Define template based on parameter pack values

Question

The methods in this class use an index sequence containing a sequence of indices covering the entire data tuple (an index for each argument passed in the parameter pack). Per other answers I've seen, I'm currently templating my methods as such:

#include <tuple>
#include <memory>
#include <vector>

using std::tuple;

template <typename... Args>
class Foo
{
    protected:
        tuple<std::shared_ptr<std::vector<Args>>...> data;

    private:

        template<size_t...Is>
        void _clear_(std::index_sequence<Is...>) {
            (std::get<Is>(data)->clear(), ...);
        };

    public:

        void clear()
        { _clear_(std::index_sequence_for<Args...>()); };
};

This seems messy, though. I was wondering if there's a way that I could define Is at a class-level scope (or, more generally, whether there's a better way to do this).

Answer 1

With C++17, you might simply use std::apply:

void clear()
{
    std::apply([](auto&... vs){ (vs.clear(), ...); }, data);
}

I was wondering if there's a way that I could define Is at a class-level scope

With extra layer:

template <typename Seq, typename...> class FooImpl;

template <std::size_t... Is, typename... Ts>
class FooImpl<std::index_sequence<Is...>, Ts...>
{
protected:
    std::tuple<std::shared_ptr<std::vector<Ts>>...> data;

public:
    void clear() { (std::get<Is>(data).clear(), ...); };
};

template <typename... Ts>
using Foo = FooImpl<std::make_index_sequence<sizeof...(Ts)>, Ts...>;