as intended” one (it just happened to work by accident). For starters, atoi() requires a NUL-terminated string, which you are not providing.

How about the following:

unsigned int GetDigit(const string& s, unsigned int pos)
{
      return s[pos] - '0';
}

This assumes that you know that s[pos] is a valid decimal digit. If you don’t, some error checking is in order.