Check if template parameter is equal to Base class of another template parameter

Question

I have a class like this :

template <class T1, class T2>
class A
{
    //Error if Base class of T2 is not T1 (At compile time)
};

I want to check if T1 is Base class of T2 . Is is possible at compile time ?

Some Examples :

class C{};
class B :public C{};

A< C, B > a;     //Ok because C is base class of B
A<B, C> b;       //Error B is not base class of C
A<char, char> c; //Error char is not base class of char 
//.....

Answer 1

std::is_base_of will get you most of the way there, but it's not quite what you want. You also want an error if the two types are the same, and is_base_of<T, T>::value is always true for any user defined type T. Combine the check with std::is_same to get the behavior you want.

template <class T1, class T2>
class A
{
    static_assert(std::is_base_of<T1, T2>::value &&
                  !std::is_same<T1, T2>::value,
                  "T1 must be a base class of T2");
};

This will result in the following:

A< C, B > a;     //Ok because C is base class of B
A<B, C> b;       //Error B is not base class of C
A<char, char> c; //Error char is not base class of char 
A<B, B> d;       //Error B is not base class of B <-- this won't fail without
                 //                                   the is_same check