检查template参数是否等于另一个模板参数的Base类

Question

我有一个这样的课:

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

我想检查一下是否T1是Base类T2.可以在编译时吗？

一些例子 :

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会让你在那里大部分时间,但它不是你想要的.如果两种类型相同,您还需要一个错误,并且is_base_of<T, T>::value始终true用于任何用户定义的类型T.结合检查std::is_same以获得您想要的行为.

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");
};

这将导致以下结果:

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