Example 5, 6, 7 of variadic templates

template<typename... args>
ostream& << (ostream os, const tuple<Args...>& t) {
	os << "[";
	PrintTuple<0, sizeof(Args...), Args...>::print();
	os << "]"
}

template<int IDX, int MAX, typename... Args>
struct PrintTuple {
	static void print(ostream os, const tuple<Args...>& t) {
		os << get<IDX>(t) << (IDX + 1 == MAX?"":",");
		PrintTuple<IDX+1, MAX, Args>::print();
	}
}
//特化类PrintTuple用于结束循环
template<int MAX, typename... Args>
struct PrintTuple<MAX, MAX, Args...>
	static void print(ostream os, const tuple<Args...>& t) {
	}
}

template<> class tuple<>{}

template<typename Head, typename... Tail>
class tuple<Head, Tail...> : private tuple<Tail...> {
 typedef tuple<Tail...> TupleTail;
 tuple(Head begin, Tail... tail) : head(begin), TupleTail(tail...){} 
 Head getHead() {return head};
 TupleTail& getTail() {return *this};
protected:
 Head head;
}

TupleTail(tail...)  //调用父类的构造函数，并传递参数tail...，并非创建临时对象

TupleTail& getTail() {return *this}; //子类强制转换为父类，转换之后便失去了子类的特性

tuple<int, float, string>  t(1, 6.3, ocean);

tuple<int, float,string>  int head(1) -> tuple<float,string>  float head(6.3) -> tuple<string>  sting head("nico") -> tuple<>

template<> class tup<>{};    //这是最后一个复合的类型
template<typename Head, typename... Tail>
class tup< Head, Tail...> {
    typename tup<Tail...> TupTail;
protected:
    TupTail tupTail;
    Head head;
public:
    tup(){}
    tup(Head begin, Tail... tail):head(begin):tupTail(tail...){}
    Head getHead( ) const {return head;}
    TupTail& getTail() const {return tupTail;}
};