Pointers and the const Qualifier

Pointers to const Objects

const double *cptr;   //cptr may point to a double that is const

Here cptr is a pointer to an object of type const double. The const qualifies the type of the object to which cptr points, not cptr itself. That is, cptr itself is not const. We need not initialize it and can assign anew value to it if we so desire. What we cannot do is use cptr to change the value to which it points:

*cptr = 42;      // error: *cptr might be const

It is also a compile-time error to assign the address of a const object to a plain, not const pointer:

const double pi = 3.14;
double *ptr=π                // error: ptr is a plain pointer, but pi is a const variable.
const double *cptr=π     // ok: cptr is a pointer to const

We cannot use a void* pointer to hold the address of a const object. Instead, we must use the type const void* to hold the address of a const object:

const int universe = 42;
const void *cpv = &universe;     // ok: cpv is const
void *pv = &universe;                // error: universe is const

A pointer to a const object can be assigned the address of a non const object, such as

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

Although dval is not a const, any attempt to modify its value through cptr result in a complie-time error. When we declared cptr, we said that it would not change the value to which it points. The fact that it happens to point to a non const object is irrelevant.

const Pointers

in addition to pointers to const, we can also have const pointer that is, pointers whose own value we may not change:

int errNumb=0;
int *const curErr = &errNumb;    // curErr is a constant pointer

curErr= curErr;                            // error: curErr is const

*curErr = 0;                               // ok: rest value of the object to which curErr is boud

const Pointer to a const Object

We can also define a constant pointer to a constant object as follows:

const double pi = 3.14159;
// pi_prt is const and points to a const object

const double *const pi_ptr = π

In this case, neither the value of the object addressed by pi_ptr nor the address itself can be changed. We read its definitions from right to left as "pi_ptr is a constants pointer to an object of type double defined as const"

Please compare and contrast these three form:

const double *cptr;                               // Point to const object

int *const curErr = &errNumb;              // Const pointer

const double *const pi_ptr = π        // Const pointer to a const object