Getting derived class specific behavior explicitly

Hi,

I have a base class and a couple of derived classes. The base class
has a common functionality and the derived class obey the base class
contract.

Now these base classes themselves have additional functionality which
might not make sens to be either in the base class or the other
derived classes.

So if the client of my code needs to access this functionality he can
then do a downcast to the required type.

Is there an object oriented way of doing this as in hiding from the
client the casting operation and doing it on our side?

In the classes below, I can know what kind of a class it is based on
the type information.

Let us say the type information is an enum in this case. Then I can
have the

enum ClassInfo {Der1, Der2};

class Base
{
public:
ClassInfo getType() = 0;
virtual void fun1();
virtual void fun2();
virtual void fun3();
virtual ~Base();
};

class Der1:public Base
{
public:
void fun1();
void fun2();
void fun3();
ClassInfo getType() { return Der1;}
//Additional functions
void fun4();
};

class Der2:public Base
{
public:
void fun1();
void fun2();
void fun3();
ClassInfo getType() { return Der2;}

//Additional functions
void fun5();
void fun6();
};

//Client code

Base* ptr = new Der1();
ptr->fun1();

//Case 1 - Client knows what he is currently working on (defeats the
whole purpose of OOP. I know :) )
//How can I do this in a better manner
Der1* ptr2 = dynamic_cast<Der1*>(ptr);
ptr2->fun4();

//Case 2
if (ptr->getType() == Der1)
{
//do Der1 specific thing
}

Is there a better way to do this without letting the clients know
about the types Der1 and Der2?

I was looking at visitor pattern which does not quite solve my problem
as the functions fun4(), fun5() and fun6() may not be exactly related
and if I have to use visitor pattern then I may have to implement all
of these in all my concrete visitors.