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

How about something with a "type list", like so:

template  struct Data;

template <> struct Data<> {};

template 
struct Data : Data
{
    std::set> item;
    bool contains(const Item & x) const { return item.find(x) != item.end(); }
};

Usage:

Data<2, 8, 19> data;   // contains sets of Item<2>, Item<8> and Item<19>