Why can't I access a protected member from an instance of a derived class?

Question

I haven't done C++ in a while and can't figure out why following doesn't work:

class A {
protected:
  int num;
};

class B : public A {
};

main () {
  B * bclass = new B ();
  bclass->num = 1;
}

Compiling this produces:

error C2248: 'A::num' : cannot access protected member declared in class 'A'

Shouldn't protected members be accessible by derived classes?

What am I missing?

Answer 1

yes protected members are accessible by derived classes but you are accessing it in the main() function, which is outside the hierarchy. If you declare a method in the class B and access num it will be fine.

Answer 2

Yes, protected members are accessible by the derived class, but only from within the class.

example:

#include <iostream>

class A { 
   protected:
   int num;
};

class B : public A {    public:
   void printNum(){
      std::cout << num << std::endl;
   }

};

main () {
   B * bclass = new B ();
   bclass->printNum();
}

will print out the value of num, but num is accessed from within class B. num would have to be declared public to be able to access it as bclass->num.

Answer 3

It is accessible within the scope of B's functions, but you are attempting to access it in main.

Answer 4

But you're not accessing it from the derived class. You're accessing it from main().