C中的双指针const正确性警告

Question

A pointer to non-const data can be implicitly converted to a pointer to const data of the same type:

int       *x = NULL;
int const *y = x;

Adding additional const qualifiers to match the additional indirection should logically work the same way:

int       *      *x = NULL;
int       *const *y = x; /* okay */
int const *const *z = y; /* warning */

Compiling this with GCC or Clang with the -Wall flag, however, results in the following warning:

test.c:4:23: warning: initializing 'int const *const *' with an expression of type
      'int *const *' discards qualifiers in nested pointer types
    int const *const *z = y; /* warning */
                      ^   ~

Why does adding an additional const qualifier "discard qualifiers in nested pointer types"?

Answer 1

The reason why const can only be added one level deep is subtle, and is explained by Question 11.10 in the comp.lang.c FAQ .

Briefly, consider this example closely related to yours:

const int i;
int *p;
int const **z = &p;
*z = &i;
/* Now p points to i */

C avoids this problem by only allowing assignment to discard qualifiers at the first pointed-to level (so the assignment to z here is not allowed).

Your exact example does not suffer from this problem, because the const the second level means that the assignment to *z would not be allowed anyway. C++ would allow it in this exact case, but C's simpler rules do not distinguish between your case and the example above.

Answer 2

The FAQ entry linked by the other answer explains why the following code is not permitted:

int **x = whatever;
const int **z = x;

However, your code const int *const *z = x; is quite different, and it does not suffer from the same flaw raised by the FAQ.

In fact, there is conceptually nothing wrong with the latter code. It is just a flaw in the C specification that it is not permitted, and it forces C programmers to include ugly casts in their code.

It would have been possible for C to use the same rules that C++ did; however the C standard committee didn't decide to do that.