const qualifier

Usage of ‘const’:

const int some_object = 5 ;
int other_object = 37;
int *const y = &fixed_object;
const volatile *const z = &some_object;
int *const volatile w = &some_object;

The variable y in these declarations is declared as a constant pointer to an int value. The value it points to can be modified, but the pointer itself must always point to the same location: the address of fixed_object. Similarly, z is a constant pointer, but it is also declared to point to an int whose value cannot be modified by the program. The additional specifier volatile indicates that although the value of the const int pointed to by z cannot be modified by the program, it could legitimately be modified by a process running concurrently with the program. The declaration of w specifies that the program cannot change the value pointed to and that the program cannot modify the pointer.

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’.

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.

Now consider the following three declarations assuming that char_A has been defined as a type char variable.:

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.


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