Error in c++

I was executing a program given in the  notes, this was the error that occured:-

Below is the code:-

#include<iostream>
using namespace std;
class B{
  public:
    virtual void m1()
    {   
    cout<<"B::m1\n";
    }   
    virtual void m2()
    {   
    cout<<"B::m2\n";
    }   
};
class D :: B
{
  public:
  void m1()
    {   
    cout<<"D::m1\n";
    }   
};
int main()
{
B *p;
B b;
D d;
p = &d; // p is set to d's address
p-> m1();
p-> m2();
p = &b; // p is set to b's address
p-> m1();
p-> m2();
}
// here is the error :-