C++14 Metafunction tools

tools.cpp

#include <utility>
#include <tuple>
#include <cassert>
 
struct type_erasure { };
 
template<class T>
struct wrapper : type_erasure {
    wrapper(T&& w) : w_(std::forward<T>(w)) { }
    T&& w_;
    decltype(auto) get() { return std::forward<T>(w_); }
};
 
template<class T>
wrapper<T> wrapper_for(T&& x) {
    return { std::forward<T>(x) };
}
 
template <typename ignore>
struct lookup;
 
template <std::size_t... ignore>
struct lookup<std::index_sequence<ignore...>> {
    template <typename nth, typename... Ts>
    static decltype(auto)
    at_position(decltype(ignore, type_erasure())..., wrapper<nth> w, Ts&&...) {
        return w.get();
    }
 
    template<typename... Ts>
    static auto
    all_after(decltype(ignore, type_erasure())..., Ts&&... args) {
        return std::forward_as_tuple(args.get()...);
    }
};
 
template<std::size_t index, typename... Args>
auto getNth(Args&&... args) {
    return lookup<std::make_index_sequence<index>>::at_position(
        wrapper_for(std::forward<Args>(args))...
    );
}
 
template<std::size_t index, typename... Args>
auto getAllAfter(Args&&... args) {
    return lookup<std::make_index_sequence<index + 1>>::all_after(
        wrapper_for(std::forward<Args>(args))...
    );
}
 
int main() 
{
    assert(getNth<0>(1, 2, 3) == 1);
    assert(getNth<1>(1, 2, 3) == 2);
    assert(getNth<2>(1, 2, 3) == 3);
 
    assert(getAllAfter<2>(2, 4, 6, 8, 10) == std::make_tuple(8, 10));
}