How can a derived class use a protected member of the base class?

Question

Say that a base class A defines a protected member. A derived class B uses this member.

class A
{
public:
  A(int v) : value(v) { }         


        

Answer 1

There is actually a loophole using member pointers (no casting, no copying):

void B::compare_and_print(const A& other) const
{
  auto max_value = std::max(value, other.*(&B::value));
  std::cout << "Max value: " << max_value << "\n";
}

Answer 2

You can bypass protected by using a helper struct:

struct A_Helper : public A
{
    static int GetProtectedValue(const A & a)
    {
        return static_cast<const A_Helper&>(a).Value;
    }
};

You can get it by using it anywhere A_Helper::GetProtectedValue(a)

In your case, you could cast other to const B& (via static_cast or reinterpret_cast) but you don't know if instance of other is of type B. With that casted value people reading the code would presume that other is of type B and could insert code that causes reads/writes to "random" memory.

Consider that class B has another memeber value_B and other is of type C. Using static_cast<const B&>(other).value_B is Undefined Behavior.