confusion on structure access

[thinhare]

Look at the comments please.

#include <stdio.h>

#define MAXLEN 20

struct proto_type {
        int i;
        char s[MAXLEN];
};

main() {
        struct proto_type stru_1, *p;
        int i;

        p = &stru_1;

        for (i = 0; i < MAXLEN-1; i++)
                (&stru_1)->s[i] = '0';
        (&stru_1)->s[i] = '\0';         /* necessary when using (&stru_1)
                                           to access a member in a structure */
        printf("before:\n%s\n", (&stru_1)->s);

        for (i = 0; i < MAXLEN-1; i++)
                p->s[i] = '0';
        /* p->s[i] = '\0'; */           /* unnecessary when using pointer 'p'
                                           to access a member in a structure */
        printf("after:\n%s\n", (&stru_1)->s);
}

[quzah]

No. You've got it wrong. You don't use the address-of operator to dereference (ie: access). And if you did, you'd use the dot operator, not the arrow operator.

struct foo instance, *pointer;
pointer = &instance;

pointer->x = 10; /* assign the value of 10 to the member variable x via a pointer*/
instance.x = 10; /* assign the value of 10 to the member variable x directly*/

(*pointer).x = 10; /* assign the value of 10 to the member variable x via a pointer*/

As you can see, there's no point to doing the latter, when you can just use the arrow. (That's what they made it for!)

Quzah.

[Salem]

> Quote:
> #define MAXLEN 20
Better than nothing, but please use the code tags, not the quote tags.

[thinhare]

> Quote:
> #define MAXLEN 20
Better than nothing, but please use the code tags, not the quote tags.

okay, I didn't notice the code button before, and I'll use it definitely afterwards.

[thinhare]

yes, exactly I do know what you mentioned about.
but my point is for the case I don't wanna define
one more variable of pointer by using "&" instead.
That might not be a good programming habit
but I'm just curious.

No. You've got it wrong. You don't use the address-of operator to dereference (ie: access). And if you did, you'd use the dot operator, not the arrow operator.

As you can see, there's no point to doing the latter, when you can just use the arrow. (That's what they made it for!)

Quzah.

[quzah]

I don't understand. Why if you only want to use an instance, aren't you simply using the dot operator? What's your fascination with the arrow operator that you feel you must use it?

Quzah.

[thinhare]

it's not that I MUST use it but for curiosity.
"&instance" and "pointer to instance" do the same thing, don't they?
could you explain what cause the difference in the code above?
Thanks!

I don't understand. Why if you only want to use an instance, aren't you simply using the dot operator? What's your fascination with the arrow operator that you feel you must use it?

Quzah.

[vim]

I think they are same,
What difference are they?

[Kleid-0]

I see what you're saying:

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

struct proto_type {
	char c;
};

int main(void) {
	struct proto_type stru_1, *p;
	p = &stru_1;
	
	p->c = 'p';

	// We get a segment fault here...
	//putchar((&stru_1)->c); <-- The -> seems to only work with "real" pointers...
	puts(NULL);

	return 0;
}

But I don't think it's possible... :(

[Salem]

Nothing wrong with the putchar - that works fine

It's the puts(NULL) which gets a segfault.