My Tuple class, C++ (gcc)

My Tuple class

#include <iostream>

struct Nil{};

template<typename T1 = Nil, typename T2 = Nil, typename T3 = Nil, typename T4 = Nil>
struct Tuple : Tuple<T2,T3,T4>
{
    
    T1 x;
    using Base = Tuple<T2,T3,T4>;
    
    Base* base() { return static_cast<Base*>(this);}
    
    Tuple(const T1& t1 = T1(), const T2& t2 = T2(), const T3& t3 = T3(), const T4& t4 = T4())
        : Base(t2,t3,t4), x(t1){}
    
};

template<typename T1>
struct Tuple<T1>
{
    
    T1 x;
    
    Tuple(const T1& t1= T1(), const Nil& = Nil(), const Nil& = Nil(), const Nil& = Nil()): x(t1){};
};

template<>
struct Tuple<>
{
    Tuple(){};
};

template<typename T1, typename T2, typename T3, typename T4>
void print_elements(std::ostream& os, Tuple<T1, T2, T3, T4>& t)
{
    os << t.x <<", ";
    print_elements(os, *t.base());
}

template<typename T1>
void print_elements(std::ostream& os, Tuple<T1>& t)
{
    os << t.x;
}

template<typename T1, typename T2, typename T3, typename T4>
std::ostream& operator << ( std::ostream& os, Tuple<T1,T2,T3,T4>& t)
{
    os << "{";
    print_elements(os,t);
    os << "}";
    return os;
}

template<int N, typename T1, typename T2, typename T3, typename T4> struct sel;

template<typename T1, typename T2, typename T3, typename T4> struct sel<0, T1, T2, T3,T4>{ using type = T1;};

template<typename T1, typename T2, typename T3, typename T4> struct sel<1, T1, T2, T3,T4>{ using type = T2;};

template<typename T1, typename T2, typename T3, typename T4> struct sel<2, T1, T2, T3,T4>{ using type = T3;};

template<typename T1, typename T2, typename T3, typename T4> struct sel<3, T1, T2, T3,T4>{ using type = T4;};

template<int N, typename T1, typename T2, typename T3, typename T4>
using Select = typename sel<N, T1,T2,T3,T4>::type;

template<typename Ret, int N>
struct SNthTuple
{
    template<typename Tup>
    static Ret& get(Tup& t)
    {
        return SNthTuple<Ret,N-1>::get(*t.base());
    }
};

template<typename Ret>
struct SNthTuple<Ret,0>
{
    template<typename Tup>
    static Ret& get(Tup& t)
    {
        return t.x;
    }
};

template<int N, typename T1, typename T2, typename T3, typename T4>
Select<N,T1,T2,T3,T4>& get(Tuple<T1,T2,T3,T4>& t)
{
    return SNthTuple<Select<N,T1,T2,T3,T4>,N>::get(t);
};

int main()
{
    
    Tuple<int,double> t1(1,2.9);
    
    Tuple<double, Tuple<int,double>, int, bool> t2(1.2, Tuple<int,double>(1,2), 1, false) ;
    
    std::cout <<  get<0>(t2) << std::endl;
    std::cout <<  get<1>(t2) << std::endl;
    std::cout <<  get<2>(t2) << std::endl;
    std::cout <<  get<3>(t2) << std::endl;
    
    std::cout <<  t2 << std::endl;
}


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