Generic list manipulation function in C?

Question

What is a generic list manipulation function in C? (I saw this when I was going through some materials.)

What is the difference between this function and a function which can accept elements of any kind?

Are they same...? How can we implement them individually if they are not same?

Answer 1

C has no concept of "generic" pointers or objects - the closest you can get is using a void* pointer. If you want one piece of code to be able to handle any data type, you pretty much have to use void* pointers. For data types with sizes no larger than a pointer, you can cast between the type and void*; for larger data types, you'll have to use dynamic memory and have the void* member point to the dynamic memory. Just watch out for memory leaks!

typedef struct list_node {
  struct list_node *next;
  void *data;
} list_node;

void list_insert(list_node *node, void *data) {
  // ...
}

On the other hand, if you want to generate code for each possible data type, you'll have to do it with macros, and then instantiate the macros for each data type you might use. For example:

#define DEFINE_LIST(type) \
  typedef struct list_node_##type { \
    struct list_node_##type *next; \
    type data; \
  }

#define IMPLEMENT_LIST_INSERT(type) \
  void list_##type##_insert(list_node_##type *node, type data) { \
    ... \
  }

DEFINE_LIST(int);     // defines struct list_node_int
DEFINE_LIST(double);  // defines struct list_node_double
IMPLEMENT_LIST_INSERT(int);    // defines list_int_insert
IMPLEMENT_LIST_INSERT(double); // defines list_double_insert