How to cascade -> operator?

Question

I have the following:

typedef struct Node_struct
{
    int number;
    Node *nextNode;
    Node *prevNode;
} Node;

and later

Node *nodeInSet = lookup(set, number);
nodeInSet->nextNode = (Node *) malloc(sizeof(Node));
nodeInSet->nextNode->prevNode

the last line is saying: "expression must have pointer-to-class type". I can't see how my code is wrong. Since nodeInSet->nextNode is a Node *, I think I should be allowed to write nodeInSet->nextNode->prevNode. What is not working here?

I also suspect that you are using the wrong compiler otherwise [Do not cast the return value from `malloc()`](http://stackoverflow.com/a/605858/1983495). You are allowed to write that, but you really shouldn't. From the sample code it's not possible to say why is the error happening, can you post a reproducible sample? Also, wouldn't `nodeInSet->nextNode->prevNode` be `nodeInSet`? if the list is correctly populated. — Iharob Al Asimi, May 17 '15 at 15:00
inside `Node_struct` the `typedef Node` is not yet defined, so you should declare `struct Node_struct *nextNode` — bolov, May 17 '15 at 15:03
a struct definition should not be typedef'd. typedef'ing clutters the code, leads to mis-understandings, adds no value, and clutters the compiler name space. Rather always use: 'struct Node_struct' — user3629249, May 18 '15 at 15:38

score 6 · Accepted Answer · answered May 17 '15 at 15:05

The prevNode and nextNode members have incomplete type, you have to write it like this

typedef struct Node_struct
{
    int number;
    struct Node_struct *nextNode;
    struct Node_struct *prevNode;
} Node;

or

typedef struct Node_struct Node;
struct Node_struct
{
    int number;
    Node *nextNode;
    Node *prevNode;
};

The reason is that you can declare a poniter to an incomplete type, but if you try to dereference the pointer, like when you use the -> operator, then the type must be known, because the size of the type is needed.

score 3 · Answer 2 · edited May 17 '15 at 15:07

3

You can change your code to this.

typedef struct Node_struct
{
    int number;
    struct Node_struct *nextNode;
    struct Node_struct *prevNode;
} Node;

edited May 17 '15 at 15:07

Iharob Al Asimi

52,653
6
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answered May 17 '15 at 15:06

Rishitha Minol

323
2
11

It's not optional, _you can change ..._ sounds as if it were. – Iharob Al Asimi May 17 '15 at 15:07

How to cascade -> operator?

2 Answers2