Virtual table sketch, C (gcc)

Virtual table sketch

//gcc 5.4.0

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

#define DEBUG(...) printf(__VA_ARGS__);

#define alloc_stack(class) ((struct class *)((char[sizeof(struct class) + sizeof(struct vtable)]){0}))
#define alloc_heap(class)  ((struct class *)malloc(sizeof(struct class) + sizeof(struct vtable)))

#define access_vtable(class, instance) ((struct vtable *)(instance + sizeof(struct class)))

struct accumulator
{
    int a;
    int b;
};

struct vtable
{
    void (*set_a)(struct accumulator *instance, int a);
    int (*get_a)(struct accumulator *instance);
    
    void (*set_b)(struct accumulator *instance, int b);
    int (*get_b)(struct accumulator *instance);
    
    int (*sum)(struct accumulator *instance);
};

void set_a(struct accumulator *instance, int a)
{
    instance->a = a;
}

int get_a(struct accumulator *instance)
{
    return instance->a;
}

void set_b(struct accumulator *instance, int b)
{
    instance->b = b;
}

int get_b(struct accumulator *instance)
{
    return instance->b;
}

int sum(struct accumulator *instance)
{
    return instance->a + instance->b;
}

void accumulator_constructor(struct accumulator *instance)
{
	instance->a = 0;
	instance->b = 0;

struct vtable *methods = (struct vtable *)(instance + sizeof(struct accumulator));
	methods->set_a = set_a;
	methods->get_a = get_a;
	methods->set_b = set_b;
	methods->get_b = get_b;
	methods->sum = sum;
}

int main(void)
{
    printf("Memmory allocation... ");
    struct accumulator *instance = alloc_heap(accumulator);
    printf("instance at %p\r\n", instance);
    
    printf("Constructor\r\n");
	accumulator_constructor(instance);

printf("Setters\r\n");
    struct vtable *methods = access_vtable(accumulator, instance);
	access_vtable(accumulator, instance)->set_a(instance, 1);
	access_vtable(accumulator, instance)->set_b(instance, 5);

printf("Getters\r\n");
	printf("a = %d\r\n", access_vtable(accumulator, instance)->get_a(instance));
	printf("b = %d\r\n", access_vtable(accumulator, instance)->get_b(instance));
    printf("a + b = %d\r\n", access_vtable(accumulator, instance)->sum(instance));
    return 0;
}


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