what does this declaration mean?

[franco ziade]

int const*const* p;

a const pointer to a constant pointer to an int?
this means that only the value of these pointers is variable however both
pointers cannot change where they point to? why would someone declare such a
pointer?

Thanks

 

[Andrey Tarasevich]

int const*const* p;

a const pointer to a constant pointer to an int?

No. A non-const pointer to a const pointer to a const int.

this means that only the value of these pointers is variable however both
pointers cannot change where they point to?

No. This means that the value of 'p' _can_ change. However, the value of
'*p' (also a pointer) and the value of '**p' (an int) cannot be changed
through 'p' (unless an explicit cast is used to change away the constness).

why would someone declare such a
pointer?

Why not?

 

[E. Robert Tisdale]

int const*const* p;

a const pointer to a constant pointer to an int?

No. A pointer to a const pointer to a const int.
It means the same thing as

const int *const *p;

This means that only the value of these pointers is variable.

Only the value of p is variable. Not *p or *(*p).

However, both pointers cannot change where they point to?

No. You *can* change p but *not* *p.

Why would someone declare such a pointer?

To ensure that neither *p or *(*p) are ever changed.

> cat main.c

#include <stdio.h>

int main(int argc, char* argv[]) {
int i = 13;
int j = 42;
const
int *const p = &i;
const
int *const q = &j;
const
int *const *pp = &p;
pp = &q; // OK
*pp = p; // error
*(*pp) = 666; // error
return 0;
}

> gcc -Wall -std=c99 -pedantic -o main main.c

main.c: In function `main':
main.c:13: error: assignment of read-only location
main.c:14: error: assignment of read-only location

 

[Luke Wu]

No. A non-const pointer to a const pointer to a const int.


No. This means that the value of 'p' _can_ change. However, the value of
'*p' (also a pointer) and the value of '**p' (an int) cannot be changed
through 'p' (unless an explicit cast is used to change away the constness).

But if you cast away the constness (and the object the pointer points
it was initially defined as consst) and you derefernce and modify the
object, you are invoking undefined behaviour.

 

[Andrey Tarasevich]

But if you cast away the constness (and the object the pointer points
it was initially defined as consst) and you derefernce and modify the
object, you are invoking undefined behaviour.
...

That's true. But that's outside the scope of the original question. The
object might be const, the object might not be const.