Polymorphic behaviour in C

duck.c

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

//
typedef struct Duck Duck;
typedef void (*quack_t)(void *self);
void Duck_quack(void *self);

// Overriding original structure members means you must include everything up.
// So if I want to override `quack`, I dont need to override anything
// else. If I want to override `quacked`, I must override `quack` also.
struct Duck {
	quack_t quack;
	int quacked;
};

Duck *Duck_create()
{
	Duck *d = malloc(sizeof(Duck));
	d->quack = Duck_quack;
	d->quacked = 0;
	return d;
}

void Duck_destroy(Duck *duck)
{
	free(duck);
}

void Duck_quack(void *self)
{
	Duck *d = self;
	if(d->quacked) {
		printf("already quacked\n");
	} else {
		printf("quack\n");
	}
	d->quacked = 1;
}

//
typedef struct Bird Bird;
void Bird_quack(void *self);

struct Bird {
	// Overrides have to align in memory so this comes first
	quack_t quack;

	// Specific to bird
	Duck *duck; // we keep this to track quacked without overriding
	int a;
};

Bird *Bird_create()
{
	Bird *b = malloc(sizeof(Bird));
	b->quack = Bird_quack;
	Duck *d = malloc(sizeof(Duck));
	b->duck = d;
	b->a = 0;
	return b;
}

void Bird_destroy(Bird *b)
{
	free(b->duck);
	free(b);
}

void Bird_quack(void *self)
{
	Bird *b = self;

	// just some sillyness on Bird alloc
	if(b->duck->quacked) {
		b->a += 20;
	} else {
		b->a++;
	}

	// here we have to account for how the super method (basically)
	// also edits `quacked` but we didn't override it. So we pass
	// in our struct Duck alloc so we can call the original method
	// and have `quacked` available for use (essentially public).
	Duck_quack(b->duck);

	// can't do this since addition of `quacked` and I intentionally
	// didn't track that in struct Bird
	//Duck_quack(self);

	// show progression of `a`
	printf("a: %d\n", b->a);
}

//
void quack_it(void *obj)
{
	Duck *d = obj;
	d->quack(d);
}

int main(int argc, char *argv[])
{
	Bird *b = Bird_create();
	quack_it(b);
	quack_it(b);
	Duck *d = Duck_create();
	quack_it(d);
	Duck_destroy(d);
	Bird_destroy(b);
	return 0;
}