Classes & pure virtual - c++

Question

I have 4 classes.
class A, class B, class C, class D
Class C includes class A and class B and reference them:
The Header File:
class C
{
private:
A &a;
B &b;
int x;
int y;
int energy;
public:
C(A &a, B &b);
void print(void);
virtual void printAt(void);
CPP File includes:
void C::printAt(void)
{
// move cursor to the current x, y coordinates
b.gotoXY(x,y);
}
In class D, I make class C a friend class by (class D : public class C...)
Then I have a void printAt(void).
This all works, but how do I access the b class attributes (b.gotoXY..) from class D?
Hopefully this makes Sence.

Answer 1

Just put them in protected section:
class C {
protected:
A &a;
B &b;
...
};
NOTE: It has nothing to do with virtual methods.

Answer 2

The reason you cannot access them from D is because they are private, which means they are only accessible from within D itself. In order to be able to access them only from D or its subclasses, you need to use the protected access modifier instead:
class C
{
private:
int x;
int y;
int energy;
protected:
A &a;
B &b;
public:
C(A &a, B &b);
void print(void);
virtual void printAt(void);
/// ...
};
Now, a bit of terminology:
When you type class C : public D you are not making it a friend, you are inheriting from it. This means C will be a base class of D. A friend is another, related concept.
A friend of some class is another class which has access to its private properties. So, if you instead had made D a friend of C, you would have had access to a and b without having to make them protected. This would be accomplished as such:
class C
{
// Some code...
friend D;
//Lots of code ...
}
Please note that, for this to work, you need to declare D before C.
Now, which of these options should you use?
Ask yourself this question: is a D logically a more specific type of C? If so, it should use inheritance. If not, it may be better to make D have a member of type C and use the friend keyword. In either case, use friend sparingly, and only if there is necessarily a very tight relationship between the two classes (perhaps if D is a factory for type C and C has a private constructor.)

Answer 3

You should make intended members protected or make their classes friend to your class.
In addition, I feel you will have a problem when you instantiating an object from C because of uninitialized references.
class C
{
private:
A &a;
B &b;
// ...
public:
C(A &a, B &b) : a(a), b(b)
^^^^^^^^^^^^
// ...
};

Answer 4

when you want other class in inherit access to your attributes .dont private them
so you can choose protected or public.
for more detail you can go http://www.learncpp.com/cpp-tutorial/115-inheritance-and-access-specifiers/
for solve problem try below code
class C
{
protected://or public
A &a;
B &b;
int x;
int y;
int energy;
public:
C(A &a, B &b);
void print(void);
virtual void printAt(void);
and in class D
class D:public C
{
public:
void printAt(void);
};