C++: Accessing variables present only in one of the derived classes

Question

void func1()
{
}

class Base
{
    public:
        virtual void memfunc() = 0;
};
class DerivedA: public Base
{
    public:
        virtual void memfunc() = 0;
};
class Derived1: public DerivedA
{
    public:
        void memfunc()
        {  
            func1();
        }
};
class Derived2: public DerivedA
{
    public:
        int* pbuf;
        int val;

        void func2(int* pbuf,int val)
        {
                /* update pbuf depending on the value of val*/
        }

        void memfunc()
        {  
            func1();
            func2(pbuf,val);
        }
};
class user{
    public:
    Base* mBase;
    void userfunc()
    {
        mBase = f(); //Object pointer to Derived1/Derived2 will be assigned based on dynamic loading;
//Before calling the below func, I Need to assign value to the 'val' variable of the class 'Derived2'
        mBase->memfunc();
    }

};
int main()
{
    user ouser;
    ouser.userfunc();
    return 0;
}

The variables val and pbuf are present in Derived2 only. How can I assign values to them in userfunc/main as I don't know if the object mBase would point to Derived1/Derived2 .

Answer 1

You can use dynamic_cast and check for NULL, which will be returned if the object isn't of type Derived2. If the cast succeeds, you can use all methods of Derived2.

Answer 2

Use a dynamic_cast in userfunc() and test for Derived2 , ie

Derived2* p = dynamic_cast<Derived2*>(mBase);
if (p) // this is NULL if the above fails.
{
  // initialize...
}

Answer 3

If val and pbuf are present only on Derived2, then you must have some way to identify them. You would need to create an enum Base::Type { Derived1, Derived2 }, set them on your derived classes, and test in main. Then, you can cast mBase to Derived2 if it is of that type, and set val.

Answer 4

If you are sure of the concrete type of your class you can use a dynamic_cast. Anyway that's a bad practice. The purpose of using a base abstract class reference is to hide internal details and specific implementations issues.

If you need to do so, maybe you can add a getProperty()/setProperty() method to your base class. Something like:

class Base
{
    public:
        virtual void memfunc() = 0;
        virtual void setProperty(String name, Property prop);
        virtual Property getProperty(String name);
};

Answer 5

Use typeid:

http://en.wikipedia.org/wiki/Typeid