CODEWITHSUNDEEP
cstructtypedef
Kenny Cason
Kenny Cason

Reputation: 12328

How to define a typedef struct containing pointers to itself?

I am writing a LinkedList in C, the below code represent my Node definition.

typedef struct {
    int value;
    struct Node* next;
    struct Node* prev;
} Node;

I understand (or think that I do) that struct Node not the same as typedef struct Node. Granted my code compiles and runs as it's supposed to, however, I get a lot of warnings when assigning next and prev (warning: assignment from incompatible pointer type). I am guessing that this has to do with how I'm defining them in the Node structure. I uploaded the full source here

So, if that is indeed the problem, how should I define next and prev inside the typedef struct Node?

I was worried this may be a repost, but couldn't quite find what I was looking for. Thanks.

Upvotes: 53

Views: 63342

Answers (2)

Arun
Arun

Reputation: 20383

Another acceptable way and with the least change to OP's code is the following:

typedef struct NodeT {
    int value;
    struct NodeT * next;
    struct NodeT * prev;
} Node;

Note the introduction of NodeT and its usage in next and prev until Node is available.

Upvotes: 38

unwind
unwind

Reputation: 399833

You need to do it in this order:

typedef struct Node Node;

struct Node
{
  int value;
  Node *next;
  Node *prev;
};

That doesn't do exactly what you asked, but it solves the problem and is how this generally is done. I don't think there's a better way.

This kind of forward declaration has a second usage, in data hiding. If the list was implemented in a library, you could have just the typedef in the public header, along with functions like:

Node * list_new(void);
Node * list_append(Node *head, Node *new_tail);
size_t list_length(const Node *head);

This way, users of the library don't have easy access to the internals of your library, i.e. the fields of the Node structure.

Upvotes: 92

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