双链表2

Question

I did a lot of linked lists already, but I didn't use them for a while and didn't really programm anything, therefore I am litterally lost. The following is my Code:

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

typedef struct
{
    int age; 
    int pers_num;
    char name[100];
    void *previous;
    void *next;
    
}Person;

int main(){
    Person first_element;
    Person last_element;
    first_element.next = (Person *)(&last_element);
    last_element.age = 19;
    printf("%d\n",(&last_element)->age);
    printf("%d\n",(first_element.next)->age);
    
    }

Could you explain to me why following error is thrown?

speicher.c: In function 'main':
speicher.c:22:39: warning: dereferencing 'void *' pointer
   23 |     printf("%d\n",(first_element.next)->age);
      |                                       ^~
speicher.c:23:39: error: request for member 'age' in something not a structure or union

As I understand it "first_element.next" should be the pointer which points at last_element. therefore you should be able to use the -> to access the data inside last_element. The line 22 Runs perfectly even thought it should have the same Output as line 23, where the error is thrown.

Answer 1

You can't dereference a void pointer because it's a pointer that references a void type:-) In other words, it doesn't really know what actual type it points at. The reason why the two lines behave differently are, keeping in mind that a->b is just syntactic sugar for (*a).b :

printf("%d\n",(&last)->age);
    // This (&last)->age, which is actually the same
    // as last.age. In both cases, the type being used
    // to get the age field is Person, so no problem.

printf("%d\n",(first.next)->age);
    // This (first.next)->age is the same as
    // (*(first.next)).age. Because first.next is of type
    // void *, it cannot be dereferenced.

What you are doing is akin to declaring a variable with void x , which is not valid (don't confuse that with void *x which is valid).

You would be better off (within the structure) pointing to the actual type you want, with something like:

typedef struct s_Person { // A
    int age; 
    int pers_num;
    char name[100];
    struct s_Person *previous, *next;
} Person; // B

Note that you cannot use the Person type definition while you're creating it, since it does not yet exist. Simplistically, you can think of it coming into existence at the B point. The named structure s_Person comes into existence at A so it can be used within the structure.