Hi I am a C++ beginner just encountered a problem I don't know how to fix

I have two class, this is the header file:

class A  
{  
public:  
  int i;  
  A(int a);  
};

class B: public A  
{  
public:  
  string str;  
  B(int a, string b);  
};    

then I want to create a vector in main which store either class A or class B

vector<A*> vec;  
A objectOne(1);  
B objectTwo(2, "hi");  
vec.push_back(&objectOne);  
vec.push_back(&objectTwo);  
cout << vec.at(1)->i; //this is fine  
cout << vec.at(1)->str; //ERROR here 

I am really confused, I checked sites and stuff but I just don't know how to fix it, please help

thanks in advance