Folding-expressionを用いてparameter pack長のindex_sequenceを構築する

再帰深度がO(1)に抑えられる．(趣味)

melpon.org

#include <type_traits>

template <int... Ns>
struct V;

template <int N, int... Ns>
struct V<N, Ns...> {
  template <int M> V<N + 1, N, Ns...> operator +(V<M>);
};

template <>
struct V<> {
  template <int M> V<0> operator +(V<M>);
};

template <typename... Types>
struct S {
    template <int... Indices>
    static V<(sizeof...(Indices) - Indices - 1)...> reverse(V<Indices...>);
    
    template <typename> struct dummy {};
        
    using type = decltype(reverse((V<>{} + ... + V<sizeof(dummy<Types>)>{})));
};

struct T;

int main() {
    static_assert(std::is_same<V<0, 1, 2, 3, 4>, typename S<T, T, T, T, T>::type>::value);
}