Using * instead of -> operator in c

Question

I created a very simple program of linked lists in C.

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

int main(){
    struct Int{
        int num;
        struct Int *ptr;
    };
    typedef struct Int NODE;
    NODE *start;
    start = (NODE *) malloc(sizeof(NODE));
    (*start).num = 100;
    (*start).ptr = (NODE *) malloc(sizeof(NODE));

    (*start).(*ptr).num = 123;
    (*start).(*ptr).ptr = NULL;

}

When I replace the last two lines as :-

start -> ptr -> num = 123;
start -> ptr -> ptr = NULL;

The error solved.

The question is that why I can't use (* start). instead of start -> .According to this answer What does this mean "->"? both of them are same.

It would be helpful to include in the question what the "error" is. — Paul Hankin, Jun 04 '16 at 08:23
**Which** error, you didn't have the compiler error in your code. — Antti Haapala -- Слава Україні, Jun 04 '16 at 08:23

Mike · Answer 1 · 2016-06-04T08:42:52.947

11

you should write the last two lines as:

(*(*start).ptr).num = 123;
(*(*start).ptr).ptr = NULL;

because (*ptr) is not a member of (*start), you should access ptr then dereference the whole expression.

edited Jun 04 '16 at 08:42

answered Jun 04 '16 at 08:23

Mike

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score 4 · Answer 2 · answered Jun 04 '16 at 08:24

*ptr is not a member of struct Int - ptr is, so you can't have *ptr following the . operator.

If you absolutely have to use * and dereference your pointers, you need to treat the entire (*strart).ptr expression as a pointer:

(*(*start).ptr).num = 123;
(*(*start).ptr).ptr = NULL;

score 2 · Answer 3 · answered Jun 04 '16 at 08:26

N3337 5.2.5/1,2

A postﬁx expression followed by a dot . or an arrow ->, optionally followed by the keyword template (14.2), and then followed by an id-expression, is a postﬁx expression.[...]

In either case, the id-expression shall name a member of the class or of one of its base classes.[...]

So dot has to be followed by name of the class member. You can't even write:

A a;
a.(some_member);//note the parentheses

Using * instead of -> operator in c

3 Answers3