求大神，这个怎么改啊，不会了。。。。。

#include <iostream>
#include <cstring>        
 using namespace std                  
 const double pi = 3.14  
 class circle
{
public:
       circle (double r);
       double virtual  area();         
       void area_message(string message);
 
private:                              
       double radius;
};
 
circle :: circle(double r) : radius(r)
{
 
}
 
double circle :: area()
{
       return pi * radius * radius;
}
 
void circle :: area_message (string message)
{
       cout << message << area() << endl;
}
 
class cylinder : public circle
{
public:
       cylinder(double r, double l);
       virtual double area();
 
private:
       double length;
}                                                     
 
cylinder :: cylinder (double r, double l) : circle(r) : length (1)            
{
 
}
 
double cylinder :: area()
{
       return 2 * pi * radius * (radius + length);
}
 
class sphere : circle                    
{
public :
       sphere (double r);
       double virtual  area();                              
private:
}
 
sphere :: sphere (double r) : circle(r)
{
 
}
 
virtual double sphere :: area()                           
{
       return 4 * pi * radius * radius ;
}
 
main()
{
       char shape;
       double radius, height;
       circle * ptr;
 
cout << "Enter a shape (1 = circle, 2 = cylinder, 3 = sphere): "<< endl;
       cin >> shape;
 
       if(shape > '0' & shape  < '4')
       {
cout << "Enter radius" << endl;
              cin >> radius;
 
              if(shape = 1 )               
              {
                     ptr= new circle(radius);
              }
              else if(shape = 2)                 
              {
                     cout << "Enter height" << endl;
                     cin >> height;
                     ptr = new cylinder(radius, height);
              }
              else if (shape = 3)
              {
                     ptr = new sphere (radius);
              }
              ptr.area_message("The area is : ");
       }
       else
              cout << "Invalid input" << endl;
}