string s = "ab";
s[0] = '1';
s[1] = '2';
cout << s << "." << s[0] << "." << s[1] << "." << endl;

string ss = "";
ss[0] = '1';
ss[1] = '2';
cout << ss << "." << ss[0] << "." << ss[1] << "." << endl;

string ss = "";
ss[0] = '1';
ss[1] = '2';
cout << ss << "." << ss[0] << "." << ss[1] << "." << endl;

string ss = "";
ss.at(0) = '1';
ss.at(1) = '2';
cout << ss << "." << ss[0] << "." << ss[1] << "." << endl;

$ ./cxx-test.exe
libc++abi.dylib: terminate called throwing an exception
Abort trap: 6

string ss = "  ";    // two blanks spaces

string ss;
ss.resize(2);