const member and assignment operator. How to avoid the undefined behavior?

Question

I answered the question about std::vector of objects and const-correctness, and received a comment about undefined behavior. I do not agree and therefore I have a question.<

Answer 1

If you definitely want to have an immutable (but assignable) member, then without UB you can lay things out like this:

#include 

class ConstC
{
    int c;
protected:
    ConstC(int n): c(n) {}
    int get() const { return c; }
};

class A: private ConstC
{
public:
    A(int n): ConstC(n) {}
    friend std::ostream& operator<< (std::ostream& os, const A& a)
    {
        return os << a.get();
    }
};

int main()
{
    A first(10);
    A second(20);
    std::cout << first << ' ' << second << '\n';
    first = second;
    std::cout << first << ' ' << second << '\n';
}