Question

What are the rules for a subclass accessing protected member objects? I thought I understood them but my code disagrees.

I have a base class, defined as

class Datum {
public:
   Datum(Datum *r, Datum *l) : right(r), left(l) {}

protected:
   Datum *right, *left;
};

I subclass Datum as follows:

class Column: public Datum
{
public:
   Column(Datum *r, Datum *l, string n, int s): Datum(r,l), name(n), size(s) {}

   void cover() {
      right->left = left;
      left->right = right;
   }

protected:
   string name;
   int size;
};

When I compile, using G++ v.4.5.1, I get the error messages pointing to the two lines in cover:

error: Datum* Datum::right is protected
error:  within this context
error: Datum* Datum::left is protected
error:  within this context

Obviously, making the section public causes the errors to go away. Why are they there when the section is protected?

Answer 1

The problem is Datum::Datum *right is protected. Pointer right is accessible / assignable because the base class is inherited publicly. But the object pointed by right has no access previliges the way you are trying to in the member function Column::cover() because the object pointed by right has no direct relation in the current scope of the derived class.

 void cover() {
      // Not posssible right->left = left;

      // But -
       right = new Datum(); // Assuming the class has default constructor.
 }

Similar is the problem for the Datum::Datum *left too.