Why should one never use auto&& for local variables?

Question

While T&& is used with templates as forwarding reference or universal reference (as Scott Meyers calls them), I have seen some blogs use auto&& in code samples. I think auto itself should be enough, however, Herb Sutter in CppCon 2014 says: Never use auto&& for local variables

Why is that?

Seeing all replies , I feel I think I should have asked the contra. The general coding guidelines notwithstanding are there any good use cases of using auto&& inside a function body for code correctness and maintainability .

Answer 1

There are cases when you need a auto&& as a local, consider:

vector<bool> vb { true, false, true };
for( auto&& b : vb )
    b = !b;

Here, a auto& wouldn't do. (if vector<bool> is specialized)

However, the usual reason you use && together with type-deduction
(ie a forwarding reference) is because you don't know or care of the
exact type, you are just going to forward it.

The term 'universal reference' is a bit misleading, you should not use
it universally, hence its changing name to 'forwarding reference'

Note that && not used with type-deduction is a rvalue reference, a different
thing altogether.