求高手帮忙、、、、

#include<stdio.h>
#include<iostream.h>
class base{
       public:
        virtual double getArea()=0;
};
class square:public base{
       public:
            square(double L){
               Long=L;
           }
           double getArea(){
              return Long*Long;
           }
       private:
          double Long;
};
class circle{
        public:
             circle(double r){
              R=r;
            }
            double getArea(){
              return 3.14*R*R;
            }
        private:
            double R;
};
int main(){
    double s1,s2;
    base *pd;
    square Sq(2.0);
    circle Cir(3.0);
    pd=&Sq;
    s1=pd->getArea();
    pd=&Cir;
    s2=pd->getArea();
   cout<<"s1="<<s1<<endl;
   cout<<"s2="<<s2<<endl;
return 0;
}

Compiling...
a.cpp
G:\BCheng\1\a.cpp(36) : error C2440: '=' : cannot convert from 'class circle *' to 'class base *'（问题就在这里，不知道怎么改）
        Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
Error executing cl.exe.

a.exe - 1 error(s), 0 warning(s)