vtable.c

// A small vtable example written in C.
// Enjoy :)

//// Interface definition ////

typedef struct {
    void (*feed)(void* self, float amount);
} FeedableTable;

typedef struct {
    FeedableTable* table;
    void* value;
} Feedable;

//// Cat implementation ////

typedef struct {
    int age;
    float hunger;
} Cat;

void Cat_feed(Cat* self, float amount) {
    self->hunger -= amount;
}

FeedableTable Cat_Feedable_table = (FeedableTable) {
    .feed = (void (*)(void*, float)) &Cat_feed
};

//// Dog implementation ////

typedef struct {
    unsigned long int playtime;
    double volume;
} Dog;

void Dog_feed(Dog* self, float amount) {
    self->volume -= amount / 2.0;
}

FeedableTable Dog_Feedable_table = (FeedableTable) {
    .feed = (void (*)(void*, float)) &Dog_feed
};

//// Usage ////

void feed_dynamic(Feedable f, float amount) {
    f.table->feed(f.value, amount);
}

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

int main(void) {
    srand(time(NULL));
    Cat my_cat = (Cat) { .age = 5, .hunger = 0.5 };
    Dog my_dog = (Dog) { .playtime = 1253, .volume = 2.0 };
    Feedable my_feedable = rand() % 2 == 0
        ? (Feedable) {
            .table = &Cat_Feedable_table,
            .value = &my_cat
        }
        : (Feedable) {
            .table = &Dog_Feedable_table,
            .value = &my_dog
        };
    feed_dynamic(my_feedable, 0.3);
    printf("my_cat.hunger = %f\n", my_cat.hunger); // prints '0.2' (if chosen) or '0.5'
    printf("my_dog.volume = %f\n", my_dog.volume); // prints '1.85' (if chosen) or '2.0'
    return 0;
}