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What is the advantage of using universal references in range-based for loops?

To iterate over the elements of a vector<int> to read them, I think the following usage of C++11 range-based for with auto is 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 something "heavier", like say std::strings, my understanding is that to iterate over them in a vector<string> the correct usage of range-based for + auto is:

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

The use of const & avoids useless deep-copies, and should be OK since the loop code is just observing the content of the vector.

Is my understanding correct so far?

Now, I saw code that uses another form of auto in range-based for loops: auto&&, e.g.:

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

What is this usage good for? What is the advantage of using r-value references (&&) with range-based for loops? In what cases should we use this style?

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