auto doesn't mean "make a copy or reference, whatever is faster". auto first and foremost means "make a copy". Then sometimes compilers can do a trick called co

auto doesn't mean "make a copy or reference, whatever is faster". auto first and foremost means "make a copy". Then sometimes compilers can do a trick called copy-elision and you might be working on a reference. But generally assume it's going to be a copy. auto& means only make a reference.