Question

I need some answers to basic questions. I’m lost again. 🙁

q1 – Is this statement valid:
Whenever we define the function to be pure virtual function,
this means that function has no body.

q2 – And what is the concept of Dynamic Binding? I mean if the Compiler optimizes the code using VTABLEs and VPTRs then how is it Run-Time Polymorphism?

q3 – What are VTABLES AND VPTRs and how do their sizes change?

q4 – Please see this code:

class base
{
    public:
        virtual void display()
        {
            cout<<"Displaying from base";
        }
};

class derived:public base
{
    public:
        void display(){cout<<"\nDisplaying from derived";}
};

int main()
{
    base b,*bptr;
    derived d;
    bptr=&b;
    bptr->display();
    bptr=&d;
    bptr->display();
} 

Output:

Displaying from base
Displaying from derieved

Please can somebody answer why a pointer of base class can point the member function of a derived class and the vice-versa is not possible, why ?

Answer 1

1. False. It just means any derived classes must implement said function. You can still provide a definition for the function, and it can be called by Base::Function().*

2. Virtual tables are a way of implementing virtual functions. (The standard doesn’t mandate this is the method, though.) When making a polymorphic call, the compiler will look up the function in the function table and call that one, enabling run-time binding. (The table is generated at compile time.)

3. See above. Their sizes change as there are more virtual functions. However, instances don’t store a table but rather a pointer to the table, so class size only has a single size increase.

4. Sounds like you need a book.

_{*A classic example of this is here:}

struct IBase
{
    virtual ~IBase(void) = 0;
};

inline IBase::~IBase(void) {}

This wouldn’t be an abstract class without a pure virtual function, but a destructor requires a definition (since it will be called when derived classes destruct.)