#include <iostream>

using namespace std;

class Abstract
{
protected:
    double a;
public:
    Abstract(const double& a):a(a) {}
    double GetA()const
    {
        return a;
    }
    void Sum(const Abstract &ab)
    {
        doSum(ab);
    }
private:
    virtual void doSum(const Abstract &ab) = 0;
};
class Derived : public Abstract
{
public:
    Derived(const double& a) :Abstract(a) {}
    void doSum(const Abstract &der)
    {
        a += der.GetA();
    }
};
int main()
{
    Derived object(2);
    Derived object2(5);
    object.Sum(object2);
    cout << object.GetA() << endl;
    system("pause");
}