self referential struct definition?

Question

I haven't been writing C for very long, and so I'm not sure about how I should go about doing these sorts of recursive things... I would like each cell to contain another cell, but I get an error along the lines of "field 'child' has incomplete type". What's up?

typedef struct Cell {
  int isParent;
  Cell child;
} Cell;

Answer 1

There is sort of a way around this:

struct Cell {
  bool isParent;
  struct Cell* child;
};

struct Cell;
typedef struct Cell Cell;

If you declare it like this, it properly tells the compiler that struct Cell and plain-ol'-cell are the same. So you can use Cell just like normal. Still have to use struct Cell inside of the initial declaration itself though.

Answer 2

A Structure which contain a reference to itself. A common occurrence of this in a structure which describes a node for a link list. Each node needs a reference to the next node in the chain.

struct node
{
       int data;
       struct node *next; // <-self reference
};

Answer 3

Another convenient method is to pre-typedef the structure with,structure tag as:

//declare new type 'Node', as same as struct tag
typedef struct Node Node;
//struct with structure tag 'Node'
struct Node
{
int data;
//pointer to structure with custom type as same as struct tag
Node *nextNode;
};
//another pointer of custom type 'Node', same as struct tag
Node *node;

Answer 4

In C, you cannot reference the typedef that you're creating withing the structure itself. You have to use the structure name, as in the following test program:

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

typedef struct Cell {
  int cellSeq;
  struct Cell* next; /* 'tCell *next' will not work here */
} tCell;

int main(void) {
    int i;
    tCell *curr;
    tCell *first;
    tCell *last;

    /* Construct linked list, 100 down to 80. */

    first = malloc (sizeof (tCell));
    last = first;
    first->cellSeq = 100;
    first->next = NULL;
    for (i = 0; i < 20; i++) {
        curr = malloc (sizeof (tCell));
        curr->cellSeq = last->cellSeq - 1;
        curr->next = NULL;
        last->next = curr;
        last = curr;
    }

    /* Walk the list, printing sequence numbers. */

    curr = first;
    while (curr != NULL) {
        printf ("Sequence = %d\n", curr->cellSeq);
        curr = curr->next;
    }

    return 0;
}

Although it's probably a lot more complicated than this in the standard, you can think of it as the compiler knowing about struct Cell on the first line of the typedef but not knowing about tCell until the last line :-) That's how I remember that rule.

Answer 5

Lets go through basic definition of typedef. typedef use to define an alias to an existing data type either it is user defined or inbuilt.

typedef <data_type> <alias>;

for example

typedef int scores;

scores team1 = 99;

Confusion here is with the self referential structure, due to a member of same data type which is not define earlier. So In standard way you can write your code as :-

//View 1
typedef struct{ bool isParent; struct Cell* child;} Cell;

//View 2
typedef struct{
  bool isParent;
  struct Cell* child;
} Cell;

//Other Available ways, define stucture and create typedef
struct Cell {
  bool isParent;
  struct Cell* child;
};

typedef struct Cell Cell;

But last option increase some extra lines and words with usually we don't want to do (we are so lazy you know ;) ) . So prefer View 2.

Answer 6

From the theoretical point of view, Languages can only support self-referential structures not self-inclusive structures.