How to test whether class B is derived from template family of classes

问题

How to test at compile time whether class B is derived from std::vector?

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

How to test at compile time whether class B is derived from template family?

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

Using:

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
}

回答1:

Try this:

#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);
};

Usage:

#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;
}

Note: In the line marked #1, you could also make your trait accept any template that has at least one, but possibly more type arguments by writint:

template <typename, typename...> class Tmpl

回答2:

I had the same situation that I needed to know if a class is derived from a vector(like)-class. Unfortunately there is no C++-11 or variadic macros allowed in my project. So my solution was a mixture of Kerrek's answer and this article with some googletest-code at the end:

#include <vector>

template <typename T>
class is_derived_from_vector
{
    typedef char Yes_t[1];
    typedef char No_t[2];

    static No_t& test(const void* const);

    template <typename U>
    static Yes_t& test(const std::vector<U>* const);

public:
    static const bool value = ((sizeof(test(static_cast<T*>(0)))) == (sizeof(Yes_t)));
};

template<class T> struct X {};
struct A : X<int> {};
struct B : std::vector<char> {};

TEST(Example, IsDerivedFrom)
{
   EXPECT_FALSE(is_derived_from_vector<A>::value);
   EXPECT_TRUE(is_derived_from_vector<B>::value);
}

A common solution for any templates I think would not be possible to define without usage of C++-11 or higher.

回答3:

I was looking for a solution to this problem not long ago and after consulting with

Modern C++ Design: Generic Programming and Design Patterns Applied

I was able to build the following which is more or less similar to what was provided in the comments.

#include <iostream>
#include <type_traits>
#include <utility>

template <typename T, template <typename...> class U>
struct is_derived
{
private:
    template <typename...Ts>
    static constexpr std::true_type check(const U<Ts...>&);
    static constexpr std::false_type check(...);


    template <typename>
    struct is_same
    {
        static constexpr bool value = false;
    };

    template <typename...Ts>
    struct is_same <U<Ts...>>
    {
        static constexpr bool value = true;
    };

  public:
    static constexpr bool value =
        std::is_same<decltype(check(std::declval<T>())),
                     std::true_type>::value &&
        !is_same<T>::value;
};

template <typename, typename>
struct X
{
};

template <typename T>
struct Y : X <T, int>
{
};


int main(int argc, char **argv) {

    std::cout << std::boolalpha << is_derived<Y<int>, X>::value << std::endl;
    std::cout << std::boolalpha << is_derived<X<int,int>, X>::value << std::endl;
    std::cout << std::boolalpha << is_derived<int, X>::value << std::endl;

  return 0;
}

来源：https://stackoverflow.com/questions/12181885/how-to-test-whether-class-b-is-derived-from-template-family-of-classes