c++ - How to access variables in different class from other class?

Question

Let just say that we have two classes, A and B.
Here is code for both of them

class A
{
public:
    int x;
};

class B
{
public:
    int y;
    void FindY() { y = x + 12; }
};

void something()
{
    A fs;
    B fd;
    fs.x = 10;
    fd.FindY();
}

the problem is that i want to access x but i don't wanna pass anything as argument to my function i look at friend and inheritance but both didn't seem to help me, correct me if i'm wrong.
some how i need to find x in function FindY().
I'm going with the static method but in my case i get this error.

Error 2 error LNK2001: unresolved external symbol "public: static class std::vector<class GUIDialog *,class std::allocator<class GUIDialog *> > Window::SubMenu" (?SubMenu@Window@@2V?$vector@PAVGUIDialog@@V?$allocator@PAVGUIDialog@@@std@@@std@@A) C:UsersOwnerdocumentsvisual studio 2010ProjectsMonopolyMonopolyWindow.obj Here is how i declared it

static vector<GUIDialog *> SubMenu;

I get that error because of this line

SubMenu.resize(3);

Answer 1

Three different approaches:

1. Make B::FindY take an A object as a parameter

   class B {
   public:
     void FindY(const A &a) { y = a.x + 12; }
   };
   

2. Make A::x static

   class A {
   public:
     static int x;
   };
   class B {
   public:
     void FindY() { y = A::x + 12; }
   };
   

3. Make B inherit from A.

   class B : public A {
   public:
     void FindY() { y = x + 12; }
   };
   

CashCow also points out more ways to do this in his answer.