c++ - 递减折叠表达式内的模板参数

Question

考虑以下代码:

template<typename...>
struct C
{ /* ... */ };

template<typename T, unsigned N>
struct B
{
    using type = /* ... */;
};

template<size_t N, typename... Ts>
struct A
{
    using type = C<typename B<Ts, N-->::type...>; // decrement N sizeof...(Ts) times
};

举个例子

typename A<5, int, long, void>::type

扩展为

C<typename B<int, 5>::type, typename B<long, 4>::type, typename B<void, 3>::type>

由于 N 是一个 const 值，因此该代码无法编译。还有其他办法吗？

Answer 1

减去 std::index_sequence来自N

namespace detail {
    template<size_t N, typename... Ts, size_t... Is>
    C<typename B<Ts, N - Is>::type...> A_impl(std::index_sequence<Is...>);
}

template<size_t N, typename... Ts>
struct A
{
    using type = decltype(detail::A_impl<N, Ts...>(std::index_sequence_for<Ts...>{}));
};