oop.c

#pragma GCC diagnostic ignored "-Wincompatible-pointer-types"
#include <stdio.h>

// base declaration

struct vtable {
    int (*bar)(void *);
    int (*baz)(void *);
    void (*destructor)(void *);
};

struct base {
    struct vtable *vpointer;
    int x;
    int y;
};

void base_constructor(struct base *, int, int);
int base_foo(struct base *);
int base_bar(struct base *);
void base_destructor(struct base *);

extern int base_g;
int base_qux();

// base definition

struct vtable base_vtable = {
    .bar = &base_bar,
    .baz = 0,
    .destructor = &base_destructor
};

void base_constructor(struct base *this, int x, int y) 
{
    this->vpointer = &base_vtable;
    this->x = x;
    this->y = y;
}

int base_foo(struct base *this) 
{
    return this->x;
}

int base_bar(struct base *this) 
{
    return this->x + this->y;
}

void base_destructor(struct base *this) 
{}

int base_g = 0;

int base_qux()
{
    return base_g;
}

// derived declaration

struct derived {
    struct vtable *vpointer;
    int x;
    int y;
    int z;
};

void derived_constructor(struct derived *, int, int, int);
int derived_bar(struct derived *);
int derived_baz(struct derived *);
void derived_destructor(struct derived *);

// derived definition

struct vtable derived_vtable = {
    .bar = &derived_bar,
    .baz = &derived_baz,
    .destructor = &derived_destructor
};

void derived_constructor(struct derived *this, int x, int y, int z) 
{
    base_constructor(this, x, y);
    this->vpointer = &derived_vtable;
    this->z = z;
}

int derived_bar(struct derived *this) 
{
    return base_bar(this) + this->z;
}

int derived_baz(struct derived *this) 
{
    return this->z;    
}

void derived_destructor(struct derived *this)
{
    base_destructor(this);
}

int main() 
{
    //struct base b;
    //base_constructor(&b, 1, 2);

    struct derived d;
    derived_constructor(&d, 3, 4, 5);

    struct base *p = &d;

    printf("%i %i %i %i\n", base_foo(p), p->vpointer->bar(p), p->vpointer->baz(p), base_qux());

    p->vpointer->destructor(p);
}