关于虚函数，错误

我想创建一个形状类，在xy和区域外绘制矩形和圆形。我必须使用虚函数，它会出现错误:

43 10 D:CppTurboC4OOPCpp Shape . Cpp[错误]无法将字段Circle::r声明为抽象类型ShapeD:CppTurboC4OOPCpp Shape . Cpp，因为以下虚拟函数在"Shape"中是纯的:18 18 D:CppTurboC4OOPCpp Shape . Cpp virtual float Shape::area()

下面是我的代码:

class Shape{
    public:
            int x,y;
    public:
            void set_data (int a,int b){
                x=a;
                y=b;
            }
            void input(){
                cout<<"x= "; cin>>x;
                cout<<"y= "; cin>>y;
            }
            virtual float area()=0;
};
class Rectangle: public Shape{
    public:
            Rectangle(){
                x=0;
                y=0;
            }
            Rectangle(int x,int y){
                this->x=x;
                this->y=y;
            }
            void input(){
                cout<<"Enter value of width and height: ";
                cin>>x;
                cin>>y;
            }
            float area(){
                return x*y;
            }
};
class Circle: public Shape{
    protected:
            Shape r;
    public:
            Circle(){
                r.x=0;
                r.y=0;
            }
            //center of Circle: default(0,0) , r is 1 point in the circle.
            Circle(int x,int y){
                r.x=x;
                r.y=y;
            }   
            void nhap(){
                        cout<<"Enter values x,y of r: ";
                        cin>>x;
                        cin>>y;
            }
            virtual float area(){
                float a=sqrt(r.x*r.x+r.y*r.y);
                return 3.14*a*a;
            }
};
class ArrayOfShape{
    private:
            int n;
            Shape **a;
    public:
            void nhap(){
                int hinh;
                cout<<"input number of shape: ";
                cin>>n;
                a=new Shape*[n];
                for (int i=0;i<n;i++){
                    cout<<"nEnter shape (1:rectangle,2:circle): ";
                    cin>>hinh;
                    if (hinh==1){
                        Rectangle *p=new Rectangle();
                        p->nhap();
                        a[i]=p;
                    }
                    else{
                        if(hinh==2){
                        Circle *e=new Circle();
                        e->input();
                        a[i]=e;
                        }
                        else{
                            cout<<"Invilid input";
                        }
                    }
                }
            }
            void area(){
                for (int i=0;i<n;i++)
                cout<<"nArea of shape"<<i+1<<" : "<<a[i]->area();
            }
};
int main(){
    ArrayOfShape a;
    a.input();
    a.area();
}