Pointer to array with const qualifier in C & C++

Question

Consider following program:

int main()
{
    int array[9];
    const int (*p2)[9] = &array;
}

It compiles fine in C++ (See live demo he

Answer 1

GCC-gnu

In GNU C, pointers to arrays with qualifiers work similar to pointers to other qualified types. For example, a value of type int (*)[5] can be used to initialize a variable of type const int (*)[5]. These types are incompatible in ISO C because the const qualifier is formally attached to the element type of the array and not the array itself.

C standard says that (section: §6.7.3/9):

If the specification of an array type includes any type qualifiers, the element type is so- qualified, not the array type.[...]

Now look at the C++ standard (section § 3.9.3/5):

[...] Cv-qualifiers applied to an array type attach to the underlying element type, so the notation “cv T,” where T is an array type, refers to an array whose elements are so-qualified. An array type whose elements are cv-qualified is also considered to have the same cv-qualifications as its elements. [ Example:

 typedef char CA[5];
 typedef const char CC;
 CC arr1[5] = { 0 };
 const CA arr2 = { 0 };

The type of both arr1 and arr2 is “array of 5 const char,” and the array type is considered to be const- qualified. —endexample]

Therefore, the initialization

const int (*p2)[9] = &array;  

is assignment of type pointer to array[9] of int to pointer to array[9] of const int. This is not similar to assigning int * to a const int * where const is applied directly to the object type the pointer points to. This is not the case with const int(*)[9] where, in C, const is applied to the elements of the array object instead of the object the pointer points to. This makes the above initialization incompatible.

This rule is changed in C++. As const is applied to array object itself, the assignment is between same types pointer to const array[9] of int instead of type pointer to array[9] of int and pointer to array[9] of const int.