$ cat oo.c
#include <stdlib.h>
#include <stdio.h>

/* ******** Abstract Class Animal ******** */

#define Animal_TEMPLATE \
	void (*makeNoise)(void*); \
	void (*destroy)(void*);
struct Animal { Animal_TEMPLATE };
typedef struct Animal Animal;

void Animal_destroy(void* this) {
	free(this);
}

#define Animal_INIT \
	new->makeNoise = NULL; \
	new->destroy = Animal_destroy;

/* ******** Class Cat ******** */

#define Cat_TEMPLATE Animal_TEMPLATE
struct Cat { Cat_TEMPLATE };
typedef struct Cat Cat;

void Cat_makeNoise(void* this) {
	printf("Meow!\n");
}

#define Cat_INIT Animal_INIT \
	new->makeNoise = &Cat_makeNoise;

Cat* new_Cat() {
	Cat* new = malloc(sizeof(Cat));
	Cat_INIT;
	return new;
}

/* ******** Class Dog ******** */

#define Dog_TEMPLATE Animal_TEMPLATE
struct Dog { Dog_TEMPLATE };
typedef struct Dog Dog;

void Dog_makeNoise(void* this) {
	printf("Woof!\n");
}

#define Dog_INIT Animal_INIT \
new->makeNoise = &Dog_makeNoise;

Dog* new_Dog() {
	Dog* new = malloc(sizeof(Dog));
	Dog_INIT;
	return new;
}

/* ******** Main ******** */

void poke(Animal* a) {
	printf("Poking Animal:\n");
	a->makeNoise(a);
}

int main() {
	Animal* a = (Animal*) new_Cat();
	Animal* b = (Animal*) new_Dog();

	poke(a);
	poke(b);

	a->destroy(a);
	b->destroy(b);

	return 0;
}
$ gcc -o oo oo.c
$ valgrind ./oo
==13725== Memcheck, a memory error detector
==13725== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==13725== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==13725== Command: ./oo
==13725== 
Poking Animal:
Meow!
Poking Animal:
Woof!
==13725== 
==13725== HEAP SUMMARY:
==13725==     in use at exit: 0 bytes in 0 blocks
==13725==   total heap usage: 2 allocs, 2 frees, 32 bytes allocated
==13725== 
==13725== All heap blocks were freed -- no leaks are possible
==13725== 
==13725== For counts of detected and suppressed errors, rerun with: -v
==13725== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 6)
$