二进制操作数无效！=（具有结构和 void *）

Question

This piece of code is throwing up the error:

invalid operands to binary !=(have 'dataframe' {aka 'struct dataframe'} and 'void *'

      if (new->bucket[i] != NULL) {

Yet I don't understand why this error is occuring, as I am attempting to assess whether the certain cell of my array is NULL or not.

My structs and the function that contains the error throwing code:

enum dfStatus {
   EMPTY = 2, FULL, REMOVED
};

typedef struct dataframe {
   void *key;
   void *data;
   enum dfStatus status;
} dataframe;

typedef struct assoc {
   dataframe *bucket;
   unsigned int buckCnt;
   unsigned int totalCnt;
   unsigned int multip;
   unsigned int keysize;
} assoc;

assoc* _assoc_resize(assoc* a)
{
   assoc *ass = a, *new;
   int size = ass->buckCnt, i, multi = ass->multip * 2;
   dataframe *df = ncalloc(size * 2, sizeof(dataframe));

   for (i = 0; i < size * SCALEFACTOR; i++) {
      df[i].status = EMPTY;
   }

   new = ncalloc(1, sizeof(assoc));
   new->multip = 1;
   new->bucket = df;
   new->buckCnt = size * 2;
   new->totalCnt = 0;

   for (i = 0; i < size; i++) {
      if (new->bucket[i] != NULL) {
         new->bucket[i / multi] = ass->bucket[i];
      }
   }
   new->totalCnt = ass->totalCnt;
   new->multip = multi;
   free(ass->bucket);
   free(ass);

   a = new;

   return a;
}

Answer 1

NULL can be used only with pointers. Even if new->bucket is pointer, when you use new->bucket[i] it will return dataframe not pointer. You can try using double pointers, instead of saving new->bucket as dataframe* save it as dataframe** so it won't be 'array' of dataframe but 'array' of dataframe* .