vtable example in C

vtable.c

#include <stdio.h>

/* class definitions */
typedef struct Base
{
    void (**vtable)();
    int _x;
} Base;

typedef struct Child
{
    void (**vtable)();
/* begin base class slice */
    int _x;
/* end base class slice */
    int _y;
} Child;

/* class method implementations */
void Base_ToString(Base const* obj) { printf("Base: (%d)\n", obj->_x); }
void Child_ToString(Child const* obj) { printf("Base: (%d,%d)\n", obj->_x, obj->_y); }

/* soley method pointer */
void (*fn_ptr)();

/* vtable implementation */
enum { Call_ToString };
void (*Base_Vtable[])() = { Base_ToString };
void (*Child_Vtable[])() = { [Call_ToString] = Child_ToString }; /* gcc designated inits */

/* virtual method implementation */
void ToString(Base const* obj)
{
    obj->vtable[Call_ToString](obj);
}

int main()
{
    /* pick the vtable for objects at compile time */
    Base base = {Base_Vtable, 123};
    Child child = {Child_Vtable, 456, 789};
    Child object = {NULL, 12, 14};
    Base* a = &base;
    Base* b = (Base*)&child;
    Base* c = (Base*)&object;
    
    /* call the virtual methods */
    ToString(a);
    ToString(b);
    
    fn_ptr = Child_ToString;
    fn_ptr(c);
    
   return 0;
}