Virtual Function Help.

**Cheeze-It** · 07-02-2003

I'm not really comfortable with these 'pure virtual' functions or 'abstract
classes' yet, so I don't know if I'm using them right.

But, I have a base class that has a few virtual functions, then I
have a derived class that has those functions defined, plus a few
more. Can I not use the extra functions that are in the derived
class?

Code:

class cl_One
{
    public:
      cl_One();
      virtual ~cl_One();

      virtual void Function1(void) = 0;
      virtual void Function2(void) = 0;

    private:
      int var1;
      int var2;
};

class cl_Two: public cl_One
{
     public:
       cl_Two();
       ~cl_Two();

       void Function1(void);
       void Function2(void);
       //extra function..
       void Function3(void);
    
    private:
       int var3;
       int var4;
};

cl_One* Cowptr = new cl_Two;

Cowptr->Function3();

I get a "Function3() is not a memober of 'cl_One' error. Do I need
to include a pure virtual Function3 in cl_One? I can do that, but
not all of my classes that derive from cl_One will use a Function3().
Is it common to just define these unused functions as functions
that do nothing ({}), or am I using these virtual functions completely
wrong?

What do I do?

Thanks,

ethic.

**cr_naik** · 07-02-2003

> cl_One* Cowptr = new cl_Two;

Here you are creating new object of derived class and use it as a pointer to base class. So it will not allow you to access Function3() which is declared and defined in derived class only.

If you want to access derived class's function through pointer to base class then you must have to define the same function in base class as virtual function. And that is wright use of virtual functions.

>am I using these virtual functions completely wrong?

No man....this is the use of virtual functions. You are doing right thing.

**Zach L.** · 07-02-2003

Creating a pointer to the base class will allow you to access all common functions of any derived class (namely, those functions which belong to the base class). If you have a pointer to a base class, then you can try to cast it to the derived type as follows:

Code:

cl_One* ptr1 = new cl_Two;
cl_One* ptr2 = new cl_Three;

cl_Two* ptr3 = dynamic_cast<cl_Two*>(ptr1);

cl_Two* ptr4 = dynamic_cast<cl_Two*>(ptr2);

if(ptr3 != 0) // Cast succeeded
   ptr3->Function3();
else
   ...

if(ptr4 != 0)
   ...
else // Cast failed, null pointer returned.
   std::cout << "ptr2 was not a pointer to an instance of cl_Two.";

**Zach L.** · 07-02-2003

One other thing, you can use dynamic_cast on references as well, but you have to be sure of what derived type you have, because you cannot check if dynamic_cast failed.

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