Virtual functions - inheritance query in c++

Hello, i'm now learning c++ and i want to ask this:
Suppose that we have these three classes:

Code:

class A {
private: 
             int sth;
             //.....
public:
             virtual int is_class_B (void) = 0;
             void print (void)  { cout << "I'm in class A" << endl; return;}
             int get_sth (void) { return sth; }
             //.......
};

class B : public A  {
private: //......
public:
             int is_class_B (void) { return 1; }
             void print (void) { cout << "I'm in class B" << endl; return; }
};

class C : public A {
private: //........
public:
             int is_class_B (void) { return 0; }
             void print (void) { cout <<"I'm in class C" << endl; return; }
};

We also have a class A * that points to either a class B object or a class C object (lets call him classptr). I want to make a function that can be described in a pseudo-language like this:

Code:

switch (classptr.get_sth ()) {
             case 0: 
                          classptr.print (); // use the print func of class A
                          break;
             default:
                          if (classptr.is_class_B ()) {
                                      classptr.print (); // use the print func of class B
                                      // .....
                          }
                          else {
                                       classptr.print (); // use the print func of class C
                                       // ........
                          }
                          break;
}

My question is: How can i choose which print will be used in each case? For example the classptr can point to a class B object but if classptr.sth == 0 i want to use the print function of class A, not this of the derived class. On the other hand, if sth != 0 i want to use the print function of the derived class. Do you have any ideas? Thanx in advance for the help & sorry for any english mispelling!