2.4.2. Pointers and const

As with references, we can define pointers that point to either const or nonconst types. Like a reference to const, a pointer to const (§ 2.4.1, p. 61) may not be used to change the object to which the pointer points. We may store the address of a const object only in a pointer to const:

const double pi = 3.14;   // pi is const; its value may not be changed
double *ptr = π        // error: ptr is a plain pointer
const double *cptr = π // ok: cptr may point to a double that is const
*cptr = 42;               // error: cannot assign to *cptr

In § 2.3.2 (p. 52) we noted that there are two exceptions to the rule that the types of a pointer and the object to which it points must match. The first exception is that we can use a pointer to const to point to a nonconst object:

double dval = 3.14;       // dval is a double; its value can be changed
cptr = &dval;             // ok: but can't change dval through cptr

Like a reference to const, a pointer to const says nothing about whether the object to which the pointer points is const. Defining a pointer as a pointer to const affects only what we can do with the pointer. It is important to remember that there is no guarantee that an object pointed to by a pointer to const won’t change.

const Pointers

Unlike references, pointers are objects. Hence, as with any other object type, we can have a pointer that is itself const. Like any other const object, a const pointer must be initialized, and once initialized, its value (i.e., the address that it holds) may not be changed. We indicate that the pointer is const by putting the const after the *. This placement indicates that it is the pointer, not the pointed-to type, that is const:

int errNumb = 0;
int *const curErr = &errNumb;  // curErr will always point to errNumb
const double pi = 3.14159;
const double *const pip = π // pip is a const pointer to a const object

As we saw in § 2.3.3 (p. 58), the easiest way to understand these declarations is to read them from right to left. In this case, the symbol closest to curErr is const, which means that curErr itself will be a const object. The type of that object is formed from the rest of the declarator. The next symbol in the declarator is *, which means that curErr is a const pointer. Finally, the base type of the declaration completes the type of curErr, which is a const pointer to an object of type int. Similarly, pip is a const pointer to an object of type const double.

The fact that a pointer is itself const says nothing about whether we can use the pointer to change the underlying object. Whether we can change that object depends entirely on the type to which the pointer points. For example, pip is a const pointer to const. Neither the value of the object addressed by pip nor the address stored in pip can be changed. On the other hand, curErr addresses a plain, nonconst int. We can use curErr to change the value of errNumb:

*pip = 2.72;     // error: pip is a pointer to const
// if the object to which curErr points (i.e., errNumb) is nonzero
if (*curErr) {
    errorHandler();
    *curErr = 0; // ok: reset the value of the object to which curErr is bound
}