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

Question

I'm wondering what's the difference between

for (auto& i : v)

and

for (auto&& i : v)

in a range-based for loop like in this code:

#include <iostream>

#include <vector>



int main() 

{

    std::vector<int> v = {0, 1, 2, 3, 4, 5};



    std::cout << "Initial values: ";



    for (auto i : v)    // Prints the initial values

        std::cout << i << ' ';

    std::cout << '\n';



    for (auto i : v)    // Doesn't modifies v because i is a copy of each value

        std::cout << ++i << ' ';

    std::cout << '\n';



    for (auto& i : v)   // Modifies v because i is a reference

        std::cout << ++i << ' ';

    std::cout << '\n';



    for (auto&& i : v)  // Modifies v because i is a rvalue reference (Am I right?)

        std::cout << ++i << ' ';

    std::cout << '\n';



    for (const auto &i : v) // Wouldn't compile without the /**/ because i is 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 to do the same thing here but I want to know what's the difference between

for (auto& i : v)

and

for (auto&& i : v)

in this code.

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Answer 1

This answer will probably answer your question, the most relevant part is the following:

auto         => will copy the element, but a reference is more efficient
auto&        => will only bind to modifiable lvalues
const auto&  => will bind to anything but make it const, giving us const_iterator
const auto&& => will bind to rvalues

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

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