Trouble with abstract base classes

[Need4Sleep]

EDIT:
Thanks for the help guys! i've just got one more question, how would i use the constructor for the Obj? i want to declare its (x,y) position but i dont know how to do so, i included what i tried in my new updated code.



ERROR:

F:\C++\AbstractBaseClass_Shapes\main.cpp||In function 'int main()':|
F:\C++\AbstractBaseClass_Shapes\main.cpp|7|error: initializer expression list treated as compound expression|
F:\C++\AbstractBaseClass_Shapes\main.cpp|7|warning: left-hand operand of comma has no effect|
F:\C++\AbstractBaseClass_Shapes\main.cpp|7|error: invalid conversion from 'int' to 'Abc*'|
||=== Build finished: 2 errors, 1 warnings ===|

main.cpp

#include <iostream>
#include "Abc.h"
using namespace std;

int main()
{
    Abc * Obj;
    //Abc * Obj(4,3); doesnt work, how to do this?
    Obj = new Circle(5); //allocate new circle object
    std::cout << "The area of the circle: " << Obj->Area() << std::endl;
    delete Obj;

}

Abc.h

#ifndef ABC_H
#define ABC_H


class Abc
{
    public:
        virtual ~Abc();                             //destructor
        virtual double Area() const = 0;            //pure virtual
        void move(int nx, int ny) {x = nx; y = ny;} //all classes use same function
        Abc(double x0 = 0, double y0 = 0): x(x0),y(y0){}
    protected:
    private:
        double x;   //x and y of origin
        double y;
};

class Ellipse : public Abc
{
    public:
        Ellipse(double a0, double b0) {a = a0;b = b0;}
        virtual double Area() const;
        ~Ellipse(){}
    protected:
    private:
        double a; //horizontal and vertical radius
        double b;
        double angle; //anlge it is at(not used yet)
};
class Circle: public Abc
{
    public:
        Circle(double ra) {r = ra;}
        virtual double Area() const;
        ~Circle(){}
    protected:
    private:
        double r; //radius
};
#endif // ABC_H

Abc.cpp

#include "Abc.h"
#include <iostream>

double Ellipse::Area() const
{
        return (3.14 * a * b);
}

double Circle::Area() const
{
    return (3.14 * r * r);
}

Abc::~Abc()
{

}

Appreciate the help!

[Peter87]

Obj is a pointer so you have to write Obj->Area() to call the Area function.

In Ellipse and Circle you have declared Area functions with extra parameters. I guess this is just a mistake because when you define them you have left out the parameters.

[Need4Sleep]

Thanks for that, corrected! but i'm getting the same errors as before, any ideas of why?

[Mathhead200]

Peter87 nailed both errors. You need to change Obj.Area() to Obj->Area() (in main), and you need to declare and define Area() without parameters in both Ellipse and Circle. In other words, you haven't overridden (abstract) ABC::Area() yet, you've declared and defined two new methods, Ellipse::Area(double, double) and Circle::Area(double). This makes Ellipse and Circle abstract classes still.

Note: shouldn't Circle extend Ellipse?

[vlad from moscow]

I think that maybe you forgot to specify qualificator const in functios Area in class Circle

virtual double Area(); // here shall be const.

In any case if you get an error you should show your updated code.

[Need4Sleep]

Thanks for the help guys! i've just got one more question, how would i use the constructor for the Obj? i want to declare its (x,y) position but i dont know how to do so, i included what i tried in my new updated code.

[chipp]

maybe:

Abc * Obj = new Abc (4,3);

the Abc constructor have to be modified too (perhaps)

CMIIW

[Need4Sleep]

im trying to declare the object as a circle, so i wouldn't allocate it as an Abc object. Still stuck

[ne555]

If you want to set the position of the circle, then you ought to provide methods to set it.

Circle::Circle( double x, double y, double r):
  Abc(x,y), r(r){}

shouldn't Circle extend Ellipse?

Nope. Look for Liskov's sustitution principle.
You can change any axis in the ellipse, but in the circle they must be equal (restriction)