模板类如何实现多态？

template<typename T>
class Temp
{
    virtual void X(T value);
    virtual void Y();
};

class Person
{
public:
    void Y()
    {
        a->Y();
    }

protected:
    Temp<????> *a;    //这里????处如何定义？

};

class Teacher: public Person
{
public:
    Teacher()
    {
        a = new Temp<int>;
    }

};

class Student: public Person
{
public:
    Student()
    {
        a = new Temp<string>;
    }

};