About rvalue reference in C++

std::vector<std::string> func(int num) {
  std::vector<std::string> vec;
  int sum = 0;
  for(int i = 0; i < num; i++) {
    sum +=i;
    std::string s = std::to_string(sum);
    vec.emplace_back(s);
  }
  return vec;
}

what's the difference between the code below about using rvalue and lvalue reference.

  std::vector<std::string> res = func(10);      // (case 1)
  std::vector<std::string> &&res = func(10);    // (case 2)
  std::vector<std::string> &res = func(10);   // I got an error!, case3
  const std::vector<std::string> &res = func(10);  // case4

Question:

1. can the rvalue reference(case 2) save a memory copy? than case1
2. why lvalue reference(case3) got an error but it work with the const (case4)?