Is it a good practice to use "const auto&" in a range for to process the elements of std::initializer_list

Question

I'm reading a code from C++ Primer 5th (6.2.6. Functions with Varying Parameters):

void error_msg(ErrCode e, initializer_list<string> il)
{
    cout << e.msg() << ": ";
    for (const auto &elem : il)
        cout << elem << " ";
    cout << endl;
}

I always use auto directly in a range for. like this:

for (auto &elem : il)

But I never saw const auto& before.I know that the elements in an initializer_list are always const values. Is it the reason to use const auto&?

• What's the difference between this way and my old way?
• Is it a good practice to use "const auto&" in this case?

Answer 1

It is a good practice because the intent is clear and prevent accidental changes to break your code.

The difference between the several forms of ranged base for loop:

• for (auto elem : il) will create a temporary copy of each element. Usually less efficient.

• for (auto& elem: il) won't create copies, and allow you to modify the elements if the underlying container allows that (e.g. not const). But it cannot bind to rvalue references, so it cannot be used on containers that return proxy objects (e.g. std::vector<bool>).

• for (const auto& elem: il) won't create copies either, and won't allow you to do any modifications. This can prevent accidental modifications and signify your intent clearly.

• for (auto&& elem: il) automatically deduces the correct type for elem, be it l-value, const l-value or r-value reference. A standard proposal N3994, already implemented by clang, simplify it into the form for (elem: il).