subtle C++ inheritance error with protected fields

Question

Below is a subtle example of accessing an instance\'s protected field x. B is a subclass of A so any variable of type B is also of type A. Why can B::foo() access b\'s x fie

Answer 1

In Public Inheritance:
All Public members of the Base Class become Public Members of the derived class &
All Protected members of the Base Class become Protected Members of the Derived Class.

As per the above rule:
protected member x from A becomes protected member of class B.

class B can access its own protected members in its member function foo but it can only access members of A through which it was derived not all A classes.

In this case, class B contains a A pointer a, It cannot access the protected members of this contained class.

Why can the B::foo() access the members of the contained class B pointer b?

The rule is:
In C++ access control works on per-class basis, not on per-object basis.
So an instance of class B will always have access to all the members of another instance of class B.

An Code Sample, which demonstrates the rule:

#include

class MyClass 
{
    public: 
       MyClass (const std::string& data) : mData(data) 
       {
       }

       const std::string& getData(const MyClass &instance) const 
       {
          return instance.mData;
       }

    private:
      std::string mData;
};

int main() {
  MyClass a("Stack");
  MyClass b("Overflow");

  std::cout << "b via a = " << a.getData(b) << std::endl;
  return 0;
}