What is std::move() used for?

template <class T>
swap1(T& a, T& b) {
    Ttmp(a); // have two copies of a.
    a = b; // become two copies of b (one of the copies of + a disappears)
    b = tmp; // 2 copies of tmp (one of the copies of +b disappears)
}

If you use //move, you can swap without copying
template <class T>
swap2(T& a, T& b) {
    T tmp(std::move(a));
    a = std::move(b);   
    b = std::move(tmp);
}