c++ – converting a base class pointer to a derived class pointer

You must change the base type to be polymorphic:

class Base {
public:
    Base() {};
    virtual ~Base(){};
};

To cast from some supertype to some derived type, you should use dynamic_cast:

Base *b = new Derived<int>(1);
Derived<int> *d = dynamic_cast<Derived<int> *>(b);

Using dynamic_cast here checks that the typecast is possible. If there is no need to do that check (because the cast cannot fail), you can also use static_cast:

Base *b = new Derived<int>(1);
Derived<int> *d = static_cast<Derived<int> *>(b);