Manual implementation of virtual dispatch and virtual tables in C.

virt.c

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

typedef struct Animal {
	void (*say)(void *this);
} Animal;

typedef struct Cat {
	Animal *vptr;
	char *name;
	int age;
} Cat;

typedef struct Dog {
	Animal *vptr;
	char *name;
	char *nickname;
} Dog;

void cat_say(Cat *this) {
	printf("I'm cat %s and im %d yrs old\n", this->name, this->age);
}

void dog_say(Dog *this) {
	printf("I'm dog %s but they call me %s\n", this->name, this->nickname);
}

Animal cat_vtable = { .say = (void*)cat_say };
Animal dog_vtable = { .say = (void*)dog_say };

void cat_init(Cat *this, char* name, int age) {
	this->vptr = &cat_vtable;
	this->name = name;
	this->age = age;
}

void dog_init(Dog *this, char *name, char *nickname) {
	this->vptr = &dog_vtable;
	this->name = name;
	this->nickname = nickname;
}

int main() {
	Cat *c = (Cat*)malloc(sizeof(Cat));
	Dog *d = (Dog*)malloc(sizeof(Dog));

	cat_init(c, "Bella", 2);
	dog_init(d, "Jasper", "Jas");

	Animal** animals[2] = { (Animal**)c, (Animal**)d };

	for (int i = 0; i < 2; i++) {
		Animal **a = animals[i];
		(*a)->say(a);
	}

	return 0;
}