Question

I’m following LCTHW tutorial and I have a task to do.
This is the data structure:

typedef struct DArray {
    int end;
    int max; 
    size_t element_size;
    size_t expand_rate;
    void **contents; 
} DArray;

I have declared a typedef:

typedef int (*DArray_compare) (const void *a, const void *b);

When I create a sorting function, I pass to it a DArray_compare, the problem is that I can’t figure out how to do an example of this comparator.

I tried to do something like this:

int compare(const void *a, const void *b)
{
  int i = (int)*a;
  int k = (int)*b;
  printf("%d %d\n", i, k);
  return i - k;
}

But I get an error:

error: operand of type 'void' where arithmetic or pointer type is required int i = (int)*a;

The question is: without changing the struct and the typedef of the comparator, I want to create a comparator that compares int, how can I do it?

Answer 1

int i = *(int*)a;
// This one has more parens to make it really obvious what your intent is.
int k = *((int*)b);

The second line (k=) is easiest to explain cos of all the brackets. You can rewrite it as follows:

// Cast b from a pointer to a void into a pointer to an int.
int *X = (int*)b; 
// k = "what X is pointing to" or "the contents of X"
int k = *X;

edit:
I think ralu’s comment is suggesting you change all the void* to int* which is a much safer solution if you have that power.

typedef int (*DArray_compare) (const int *a, const int *b);

int compare(const int *a, const int *b)
{
    int i = *a;
    int k = *b;
    ...