Just like normal variables, pointers can be declared constant. There are two different ways that pointers and const can be intermixed, and they are very easy to mix up.
To declare a const pointer, use the const keyword between the asterisk and the pointer name:
int nValue = 5; int *const pnPtr = &nValue;
Just like a normal const variable, a const pointer must be initialized to a value upon declaration, and its value can not be changed. This means a const pointer will always point to the same value. In the above case, pnPtr will always point to the address of nValue. However, because the value being pointed to is still non-const, it is possible to change the value being pointed to via dereferencing the pointer:
*pnPtr = 6; // allowed, since pnPtr points to a non-const int
It is also possible to declare a pointer to a constant variable by using the const before the data type.
int nValue = 5; const int *pnPtr = &nValue;
Note that the pointer to a constant variable does not actually have to point to a constant variable! Instead, think of it this way: a pointer to a constant variable treats the variable as constant when it is accessed through the pointer.
Thus, the following is okay:
nValue = 6; // nValue is non-const
But the following is not:
*pnPtr = 6; // pnPtr treats its value as const
Because a pointer to a const value is a non-const pointer, the pointer can be redirected to point at other values:
int nValue = 5; int nValue2 = 6; const int *pnPtr = &nValue; pnPtr = &nValue2; // okay
Confused? To summarize:
- A non-const pointer can be redirected to point to other addresses.
- A const pointer always points to the same address, and this address can not be changed.
- A pointer to a non-const value can change the value it is pointing to.
- A pointer to a const value treats the value as const (even if it is not), and thus can not change the value it is pointing to.
Finally, it is possible to declare a const pointer to a const value:
const int nValue; const int *const pnPtr = &nValue;
A const pointer to a const value can not be redirected to point to another address, nor can the value it is pointing to be changed.
Const pointers are primarily used for passing variables to functions. We will discuss this further in the section on functions.
|6.11 — References|
|6.9 — Dynamic memory allocation with new and delete|