About Virtual function, errors

Question

I want to create a shape class to draw rectangles and circles in Oxy and out of the area. I have to use virtual function, and it goes to errors:

43 10 D:\Cpp\TurboC4\OOPCpp\shape.cpp [Error] cannot declare field 'Circle::r' to be of abstract type 'Shape' 6 7 D:\Cpp\TurboC4\OOPCpp\shape.cpp because the following virtual functions are pure within 'Shape': 18 18 D:\Cpp\TurboC4\OOPCpp\shape.cpp virtual float Shape::area()

Here's my code:

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();
}

Answer 1

The problem is here:

class Circle: public Shape{
    protected:
            Shape r; // Wrong

Because Shape is a pure virtual function, you cannot create an instance of it (which is precisely what Shape r tries to do).

Possible solution:

class Circle: public Shape{
protected:
    int x, y
    ...
    public Circle() {
      x = y = 0;  // Do you really need "r" at all?  Why not just x and y?
      ...

ALSO:

If you're really using Turbo C++ ... please don't. It's 20 years out of date ... and it matters. Especially if you're learning. There are plenty of free and/or Open Source C++ compilers that would probably serve you much better...

Answer 2

When you declare the member variable r in Circle, then you declare an instance of the Shape class. Since Shape is an abstract class, that is not possible.

You need to make the variable r either a reference or a pointer.

---

After reading your code a little more, you probably should not use a member variable here at all, but use the x and y members you inherit from the Shape base class. Like you already do in the Rectangle class.