C++11 range based loop: get item by value or reference to const

If you don't want to change the items as well as want to avoid making copies, then auto const & is the correct choice:

for (auto const &x : vec)

Whoever suggests you to use auto & is wrong. Ignore them.

Here is recap:

• Choose auto x when you want to work with copies.
• Choose auto &x when you want to work with original items and may modify them.
• Choose auto const &x when you want to work with original items and will not modify them.

If you have a std::vector<int> or std::vector<double>, then it's just fine to use auto (with value copy) instead of const auto&, since copying an int or a double is cheap:

for (auto x : vec)
    ....

But if you have a std::vector<MyClass>, where MyClass has some non-trivial copy semantics (e.g. std::string, some complex custom class, etc.) then I'd suggest using const auto& to avoid deep-copies:

for (const auto & x : vec)
    ....

When we don't need changing vec items, Examples suggest to use first version.

Then they give a wrong suggestion.

Why they don't suggest something which const references

Because they give a wrong suggestion :-) What you mention is correct. If you only want to observe an object, there is no need to create a copy, and there is no need to have a non-const reference to it.

EDIT:

I see the references you link all provide examples of iterating over a range of int values or some other fundamental data type. In that case, since copying an int is not expensive, creating a copy is basically equivalent to (if not more efficient than) having an observing const &.

This is, however, not the case in general for user-defined types. UDTs may be expensive to copy, and if you do not have a reason for creating a copy (such as modifying the retrieved object without altering the original one), then it is preferable to use a const &.