Question

If I create a pointer out of my Shape base class, how would I got about making it behave like the circle (derived) class? Here are my class definitions:

    //CShape.h class definition
    //Shape class definition
    #ifndef CSHAPE_H
    #define CSHAPE_H

    class CShape
    {
    protected:
        float area;
        virtual void calcArea();
    public:
        float getArea()
        {
            return area;
        }
    };


    class CCircle : public CShape
    {
    protected:
        int centerX;
        int centerY;
        float radius;
        void calcArea()
        {
            area = float(M_PI * (radius * radius));
        }
    public:
        CCircle(int pCenterX, int pCenterY, float pRadius)
        {
            centerX = pCenterX;
            centerY = pCenterY;
            radius = pRadius;
        }
        float getRadius()
        {
            return radius;
        }
    };

In my project file where I’m calling these objects, I have the following code:

            CShape *basePtr = new CCircle(1, 2, 3.3);
            basePtr->getRadius();

It seems to me like this should work, however I’m told CShape has no member “getRadius().”

EDIT
Based on the response below I tried to dynamic_cast the basePtr object to CCircle like so:

CCircle *circle = new CCircle(1, 2, 3.3);
basePtr = dynamic_cast<CCircle *>(circle);

This failed too, however. I’ve never done a dynamic_cast and am unfamiliar with most of the syntax in C++, so any help is greatly appreciated.

Answer 1

To get to getRadius function you must declare it of type CCircle:

        CCircle *basePtr = new CCircle(1, 2, 3.3);
        basePtr->getRadius();

Otherwise, if you want to declare basePtr as CShape, use dynamic binding with dynamic_cast or use c-style cast directly, as @Janisz pointed out.

       CShape *basePtr = new CCircle(1, 2, 3.3);
       CCircle *child
       // choose one of the two ways, either this:
       child = (CCircle*)basePtr; // C-style cast
       // or this:
       child = dynamic_cast<CCircle*>(basePtr); // C++ version
       child->getRadius();