How can you perform arithmetic on an element of a non-type parameter pack?

Question

I want a method that returns an object with a modified parameter pack. For example, the Nth template parameter is increased by 1.

template<size_t... Ni>
struct Test
{
    template<size_t Nth>
    constexpr auto RunTest()
    {
        // What should be written here?
    }
};

int main()
{
    Test<2, 2, 2> t1;
    Test<2, 3, 2> t2 = t1.RunTest<1>(); // How to make this work?
}

Answer 1

If you can afford a helper method, the following way should work starting from C++14:

#include <utility>
#include <type_traits>

template <std::size_t... Ni>
struct PackHolder
 {
   template <std::size_t Nth, std::size_t ... Is>
   constexpr auto IncrementHelper (std::index_sequence<Is...>)
      -> PackHolder<(Ni + (Nth == Is))...>
    { return {}; }

   template <std::size_t Nth>
   constexpr auto IncrementPackElement()
    { return IncrementHelper<Nth>(std::make_index_sequence<sizeof...(Ni)>{}); }
 };

int main()
 {
   PackHolder<2, 2, 2> t1;

   auto t2 = t1.IncrementPackElement<1>();

   static_assert( std::is_same<decltype(t2), PackHolder<2, 3, 2>>::value, "!" );
 }