Poor Man's Polymorphism in C

polymorphism.c

/**
 *
 * Poor mans polymorphism
 *
 * c0der78 (www.arg3.com)
 */
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

typedef struct vehicle Vehicle;
typedef struct car Car;
typedef struct plane Plane;

/**
 * The "Base" structure
 * sub structures must implement the exact type and order to cast it
 */
struct vehicle
{
	int key;
	void (*onStart)(Vehicle *);
};

/**
 * Has the same field order as Vehicle plus additional info
 */
struct car
{
	int key;
	void (*onStart)(Vehicle *);

	int doors;
	int tires;
};

/**
 * Has the same field order as Vehicle plus additional info
 */
struct plane
{
	int key;
	void (*onStart)(Vehicle *);

	int engines;
};

/**
 * This handles common code between a Car and a Plane
 */
void start_vehicle(Vehicle *this, int key, const char *vehicleName, const char *keyName)
{
        // check for the right key
	if(this->key == key) {
		
		printf("%s fits %s\n", keyName, vehicleName);
		
		this->onStart(this);
	}
	else
	{
		printf("%s does not fit %s\n", keyName, vehicleName);
	}

}

/**
 * callback when a car is started
 */
void car_started(Vehicle *this)
{
	// cast back to a car
	Car *car = (Car*) this;

	printf("the car has %d doors and %d tires\n", car->doors, car->tires);
}

/**
 * callback when a plane is started
 */
void plane_started(Vehicle *this)
{
	// cast back to a plane
	Plane *plane = (Plane *) this;

	printf("the plane has %d engines\n", plane->engines);
}

/**
 * Output:
 *
 * key1 fits volkswagon
 * the car has 4 doors and 4 tires
 * key2 does not fit volkswagon
 * key1 does not fit b52
 * key2 fits b52
 * the plane has 6 engines
 */
int main(int argc, char *argv[])
{
	srand(time(0));

	int key1 = rand() % 10000 + 1;
	int key2 = rand() % 10000 + 1;

	// init a car
	Car volkswagon;
	volkswagon.key = key1;
	volkswagon.doors = 4;
	volkswagon.tires = 4;
	volkswagon.onStart = car_started;

	// init a plane
	Plane b52;
	b52.key = key2;
	b52.engines = 6;
	b52.onStart = plane_started;

	// test. this is really what the magic is all about.  We can cast a Car structure
	// to a Vehicle structure because the field type and order is the same
	start_vehicle((Vehicle*) &volkswagon, key1, "volkswagon", "key1");
	start_vehicle((Vehicle*) &volkswagon, key2, "volkswagon", "key2");

	// again we can cast a Plane structure to a Vehicle structure because
	// the field type and order is the same
	start_vehicle((Vehicle*) &b52, key1, "b52", "key1");
	start_vehicle((Vehicle*) &b52, key2, "b52", "key2");

	return 0;
}