In a project I'm writing code for, I have a void pointer, "implementation", which is a member of a "Hash_map" struct, and points to an "Array_hash_map" struct. The concepts behind this project are not very realistic, but bear with me. The specifications of the project ask that I cast the void pointer "implementation" to an "Array_hash_map" before I can use it in any functions.

My question, specifically is, what do I do in the functions to cast the void pointers to the desired struct? Is there one statement at the top of each function that casts them or do I make the cast every time I use "implementation"?

Here are the typedefs the structs of a Hash_map and Array_hash_map as well as a couple functions making use of them.

typedef struct {
  Key_compare_fn key_compare_fn;
  Key_delete_fn key_delete_fn;
  Data_compare_fn data_compare_fn;
  Data_delete_fn data_delete_fn;
  void *implementation;
} Hash_map;

typedef struct Array_hash_map{
  struct Unit *array;
  int size;
  int capacity;
} Array_hash_map;

typedef struct Unit{
  Key key;
  Data data;
} Unit;

functions:

/* Sets the value parameter to the value associated with the
   key parameter in the Hash_map. */
int get(Hash_map *map, Key key, Data *value){
  int i;
  if (map == NULL || value == NULL)
    return 0;
  for (i = 0; i < map->implementation->size; i++){
    if (map->key_compare_fn(map->implementation->array[i].key, key) == 0){
      *value = map->implementation->array[i].data;
      return 1;
    }
  }
  return 0;
}

/* Returns the number of values that can be stored in the Hash_map, since it is
   represented by an array. */
int current_capacity(Hash_map map){
  return map.implementation->capacity;
}