virtual function

Question

what are virtual functions? what is pure virtual function? please explain with example

Answer 1

There's an excellent explanation on wikipedia.

Difference between a virtual function and a pure virtual function is that a virtual function has an implementation and you have the option to override it. A pure virtual function (abstract in Java) has no implementation and you have to implement it in a subclass (unless your subclass is also abstract).

Answer 2

/**
Virtual Function : A virtual function is a member function that is declared
within a base class and redefined by a derived class.

A base class pointer can be used to point an object of any class derived from
that base. When a base pointer points to a derived object that contains a 
virtual function, C++ determines which version of that function to call based
upon the type of object pointed by the pointer.

*/

#include < iostream >
using namespace std;

class baseclass 
{
    public :
    virtual void vfunc() { cout << "This is base class vfunc\n"; };
};

class derived : public baseclass {
    public :
    void vfunc() { cout << "This is derived class vfunc\n"; };
};

class derived2 : public baseclass {};

int main () 
{
    class baseclass * ptr, b;   // baseclass pointer and its object
    class derived d;        // object of derived class
    class derived2 d2;      // object of derived2 class
    ptr = &b;           // ptr points to baseclass
    ptr->vfunc();       // vfunc of base class will execute
    ptr = &d;           // ptr points to derived class
    ptr->vfunc();       // vfunc of derived class will execute
    ptr = &d2;      // ptr points to derived2 class
    ptr->vfunc();       // vfunc of base class will execute
    return 0;
}    

******************************************************************************

/*
In a Pure Virtual Function, base class have no definition of virtual function
*/

#include < iostream >
using namespace std;

class baseclass 
{
    public :
    virtual void vfunc() = 0 ;  // Pure virtual function
};

class derived : public baseclass {
    public :
    void vfunc() { cout << "This is derived class vfunc\n"; };
};

class derived2 : public baseclass {
    public :
    void vfunc() { cout << "This is derived2 class vfunc\n"; };
};

int main () 
{
    class derived d;        // object of derived class
    class derived2 d2;      // object of derived2 class
    d.vfunc();      // vfunc of derived class will execute
    d2.vfunc();     // vfunc of derived2 class will execute
    return 0;
}