Cannot initialize a variable of type 'int *const' with an rvalue of type 'const int *'

Why I have the error for the following code:

const int r = 3;
int *const ptr = &r;

However it works normaly if I define r as a plain int. As I understand, the second line only defines the pointer ptr as a const, which means that the value of this pointer cannot be changed. But why I a const pointer cannot point to a const int?

>Solution :

The clockwise/spiral rule says that the definition

int *const ptr = &r;

makes ptr a constant pointer to non-constant int.

That is, while the variable ptr itself can’t be modified (you can’t assign to ptr), what it points to can. And that doesn’t match the type of r which is constant.

If you want a constant pointer to a constant int you need:

int const* const ptr = &r;

Now neither the variable ptr can be modified, nor the data it points to.