How to mimic template variable declaration

Question

I have a basic type Item which depends on an integer template parameter N and class Data which holds instances of Item

Answer 1

Following may help:

struct Data
{
    std::tuple>,
               std::set>,
               std::set>,
               std::set>,
               std::set>> items;

    template 
    bool contains(const Item& x) const {
        static_assert(0 < N && N < 6, "N out of range");
        return std::get(items).find(x) != std::get(items).end();
    }
};