Member function of abstract class which is a derived class of another abstract class

Question

I want to define the member function area() and volume() separately in two abstract classes Shapes_2D and Shapes_3D, which are derive class of another abstract base class Shapes. But I got the error:

no member named 'area' in 'Shapes'

Here's my code:

class Shapes
{
protected:
    unsigned int dim;       
public:
    virtual ~Shapes() {}
    Shapes(unsigned int x) : dim{x} {}
    unsigned int getDim() { return dim; }
};

class Shapes_2D : public Shapes
{
public:
    Shapes_2D() : Shapes{2} {}
    virtual double area() = 0;  
};

class Shapes_3D : public Shapes
{
public:
    Shapes_3D() : Shapes{3} {}
    virtual double volume() = 0;
};

class Rectangle : public Shapes_2D
{
private:
    double width;
    double height;
public:
    Rectangle(double w, double h) : width{w}, height{h} {}
    double area() {return width * height;};
};

Here's how I tried to use area()

Shapes *rectangle = new Rectangle(2, 3);
rectangle->area();

I can fix this by defining volume() in the 2D base class:

class Shapes
{
public:
    // ...
    virtual double area() = 0;
    virtual double volume() = 0;
};
class Shapes_2D : public Shapes
{
public:
    // ...
    double area() { return 0; }  
    double volume() { return 0; }
};

Is there a way to avoid define the volume() in the 2D class since it doesn't make sense.

Answer 1

If you intent to calculate the area of rectangle, then the type of rectangle must be Shapes_2D* or D* where D is derived from Shapes_2D. If you declare rectangle with type Shapes*, then how do you know it actually is a 2D shape that has an area? It could be pointing to something that is a 3D shape and doesn't have an area.

So:

Shapes_2D *rectangle = new Rectangle(2, 3);

(Also, from a mathematical viewpoint, there are several possibilities to interpret "area" and "volume", so that maybe your separation into distinct properties for 2D and 3D doesn't really make sense.)