c++ - std::is_base_of for template classes

Question

Is there a way to test std::is_base_of<A, B> when A is a template class?

template <typename X, typename Y> class A {};

template <typename X> class B : public A<X, char> {};

I want to statically test something like, std::is_base_of<A, B<int>> meaning, B is derived from any specialization of A. (To make it more general, let's say we don't know the way B specializes A, i.e. B<X> derives from A<X, char>)

One way to solve would be to derived A from a (non-template) class say C, and then check std::is_base_of<C, B<int>>. But is there another way to do this?

Answer 1

You may do the following:

template <template <typename...> class C, typename...Ts>
std::true_type is_base_of_template_impl(const C<Ts...>*);

template <template <typename...> class C>
std::false_type is_base_of_template_impl(...);

template <typename T, template <typename...> class C>
using is_base_of_template = decltype(is_base_of_template_impl<C>(std::declval<T*>()));

Live Demo

but will fail with multiple inheritance or private inheritance from A.

With Visual Studio 2017 this will fail when the base class template has more than one template parameter, and it is unable to deduce Ts...

Demo

VS Bug Report

Refactoring solves the problem for VS.

template < template <typename...> class base,typename derived>
struct is_base_of_template_impl
{
    template<typename... Ts>
    static constexpr std::true_type  test(const base<Ts...> *);
    static constexpr std::false_type test(...);
    using type = decltype(test(std::declval<derived*>()));
};

template < template <typename...> class base,typename derived>
using is_base_of_template = typename is_base_of_template_impl<base,derived>::type;

Live Demo