Why doesn't C++ have virtual variables?

This might have been asked a million times before or might be incredibly stupid but why is it not implemented?

class A
{
      public:
             A(){ a = 5;}
             int a;
};

class B:public A
{
      public:
             B(){ a = 0.5;}
              float a;
};

int main()
{
    A * a = new B();

    cout<<a->a;
    getch();
    return 0;
}

This code will access A::a. How do I access B::a?