Inheritance

class Point {
protected:
        double x,y;
};

class DataPoint: public Point {
protected:
        double e;
};

int main() {}; 

class Function {
public:
    double operator() (const double x);
};

class Function1 : public Function {
public:
    double operator() (const double x) {
        return x;
    }
};

class Function2 : public Function {
public:
    double operator() (const double x) {
        return x*x;
    }
};

double callFunction(Function& F, double x) {
        return F(x);
}

int main() {

}