Concatenate two typed variadic template packs

Question

Consider this struct

template<std::size_t... a>
struct A{};

How can I concatenate two typed variadic template arguments?

concat<A<1,2,3>, A<4,5,6,7>> // should be of type A<1,2,3,4,5,6,7>

Answer 1

You can implement this with a specialization that handles instantiations of A.

template<typename...>
struct concat_impl;   // just a declaration, to allow for specializations

template<std::size_t... s1, std::size_t... s2>
struct concat_impl<A<s1...>, A<s2...>> {        // specialize   
    using type = A<s1..., s2...>;               // concatenate
};

and a convenience alias to avoid having to say typename everywhere.

template<typename A1, typename A2>
using concat = typename concat_impl<A1,A2>::type;

Here's a demo