C++ comparing the index of a string to another string?

C++ comparing the index of a string to another string?

How can I compare a single character from a string, and another string
(which may or may not be greater than one character)
This program gives me almost 300 lines of random errors. The errors don't
reference a specific line number either, just a lot of stuff about "char*
", "", or "std::to_string".
#include <iostream>
#include <string>
using std::cout;
using std::string;
int main() {
string str = "MDCXIV";
string test = "D";
if (test == str[4]) { // This line causes the problems
cout << test << endl;
}
return 0;
}