Question

How this code modifies a value in a const function:

#include<iostream>
using namespace std;
class Y;
class X{
  public:
    void access(Y &y) const;
};

class Y{
    int d;
  public:
    friend void X::access(Y &y) const;
    Y(){d=0;}
}; 

void X::access(Y &y) const 
{
    cout<<"Y is "<<y.d<<endl;
    y.d=1;
    cout<<"Y is "<<y.d<<endl;
}

int main() {
    X x;
    Y y;
    x.access(y);
}

And this code gives an error:

#include<iostream>

using namespace std;

class Y{
    int d;
  public:
    void set() const{d=0;}
};                          

int main() {
    Y y1;
    return 0;
}

Answer 1

Because inside a const function, only *this is const, not the parameters passed to the function or anything else.