检查字符串是否可转换为std :: tuple的第n个元素

Question

我有一个std :: tuple类型，需要检查字符串是否可以反序列化为其中一个元素。元素和字符串的索引仅在运行时已知。

我的解决方案（如下所示）几乎可以正常工作，但在使用std::tuple_element时会产生编译器错误，并给出：

  

错误：无效使用不完整类型'struct std :: tuple_element＆lt; 0ul，std :: tuple＆amp;＆gt;

#include <sstream>
#include <tuple>

template<class Tuple, std::size_t N>
struct IsDeserializable {
  static void check(std::size_t idx, std::stringstream& in) {
    IsDeserializable<Tuple, N-1>::check(idx, in);
    if (idx == N-1) {
      typename std::tuple_element<N-1,Tuple>::type tmp;
      in >> tmp;
    }
  }
};

template<class Tuple>
struct IsDeserializable<Tuple, 1> {
  static void check(std::size_t idx, std::stringstream& in) {
    if (idx == 0) {
      typename std::tuple_element<0,Tuple>::type tmp;
      in >> tmp;
    }
  }
};

template<class Tuple>
bool is_deserializable(std::size_t idx, const std::string& val) {
  std::stringstream in;
  in << val;
  try {
    IsDeserializable<Tuple, std::tuple_size<Tuple>::value>::check(idx, in);
    return true;
  }
  catch(...) {return false;}
} 


// Example Usage
std::tuple<int,double,char> MyTuple;

assert(is_deserializable<MyTuple>(0,"4"));
assert(is_deserializable<MyTuple>(1,"56.5"));
assert(is_deserializable<MyTuple>(2,"h"));

assert(!is_deserializable<MyTuple>(0,"4.3"));
assert(!is_deserializable<MyTuple>(1,"Hello"));
assert(!is_deserializable<MyTuple>(2,"42"));