c++ - effective way to select last parameter of variadic template

Question

I know how to select first parameter of variadic template:

template< class...Args> struct select_first;
template< class A, class ...Args> struct select_first<A,Args...>{  using type = A;};

It's a very simple. However, select_last is not similar:

template< class ...Args> struct select_last;
template< class A> struct select_last<A> { using type = A; };
template< class A, class Args...> struct select_last<A,Args...>{ 
        using type = typename select_last<Args...>::type;
};

This solution needed deep recursive template instantinations. I try to solve this with as:

template< class A, class Args...>
struct select_last< Args ... , A>{  using type = A; }; // but it's not compiled.

Q: exist more effective way to selecting last parameter of variadic templates?

Answer 1

With C++17, the cleanest way is

template<typename T>
struct tag
{
    using type = T;
};

template<typename... Ts>
struct select_last
{
    // Use a fold-expression to fold the comma operator over the parameter pack.
    using type = typename decltype((tag<Ts>{}, ...))::type;
};

with O(1) instantiation depth.