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          


        

Answer 1

When utilizing a class, there really is no difference between protected and private members. Neither are accessible to anything that utilizes the class.

class A {
    private: int privateNum;

    protected: int protectedNum;

    public:  int publicNum;

    void SetNumbers(int num) {
        privateNum = num; //valid, private member can be accessed in member function
        protectedNum = num; //valid, protected member can be accessed in member function
    }
};

void main() {
    A classA;
    classA.privateNum = 1; //compile error can't access private member
    classA.protectedNum = 1; //compile error can't access protected member
    classA.publicNum = 1; //this is OK
    classA.SetNumbers(1);  //this sets the members not accessible directly
 }

The difference comes into effect when you inherit from a class with protected members.

class B : public A {
};

All private members of a base class are still private, and will not be accessible to the derived class. The protected members, on the other hand, are accessible to the inherited class, but are still not accessible outside of the inherited class.

class B : public A {
 public:
   void SetBNumbers(int num) {
       privateNum = num; //compile error, privateNum can only be accessed by members of A, not B
       protectedNum = num; //this works, as protected members can be accessed by A and B
   }
};

void main() {
  B classB;
  classB.publicNum = 1; //valid, inherited public is still public
  classB.protectedNum = 1; //compile error, can't access protected member
  classB.privateNum = 1; //compile error, B doesn't know that privateNum exists
  classB.SetBNumbers(1); //this sets the members not accessible directly
}