C++ 校验指针&引用类型

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

template<typename T>
class TypeTrait{
private:

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


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

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

void Test(){
    std::cout<< TypeTrait<int>::isPointer<<std::endl;
    std::cout<< TypeTrait<int*>::isPointer<<std::endl;

    //TypeTrait<int>::PointerType var; // error, is NullType , not defined;
    TypeTrait<int*>::PointerType var1 = 10;
    int var2;
    bool isInt = typeid(var1).hash_code() == typeid(var2).hash_code(); 
    std::cout<<isInt<<std::endl; // 1, is int type


    std::cout<< TypeTrait<int>::isReference<<std::endl; // 0
    std::cout<< TypeTrait<int*>::isReference<<std::endl; // 0
    std::cout<< TypeTrait<int&>::isReference<<std::endl; // 1

    std::cout<< TypeTrait<std::vector<int>::iterator>::isPointer<<std::endl;// 0
}


int main(){
    Test();
}