Question

So say I define a tuple as such:

template<typename... Args>
class Tuple 
{
    Method () {...};
};

How do I define and access the instance variables for Tuple considering it can have an undefined number of them?

Answer 1

There are a couple of ways. The easiest one is to use structural recursion the way LISP does: a tuple is either

• an empty tuple, or
• a pair (head, tail) where head is the first element of the tuple and tail is a tuple containing the rest of the elements.

In C++, this would look like the following:

template <typename... Elems>
struct tuple; // undefined

template <>
struct tuple<> { }; // empty tuple

template <typename Head, typename... Tail>
struct tuple<Head, Tail...> {
    Head first_elem;
    tuple<Tail...> rest;
};

Then you need a get<n> function template to actually access the elements by index; it should be rather easy to implement if you grok how the tuple itself is recursively defined.

As I said, there are other, trickier, implementation methods – for various reasons the above is not how most real-world std::tuple implementations do it.