Linked lists when to use -> and when .?

Question

Let's say for a structure called Point with x, y and next; when do I write P.y and when P -> next because for this code:

typedef struct
{
    int x, y;
    struct Point *next;
} Point;

I cannot use P -> next nor can I try if (P == NULL).

Can I have combination of P.x and P -> next?
When must I use -> and when . ?

This question might be a duplicate. – Stefan May 03 '14 at 23:37 — Stefan, May 03 '14 at 23:37

score 3 · Accepted Answer · edited May 23 '17 at 10:32

3

If P has type Point, then you always use ..

 Point p;
 p.next;

If P has type Point*, then you always use ->.

 Point* p
 p->next;

If you really want to use . in the latter case, you have to deference first.

Point *p;
(*p).next;

Only in the latter case, it makes sense to check p == NULL, because a pointer can be NULL but a structure generally is not.

Since next is itself declared as a pointer, then you should use -> for following the pointer to its next pointer.

Point p;
p.next->next; // Will only work if p.next is NOT NULL.

Point* p;
p->next->next; // Will only work if p->next is NOT NULL.

Update: To remove the warning, change your declaration to the following.

typedef struct Point
{
    int x, y;
    struct Point *next;
} Point;

The problem is that the outside declaration is using the typedef and not the struct's tag, while the inside is using struct Point. See this question for details about why you get this behavior.

edited May 23 '17 at 10:32

Community

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answered May 03 '14 at 23:35

merlin2011

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but for *next I cant use -> next, it says invalid type – Mihai Bratulescu May 03 '14 at 23:36
@MihaiBratulescu, That is because `p->` is shorthand for `(*p).` – merlin2011 May 03 '14 at 23:37
@MihaiBratulescu, See the updates to my answer. – merlin2011 May 03 '14 at 23:41
why am I getting a warning for somePoint -> next = otherPoint when they are bouth Point * and next is struct Point *next ? – Mihai Bratulescu May 03 '14 at 23:57
@MihaiBratulescu, What does the warning say? – merlin2011 May 03 '14 at 23:58
warning: assignment from incompatible pointer type [enabled by default] current-> next = newPoint; – Mihai Bratulescu May 04 '14 at 00:01
@MihaiBratulescu, Please show the declaration of `newPoint`. – merlin2011 May 04 '14 at 00:02
Point *newPoint = (Point *) malloc ( sizeof(Point) ); newPoint -> next = NULL; – Mihai Bratulescu May 04 '14 at 00:03

score 3 · Answer 2 · answered May 03 '14 at 23:36

3

You use P->next when P is a pointer type Point*, and you use P.next when it's the actual struct of type Point.

answered May 03 '14 at 23:36

Leigh

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Linked lists when to use -> and when .?

2 Answers2