Constant Pointers and Pointers to Constants

In the CodeGuru newsletter, I brought up the topic of constant pointers and pointers to constants. While this is a beginning level topic, it is one that some advanced-level people goof up in their code.

Pointer contants and contant pointers are also something that many people simply don't use. If you have a value in your program and it should not change, or if you have a pointer and you don't want it to be pointed to a different value, you should make it a constant with the const keyword.

There are generally two places that the const keyword can be used when declaring a pointer. Consider the following declaration:

char A_char = 'A';
char * myPtr = &A_char;

This is a simple declaration of the variable myPtr. myPtr is a pointer to a character variable and in this case points to the character 'A'.

Now consider the following three declarations assuming that char_A has been defined as a type char variable.:Don't be confused about the fact that a character pointer is being used to point to a single character—this is perfectly legal! Not every character pointer has to point to a string.

const char * myPtr = &char_A;
char * const myPtr = &char_A;
const char * const myPtr = &char_A;

What is the difference between each of the valid ones? Do you know?

They are all three valid and correct declarations. Each assigns the addres of char_A to a character pointer. The difference is in what is constant.

The first declaration:

const char * myPtr

declares a pointer to a constant character. You cannot use this pointer to change the value being pointed to:

char char_A = 'A';
const char * myPtr = &char_A;
*myPtr = 'J';    // error - can't change value of *myPtr

The second declaration,

char * const myPtr

declares a constant pointer to a character. The location stored in the pointer cannot change. You cannot change where this pointer points:

char char_A = 'A';
char char_B = 'B';

char * const myPtr = &char_A;
myPtr = &char_B;    // error - can't change address of myPtr

The third declares a pointer to a character where both the pointer value and the value being pointed at will not change.

Pretty simple, but as with many things related to pointers, a number of people seem to have trouble.