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 const A a1 . So why line a1.display() is giving an error ?

CodePudding user response：

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.