"Identifier is undefined" error in accessing "protected" data in sub class - c++

Question

Please have a look at the following code
GameObject.h
#pragma once
class GameObject
{
protected:
int id;
public:
int instances;
GameObject(void);
~GameObject(void);
virtual void display();
};
GameObject.cpp
#include "GameObject.h"
#include <iostream>
using namespace std;
static int value=0;
GameObject::GameObject(void)
{
value++;
id = value;
}
GameObject::~GameObject(void)
{
}
void GameObject::display()
{
cout << "Game Object: " << id << endl;
}
Round.h
#pragma once
#include "GameObject.h"
class Round :
public GameObject
{
public:
Round(void);
~Round(void);
};
Round.cpp
#include "Round.h"
#include "GameObject.h"
#include <iostream>
using namespace std;
Round::Round(void)
{
}
Round::~Round(void)
{
}
void display()
{
cout << "Round Id: " << id;
}
I am getting 'id' : undeclared identifier error in Round class. Why is this? Please help!

Answer 1

In this function:
void display()
{
cout << "Round Id: " << id;
}
You are trying to access a variable named id inside a non-member function. The compiler cannot resolve that name, because id is not the name of any global nor local variable, therefore you get an error complaining that the identifier was not declared.
If you meant to make display() a member function of Round(), you should have declared it as such:
class Round : public GameObject
{
public:
Round(void);
~Round(void);
void display(); // <==
};
And defined it this way:
void Round::display()
// ^^^^^^^
{
...
}
This way, function Round::display() would override the virtual function GameObject::display().

Answer 2

You declared a globally scoped method named display in your Round.cpp file. Edit your header and cpp like this:
Round.h
#pragma once
#include "GameObject.h"
class Round :
public GameObject
{
public:
Round(void);
virtual ~Round(void);
virtual void display(void);
};
Round.cpp
#include "Round.h"
#include "GameObject.h"
#include <iostream>
using namespace std;
Round::Round(void)
{
}
Round::~Round(void)
{
}
void Round::display()
{
cout << "Round Id: " << id;
}
Note - you should make the destructor in GameObject virtual