Pointer to constant in C

Question

Im having a problem with the pointer to constant. Here are my code:

#include <stdio.h>

typedef struct _duong                   
{
    int tuoi;
    int chieucao;
}duong;             

struct _duong DUONG;

typedef struct _duong *pduong;

const pduong p;

int main(void)
{
    p = &DUONG;
    p->tuoi = 25;
    p->chieucao = 165;
    printf("tuoi : %d\n",DUONG.tuoi);
    printf("chieucao : %d\n",DUONG.chieucao);
    return 1;
}

compiler throw an error: Cannot assign to variable 'p' with const-qualified type 'const pduong'. I can see that the problem come from

const pduong p;

and

p = &DUONG;

for my understanding const pduong p is a pointer to constant so that we can not change the value of the pointed variable but we can change the address it point to. So can anyone help me why the compiler showed this error? Thank you.

https://stackoverflow.com/questions/2253738/c-typedef-interpretation-of-const-pointers — AnT stands with Russia, Jul 26 '18 at 14:59
i know the differences between the constant pointer and pointer to constant, but can you explain more specific for this case, why `const pduong p` here is a constant pointer? @AnT — Dương, Jul 26 '18 at 15:43
@Dương: Follow the link above. Or do a search here. The question has been explained, explained and overexplained many times on SO. Honestly, I'd say that it is you who have to explain first why you expected it to be a pointer to constant. Why really? It is hard to answer your question without understanding the source of your confusion. — AnT stands with Russia, Jul 26 '18 at 16:00

Ed Heal · Answer 1 · 2018-07-26T14:53:40.270

2

No the pointer itself is a constant.

I.e you cannot change the pointer. But whatever that pointer points to you can change.

To correct this change

const pduong p;

to

const pduong p = &DUONG;

And then drop the line

 p = &DUONG;

EDIT

Perhaps you want

const duong p_contents = {25, 165};
const pduong * const p = &p_contents;

edited Jul 26 '18 at 14:53

answered Jul 26 '18 at 14:45

Ed Heal

59,252
17
87
127

2

would be worth adding the syntax of a 'pointer to const DUONG` if that's what op really wanted to have – Ingo Leonhardt Jul 26 '18 at 14:48
Add `const pduong const p` as well? – Richard Chambers Jul 26 '18 at 14:49

score 1 · Accepted Answer · answered Jul 26 '18 at 15:51

And this is one of the reasons why I strongly recommend against typedef-ing pointer types - the const semantics apply to the pointer, not to what is being pointed to.

const pduong p;

means

struct _duong * const p; // p is a const pointer to non-const type

not

const struct _duong *p;  // p is a non-const pointer to const type

In other words, you are declaring p as a const pointer to struct _duong, not as a pointer to const struct _duong.

Personally, I would recommend you ditch the typedef altogether and declare the pointer explicitly:

const struct _duong *p;

Typedefs are great for abstracting away details, and typedef-ing pointer types is fine if the user doesn't have to be aware of the "pointer-ness" of the type. In this case, the user does have to be aware of p's "pointer-ness" to use it correctly, so its type should not be hidden behind a typedef.

score 0 · Answer 3 · answered Jul 26 '18 at 14:52

0

No, the address of a const pointer can't be changed. Only the value it is pointing to.

answered Jul 26 '18 at 14:52

Jakob Herk

157
11

Pointer to constant in C

3 Answers3