C++ 校验指针&引用类型

#include 
#include 
class NullType; // 只有声明，未定义，当做类型占位符

template
class TypeTrait{
private:

    // check template U param is pointer
    template class IsPointer{
        public:
            enum {isResult = false};
            typedef NullType PointerType;
    };
    template class IsPointer {
        public:
            enum {isResult = true};
            typedef U PointerType;
    };


    // check template U param is reference
    template class IsReference{
        public:
            enum {isResult = false};
            typedef NullType PointerType;
    };
    template class IsReference{
        public:
            enum {isResult = true};
            typedef U PointerType;
    };

public:
    enum { isPointer=IsPointer::isResult, isReference=IsReference::isResult};
    typedef typename IsPointer::PointerType PointerType;
};

void Test(){
    std::cout<< TypeTrait::isPointer<::isPointer<::PointerType var; // error, is NullType , not defined;
    TypeTrait::PointerType var1 = 10;
    int var2;
    bool isInt = typeid(var1).hash_code() == typeid(var2).hash_code(); 
    std::cout<::isReference<::isReference<::isReference<::iterator>::isPointer<
    Test();
}