Why do we use const qualifier in C++?

We use the const qualifier to declare a variable as constant. That means that we cannot change the value once the variable has been initialized. Using const has a very big benefit. For example, if you have a constant value of the value of PI, you wouldn't like any part of the program to modify that value. So you should declare that as a const.

Objects declared with const-qualified types may be placed in read-only memory by the compiler, and if the address of a const object is never taken in a program, it may not be stored at all.  For example,

#include<iostream>
using namespace std;
int main() {
   const int x = 10;
   x = 12;
   return 0;
}

This program would produce an error as we have tried to reassign a const value.