Question

Possible Duplicate:
Reference collapsing?

template<
    class T = const std::vector<int> &
> void f(const T &);

If T is already const and a reference, what will happen? Why does this code compile then?

Answer 1

That would be the equivalent of const (const std::vector<int>&)&, and in this case, the const would be ignored since you cannot have a const reference to const T, only reference to const T. Since references cannot be reseated, the const would be redundant anyway. Also, ignoring the const you have (T&) &, and due to reference collapsing rules in C++11, this becomes T&. So the end result is const T& or const std::vector<int>& in your case.

If you had const T*, then it would make a difference since that would be const (const std::vector<int>&)* p which would make the pointer const.