Accessing protected members in a derived class

Question

I ran into an error yesterday and, while it\'s easy to get around, I wanted to make sure that I\'m understanding C++ right.

I have a base class with a protected memb

Answer 1

A class can only access protected members of instances of this class or a derived class. It cannot access protected members of instances of a parent class or cousin class.

In your case, the Derived class can only access the b protected member of Derived instances, not that of Base instances.

Changing the constructor to take a Derived instance will solve the problem.

Answer 2

As mentioned, it's just the way the language works.

Another solution is to exploit the inheritance and pass to the parent method:

class Derived : public Base
{
  protected:
    int d;
  public:
    void DoSomething(const Base& that)
    {
      Base::DoSomething(that);
      d=0;
    }
};