如何测试B类是否来自模板族类

Question

如何在编译时测试B类是否来自std :: vector？

template<class A>
struct is_derived_from_vector {
  static const bool value = ????;
};

如何在编译时测试B类是否来自模板系列？

template<class A, template< class > class Family>
struct is_derived_from_template {
  static const bool value = ????;
};

使用:

template<class T> struct X {};

struct A : X<int> {}
struct B : std::vector<char> {}
struct D : X<D> {}

int main() {
   std::cout << is_derived_from_template<A, X>::value << std::endl; // true
   std::cout << is_derived_from_template<D, X>::value << std::endl; // true
   std::cout << is_derived_from_vector<A>::value << std::endl; // false
   std::cout << is_derived_from_vector<B>::value << std::endl; // true
}

Answer 1

试试这个:

#include <type_traits>

template <typename T, template <typename> class Tmpl>  // #1 see note
struct is_derived
{
    typedef char yes[1];
    typedef char no[2];

    static no & test(...);

    template <typename U>
    static yes & test(Tmpl<U> const &);

    static bool const value = sizeof(test(std::declval<T>())) == sizeof(yes);
};

用法:

#include <iostream>

template<class T> struct X {};

struct A : X<int> {};

int main()
{
    std::cout << is_derived<A, X>::value << std::endl;
    std::cout << is_derived<int, X>::value << std::endl;
}

注意:在标记的行中#1,您还可以使您的特征接受任何具有至少一个但可能更多类型参数的模板:writint:

template <typename, typename...> class Tmpl