pointer to const and const pointer to something

Question

what is the difference between

const int d=1;
const int *p = &d;

and

const int d=1;
int const *p = &d;

What can I do with the former and the latter ?

Answer 1

const int *p;

p is a pointer to a constant integer. You can change the value stored in p (thus it point somewhere else) but you can't change the value where p points to.

int* const p;

p is a constant pointer to a non constant integer. You can't change the value of p, but change the integer where it points to

Answer 2

const int *p;

The declaration above declares a pointer p that points to a constant int. In other words, you can't change the value of the referand but you can change p itself.

int* const p;

The declaration above declares a constant pointer p that points to an int. In other words, you can change the value of the referand, but you can't change p. Also,

const int d = 1;
int * const p = &d;

is not legal. Taking the address of d yields const int*, and coversion from const int* to int* is not allowed (you could inadvertedly change the value of a constant object if it were).

You could make the conversion explicitly by using const_cast:

const int d = 1;
int * const p = const_cast<int*>(&d);

but it would still be illegal (undefined behaviour) to change the value of d through p.

Answer 3

There is no difference, they're completely identical.

The grammar of the language simply allows a certain amount of freedom for certain constructions, and CV-qualification of types is one of those situations. There are other examples (e.g. declarations like foo typedef int;).