Making an index sequence for tuple implementation - c++

Question

Suppose I want to implement something like std::tuple myself, just the basics. I want to show a failed attempt first.
#include <utility>
#include <iostream>
template <std::size_t I>
struct tuple_index_leaf {
using Index = std::integral_constant<std::size_t, I>;
std::size_t i = Index::value;
};
template <std::size_t... Is>
struct tuple_index : tuple_index_leaf<Is>...
{};
template <std::size_t I, std::size_t... Is>
constexpr auto get_index(tuple_index<Is...> const &i) {
return static_cast<const tuple_index_leaf<I>*>(&i)->i;
}
template <std::size_t I, typename T>
struct tuple_leaf : tuple_index_leaf<I> {
T elem;
};
template<typename... Ts>
struct tuple : tuple_leaf<sizeof...(Ts), Ts>... {
};
template <std::size_t I, typename... Ts>
auto& get(tuple<Ts...> &t) {
return static_cast<tuple_leaf<I, float>*>(&t)->elem;
}
int main() {
tuple_index<0, 1, 2> ti;
std::cout << get_index<0>(ti) << "\n";
tuple<int, float> t;
get<2>(t) = 3.14;
}
Now, take a look at get function. I hard coded the last type float and I can only call this with index 2, like get<2>. This is because the deficiency in my tuple constructor. If you look there, you will see that I am passing sizeof...(Ts) to tuple_leaf. For example, in this case, all my tuple leaves will be like tuple_leaf<2, int>, tuple_leaf<2, float>. What I wanted was an expansion like tuple_leaf<0, int>, tuple_leaf<1, float>.... The expansion I used, tuple_leaf<sizeof...(Ts), Ts>... does not give me these, I know. I need some sort of index sequence I figured and started implementing something like tuple_index. But that one requires me to pass std::size_t... and I don't know how to do that. So the question is, how can I get an expansion like tuple_leaf<0, int>, tuple_leaf<1, float>...?

Answer 1

It is not hard. Here is one example how to do this (not claiming the only one way, this was something I put together quickly):
#include <utility>
#include <cstddef>
template <std::size_t I, typename T>
struct tuple_leaf {
T elem;
};
template<class SEQ, class... TYPE> struct tuple_impl;
template<size_t... Ix, class... TYPE>
struct tuple_impl<std::index_sequence<Ix...>, TYPE...> : tuple_leaf<Ix, TYPE>... { };
template<typename... Ts>
struct tuple : tuple_impl<std::make_index_sequence<sizeof...(Ts)>, Ts...> { };
// below lines are for testing
tuple<int, double, char*> tup;
// the fact that this compiles tells us char* has index 2
auto& z = static_cast<tuple_leaf<2, char*>&>(tup);