C++ 校验类型是否相互转换

#include <iostream>

template<class T, class U>
class IsConvertable{

private:
    typedef char Small;
    class Big { char dummy[2];};
    static Small Test(U);
    static Big Test(...);
    static T MakeT(); // 防止T构造函数是private时构造失败

public:
    enum {exists = sizeof(Test(MakeT())) == sizeof(Small)};
    enum {exists2Way = exists && IsConvertable<U,T>::exists};
    
};


struct Base{};
struct Derived: public Base{
    int a;
};
void Test(){
    std::cout<<IsConvertable<double,int>::exists<<std::endl; // 1
    std::cout<<IsConvertable<int,double>::exists<<std::endl; // 1
    std::cout<<IsConvertable<Derived,Base>::exists<<std::endl;// 1
    std::cout<<IsConvertable<Base,Derived>::exists<<std::endl; // 0
    std::cout<<IsConvertable<std::string,int>::exists<<std::endl;// 0

    std::cout<<"<<<<<<<<<<<<<<<<<"<<std::endl;
    std::cout<<IsConvertable<double,int>::exists2Way<<std::endl; // 1
    std::cout<<IsConvertable<int,double>::exists2Way<<std::endl; // 1
    std::cout<<IsConvertable<Derived,Base>::exists2Way<<std::endl;// 0
    std::cout<<IsConvertable<Base,Derived>::exists2Way<<std::endl; // 0
    std::cout<<IsConvertable<std::string,int>::exists2Way<<std::endl;// 0


}

int main(){
    Test();
}