typetraitsのstd::is_base_ofにパラメータパックを入れられるare_derived_fromを作る

// OK
template<class Base,class Class>
class MyClass {
  static_assert(std::is_base_of<Base, Class>::value, " Class is invalid");
};

// NG
template<class Base, class... Classes>
class MyClass2 {
  static_assert(std::is_base_of<Base, Classes...>::value, " Class are invalid");
};

class MyBase {
  int a;
};
class MyDeri1 :public MyBase{
  int b;
};
class MyDeri2 :public MyBase {
  int c;
};
class Independent{
  int d;
};

  std::cout << 
    std::is_base_of<
      std::true_type, 
      std::is_base_of<MyBase, MyDeri1>>::value;

template<class Base, class CHead, class ...Classes>
struct are_derived_from : public
  std::conditional<
    std::is_base_of<Base, CHead>::value,  //CHeadに対して評価
      are_derived_from<Base, Classes...>, // 上評価がtrueならそれ以外の型についても調べる
      std::false_type                       // 一つでもfalseなら全部falseになる
  >::type
{
};


//末尾の特殊化
template<class Base, class CTail>
struct are_derived_from<Base, CTail> : public std::is_base_of<Base, CTail>
{
};

int main()
{

  std::cout << are_derived_from<MyBase, MyDeri1, MyDeri2>::value << std::endl;
  std::cout << are_derived_from<MyBase, MyDeri1, Independent>::value << std::endl;

}