OOP using plane C

oop_class_demo.c

#include <stdlib.h>
#include <stdio.h>


/*
 * Base class. It defines int area() method.
 * The class is abstract so we did not made constructor.
 * Calling the area() method will probably core dump.
 */

typedef struct{
	char	*_classname;
	int	(*area)(void *);
} Figure;


/*
 * Circle class. It "extends" Figure class.
 * _newCircle() is the constructor.
 * it allocates the memory, initialize the fields and maps the area() method.
 */

typedef struct{
	char	*_classname;
	int	(*area)(void *);
	int	radius;
	float	(*areaFl)(void *);
} Circle;

float CircleAreaFL(Circle *self){
	return 3.14 * self->radius * self->radius;
}

int CircleArea(Circle *self){
	return (int) CircleAreaFL(self);
}

Circle *_newCircle(int radius){
	Circle *circ = malloc(sizeof(*circ));

	circ->_classname	= "Circle";
	circ->radius		= radius;
	circ->area		= (int(*)(void *)) CircleArea;
	circ->areaFl		= (float(*)(void *)) CircleAreaFL;

	return circ;
}


/*
 * Rectangle class. It "extends" Figure class.
 * _newRectangle() is the constructor.
 * it allocates the memory, initialize the fields and maps the area() method.
 */

typedef struct{
	char	*_classname;
	int	(*area)(void *);
	int	a;
	int	b;
} Rectangle;

int RectangleArea(Rectangle *self){
	return self->a * self->b;
}

Rectangle *_newRectangle(int a, int b){
	Rectangle *rect = malloc(sizeof(*rect));

	rect->_classname	= "Rectangle";
	rect->a			= a;
	rect->b			= b;
	rect->area		= (int(*)(void *)) RectangleArea;

	return rect;
}


/*
 * Square class. It "extends" Rectangle class.
 * _newSquaree() is the constructor.
 * it uses parent constructor for instanciate the class.
 */

typedef struct{
	char	*_classname;
	int	(*area)(void *);
	int	a;
	int	b;
} Square;

Square *_newSquare(int a){
	Square *sq = (Square *) _newRectangle(a, a);
	sq->_classname	= "Square";

	return sq;
}


/*
 * print_area() knows to work only with the base Figure class.
 */

void print_area(Figure *f){
	printf("%-30s %d\n", f->_classname, f->area(f));
}


/*
 * main() function
 */

int main(){
	/* Create 10 x 15 rectangle */
	Rectangle *rect = _newRectangle(10, 15);
	/* Create 10 x 10 square */
	Square    *sq   = _newSquare(10);
	/* Create 5R circle */
	Circle    *circ = _newCircle(5);

	/* print the area */
	print_area((Figure *) rect);
	print_area((Figure *) sq);
	print_area((Figure *) circ);

	printf("Circle->areaFl = %.4f\n", circ->areaFl(circ));

	/*
	 * free the resources,
	 * note did is not the same as destructor.
	 * destructor must be implemented in a way similar to the constructor.
	 */

	free(rect);
	free(sq);
	free(circ);

	return 0;
}