Question

I want to create a hash table that relies on an independent vector data structure in C99. I can do this in C++ with the help of OO, but I’m unsure how to approach this using structs and unions.

I would prefer that any linked examples do not include hash table implementations that have highly complex hashing functions. I do not particularly care about collisions or efficiency of storage. I just want either advice as to how to proceed or a simple example that exemplifies the form rather than function of the respective data structures.

Answer 1

If I infer correctly that you want to implement growing hash tables in a fully generic way, then you’ll need a lot of void pointers. A vector isn’t too hard, it just takes a lot of typing:

typedef struct {
    size_t capacity, nelems;
    void **contents;
} Vector;

enum { INITIAL_CAPACITY = 256 };

Vector *make_vector()
{
    Vector *v = malloc(sizeof(Vector));
    if (v == NULL)
        return NULL;
    v->capacity = INITIAL_CAPACITY;
    v->contents = malloc(sizeof(void *) * v->capacity);
    if (v->contents == NULL) {
        free(v);
        return NULL;
    }
    v->nelems = 0;
    return v;
}

// exercise for the reader
int vector_append(Vector *, void *);
void *vector_at(Vector const *);

Keep in mind that a generic hash function would have prototype size_t hash(void const *, size_t), i.e. you need to pass in the size.

(Side note: it’s not C++’s OOP features that you’re going to miss; it’s templates, the type safety that they buy, and syntactic sugar such as operator overloading. Take a look at OpenBSD’s ohash library for more examples.)