Question

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

typedef struct dict_pair {
  void *key;
  void *value;
  struct dict_pair *tail;
} dict;


dict* NewDictionary(void) {
  dict *dictionary = malloc(sizeof(dict)); //or we can malloc(sizeof(struct dict_pair))
  dictionary->tail = NULL;
}

//dict operations
void put(dict *dictionary, void *key, void *value) {
  //new pair
  dict *new_pair = malloc(sizeof(dict));
  new_pair->key = key;
  new_pair->value = value;
  //chaining
  new_pair->tail = NULL;
  dict *last_node = dictionary;
  while (last_node->tail != NULL) {
    last_node = last_node->tail;
  }

  last_node->tail = new_pair;
}

void* get(dict *dictionary, void *key) {
  dict *current_dict = dictionary;
  while (1) {
    if (current_dict->key == key) {
      return current_dict->value;
    }
    else if (dictionary->tail != NULL) {
      current_dict = current_dict->tail;
    } else break;
  }
  return NULL;
}
//end operations

int main(void) {
  dict *dictionary = NewDictionary();
  put(dictionary,(void *) "buffer1",(void *) "Fake1");
  put(dictionary,(void *) "buffer2",(void *) "Fake2");
  put(dictionary,(void *) "key",(void *) "This is the value.");
  char *result = (char *) get(dictionary, (void *) "key");
  printf("%s\n",result);
}

So I managed to write the above code to implement a dictionary. While I was able to write the code and compile and get it to work expectedly, there are some stuff which I am not clear about. Mostly regarding pointers:

dict *current_dict = dictionary;

Lets take this line for example. We are declaring a variable which holds dict type. current_dict is a pointer, right? and dictionary is a pointer. However, *current_dict is not a pointer, how can it be assigned to a pointer?

Or do I have to explicitly type this to make it error?

dict (*current_dict) = dictionary;

If so, would this mean that the above line mean that we are declaring a current_dict variable with a dict type, and it is a pointer. Wouldnt that declaration be

(dict*) current_dict = dictionary;

As you can see, the spacing and positioning is confusing me.

Can someone help with explaining the difference in the * positioning?

Thanks!

Answer 1

While dict *dictionary and dict* dictionary have the same meaning in C, I prefer the former.

I prefer to think of pointer declarations in these terms:

int   x; //  x is an int
int  *y; // *y is an int
int **z; //**z is an int

If you remember that *y is the object that y points to, then it follows that y must be a pointer-to-an-int. And similarly z must be a pointer-to-a-pointer-to-an-int.