How does c++ compare two string using ==?

I came across a very simple but confusing problem today.

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

int main(){
    string str = "123";
    string a = "1";
    string b = "1";

    cout << ((str[0]+"") == a) << endl;
    cout << (a==str.substr(0,1)) << endl;
    cout << (a==b) << endl;

}

The output is: 0 1 1

Why the first compare statement is false? How does c++ compare two string when using == operator?