Difference in incomplete array pointer conversion rules between C and C++

Question

When I compile the following code with gcc and g++ , g++ gives error and not gcc . Note that the code converts from int (*)[4] to int (*)[] (which is pointer to incomplete array type).

int arr[4];
int (*p_arr)[] = &arr;

As discussed in Incomplete array type? , C language allows this conversion. But why does C++ disallow this and gives the error error: cannot convert 'int (*)[4]' to 'int (*)[]' in assignment . I know C++ is more type-safe than C, but does this assignment is really type-unsafe, because the later dereference of the pointer (eg sizeof(*p_arr) ) anyway gives error in C as well?

Answer 1

The conversion by itself is safe, but keep in mind that the same rule in C that allows this conversion also allows it in the opposite direction.

int main() {
  int array[4] = {0};
  int (*ptr4)[4] = &array;
  int (*ptrN)[]  = ptr4;
  int (*ptr5)[5] = ptrN; /* oh dear */
}

This is clearly bad. C++ has removed this rule, the rule that says int[4] and int[] are compatible types, and pretty much got rid of the concept of compatible type.

Some specific conversions that are safe are being considered for inclusion in a future version of C++ . They include your conversion as well.

Answer 2

Yes, this assignment does is really type-unsafe.

In C++, array types include the dimension. Always.

(Mostly.)