Const Qualifiers

Const qualifiers are used to specify how a value can change, if at all. You can write your code without ever using a const qualifier, but using them will help protect values from being changed in a way you didn’t intend.

Using Const with Primitives Types

The first and most basic way to use const is to use it with primitive types.

int main()
{
    const int num = 5;
    
    num = 4;
    
    return 0;
}

Try to compile this. You will get an error that resembles something like the following:

prog.c: In function 'main':
prog.c:5:14: error: assignment of read-only variable 'num'
num = 4;
^

Your compiler threw an error because you specified that you wanted num to be a constant. Because the compiler saw the const qualifier, when we tried to reassign num, it knew you were trying to change a constant. Had we not declared int as a const, your compiler would have no idea that num was supposed to be a constant.

Using Const with Pointers

Using const qualifiers with pointers is a bit more complicated. There are 4 options:

<type> * – The pointer address can be reassigned, and the value at that address can also be changed.

int num = 5;
int num2 = 6;
int * ptr = #
ptr = &num2; // This is perfectly legal
*ptr = 4; // This is also legal 

const <type> * – The pointer address can be reassigned, but the value at that address cannot be changed.

int num = 5;
int num2 = 6;
const int * ptr = #
ptr = &num2; // This is perfectly legal
*ptr = 4; // Illegal, cannot modify the value in the block 

<type> * const – The pointer address cannot be changed, but the memory block it points to can be modified.

int num = 5;
int num2 = 6;
int * const ptr = #
ptr = &num2; // Illegal, we cannot reassign the pointer
*ptr = 4; // Legal, we can modify the value in the block 

const <type> * const – Nothing can be changed.

int num = 5;
int num2 = 6;
const int * const ptr = #
ptr = &num2; // Illegal, we cannot reassign the pointer
*ptr = 4; // Illegal, cannot modify the value in the block 

If that didn’t make sense, that’s fine, there is a helpful table for us to reference below:

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