auto& in c++
Use auto && for the ability to modify and discard values of the sequence within the loop. (That is, unless the container provides a read-only view, such as std::initializer_list, in which case it will be effectively an auto const &.)
Use auto & to modify the values of the sequence in a meaningful way.
Use auto const & for read-only access.
Use auto to work with (modifiable) copies.