rvalue reference — I'm losing the value

Question

I'm a bit confused about the rvalue reference so I have tried the following code:

std::vector<char> stringToChar(std::string& str)
{
    std::vector<char> rep(str.begin(), str.end());
    return rep;
}

std::vector<char>& cryptPassword(std::vector<char>&& password)
{
    // static const char _key = 56; //crypt password
    // for (char &c : password)
    //   c = c ^ _key;
    return (password);
}

std::vector<char>& stringToPassword(std::string& str)
{
    return cryptPassword(stringToChar(str));
}

In the first case I'm getting the correct output, but I the second case I'm getting garbage.

std::string str("Hello");
std::vector<char> tmp = cryptPassword(stringToChar(str));

//correct output
for (char c : tmp)
   std::cout << c;
std::cout << std::endl;

//garbage
tmp = stringToPassword(str);
for (char c : tmp)
   std::cout << c;
std::cout << std::endl;

Answer 1

The result of stringToChar(str) is an rvalue that will bind to the rvalue reference taken by cryptPassword(). When returning password the vector is copied and bound to the reference (cryptPassword() returns an lvalue reference). The problem is that the return value is a temporary copy that has since died after the call to temp = stringToPassword(). Using it is Undefined Behavior.

Instead of returning a reference, return by value and std::move() password out:

std::vector<char> cryptPassword(std::vector<char>&& password)
{
    // ...  
    return std::move(password);
}