class Base{
public:
virtual int get_data();
virtual void set_data(int a);
};

class ChildA : public Base
{
private:
int a;
ChildA * child;
public:
void set_object(ChildA& childa){   child = childa;  }
void set_data(int a){ this.a = a; }
int get_data(){ return a; }
}

class ChildB : public Base
{
...
}
int main(){
ChildA * childa = new ChildA;
childa.set_object(childa);
childa.set_data(12);
ChildB * childb = new ChildB;
childb.get_data();
};