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?
}
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, "!" );
}