C++ define sub tuple types given base tuple type and variadic index sequence

Question

Assume I have a tuple, e.g. using BaseTuple = std::tuple<int, double, string>;, I'd like to define new types from a given variadic index sequence, and methods to convert to and from BaseTuple. For example, CustomTuple<0,1> should derive from tuple<int, double>.

using BaseTuple = tuple<int, double, string>;

template <size_t... I>
struct CustomTuple: public tuple<tuple_element_t<I, BaseTuple>...>{
    using tuple<tuple_element_t<I, BaseTuple>...>::tuple;

    static CustomTuple fromBaseTuple(const BaseTuple& key){
        return {get<I>(key)...};
    };

    BaseTuple toBaseTuple(){
        BaseTuple t;
        ((get<tuple_element_t<I, BaseTuple>>(t) = get<tuple_element_t<I, BaseTuple>>(t)), ...);
        return t;
    }
};

The above code works if tuple has unique types, but I wonder if there's any way for duplicate types. I can't find a way to iterate I... with its index.

Answer 1

Something along these lines:

template <std::size_t... S>
BaseTuple toBaseTuple(std::index_sequence<S...>) {
    BaseTuple t;
    ((std::get<I>(t) = std::get<S>(*this)), ...);
    return t;
}
BaseTuple toBaseTuple(){
    return toBaseTuple(std::make_index_sequence<sizeof...(I)>{});
}

Demo