const_cast setting a rule and breaking it for function const

Question

In the following example I found on the net, it is mentioned that one of the advantages of const_cast is that it allow a constant function changes the class members. It is a question to me. Why should we set a rule for a function by const and then breaking that rule with const_cast? Isn't it like a cheating? Wouldnt it better to not set const for the function at all?

#include <iostream>
using namespace std;

class student
{
private:
    int roll;
public:

    student(int r):roll(r) {}

    // A const function that changes roll with the help of const_cast
    void fun() const
    {
        ( const_cast <student*> (this) )->roll = 5;
    }

    int getRoll()  { return roll; }
};

int main(void)
{
    student s(3);
    cout << "Old roll number: " << s.getRoll() << endl;

    s.fun();

    cout << "New roll number: " << s.getRoll() << endl;

    return 0;
}

reference

Answer 1

That is indeed a bad idea. As well as lying about the function's behaviour, it allows you to modify a member of a constant student object, giving undefined behaviour.

In general, a member function should be const if, and only if, it doesn't modify the object's observable state. So in this case, it should not be const.

Sometimes, you might want specific members to be modifiable in a function that doesn't otherwise cause an observable change; for example, locking a mutex to access shared data, or caching the result of a complex calculation. In that case, declare those members mutable, so that the rest of the class is still protected by const-correctness.