C ++ 11 for use with auto

Question

Possible duplicate:
What is the advantage of using universal references in range-based loops?

To iterate over a vector<int> elements to read them, I think the following C ++ 11 use for a range with auto correct:

 std::vector<int> v; .... for (auto n : v) { // Do something on n ... .... } 

If the elements stored in the container are not simple integers, but are somewhat "heavier", for example, "t23> s", I understand that to repeat them in vector<string> correct range is used for + auto :

 std::vector<std::string> v; .... for (const auto& s : v) { // Do something on s ... .... } 

Using const & avoids unnecessary deep copies and should be OK, because the loop code just watches the contents of the vector.

As far as I understand, still?

Now I saw code that uses a different form of auto in a loop for a loop: auto&& , for example:

 for (auto&& elem : container) .... 

What is this good for? What is the advantage of using r-value ( && ) references with a range for loops? When should we use this style?