Calling member function through const qualified object gives error as function is not marked as const?

Question

Code

#include <iostream>
class A
{
public:
    mutable int x;
    mutable int y;

    A(int k1 = 0, int k2 = 0) :x(k1), y(k2) {}

    void display()
    {
        std::cout << x << "," << y << "\n";
    }
};

int main()
{
    const A a1;
    a1.x = 3;
    a1.y = 8;
    a1.display();
    return 0;
}

Output

Error: 'this' argument to member function 'display' 
        has type 'const A', but function is not marked const

I am just calling member function A::display() through const qualified object a1. So why line a1.display() is giving an error ?

Answer 1

Why line a1.display() is giving an error ?

The mutable variable let you modify the member variables inside a const qualified function.

It does not allow you to be able to call the non-const qualified member function to be called via a const qualified instance. Therefore, you need a const member function there.