当成为朋友的类具有相同的名称成员变量时会发生什么

Question

In C++, what happens when I have the following

class House
{
public:
    House();
    ~House();

private:
    int* m_peopleInside;

friend class Room;
}; 

and then in the constructor of House this is set

m_peopleInside = new int[5];
m_peopleInside[4] = 2;

and

class Room
{
public:
    Room();
    ~Room();

    Update();

private:
    int* m_peopleInside;
}; 

Then in the Room.Update() I use m_peopleInside something like this.

&m_peopleInside[4];

It's my understanding that the friend class will allow the Room class to access private members of the House class. So which m_peopleInside would be used?

I should add that in this case, m_peopleInside is being used as an array.

Answer 1

It's an instance variable. So it needs an instance to act on. If no instance is provided, then it is the same as this->m_peopleInside , which means it refers to the instance on which the function was called. So, for example, if this is your function:

void Room::Update() {
    // these two are the same, they null the member of the Room object
    m_peopleInside = nullptr;
    this->m_peopleInside = nullptr;

    House h;
    // should be pretty obvious what this does
    h.m_peopleInside = nullptr;
}

Answer 2

It's my understanding that the friend class will allow the Room class to access private members of the House class.

That is correct.

So which m_peopleInside would be used?

To access the m_peopleInside member of a House object, you will need an object or pointer of type House .

In Room::update() , if you simply use m_peopleInside , it will be member variable of Room , not House .

Answer 3

When you use "m_peopleInside" inside "Room.Update()" you will definitely use the data member of "Room". Your understanding of "friend" classes is not so correct. To make it clear, suppose that you have an object "x" from the class "House" in one of the methods of the class "Room', like "Update()" for example. Then, the following code is correct in this method:

cout << x.m_peopleInside;

Although "m_peopleInside" is private in "House", it is accessible from Room's methods, because the class "House" declares that "Room" is a friend of his.