Thread: Segmentation fault on accessing virtual function of base class by base class pointer

Code:

#include<iostream>
using namespace std;
class A
{
        public:
        virtual void s()
        {
                cout<<"I am in A"<<endl;
        }
};
class B:public A
{
        public:
        void s()
        {
                cout<<"I am in B"<<endl;
        }
};
int main()
{
        A *o;
        o->s();                                  //Problem
        B ob;
        o=&ob;
        o->s();
        return 0;
}

Though that virtual function is accessible if I call it inside derived class' s(); using scope resolution, i.e.:

Code:

 void s()
        {
                A::s();
                cout<<"I am in B"<<endl;
        }

Why can't I call base class (A's) virtual function by base class pointer?