vtable.c

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

typedef struct _Animal { void (**vtable)(); } Animal;

typedef struct _Dog    { Animal base; } Dog;
typedef struct _Cat    { Animal base; } Cat;

// To strings
void animal_to_string(Animal *this) { printf(" Base animal\n"); }
void dog_to_string   (Dog *this)    { printf(" Dog\n");         }
void cat_to_string   (Cat *this)    { printf(" Cat\n");         }

// Actions
void animal_action(Animal *this) { printf(" ...\n");   }
void dog_action   (Dog *this)    { printf(" Bark!\n"); }
void cat_action   (Cat *this)    { printf(" Meow!\n"); }

enum { Call_To_String, Call_Action };

void (*Animal_Vtable[])() = { &animal_to_string, &animal_action };
void (*Dog_Vtable[])()    = { &dog_to_string, &dog_action       };
void (*Cat_Vtable[])()    = { &cat_to_string, &cat_action       };

void to_string(Animal *this) { this->vtable[Call_To_String](this); }
void action   (Animal *this) { this->vtable[Call_Action](this);    }

int main(int argc, char **argv)
{
    Animal a   = { Animal_Vtable };
    Dog    d   = { Dog_Vtable    };
    Cat    c   = { Cat_Vtable    };

    Animal *animal = (Animal *)&a;
    Animal *dog    = (Animal *)&d;
    Animal *cat    = (Animal *)&c;

    printf("\n To String\n");
    printf(" ----------\n");

    to_string(animal);
    to_string(dog);
    to_string(cat);

    printf("\n Actions\n");
    printf(" --------\n");

    action(animal);
    action(dog);
    action(cat);

    return 0;
}