8.6.3. Constant Pointer to Nonconstant Data

A constant pointer to nonconstant data is a pointer that always points to the same memory location, and the data at that location can be modified through the pointer. Pointers that are declared const must be initialized when they’re declared, but if the pointer is a function parameter, it’s initialized with the pointer that’s passed to the function.

The program of Fig. 8.11 attempts to modify a constant pointer. Line 11 declares pointer ptr to be of type int * const. The declaration is read from right to left as “ptr is a constant pointer to a nonconstant integer.” The pointer is initialized with the address of integer variable x. Line 14 attempts to assign the address of y to ptr, but the compiler generates an error message. No error occurs when line 13 assigns the value 7 to *ptr—the nonconstant value to which ptr points can be modified using the dereferenced ptr, even though ptr itself has been declared const.