template<typename T, class Derived>

class Base {

public:

    Base(T x) : x_(x) { };

    Derived half() {

        Derived d(x_ / 2);

        return d;

    };

    virtual T calculate() = 0;

    virtual ~Base() { };

protected:

    T x_;

};

template<typename T>

class Square : public Base<T, Square<T> > {

public:

    Square(T x) : Base<T, Square<T> >(x) { };

    T calculate() { return this->x_ * this->x_; }

};

template<typename T>

class Cube : public Base<T, Cube<T> > {

public:

    Cube(T x) : Base<T, Cube<T> >(x) { };

    T calculate() { return this->x_ * this->x_ * this->x_; }

};