Auto Type

Summary

In this post, I will introduce auto, const auto, auto&, const auto&, auto&&, and const auto&&.

Conclusion

We provide the conclusion first.

1. When you want a read-only access (generic or non-generic codes), use auto const &, which will give you a reference rather than a copy.
2. When you want a read-write access in generic codes, use auto &&.
3. When you want a read-write access in non-generic codes, use auto &.

Details

auto

Auto will create a copy of each element in the range so use this variant when you want to work with a copy.

Note the following situations.
1. Containers return proxy objects upon dereferencing of their iterators.

std::vector<bool> vec{false, false, false};
for (auto x : vec) {
    x = true; // now vec is {true, true, true}
}

const auto

It will provide an immutable copy of each element.
However we should avoid using const auto because there is no reason we create a copy. Thus, use const auto & instead.

const auto& or auto const &

This is the number one choice for iterating over a range when all you want to is read its elements. No copies are made and the compiler can verify that you indeed do not modify the elements.

auto&

Use auto& when you want to modify elements in the range in non-generic codes. & means it will make references to the original elements in the range. See special situations (eg, inside templates) in reference.

auto&&

Use auto&& when you want to modify elements in the range in generic codes. Actually it also works in non-generic codes, but it unnecessarily confuses people. Details are in reference.

const auto&&

There is no reason for choosing this variant over const auto&.

Reference

See this blog for more details and some special situations.