运行期以索引获取tuple元素-C++17

//运行期以索引获取tuple元素-C++17
//需支持C++17及以上标准的编译器，VS2017 15.5.x、CodeBlocks 16.01 gcc 7.2
//参见：http://purecpp.org/?p=1581

#include <iostream>
#include <tuple>
using namespace std;

template <size_t Idx, typename T>
void visit(T& tup, size_t i)
{
    if (i == Idx)
        cout << get<Idx>(tup) << endl;
}

//template <typename Tuple, size_t... Idx>
//void visit(size_t i, Tuple& tp, index_sequence<Idx...>)
//{
//    (visit<Idx>(tp, i), ...);
//}

template <class Tuple, size_t...Idx>
void visit1(size_t i, Tuple& tp, index_sequence<Idx...>)
{
    ((i == Idx && (visit<Idx>(tp, i), false)), ...);
}

template <typename Tuple>
void visit_help(size_t i, Tuple& tp)
{
    //visit(i, tp, make_index_sequence<tuple_size<Tuple>::value>{});
    visit1(i, tp, make_index_sequence<tuple_size<Tuple>::value>{});
}

int main()
{
    auto tp = make_tuple(, "The test", true);
    ;
    visit_help(i, tp);  //The test

    ;
}