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C++: Base Shape class

class shape {
	public:
		shape();
		virtual double area() const;
		virtual double perimeter() const;
	private:
		coordinates position;
		color outline, fill;
};

C Equivalent:

typedef struct shape shape;
typedef struct shape_vtbl shape_vtbl;

struct shape_vtbl {
	double (*area)(shape const *s);
	double (*perimeter)(shape const *s);
};
struct shape {
	shape_vtbl *vptr;
	coordinates position;
	color outline, fill;
};

C++: circle class derived from shape looks like:

class circle: public shape {
	public:
		circle(double r); 
		virtual double area() const;
		virtual double perimeter() const;
	private:
		double radius;
};

#include "shape.h"
typedef struct circle circle;
struct circle {
	shape base; // the base class sub-object
	double radius;
};
void circle_construct(circle *c, double r);

In C++, a call to the virtual area function applied to a shape looks exactly like a non-virtual call, as in:

shape *s;
~~~
s->area();

In C, virtual function calls look unlike any other kind of function call. For example, a call to the virtual area function applied to a shape looks like:

shape *s;
~~~
s->vptr->area(s);

In this case, if s points to a circle (the dynamic type of *s is circle), then the call above calls circle_area. If s points to a rectangle, then the call above calls rectangle_area.

Source: forum.eetindia.co.in/BLOG_ARTICLE_13544.HTM

Just use a table of function pointers.

In C++, a virtual function can be redefined in a derived class

class Shape {
public:
virtual int area (void) =0;
};

class Square: public Shape {
public:
int width;
int area (void)
{ return (width * width); }
};

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