C++ tolower on special characters such as ü

Question

I have trouble transforming a string to lowercase with the tolower() function in C++. With normal strings, it works as expected, however special characters are not converted

Answer 1

use ASCII

string NotLowerCase = "Grüßen";
string LowerCase = "";
for (unsigned int i = 0; i < NotLowerCase.length(); i++) {
    if(NotLowerCase[i]<65||NotLowerCase[i]>122)
    {
        LowerCase+='?';
    }
    else
        LowerCase += tolower(NotLowerCase[i]);
}