Get Latin Character in C++

I am the rookie with C++. I have a string "tỏa" but i can't get the character "ỏ" and why the length of that string is 5?

void test() {
    std::string str ("tỏa");
    for(int i=0; i<str.length(); ++i){
        std::cout << str[i] << std::endl;
    }
}

and the output of that code is:

t
�
�
�
a

Anyone can help me? Thank in advance