Polymorphism

[omerd]

maybe
I changed the code a little bit. Now it is more specific, but I also tried to keep it simple.
Actually, B and C manage a list of objects of type D, and in some cases B need to get C's D objects. B and C create objects of D in different ways.
If it's help you, C holds a list of object of type B.

 class D
{
    public:
      D(string s): name(s)
        {another_data =-1;}
      D(string s,int data): name(s),another_data(data)
        {}
      D(const D& d)
      {/*code*/}
      string GetName() {return name;}
      int GetData() {return another_data;}

    private:
      string name;
      int another_data;
};

class A
{
    public:
       A(int b=0) {a=b;}
       virtual ~A()
       {
           //code;
        }
    protected:
       virtual D* FindD(string)=0;
    private:
       int a;
};
class B: public A
{
    friend class A;
    public:
       B(int b=0): A(b){}

    D* FindD(string name)
    {
        //code that find a specific D
        //and return it
    }
       void Do(A& a, string name)
       {
           a.FindD(name); //error: `virtual D A::FindD(std::string)' is protected|
       }

    private:
    list<D> names;

};
class C: public A
{
    C(int b=0): A(b){}

    D* FindD(string name)
    {
        //code that find a specific D
        //and return it
    }

    list<D> names;
};