c++ - What's the difference between & and && in a range-based for loop? -

i'm wondering what's difference between for (auto& : v) , for (auto&& : v) in range-based loop in code:

#include <iostream> #include <vector>  int main()  {     std::vector<int> v = {0, 1, 2, 3, 4, 5};      std::cout << "initial values: ";      (auto : v)    // prints initial values         std::cout << << ' ';     std::cout << '\n';      (auto : v)    // doesn't modifies v because copy of each value         std::cout << ++i << ' ';     std::cout << '\n';      (auto& : v)   // modifies v because reference         std::cout << ++i << ' ';     std::cout << '\n';      (auto&& : v)  // modifies v because rvalue reference (am right?)         std::cout << ++i << ' ';     std::cout << '\n';      (const auto &i : v) // wouldn't compile without /**/ because const         std::cout << /*++*/i << ' ';     std::cout << '\n';  } 

the output:

initial values: 0 1 2 3 4 5
1 2 3 4 5 6
1 2 3 4 5 6
2 3 4 5 6 7
2 3 4 5 6 7

both seem same thing here want know what's difference between for (auto& : v) , for (auto&& : v) in code.

this answer answer question, relevant part following:

auto         => copy element, reference more efficient auto&        => bind modifiable lvalues const auto&  => bind make const, giving const_iterator const auto&& => bind rvalues