8.6.4. Constant Pointer to Constant Data

The minimum access privilege is granted by a constant pointer to constant data. Such a pointer always points to the same memory location, and the data at that location cannot be modified via the pointer. This is how a built-in array should be passed to a function that only reads from the built-in array, using array subscript notation, and does not modify the built-in array. The program of Fig. 8.12 declares pointer variable ptr to be of type const int * const (line 13). This declaration is read from right to left as “ptr is a constant pointer to an integer constant.” The figure shows the Xcode LLVM compiler’s error messages that are generated when an attempt is made to modify the data to which ptr points (line 17) and when an attempt is made to modify the address stored in the pointer variable (line 18)—these show up on the lines of code with the errors in the Xcode text editor. In line 15, no errors occur when the program attempts to dereference ptr, or when the program attempts to output the value to which ptr points, because neither the pointer nor the data it points to is being modified in this statement.